fix(align): calibrated fit (precision, lift, beat-shift check, baseline) and aligner guards (A7, A8)

Fit flags on precision (<0.6) and chance-corrected lift (<0.25), null with <8 recording onsets; old match rate kept as audibleRate (info). Symbolic-chroma beat/bar-shift check per window. Constant-offset baseline (-2..+2 s) replaces DTW without manual anchors unless DTW precision is better by >=0.02. Stretch warning counts in the fit summary. DTW: slope-limited steps, open begin/end options; per-bar outlier rejection (1/4 beat, Theil-Sen-transported +-3 bar median); no anchor in the final partial bar. bench real: saved/computed source, baseline + method, corrupt alignment.json skipped. Parabolic onset interpolation; warp extrapolation documented; spec amended. Co-Authored-By: Claude Opus 5.5 <[email protected]>
2026-10-30 19:50:48 +01:00
parent 3b002059e9
commit 3db87c0317
19 changed files with 1016 additions and 252 deletions
@@ -77,5 +77,6 @@ This writes `results/real-<id>.md` and `.json` (committed) and per-song output u

 Caveats:

- The fit summary (matched MIDI onsets, median residual, flagged windows) is a consistency measure between the MIDI and the audio, not ground truth; a high match rate does not prove the alignment is right.
+- The fit summary is a consistency measure between the MIDI and the audio, not ground truth: *precision* (share of the recording's detected attacks with a warped MIDI onset within ±70 ms; a 30 s window is flagged below 60 %), *lift* (precision minus its off-grid chance level; flagged below 0.25), a chroma check that flags suspected whole-beat/bar shifts, "MIDI onsets audible as attacks" (information only: a full mix exposes far fewer attacks than the score has notes), and the number of warned stretches (> 5 % and > 15 % tempo change). A high precision does not prove the alignment is right.
+- `align auto` also reports a constant-offset baseline (best single offset, −2..+2 s); with no manual anchors it keeps the baseline (one auto anchor) unless the DTW alignment beats its precision by 2 points.
 - The page plays Ogg Opus; Safari needs 18.4 or later.
@@ -93,7 +93,7 @@ aligned MIDI.
  starting from the raw MIDI; a "reset to raw MIDI" action exists.
 - **Fit quality:** median absolute onset residual (ms) between matched recording
  onsets and warped MIDI onsets, overall and per 30 s window; shown on the page
-  and in benchmark output.
+  and in benchmark output. (Amended 2026-10-30: see "Fit quality — Amended" below.)
 - **Playback of the warped MIDI:** the server re-renders the warped MIDI audio
  when anchors change (alphaTab renders a 4-minute song in ~1–2 s); the page
  requests it on Play. Waveforms are sent as precomputed peaks, not full audio.
@@ -118,6 +118,49 @@ aligned MIDI.
  match rate = share of warped MIDI onsets with a recording onset within ±70 ms,
  overall and per 30 s window. A window is flagged "don't trust" when its match
  rate < 0.5 or median residual > 60 ms; the page highlights flagged windows.
+  *(Superseded by the amendment below.)*
+- **Fit quality — Amended 2026-10-30 after real-song evidence (rulings A7, A8).**
+  On real full mixes the onset picker finds ~1.4 attacks/s while a scored MIDI
+  has ~13 unique onsets/s, so the match rate above caps near 40 % even when
+  perfectly aligned (every window was flagged on every song), and onset metrics
+  cannot see whole-beat shifts (they alias onto the grid).
+  - **Flag metric = precision:** share of recording onsets (onset flux, k = 4,
+    parabolic sub-frame peak times) with a warped scored-role MIDI onset within
+    ±70 ms; per 30 s window by recording time. A window with < 8 recording
+    onsets is null and never flagged; otherwise flagged when precision < 0.6.
+  - **Chance-corrected lift:** precision minus the mean precision with the
+    warped MIDI shifted by +0.11, +0.23 and +0.37 s (off-grid); reported per
+    window and overall; a window is also flagged when lift < 0.25.
+  - **Beat/bar-shift check:** per window, mean chroma cosine between the
+    recording (features chroma, 10 Hz) and a symbolic MIDI chroma (piano roll of
+    the warped pitched scored-role notes, no render) at the current alignment
+    vs the MIDI moved by ±1, ±2 beats and ±1 bar along the MIDI's own
+    tempo/time-signature grid. Flagged "beat shift suspected: ±k beat(s)/bar"
+    when a shift beats the current alignment by > 0.05 (the margin keeps
+    repetitive harmony, where shifts tie, from flagging). "+1 beat" means the
+    MIDI content belongs one beat later in the recording.
+  - **Information only:** the old match rate, renamed "MIDI onsets audible as
+    attacks" (JSON `audibleRate`), and the median residual (recording onsets →
+    nearest warped MIDI onset).
+  - **Stretch warnings:** the fit summary (CLI, page, real report) includes the
+    number of warned stretches (> 5 % tempo change) and of stretches with
+    |tempo change| > 15 %.
+  - **Constant-offset baseline:** grid search −2..+2 s in 10 ms steps for the
+    offset maximising overall precision; precision peaks within 0.1 of the best
+    are told apart by the symbolic chroma cosine (beat aliasing), and within a
+    peak the offset with the smallest mean residual is taken. Reported with its
+    precision. Without manual anchors, `autoAlign` returns the baseline as one
+    auto anchor (`chosen: "baseline"`) unless the DTW alignment's precision
+    beats it by ≥ 0.02 (`chosen: "dtw"`); with manual anchors there is never a
+    substitution.
+  - **Aligner guards:** both DTW passes use slope-limited steps (Sakoe–Chiba
+    P = 1: (1,1), (1,2), (2,1)), so the path cannot jump; the first segment may
+    start late in the recording (open begin) and the final segment uses an open
+    end (the path may stop before the recording ends, or before the MIDI ends,
+    each unconsumed MIDI frame costing 1), so a length mismatch is not absorbed
+    as a tempo ramp. Per-bar offsets more than ¼ beat from the median of their
+    ±3 neighbouring bars are rejected (iterated until stable), and no auto
+    anchor is placed in the MIDI's final partial bar.
 - **Editing model:** dragging an anchor only re-fits its two neighbouring
  stretches. Adding or moving a manual anchor deletes the auto anchors in the two
  affected stretches (those stretches become plain uniform scaling) until the user
@@ -2,12 +2,12 @@
 // Pure: no I/O, no Bun/process/fs. The SoundFont bytes are passed in.
 import type { Audio, Role } from "../core/types.ts";
 import type { Smf } from "./smf.ts";
-import { makeWarp, type Anchor, type Warp } from "./warp.ts";
+import { makeWarp, stretches, type Anchor, type Warp } from "./warp.ts";
 import { renderMidi } from "./render.ts";
 import { features, pickOnsets, type Features } from "./features.ts";
-import { dtw } from "./dtw.ts";
+import { dtw, type DtwOptions } from "./dtw.ts";
 import { trackKeys } from "./roles.ts";
-import { fitQuality, type FitReport } from "./fit.ts";
+import { MATCH_TOL_SEC, fitQuality, meanCosine, precisionOf, uniqueSorted, type FitReport, type ShiftInputs, type StretchCounts } from "./fit.ts";

 const FINE_RATE = 50;
 const COARSE_FACTOR = 5; // coarse pass at 10 Hz
@@ -16,10 +16,23 @@ const FINE_BANDS_SEC = [0.5, 1, 2]; // fine pass band half-widths tried in turn
 const ANCHOR_HALF_WIN_SEC = 1; // path points within ±1 s (warped time) of a bar estimate its offset
 const MIN_POINTS = 10; // diagonal path points required to place a bar anchor
 const MANUAL_CLEARANCE_SEC = 1; // no auto anchor within 1 s (MIDI time) of a manual anchor
+const OUTLIER_BEATS = 0.25; // per-bar offsets further than ¼ beat from the robust median of ±3 neighbours are rejected
+const OUTLIER_NEIGHBOURS = 3;
+const BASELINE_RANGE_SEC = 2, BASELINE_STEP_SEC = 0.01; // constant-offset baseline grid
+const BASELINE_MARGIN = 0.02; // DTW must beat the baseline's precision by this much to be kept (no manual anchors)
+const BASELINE_PLATEAU = 0.05; // within a precision peak, offsets this close to its best are refined by residual
+const BASELINE_TIE = 0.1; // precision peaks this close to the best are told apart by symbolic chroma (beat aliasing)
+const OVER_TEMPO = 0.15; // stretches with |tempo change| above this are counted separately in the fit
 const ZERO = 1e-9;

-/** Bar start times (MIDI seconds) from the tempo map and time signatures (4/4 when none), up to durationSec. */
-export function barTimes(smf: Smf): number[] {
+export interface MeterGrid { beats: number[]; bars: number[]; /** end of the last bar */ end: number; /** the MIDI ends before its last bar does */ lastBarPartial: boolean }
+
+/**
+ * Beat and bar start times (MIDI seconds) from the tempo map and time signatures (4/4 when none; a beat is one
+ * denominator note), for bars starting before `untilSec` (default: the MIDI duration). A signature change mid-bar
+ * restarts the bar there.
+ */
+export function meterGrid(smf: Smf, untilSec = smf.durationSec): MeterGrid {
  const tempos = smf.tempoMap.length ? [...smf.tempoMap].sort((a, b) => a.sec - b.sec) : [{ sec: 0, bpm: 120 }];
  const sigs = [...smf.timeSigs].sort((a, b) => a.sec - b.sec);
  const bpmAt = (t: number) => { let b = tempos[0]!.bpm; for (const p of tempos) if (p.sec <= t + ZERO) b = p.bpm; return b; };
@@ -34,19 +47,78 @@ export function barTimes(smf: Smf): number[] {
    }
    return t;
  };
-  const out: number[] = [];
+  const beats: number[] = [], bars: number[] = [];
  let t = 0;
-  while (t < smf.durationSec - ZERO && out.length < 100000) {
-    out.push(t);
+  while (t < untilSec - ZERO && bars.length < 100000) {
+    bars.push(t);
    let num = 4, den = 4;
    for (const s of sigs) if (s.sec <= t + ZERO) { num = s.num; den = s.den; }
-    let next = advance(t, (num * 4) / den);
-    const change = sigs.find((s) => s.sec > t + ZERO && s.sec < next - ZERO); // signature change mid-bar: restart there
-    if (change) next = change.sec;
-    if (!(next > t)) break;
-    t = next;
+    const change = sigs.find((s) => s.sec > t + ZERO)?.sec ?? Infinity;
+    let b = t;
+    for (let k = 0; k < num; k++) {
+      beats.push(b);
+      const nb = advance(b, 4 / den);
+      if (nb >= change - ZERO) { b = change; break; } // signature change mid-bar: restart there
+      b = nb;
+    }
+    if (!(b > t)) break;
+    t = b;
+  }
+  return { beats, bars, end: t, lastBarPartial: bars.length > 0 && t > smf.durationSec + 1e-3 };
+}
+
+/** Bar start times (MIDI seconds), up to durationSec. */
+export const barTimes = (smf: Smf): number[] => meterGrid(smf).bars;
+/** Beat start times (MIDI seconds) of the bars starting before durationSec. */
+export const beatTimes = (smf: Smf): number[] => meterGrid(smf).beats;
+
+/** Time `k` grid steps after `t` on a sorted grid (fractional positions; linear extrapolation outside it). */
+function gridShift(grid: number[], t: number, k: number): number {
+  const n = grid.length;
+  if (n < 2) return t;
+  const posOf = (x: number): number => {
+    if (x <= grid[0]!) return (x - grid[0]!) / (grid[1]! - grid[0]!);
+    if (x >= grid[n - 1]!) return n - 1 + (x - grid[n - 1]!) / (grid[n - 1]! - grid[n - 2]!);
+    let lo = 0, hi = n - 1;
+    while (hi - lo > 1) { const m = (lo + hi) >> 1; if (grid[m]! <= x) lo = m; else hi = m; }
+    return lo + (x - grid[lo]!) / (grid[hi]! - grid[lo]!);
+  };
+  const timeOf = (p: number): number => {
+    if (p <= 0) return grid[0]! + p * (grid[1]! - grid[0]!);
+    if (p >= n - 1) return grid[n - 1]! + (p - n + 1) * (grid[n - 1]! - grid[n - 2]!);
+    const i = Math.floor(p);
+    return grid[i]! + (p - i) * (grid[i + 1]! - grid[i]!);
+  };
+  return timeOf(posOf(t) + k);
+}
+
+const med = (xs: number[]): number => { const s = [...xs].sort((a, b) => a - b), h = s.length >> 1; return s.length % 2 ? s[h]! : (s[h - 1]! + s[h]!) / 2; };
+
+/**
+ * Per-bar offsets (recording − warped, s) at MIDI time `midiSec`, with each bar's beat length (s): iteratively drops
+ * the bar deviating most from the robust median of its ±3 kept neighbours' offsets, while that deviation exceeds ¼
+ * beat. The neighbours' offsets are transported to the bar along their median pairwise (Theil–Sen) slope, so a smooth
+ * ramp (a genuine tempo difference) is kept whole instead of being eroded from its ends, where a plain one-sided median
+ * lags the ramp; beat/bar jumps are still rejected.
+ */
+export function rejectOffsetOutliers<T extends { midiSec: number; off: number; beatSec: number }>(bars: T[]): T[] {
+  const kept = [...bars];
+  for (;;) {
+    let worst = -1, worstDev = 0;
+    for (let k = 0; k < kept.length; k++) {
+      const nb = [...kept.slice(Math.max(0, k - OUTLIER_NEIGHBOURS), k), ...kept.slice(k + 1, k + 1 + OUTLIER_NEIGHBOURS)];
+      if (!nb.length) continue;
+      const slopes: number[] = [];
+      for (let a = 0; a < nb.length; a++) for (let b = a + 1; b < nb.length; b++) slopes.push((nb[b]!.off - nb[a]!.off) / (nb[b]!.midiSec - nb[a]!.midiSec));
+      const slope = slopes.length ? med(slopes) : 0;
+      const x = kept[k]!.midiSec;
+      const pred = med(nb.map((b) => b.off + slope * (x - b.midiSec)));
+      const dev = Math.abs(kept[k]!.off - pred) / kept[k]!.beatSec;
+      if (dev > OUTLIER_BEATS && dev > worstDev) { worst = k; worstDev = dev; }
+    }
+    if (worst < 0) return kept;
+    kept.splice(worst, 1);
  }
-  return out;
 }

 /** Aggregate 50 Hz features by `k` frames (centred): chroma energy summed and re-normalised, onset max, re-z-scored. */
@@ -91,19 +163,38 @@ function costFn(A: Features, B: Features): (i: number, j: number) => number {
  };
 }

-type Path = [number, number][];

-/** Align rendered frames [i0, i1] with recording frames [j0, j1] (inclusive, fine rate), coarse then banded fine. */
-function alignSegment(fine: [Features, Features], coarse: [Features, Features], i0: number, i1: number, j0: number, j1: number, bandsSec: number[]): Path {
+type Path = [number, number][];
+type Open = { openBegin: boolean; openEnd: boolean };
+
+/** Slope-limited DTW; falls back to the unconstrained symmetric steps when the slope limit makes the end unreachable. */
+function dtwSlope(costAt: (i: number, j: number) => number, n: number, m: number, open: Open, band?: { center: (i: number) => number; width: number }): Path {
+  const opts: DtwOptions = { steps: "slope", ...open };
+  try {
+    return dtw(costAt, n, m, band, opts);
+  } catch (e) {
+    if (band || !(e instanceof Error) || !/unreachable/.test(e.message)) throw e;
+    return dtw(costAt, n, m, undefined, { ...open, steps: "symmetric" });
+  }
+}
+
+/**
+ * Align rendered frames [i0, i1] with recording frames [j0, j1] (inclusive, fine rate), coarse then banded fine, both
+ * slope-limited. `open.openBegin`/`openEnd` let the path start after j0 / stop before j1 (or before i1).
+ */
+function alignSegment(fine: [Features, Features], coarse: [Features, Features], i0: number, i1: number, j0: number, j1: number, bandsSec: number[], open: Open): Path {
  const K = COARSE_FACTOR;
  const ci0 = Math.round(i0 / K), ci1 = Math.max(ci0, Math.min(coarse[0].chroma.length - 1, Math.round(i1 / K)));
  const cj0 = Math.round(j0 / K), cj1 = Math.max(cj0, Math.min(coarse[1].chroma.length - 1, Math.round(j1 / K)));
  const cc = costFn(coarse[0], coarse[1]);
-  const cp = dtw((i, j) => cc(ci0 + i, cj0 + j), ci1 - ci0 + 1, cj1 - cj0 + 1);
-  // coarse map: mean recording frame per rendered frame, in segment-relative coarse units
-  const sum = new Float64Array(ci1 - ci0 + 1), cnt = new Float64Array(ci1 - ci0 + 1);
+  const cp = dtwSlope((i, j) => cc(ci0 + i, cj0 + j), ci1 - ci0 + 1, cj1 - cj0 + 1, open);
+  // coarse map: mean recording frame per rendered frame, in segment-relative coarse units; rows the path does not
+  // reach (open end on the last column) continue at slope 1
+  const nRows = ci1 - ci0 + 1;
+  const sum = new Float64Array(nRows), cnt = new Float64Array(nRows);
  for (const [i, j] of cp) { sum[i]! += j; cnt[i]! += 1; }
-  const cmap = Array.from(sum, (s, i) => s / cnt[i]!);
+  const cmap = Array.from(sum, (v, i) => (cnt[i] ? v / cnt[i]! : NaN));
+  for (let i = 1; i < nRows; i++) if (Number.isNaN(cmap[i]!)) cmap[i] = cmap[i - 1]! + 1;
  const centre = (i: number): number => { // fine row (segment-relative) -> fine column (segment-relative)
    const x = Math.min(cmap.length - 1, Math.max(0, (i0 + i) / K - ci0));
    const a = Math.floor(x), b = Math.min(cmap.length - 1, a + 1);
@@ -111,7 +202,7 @@ function alignSegment(fine: [Features, Features], coarse: [Features, Features],
    return (y + cj0) * K - j0;
  };
  const fc = costFn(fine[0], fine[1]);
-  const p = bandedWithRetry((i, j) => fc(i0 + i, j0 + j), i1 - i0 + 1, j1 - j0 + 1, centre, bandsSec);
+  const p = bandedWithRetry((i, j) => fc(i0 + i, j0 + j), i1 - i0 + 1, j1 - j0 + 1, centre, bandsSec, open);
  if (p) return p.map(([i, j]) => [i0 + i, j0 + j]);
  return densify(cp, ci0, cj0, i0, i1, j0, j1);
 }
@@ -119,19 +210,20 @@ function alignSegment(fine: [Features, Features], coarse: [Features, Features],
 /**
 * Fallback: the coarse path expanded to a contiguous fine path from (i0, j0) to (i1, j1). Each coarse step becomes K
 * fine steps in the same direction (so coarse diagonals stay diagonal and still yield anchors), clamped to the
- * segment; any remainder at the end is closed with horizontal/vertical steps.
+ * segment; the lead-in to the coarse path's first cell and any remainder at the end use horizontal/vertical steps
+ * (which carry no timing).
 */
 function densify(cp: Path, ci0: number, cj0: number, i0: number, i1: number, j0: number, j1: number): Path {
  const K = COARSE_FACTOR;
  const out: Path = [[i0, j0]];
  let i = i0, j = j0;
-  // the coarse grid origin may sit up to K/2 fine frames off (i0, j0); align to it first
  const step = (di: number, dj: number) => {
    const ni = Math.min(i1, i + di), nj = Math.min(j1, j + dj);
    if (ni !== i || nj !== j) { i = ni; j = nj; out.push([i, j]); }
  };
-  while (i < Math.min(i1, ci0 * K)) step(1, 0);
-  while (j < Math.min(j1, cj0 * K)) step(0, 1);
+  const [si, sj] = cp[0] ?? [0, 0]; // the coarse path may start off the corner (open begin)
+  while (i < Math.min(i1, (ci0 + si) * K)) step(1, 0);
+  while (j < Math.min(j1, (cj0 + sj) * K)) step(0, 1);
  for (let k = 1; k < cp.length; k++) {
    const di = cp[k]![0] - cp[k - 1]![0], dj = cp[k]![1] - cp[k - 1]![1];
    for (let q = 0; q < K; q++) step(di, dj);
@@ -140,13 +232,14 @@ function densify(cp: Path, ci0: number, cj0: number, i0: number, i1: number, j0:
  return out;
 }

-/** Banded fine DTW, widening the band (±0.5 s, ±1 s, ±2 s) when the end cell is unreachable; null if all fail. */
+/** Banded fine DTW (slope-limited), widening the band (±0.5 s, ±1 s, ±2 s) when the end is unreachable; null if all fail. */
 export function bandedWithRetry(
  costAt: (i: number, j: number) => number, n: number, m: number, centre: (i: number) => number, bandsSec = FINE_BANDS_SEC,
+  open: Open = { openBegin: false, openEnd: false },
 ): Path | null {
  for (const sec of bandsSec) {
    try {
-      return dtw(costAt, n, m, { center: centre, width: Math.round(sec * FINE_RATE) });
+      return dtw(costAt, n, m, { center: centre, width: Math.round(sec * FINE_RATE) }, { steps: "slope", ...open });
    } catch (e) {
      if (!(e instanceof Error) || !/unreachable/.test(e.message)) throw e;
    }
@@ -189,38 +282,153 @@ function compose(manual: Anchor[], auto: Anchor[]): Anchor[] {

 const durationOf = (a: Audio) => (a.channels[0]?.length ?? 0) / a.sampleRate;

-/** Fit of `warp` against recording onsets: warped onsets of scored-role notes (roles keyed by `trackKeys`). */
-function fitFor(recOnsets: number[], smf: Smf, roles: Record<string, Role | null>, warp: Warp, durSec: number): FitReport {
+/** What the fit needs from the recording: onset-flux peaks (s) and 10 Hz chroma. Compute once per song. */
+export interface RecordingCues { onsets: number[]; chroma: Float32Array[]; chromaRate: number; durSec: number }
+
+function cuesFrom(fA: Features, cA: Features, durSec: number): RecordingCues {
+  return { onsets: pickOnsets(fA.onset, FINE_RATE), chroma: cA.chroma, chromaRate: cA.rate, durSec };
+}
+
+/** Onset-flux peaks and chroma of the raw recording. Compute once and reuse with `fitFromCues` to re-score edits cheaply. */
+export function recordingCues(audio: Audio): RecordingCues {
+  const fA = features(audio, FINE_RATE);
+  return cuesFrom(fA, downsample(fA, COARSE_FACTOR), durationOf(audio));
+}
+
+/** Scored-role tracks (by `trackKeys`). */
+function scoredTracks(smf: Smf, roles: Record<string, Role | null>) {
  const keys = trackKeys(smf.tracks);
-  const midi = smf.tracks.flatMap((t, i) => (roles[keys[i]!] ? t.notes.map((n) => warp.toAudio(n.onset)) : []));
-  return fitQuality(recOnsets, midi, durSec);
+  return smf.tracks.filter((_, i) => roles[keys[i]!]);
 }

-/** Onset-flux peaks of the raw recording (seconds). Compute once and reuse with `fitFromOnsets` to re-score edits cheaply. */
-export function recordingOnsets(audio: Audio): number[] {
-  return pickOnsets(features(audio, FINE_RATE).onset, FINE_RATE);
+/** Piano-roll chroma (pitch classes of sounding pitched notes, 1 per note) with MIDI time placed by `place`. */
+function symbolicChroma(tracks: Smf["tracks"], place: (m: number) => number, nFrames: number, rate: number): Float32Array[] {
+  const out = Array.from({ length: nFrames }, () => new Float32Array(12));
+  for (const t of tracks) {
+    if (t.isDrums) continue;
+    for (const n of t.notes) {
+      const a = Math.max(0, Math.round(place(n.onset) * rate));
+      const b = Math.min(nFrames, Math.max(a + 1, Math.round(place(n.offset) * rate)));
+      for (let f = a; f < b; f++) out[f]![n.pitch % 12]! += 1;
+    }
+  }
+  return out;
 }

-/** `alignmentFit` with precomputed recording onsets; `durSec` = recording duration. */
-export function fitFromOnsets(onsets: number[], smf: Smf, roles: Record<string, Role | null>, anchors: Anchor[], durSec: number): FitReport {
-  return fitFor(onsets, smf, roles, makeWarp(anchors), durSec);
+/** Warped MIDI onsets of scored-role notes. */
+const scoredOnsets = (tracks: Smf["tracks"], place: (m: number) => number) => tracks.flatMap((t) => t.notes.map((n) => place(n.onset)));
+
+/** Fit of `anchors`: precision/lift/audible rate, the beat/bar-shift chroma check and stretch-warning counts. */
+export function fitFromCues(cues: RecordingCues, smf: Smf, roles: Record<string, Role | null>, anchors: Anchor[]): FitReport {
+  const warp = makeWarp(anchors);
+  const tracks = scoredTracks(smf, roles);
+  const nFrames = cues.chroma.length, rate = cues.chromaRate;
+  const g = meterGrid(smf, smf.durationSec + 30);
+  const barSteps = (m: number, k: number) => gridShift(g.bars, m, k);
+  const beatSteps = (m: number, k: number) => gridShift(g.beats, m, k);
+  // "+k beat(s)": the MIDI content belongs k beats later in the recording than the current alignment puts it
+  const cands: [string, (m: number) => number][] = [
+    ["+1 beat", (m) => beatSteps(m, 1)], ["−1 beat", (m) => beatSteps(m, -1)],
+    ["+2 beats", (m) => beatSteps(m, 2)], ["−2 beats", (m) => beatSteps(m, -2)],
+    ["+1 bar", (m) => barSteps(m, 1)], ["−1 bar", (m) => barSteps(m, -1)],
+  ];
+  const shift: ShiftInputs = {
+    rate, recording: cues.chroma,
+    current: symbolicChroma(tracks, warp.toAudio, nFrames, rate),
+    shifts: cands.map(([label, f]) => ({ label, chroma: symbolicChroma(tracks, (m) => warp.toAudio(f(m)), nFrames, rate) })),
+  };
+  const st = stretches(anchors, smf.durationSec);
+  const counts: StretchCounts = { total: st.length, warned: st.filter((s) => s.warn).length, over15: st.filter((s) => Math.abs(1 / s.ratio - 1) > OVER_TEMPO).length };
+  return fitQuality(cues.onsets, scoredOnsets(tracks, warp.toAudio), cues.durSec, { shift, stretches: counts });
 }

-/** Fit quality of any alignment (e.g. a hand-made one): onset-flux peaks of the raw recording vs the warped MIDI. */
+/** Fit quality of any alignment (e.g. a hand-made one) against the raw recording. */
 export function alignmentFit(audio: Audio, smf: Smf, roles: Record<string, Role | null>, anchors: Anchor[]): FitReport {
-  return fitFromOnsets(recordingOnsets(audio), smf, roles, anchors, durationOf(audio));
+  return fitFromCues(recordingCues(audio), smf, roles, anchors);
 }

+/** Best constant offset (recording − MIDI, s) and its precision; `cosine` = mean symbolic-chroma cosine there. */
+export interface Baseline { offsetSec: number; precision: number | null; cosine: number | null }
+
+/**
+ * Constant-offset baseline: grid search of −2..+2 s in 10 ms steps maximising precision. Whole-beat shifts alias onto
+ * the onset grid (equal precision), so the precision peaks within 0.1 of the best are told apart by the mean chroma
+ * cosine of the symbolic MIDI vs the recording. Within a peak precision is flat across the ±70 ms tolerance, so the
+ * offset with the smallest mean residual (among those within 0.05 of the peak's precision) is reported.
+ */
+export function constantOffsetBaseline(cues: RecordingCues, smf: Smf, roles: Record<string, Role | null>): Baseline {
+  const tracks = scoredTracks(smf, roles);
+  const midi = uniqueSorted(scoredOnsets(tracks, (m) => m));
+  const steps = Math.round(BASELINE_RANGE_SEC / BASELINE_STEP_SEC);
+  const offs = Array.from({ length: 2 * steps + 1 }, (_, k) => Math.round((k - steps) * BASELINE_STEP_SEC * 100) / 100);
+  const prec = offs.map((o) => precisionOf(cues.onsets, midi, o));
+  if (prec.every((p) => p === null)) return { offsetSec: 0, precision: null, cosine: null };
+  const best = Math.max(...prec.map((p) => p ?? -1));
+  // peaks: runs of consecutive offsets within the tie margin
+  const runs: number[][] = [];
+  for (let k = 0; k < offs.length; k++) {
+    if ((prec[k] ?? -1) < best - BASELINE_TIE) continue;
+    if (k > 0 && (prec[k - 1] ?? -1) >= best - BASELINE_TIE) runs[runs.length - 1]!.push(k); else runs.push([k]);
+  }
+  // within a run, precision is flat across the ±70 ms tolerance (with spikes at its edges, where neighbouring notes
+  // start to match): take the offset with the smallest mean residual among those within 0.05 of the run's best
+  const refine = (run: number[]): number => {
+    const top = Math.max(...run.map((k) => prec[k]!));
+    let pick = run[0]!, bestRes = Infinity;
+    for (const k of run) {
+      if (prec[k]! < top - BASELINE_PLATEAU) continue;
+      const r = meanResidual(cues.onsets, midi, offs[k]!);
+      if (r < bestRes) { bestRes = r; pick = k; }
+    }
+    return pick;
+  };
+  const cosAt = (o: number) => meanCosine(cues.chroma, symbolicChroma(tracks, (m) => m + o, cues.chroma.length, cues.chromaRate), 0, cues.chroma.length, 0);
+  let pick = refine(runs[0]!), pickCos = runs.length > 1 ? cosAt(offs[pick]!) : null;
+  for (const run of runs.slice(1)) {
+    const k = refine(run), c = cosAt(offs[k]!);
+    if (c !== null && (pickCos === null || c > pickCos)) { pick = k; pickCos = c; }
+  }
+  return { offsetSec: offs[pick]!, precision: prec[pick]!, cosine: pickCos ?? cosAt(offs[pick]!) };
+}
+
+/** Mean |recording onset − nearest (MIDI + shift)| over recording onsets matched within ±70 ms. */
+function meanResidual(rec: number[], midi: number[], shift: number): number {
+  let s = 0, n = 0;
+  for (const t of rec) {
+    const x = t - shift;
+    let lo = 0, hi = midi.length;
+    while (lo < hi) { const m = (lo + hi) >> 1; if (midi[m]! < x) lo = m + 1; else hi = m; }
+    const d = Math.min(lo < midi.length ? Math.abs(midi[lo]! - x) : Infinity, lo > 0 ? Math.abs(midi[lo - 1]! - x) : Infinity);
+    if (d <= MATCH_TOL_SEC + 1e-9) { s += d; n++; }
+  }
+  return n ? s / n : Infinity;
+}
+
+/** A single auto anchor realising a constant offset: at the bar nearest the middle of the MIDI (positive audio time). */
+function baselineAnchors(smf: Smf, offsetSec: number): Anchor[] {
+  const bars = barTimes(smf).filter((b) => b + offsetSec > 0.01);
+  const mid = smf.durationSec / 2;
+  const m = bars.length ? bars.reduce((a, b) => (Math.abs(b - mid) < Math.abs(a - mid) ? b : a)) : Math.max(0, -offsetSec) + 1;
+  return [{ midiSec: m, audioSec: m + offsetSec, kind: "auto" }];
+}
+
+/** `dtwAnchors`/`dtwPrecision`: the DTW alignment and its precision, also when the baseline replaced it. */
+export interface AutoResult { anchors: Anchor[]; fit: FitReport; baseline: Baseline; chosen: "baseline" | "dtw"; dtwAnchors: Anchor[]; dtwPrecision: number | null }
+
 /**
 * Align `smf` to `audio`, honouring `manual` anchors exactly. The MIDI is rendered under the manual warp, then each
- * stretch between consecutive manual anchors (plus start/end) is aligned by a 10 Hz DTW refined at 50 Hz in a band
- * around it. One auto anchor is placed per bar (skipping bars within 1 s of a manual anchor). `roles` is keyed by
- * `trackKeys`; tracks without a role are rendered but excluded from the fit.
+ * stretch between consecutive manual anchors (plus start/end) is aligned by a slope-limited 10 Hz DTW refined at 50 Hz
+ * in a band around it; the first stretch may start late in the recording (open begin) and the last one may end early
+ * on either axis (open end), so extra intro/outro material does not force the path. One auto anchor is placed per bar
+ * (skipping bars within 1 s of a manual anchor and the final partial bar); per-bar offsets more than ¼ beat from the
+ * median of their ±3 neighbours are rejected. Without manual anchors the result is replaced by the constant-offset
+ * baseline (one auto anchor) unless it beats the baseline's precision by ≥ 0.02. `roles` is keyed by `trackKeys`;
+ * tracks without a role are rendered but excluded from the fit.
 */
 export function autoAlign(
  audio: Audio, smf: Smf, roles: Record<string, Role | null>, manual: Anchor[], sf2: Uint8Array,
  opts: { fineBandsSec?: number[] } = {}, // band half-widths for the fine pass; [] forces the coarse-path fallback
-): { anchors: Anchor[]; fit: FitReport } {
+): AutoResult {
  const man = makeWarp(manual).anchors.map((a) => ({ ...a, kind: "manual" as const }));
  const W0 = makeWarp(man);
  const audioDur = durationOf(audio);
@@ -229,17 +437,18 @@ export function autoAlign(
  const fR = features(rendered, FINE_RATE), fA = features(audio, FINE_RATE);
  const cR = downsample(fR, COARSE_FACTOR), cA = downsample(fA, COARSE_FACTOR);
  const lastR = fR.chroma.length - 1, lastA = fA.chroma.length - 1;
+  const midiEnd = Math.max(1, Math.min(lastR, Math.round(W0.toAudio(smf.durationSec) * FINE_RATE))); // last rendered frame with MIDI content

  // segment boundaries: manual anchors map warped time == recording time, so both axes split at the same instants
-  const cuts = man.map((a) => Math.round(a.audioSec * FINE_RATE)).filter((f) => f > 0 && f < Math.min(lastR, lastA));
+  const cuts = man.map((a) => Math.round(a.audioSec * FINE_RATE)).filter((f) => f > 0 && f < Math.min(midiEnd, lastA));
  const bounds = [...new Set(cuts)].sort((a, b) => a - b);
-  // per segment: warped-time span and diagonal path steps (they carry timing; horizontal/vertical runs are stalls,
-  // silence or unmatched material and do not)
+  // per segment: warped-time span and diagonal path steps (they carry timing; horizontal/vertical steps do not)
  const segs: { lo: number; hi: number; diag: { w: number; off: number }[] }[] = [];
  let prev = 0;
  for (const b of [...bounds, -1]) {
-    const i1 = b < 0 ? lastR : b, j1 = b < 0 ? lastA : b;
-    const path = alignSegment([fR, fA], [cR, cA], prev, i1, prev, j1, opts.fineBandsSec ?? FINE_BANDS_SEC);
+    const i1 = b < 0 ? midiEnd : b, j1 = b < 0 ? lastA : b;
+    const open = { openBegin: prev === 0, openEnd: b < 0 };
+    const path = alignSegment([fR, fA], [cR, cA], prev, i1, prev, j1, opts.fineBandsSec ?? FINE_BANDS_SEC, open);
    const diag: { w: number; off: number }[] = [];
    for (let k = 1; k < path.length; k++) {
      const [i, j] = path[k]!, [pi, pj] = path[k - 1]!;
@@ -249,8 +458,12 @@ export function autoAlign(
    prev = b;
  }

-  const auto: Anchor[] = [];
-  for (const m of barTimes(smf)) {
+  const grid = meterGrid(smf);
+  const lastBar = grid.lastBarPartial ? grid.bars[grid.bars.length - 1] : undefined;
+  const beatAt = (m: number) => { const k = grid.beats.findIndex((b) => b > m + ZERO); return k > 0 ? grid.beats[k]! - grid.beats[k - 1]! : 0.5; };
+  const perBar: { midiSec: number; w: number; off: number; beatSec: number }[] = [];
+  for (const m of grid.bars) {
+    if (m === lastBar) continue; // never anchor the final partial bar
    if (man.some((a) => Math.abs(a.midiSec - m) < MANUAL_CLEARANCE_SEC)) continue;
    const w = W0.toAudio(m);
    const seg = segs.find((s) => w < s.hi) ?? segs[segs.length - 1]!;
@@ -258,9 +471,25 @@ export function autoAlign(
    if (off === null) continue;
    const a = w + off;
    if ((m > 0 ? a <= 0 : a < 0) || a > audioDur) continue; // no degenerate first stretch
-    auto.push({ midiSec: m, audioSec: a, kind: "auto" });
+    perBar.push({ midiSec: m, w, off, beatSec: beatAt(m) * (W0.toAudio(m + 1) - w) });
  }
+  const auto = rejectOffsetOutliers(perBar).map((p): Anchor => ({ midiSec: p.midiSec, audioSec: p.w + p.off, kind: "auto" }));
  const anchors = compose(man, auto);
-  const fit = fitFor(pickOnsets(fA.onset, FINE_RATE), smf, roles, makeWarp(anchors), audioDur);
-  return { anchors, fit };
+  const cues = cuesFrom(fA, cA, audioDur);
+  const fit = fitFromCues(cues, smf, roles, anchors);
+  const baseline = constantOffsetBaseline(cues, smf, roles);
+  const beats = fit.precision !== null && baseline.precision !== null && fit.precision >= baseline.precision + BASELINE_MARGIN;
+  if (man.length || baseline.precision === null || beats) return { anchors, fit, baseline, chosen: "dtw", dtwAnchors: anchors, dtwPrecision: fit.precision };
+  const b = baselineAnchors(smf, baseline.offsetSec);
+  return { anchors: b, fit: fitFromCues(cues, smf, roles, b), baseline, chosen: "baseline", dtwAnchors: anchors, dtwPrecision: fit.precision };
+}
+
+/** e.g. "DTW (precision 93.5%) · constant-offset baseline +0.06 s: precision 88.0%"; with "chosen: baseline" the
+ * single-anchor constant offset replaced a DTW alignment that was not better by 2 points. */
+export function autoSummary(r: { chosen: "baseline" | "dtw"; baseline: { offsetSec: number; precision: number | null }; dtwPrecision: number | null }): string {
+  const pc = (x: number | null) => (x === null ? "n/a" : `${(x * 100).toFixed(1)}%`);
+  const off = `${r.baseline.offsetSec >= 0 ? "+" : "−"}${Math.abs(r.baseline.offsetSec).toFixed(2)} s`;
+  const base = `constant-offset baseline ${off}: precision ${pc(r.baseline.precision)}`;
+  return r.chosen === "dtw" ? `chosen: DTW (precision ${pc(r.dtwPrecision)}) · ${base}`
+    : `chosen: baseline (DTW precision ${pc(r.dtwPrecision)} not better by ≥ 2 points) · ${base}`;
 }
@@ -1,7 +1,7 @@
 import { parseArgs } from "node:util";
 import { addSong, loadSong, readAlignment, trackKeys, writeAlignment } from "../ref/manifest.ts";
 import { parseSmf } from "./smf.ts";
-import { autoAlign } from "./auto.ts";
+import { autoAlign, autoSummary } from "./auto.ts";
 import { fitSummary } from "./fit.ts";
 import { SF2_PATH, startServer } from "./server.ts";

@@ -51,8 +51,9 @@ async function auto(argv: string[]) {
  const sf2 = new Uint8Array(await Bun.file(SF2_PATH).arrayBuffer());
  const t0 = performance.now();
  const r = autoAlign(s.audio, s.smf, roles, manual, sf2);
-  await writeAlignment(id, { anchors: r.anchors, savedAt: new Date().toISOString(), fit: r.fit }, root);
+  await writeAlignment(id, { anchors: r.anchors, savedAt: new Date().toISOString(), fit: r.fit, auto: { chosen: r.chosen, baseline: r.baseline, dtwPrecision: r.dtwPrecision } }, root);
  console.log(`${id}: ${r.anchors.length} anchors (${manual.length} manual kept) in ${((performance.now() - t0) / 1000).toFixed(1)} s -> ${root}/${id}/alignment.json`);
+  console.log(`method: ${autoSummary(r)}`);
  console.log(`fit: ${fitSummary(r.fit)}`);
 }

@@ -1,18 +1,33 @@
-// Pure (banded) dynamic time warping. Only the two latest accumulated-cost rows are kept; backpointers are one byte
-// per cell (O(n·band) when banded), so a 3000×3000 unbanded run uses ≈ 9 MB.
+// Pure (banded) dynamic time warping. Only the latest accumulated-cost rows are kept; backpointers are one byte per
+// cell (O(n·band) when banded), so a 3000×3000 unbanded run uses ≈ 9 MB.

 export interface DtwBand { center: (i: number) => number; width: number }
+/**
+ * steps: "symmetric" (default) — steps (1,0), (0,1), (1,1), each paying the cost of the cell entered.
+ *        "slope" — slope-limited (Sakoe–Chiba P = 1, asymmetric weights): (1,1), (1,2) via (i, j−1), (2,1) via
+ *        (i−1, j). The local slope stays within [1/2, 2], so the path cannot make abrupt jumps; each row (query frame)
+ *        carries weight 1, so paths that consume all rows are comparable whatever columns they cover.
+ * openBegin: the path may start anywhere on row 0 (the target may have extra material before the query).
+ * openEnd: the path may end anywhere on the last row or the last column (target or query has extra material at the
+ *          end); candidates are compared by mean cost per query row, each query row left unconsumed costing 1 (the
+ *          cost of sound vs silence), so a trivial path into a cheap corner cannot win.
+ */
+export interface DtwOptions { steps?: "symmetric" | "slope"; openBegin?: boolean; openEnd?: boolean }

-const DIAG = 0, UP = 1, LEFT = 2; // from (i-1,j-1), (i-1,j), (i,j-1)
+const DIAG = 0, UP = 1, LEFT = 2, S12 = 3, S21 = 4; // from (i-1,j-1), (i-1,j), (i,j-1), (i-1,j-2) via (i,j-1), (i-2,j-1) via (i-1,j)
+const START = 5;
+const UNCONSUMED_ROW_COST = 1;

-/** Minimum-cost monotone path from (0,0) to (n-1,m-1) with steps (1,0), (0,1), (1,1). */
+/** Minimum-cost monotone path from (0,0) to (n-1,m-1) (or open ends, see `DtwOptions`). */
 export function dtw(
  costAt: (i: number, j: number) => number,
  n: number,
  m: number,
  band?: DtwBand,
+  opts: DtwOptions = {},
 ): [number, number][] {
  if (n <= 0 || m <= 0) return [];
+  const slope = opts.steps === "slope", openBegin = !!opts.openBegin, openEnd = !!opts.openEnd;
  const lo = new Int32Array(n);
  const hi = new Int32Array(n);
  const off = new Int32Array(n + 1);
@@ -22,8 +37,8 @@ export function dtw(
      const c = Math.round(band.center(i));
      a = Math.max(0, c - band.width);
      b = Math.min(m - 1, c + band.width);
-      if (i === 0) a = 0;
-      if (i === n - 1) b = m - 1;
+      if (i === 0 && !openBegin) a = 0;
+      if (i === n - 1 && !openEnd) b = m - 1;
      if (a > b) { // centre outside the matrix: clamp to the nearest edge
        a = b = Math.min(m - 1, Math.max(0, c));
      }
@@ -32,36 +47,71 @@ export function dtw(
    off[i + 1] = off[i]! + (b - a + 1);
  }
  const bp = new Uint8Array(off[n]!);
-  let prev = new Float64Array(m).fill(Infinity);
-  let cur = new Float64Array(m).fill(Infinity);
+  // accumulated cost rows (row i in D[i % 3]) and cell-cost rows (row i in C[i % 2]); full width, cleared by range
+  const D = [0, 1, 2].map(() => new Float64Array(m).fill(Infinity));
+  const C = [0, 1].map(() => new Float64Array(m));
+  const inRow = (r: number, j: number) => r >= 0 && j >= lo[r]! && j <= hi[r]!;
+  const acc = (r: number, j: number) => (inRow(r, j) ? D[r % 3]![j]! : Infinity);
+  // open-end candidates: mean cost per query row, each row left unconsumed (ending on the last column) costing 1
+  let endScore = Infinity, endI = n - 1, endJ = m - 1;
+  const consider = (i: number, v: number, j: number) => {
+    const score = (v + (n - 1 - i) * UNCONSUMED_ROW_COST) / n;
+    if (v < Infinity && score < endScore - 1e-12) { endScore = score; endI = i; endJ = j; }
+  };
  for (let i = 0; i < n; i++) {
    const a = lo[i]!, b = hi[i]!, base = off[i]! - a;
-    const pa = i > 0 ? lo[i - 1]! : 0, pb = i > 0 ? hi[i - 1]! : -1;
-    cur.fill(Infinity, Math.max(0, a - 1), Math.min(m, b + 2));
+    const cur = D[i % 3]!, cc = C[i % 2]!, pc = C[(i + 1) % 2]!;
+    if (i >= 3) cur.fill(Infinity, lo[i - 3]!, hi[i - 3]! + 1);
+    for (let j = a; j <= b; j++) cc[j] = costAt(i, j);
    for (let j = a; j <= b; j++) {
      let best = Infinity, dir = DIAG;
-      if (i === 0 && j === 0) best = 0;
-      else {
-        if (i > 0 && j > 0 && j - 1 >= pa && j - 1 <= pb) { best = prev[j - 1]!; dir = DIAG; }
-        if (i > 0 && j >= pa && j <= pb && prev[j]! < best) { best = prev[j]!; dir = UP; }
+      const d = cc[j]!;
+      if (i === 0 && (j === 0 || openBegin)) { best = 0; dir = START; }
+      else if (slope) {
+        const dg = acc(i - 1, j - 1);
+        if (dg + d < best) { best = dg + d; dir = DIAG; }
+        const h = acc(i - 1, j - 2);
+        if (h < Infinity) {
+          const v = h + ((j - 1 >= a ? cc[j - 1]! : costAt(i, j - 1)) + d) / 2;
+          if (v < best) { best = v; dir = S12; }
+        }
+        const u = acc(i - 2, j - 1);
+        if (u < Infinity) {
+          const v = u + (inRow(i - 1, j) ? pc[j]! : costAt(i - 1, j)) + d;
+          if (v < best) { best = v; dir = S21; }
+        }
+      } else {
+        const dg = acc(i - 1, j - 1);
+        if (dg < best) { best = dg; dir = DIAG; }
+        const up = acc(i - 1, j);
+        if (up < best) { best = up; dir = UP; }
        if (j > a && cur[j - 1]! < best) { best = cur[j - 1]!; dir = LEFT; }
+        if (best < Infinity) best += d;
      }
      if (best === Infinity) { cur[j] = Infinity; continue; }
-      cur[j] = best + costAt(i, j);
+      cur[j] = dir === START ? d : best;
      bp[base + j] = dir;
    }
-    // clear stale values outside this row's range so the next row never reads them
-    if (i > 0) prev.fill(Infinity, pa, pb + 1);
-    [prev, cur] = [cur, prev];
+    if (openEnd && i < n - 1 && b === m - 1) consider(i, cur[m - 1]!, m - 1);
  }
-  if (prev[m - 1]! === Infinity) throw new Error("dtw: end cell unreachable within band");
+  // choose the end cell (last-column candidates were collected during the sweep)
+  let ei = n - 1, ej = m - 1;
+  if (openEnd) {
+    for (let j = lo[n - 1]!; j <= hi[n - 1]!; j++) consider(n - 1, acc(n - 1, j), j);
+    if (!(endScore < Infinity)) throw new Error("dtw: end cell unreachable within band");
+    ei = endI; ej = endJ;
+  } else if (acc(n - 1, m - 1) === Infinity) throw new Error("dtw: end cell unreachable within band");
  const path: [number, number][] = [];
-  let i = n - 1, j = m - 1;
+  let i = ei, j = ej;
  for (;;) {
    path.push([i, j]);
-    if (i === 0 && j === 0) break;
    const d = bp[off[i]! - lo[i]! + j]!;
-    if (d === DIAG) { i--; j--; } else if (d === UP) i--; else j--;
+    if (d === START) break;
+    if (d === DIAG) { i--; j--; }
+    else if (d === UP) i--;
+    else if (d === LEFT) j--;
+    else if (d === S12) { path.push([i, j - 1]); i--; j -= 2; }
+    else { path.push([i - 1, j]); i -= 2; j--; }
  }
  return path.reverse();
 }
@@ -108,7 +108,11 @@ export function features(audio: Audio, rate: number): Features {
  return { rate, onset, chroma };
 }

-/** Peak-pick an onset-strength curve (adaptive threshold median + k·MAD, 50 ms minimum gap). Times in seconds. */
+/**
+ * Peak-pick an onset-strength curve (adaptive threshold median + k·MAD, 50 ms minimum gap, the stronger peak wins).
+ * Peak times are refined to sub-frame precision by parabolic interpolation over the peak and its two neighbours.
+ * Times in seconds.
+ */
 export function pickOnsets(onset: Float32Array, rate: number, k = 4): number[] {
  const n = onset.length;
  if (n === 0) return [];
@@ -118,15 +122,20 @@ export function pickOnsets(onset: Float32Array, rate: number, k = 4): number[] {
  const mad = dev[n >> 1]! * 1.4826;
  const thr = med + k * Math.max(mad, 1e-3);
  const minGap = 0.05;
-  const picked: number[] = [];
+  const frames: number[] = [];
  for (let i = 0; i < n; i++) {
    const v = onset[i]!;
    if (v <= thr || v < (onset[i - 1] ?? -Infinity) || v <= (onset[i + 1] ?? -Infinity)) continue;
-    const t = i / rate;
-    const last = picked.length - 1;
-    if (last >= 0 && t - picked[last]! < minGap - 1e-9) {
-      if (v > onset[Math.round(picked[last]! * rate)]!) picked[last] = t;
-    } else picked.push(t);
+    const last = frames.length - 1;
+    if (last >= 0 && (i - frames[last]!) / rate < minGap - 1e-9) {
+      if (v > onset[frames[last]!]!) frames[last] = i;
+    } else frames.push(i);
  }
-  return picked;
+  return frames.map((i) => {
+    if (i === 0 || i === n - 1) return i / rate;
+    const a = onset[i - 1]!, b = onset[i]!, c = onset[i + 1]!;
+    const den = a - 2 * b + c;
+    const d = den < 0 ? Math.max(-0.5, Math.min(0.5, (0.5 * (a - c)) / den)) : 0;
+    return (i + d) / rate;
+  });
 }
@@ -1,13 +1,58 @@
-// Fit quality of an alignment: how well warped MIDI onsets land on onsets picked from the raw recording.
+// Fit quality of an alignment: how well onsets picked from the raw recording land on the warped MIDI onsets.
 // Pure: no I/O. Recording onsets must come from the audio itself (onset flux), never from transcription.
+//
+// Calibrated on real mixes (ruling A7, spec "Fit quality", amended 2026-10-30): the onset picker finds ~1.4 attacks/s
+// in a full mix while a scored MIDI has ~13 unique onsets/s, so the share of MIDI onsets that are heard ("audible
+// rate", the old match rate) is capped near 40% even when perfectly aligned and carries no information. The flag
+// metric is therefore *precision*: the share of recording onsets with a warped MIDI onset within ±70 ms. Because a
+// dense score matches almost anything, precision is chance-corrected by the precision at off-grid shifts (lift), and a
+// symbolic chroma check catches whole-beat / whole-bar shifts that onset metrics cannot see (they alias onto the grid).

-export interface FitWindow { startSec: number; matchRate: number | null; medianResidualMs: number | null; flagged: boolean }
-export interface FitReport { matchRate: number | null; medianResidualMs: number | null; windows: FitWindow[] }
+/** A beat/bar shift that explains the recording's chroma better than the current alignment. */
+export interface BeatShift { label: string; gain: number }
+export interface FitWindow {
+  startSec: number;
+  /** Recording onsets in this window (by recording time). */
+  recOnsets: number;
+  /** Share of this window's recording onsets with a warped MIDI onset within ±70 ms; null with < 8 recording onsets. */
+  precision: number | null;
+  /** precision − mean precision at the off-grid shifts (chance-corrected); null when precision is null. */
+  lift: number | null;
+  /** Information only (JSON `audibleRate`): share of unique MIDI onsets (in this window by warped time) heard as attacks. */
+  audibleRate: number | null;
+  /** Median |recording onset − nearest warped MIDI onset| over matched recording onsets (ms). */
+  medianResidualMs: number | null;
+  beatShift: BeatShift | null;
+  flagged: boolean;
+  /** Why the window is flagged, e.g. "precision 41%", "lift 0.12", "beat shift suspected: +1 beat". */
+  reasons: string[];
+}
+export interface StretchCounts { total: number; warned: number; over15: number }
+export interface FitReport {
+  precision: number | null;
+  lift: number | null;
+  /** Information only: share of unique MIDI onsets heard as attacks ("MIDI onsets audible as attacks"). */
+  audibleRate: number | null;
+  medianResidualMs: number | null;
+  windows: FitWindow[];
+  /** Stretches of the alignment: total, warned (> 5% tempo change) and with |tempo change| > 15%. */
+  stretches?: StretchCounts;
+}
+
+/** Symbolic-chroma inputs for the beat/bar-shift check: frames at `rate` Hz in recording time, 12-d (any scale). */
+export interface ShiftInputs { rate: number; recording: Float32Array[]; current: Float32Array[]; shifts: { label: string; chroma: Float32Array[] }[] }

 export const MATCH_TOL_SEC = 0.07;
 export const WINDOW_SEC = 30;
-const FLAG_MIN_RATE = 0.5;
-const FLAG_MAX_RESIDUAL_MS = 60;
+export const FLAG_MIN_PRECISION = 0.6;
+export const FLAG_MIN_LIFT = 0.25;
+export const MIN_REC_ONSETS = 8;
+/** Off-grid shifts (s) of the warped MIDI used as the chance level of precision. */
+export const CHANCE_SHIFTS_SEC = [0.11, 0.23, 0.37];
+/** A shifted alignment must beat the current one by this much mean chroma cosine to flag the window. */
+export const SHIFT_MARGIN = 0.05;
+/** The current symbolic chroma must cover at least this share of the window's sounding frames for the check to run. */
+const SHIFT_MIN_COVER = 0.25;
 const UNIQUE_EPS = 1e-6;

 function median(xs: number[]): number | null {
@@ -16,52 +61,126 @@ function median(xs: number[]): number | null {
  return s.length % 2 ? s[m]! : (s[m - 1]! + s[m]!) / 2;
 }

-/** |residual| in seconds to the nearest recording onset, or null when none lies within ±70 ms. */
-function nearest(sorted: number[], t: number): number | null {
+/** |distance| from `t` to the nearest value in `sorted` (Infinity when empty). */
+function nearestDist(sorted: number[], t: number): number {
  let lo = 0, hi = sorted.length;
  while (lo < hi) { const m = (lo + hi) >> 1; if (sorted[m]! < t) lo = m + 1; else hi = m; }
  let best = Infinity;
  if (lo < sorted.length) best = Math.min(best, Math.abs(sorted[lo]! - t));
  if (lo > 0) best = Math.min(best, Math.abs(sorted[lo - 1]! - t));
-  return best <= MATCH_TOL_SEC + 1e-9 ? best : null;
+  return best;
+}
+const hit = (d: number) => d <= MATCH_TOL_SEC + 1e-9;
+
+/** Sorted, de-duplicated copy. */
+export function uniqueSorted(xs: number[]): number[] {
+  const out: number[] = [];
+  for (const t of [...xs].sort((a, b) => a - b)) if (!out.length || t - out[out.length - 1]! > UNIQUE_EPS) out.push(t);
+  return out;
 }

-function summarise(residuals: (number | null)[]): { matchRate: number | null; medianResidualMs: number | null } {
-  if (!residuals.length) return { matchRate: null, medianResidualMs: null };
-  const hits = residuals.filter((r): r is number => r !== null);
-  const med = median(hits);
-  return { matchRate: hits.length / residuals.length, medianResidualMs: med === null ? null : med * 1000 };
+/** Share of `rec` (any order) with a value of sorted `midi` + `shift` within ±70 ms; null when `rec` or `midi` is empty. */
+export function precisionOf(rec: number[], midi: number[], shift = 0): number | null {
+  if (!rec.length || !midi.length) return null;
+  let n = 0;
+  for (const t of rec) if (hit(nearestDist(midi, t - shift))) n++;
+  return n / rec.length;
+}
+
+function cos(a: Float32Array, b: Float32Array): number {
+  let d = 0, na = 0, nb = 0;
+  for (let q = 0; q < 12; q++) { d += a[q]! * b[q]!; na += a[q]! * a[q]!; nb += b[q]! * b[q]!; }
+  return na > 1e-12 && nb > 1e-12 ? d / Math.sqrt(na * nb) : 0;
+}
+
+/** Mean chroma cosine over the sounding recording frames of [f0, f1); null when the candidate covers too little. */
+export function meanCosine(rec: Float32Array[], cand: Float32Array[], f0: number, f1: number, minCover = SHIFT_MIN_COVER): number | null {
+  let sum = 0, sounding = 0, covered = 0;
+  for (let t = f0; t < Math.min(f1, rec.length); t++) {
+    const r = rec[t]!;
+    if (!r.some((v) => v !== 0)) continue;
+    sounding++;
+    const c = cand[t];
+    if (!c || !c.some((v) => v !== 0)) continue;
+    covered++;
+    sum += cos(r, c);
+  }
+  if (!sounding || covered / sounding < minCover) return null;
+  return sum / sounding;
+}
+
+function shiftCheck(s: ShiftInputs, f0: number, f1: number): BeatShift | null {
+  const cur = meanCosine(s.recording, s.current, f0, f1);
+  if (cur === null) return null;
+  let best: BeatShift | null = null;
+  for (const c of s.shifts) {
+    const v = meanCosine(s.recording, c.chroma, f0, f1, 0);
+    if (v !== null && v - cur > SHIFT_MARGIN && (!best || v - cur > best.gain)) best = { label: c.label, gain: v - cur };
+  }
+  return best;
+}
+
+const pct = (x: number) => `${Math.round(x * 100)}%`;
+
+/**
+ * Recording onsets are matched to the nearest warped MIDI onset within ±70 ms (non-exclusive). Overall and per 30 s
+ * window (by recording onset time): precision, lift (precision − mean precision with the MIDI shifted by +0.11, +0.23,
+ * +0.37 s), median residual; plus the audible rate (info) and, with `extra.shift`, the beat/bar-shift chroma check.
+ * A window with < 8 recording onsets is null and unflagged; otherwise it is flagged when precision < 0.6, lift < 0.25,
+ * or a shifted alignment beats the current one by > 0.05 mean chroma cosine.
+ */
+export function fitQuality(audioOnsets: number[], midiOnsets: number[], durSec: number, extra: { shift?: ShiftInputs; stretches?: StretchCounts } = {}): FitReport {
+  const rec = [...audioOnsets].sort((a, b) => a - b);
+  const midi = uniqueSorted(midiOnsets);
+  const nWin = Math.max(1, Math.ceil(Math.max(durSec, (rec[rec.length - 1] ?? 0) + 1e-9) / WINDOW_SEC));
+  const winOf = (t: number) => Math.min(nWin - 1, Math.max(0, Math.floor(t / WINDOW_SEC)));
+  const recWin: number[][] = Array.from({ length: nWin }, () => []);
+  for (const t of rec) recWin[winOf(t)]!.push(t);
+  const midiHit = midi.map((t) => hit(nearestDist(rec, t)));
+
+  const stats = (r: number[]) => {
+    const precision = midi.length ? precisionOf(r, midi) : null;
+    if (precision === null) return { precision: null, lift: null, medianResidualMs: null };
+    const chance = CHANCE_SHIFTS_SEC.map((s) => precisionOf(r, midi, s)!);
+    const dists = r.map((t) => nearestDist(midi, t)).filter(hit);
+    const med = median(dists);
+    return { precision, lift: precision - chance.reduce((a, b) => a + b, 0) / chance.length, medianResidualMs: med === null ? null : med * 1000 };
+  };
+  const audible = (lo: number, hi: number) => {
+    let n = 0, h = 0;
+    midi.forEach((t, k) => { if (t >= lo && t < hi) { n++; if (midiHit[k]) h++; } });
+    return n ? h / n : null;
+  };
+
+  const windows = recWin.map((r, w): FitWindow => {
+    const startSec = w * WINDOW_SEC;
+    const lo = w === 0 ? -Infinity : startSec, hi = w === nWin - 1 ? Infinity : startSec + WINDOW_SEC;
+    const s = r.length >= MIN_REC_ONSETS ? stats(r) : { precision: null, lift: null, medianResidualMs: null };
+    const beatShift = extra.shift && s.precision !== null
+      ? shiftCheck(extra.shift, Math.round(startSec * extra.shift.rate), w === nWin - 1 ? Infinity : Math.round((startSec + WINDOW_SEC) * extra.shift.rate))
+      : null;
+    const reasons: string[] = [];
+    if (s.precision !== null && s.precision < FLAG_MIN_PRECISION) reasons.push(`precision ${pct(s.precision)}`);
+    if (s.lift !== null && s.lift < FLAG_MIN_LIFT) reasons.push(`lift ${s.lift.toFixed(2)}`);
+    if (beatShift) reasons.push(`beat shift suspected: ${beatShift.label}`);
+    return { startSec, recOnsets: r.length, ...s, audibleRate: audible(lo, hi), beatShift, flagged: reasons.length > 0, reasons };
+  });
+  const all = rec.length && midi.length ? stats(rec) : { precision: null, lift: null, medianResidualMs: null };
+  return { ...all, audibleRate: audible(-Infinity, Infinity), windows, ...(extra.stretches ? { stretches: extra.stretches } : {}) };
 }

 /**
- * Match each unique MIDI onset (already warped to audio time) to the nearest recording onset within ±70 ms
- * (non-exclusive). Reports match rate and median |residual| overall and per 30 s window (by MIDI onset time).
- * A window is flagged when its match rate < 0.5 or its median residual > 60 ms; empty windows are null, unflagged.
+ * One line, e.g. "precision 91.2% (lift +0.70) · MIDI onsets audible as attacks 38.1% · median residual 14.2 ms ·
+ * 1/8 windows flagged (0:30 beat shift suspected: +1 beat) · stretches: 3/40 warned, 1 > 15%".
 */
-export function fitQuality(audioOnsets: number[], midiOnsets: number[], durSec: number): FitReport {
-  const audio = [...audioOnsets].sort((a, b) => a - b);
-  const midi: number[] = [];
-  for (const t of [...midiOnsets].sort((a, b) => a - b)) if (!midi.length || t - midi[midi.length - 1]! > UNIQUE_EPS) midi.push(t);
-  const res = midi.map((t) => nearest(audio, t));
-
-  const lastOnset = midi.length ? midi[midi.length - 1]! : 0;
-  const nWin = Math.max(1, Math.ceil(Math.max(durSec, lastOnset + 1e-9) / WINDOW_SEC));
-  const buckets: (number | null)[][] = Array.from({ length: nWin }, () => []);
-  midi.forEach((t, k) => buckets[Math.min(nWin - 1, Math.max(0, Math.floor(t / WINDOW_SEC)))]!.push(res[k]!));
-  const windows = buckets.map((b, w): FitWindow => {
-    const s = summarise(b);
-    const flagged = s.matchRate !== null && (s.matchRate < FLAG_MIN_RATE || (s.medianResidualMs ?? 0) > FLAG_MAX_RESIDUAL_MS);
-    return { startSec: w * WINDOW_SEC, ...s, flagged };
-  });
-  return { ...summarise(res), windows };
-}
-
-/** One-line summary, e.g. "82.3% of MIDI onsets matched · median residual 14.2 ms · 2/9 windows flagged (0:30, 2:00)". */
 export function fitSummary(f: FitReport): string {
-  if (f.matchRate === null) return "no scored MIDI onsets";
+  if (f.precision === null) return f.audibleRate === null ? "no scored MIDI onsets" : "no recording onsets";
  const mmss = (s: number) => `${Math.floor(s / 60)}:${String(Math.floor(s % 60)).padStart(2, "0")}`;
  const flagged = f.windows.filter((w) => w.flagged);
  const med = f.medianResidualMs === null ? "n/a" : `${f.medianResidualMs.toFixed(1)} ms`;
-  const list = flagged.length ? ` (${flagged.map((w) => mmss(w.startSec)).join(", ")})` : "";
-  return `${(f.matchRate * 100).toFixed(1)}% of MIDI onsets matched · median residual ${med} · ${flagged.length}/${f.windows.length} windows flagged${list}`;
+  const lift = f.lift === null ? "" : ` (lift ${f.lift >= 0 ? "+" : "−"}${Math.abs(f.lift).toFixed(2)})`;
+  const aud = f.audibleRate === null ? "n/a" : `${(f.audibleRate * 100).toFixed(1)}%`;
+  const list = flagged.length ? ` (${flagged.map((w) => `${mmss(w.startSec)} ${w.reasons.join(", ")}`).join("; ")})` : "";
+  const st = f.stretches ? ` · stretches: ${f.stretches.warned}/${f.stretches.total} warned, ${f.stretches.over15} > 15%` : "";
+  return `precision ${(f.precision * 100).toFixed(1)}%${lift} · MIDI onsets audible as attacks ${aud} · median residual ${med} · ${flagged.length}/${f.windows.length} windows flagged${list}${st}`;
 }
@@ -255,14 +255,23 @@ async function runBusy(label: string, f: () => Promise<void>) {
  try { await f(); } catch (e) { setStatus((e as Error).message, true); } finally { busy = false; showOverlay(null); updateUi(); }
 }

+interface AutoInfo { chosen: "baseline" | "dtw"; baseline: { offsetSec: number; precision: number | null }; dtwPrecision: number | null }
+const pctOf = (x: number | null) => (x === null ? "n/a" : `${(x * 100).toFixed(1)}%`);
+/** "DTW, precision 91.0% vs constant offset +0.06 s 88.0%" or "constant offset +0.06 s (precision 91.0%; DTW 89.5% was not better)". */
+function autoNote(a: AutoInfo): string {
+  const off = `${a.baseline.offsetSec >= 0 ? "+" : "−"}${Math.abs(a.baseline.offsetSec).toFixed(2)} s`;
+  return a.chosen === "dtw" ? `DTW, precision ${pctOf(a.dtwPrecision)} vs constant offset ${off} ${pctOf(a.baseline.precision)}`
+    : `constant offset ${off} (precision ${pctOf(a.baseline.precision)}; DTW ${pctOf(a.dtwPrecision)} was not better by 2 points)`;
+}
+
 const autoAlign = () => runBusy("Auto-aligning around your anchors… (a few seconds to half a minute)", async () => {
  const manual = anchors.filter((a) => a.kind === "manual");
-  const j = await http<{ anchors: Anchor[]; fit: FitReport }>(songUrl("/auto"), send("POST", { anchors: manual }));
+  const j = await http<{ anchors: Anchor[]; fit: FitReport; auto: AutoInfo }>(songUrl("/auto"), send("POST", { anchors: manual }));
  selected = -1;
  commit(j.anchors, anchors, true);
  fit = j.fit; fitKey = keyOf(j.anchors);
  updateUi();
-  setStatus(`Auto-aligned: ${j.anchors.length - manual.length} automatic anchors around your ${manual.length}. Not saved yet.`);
+  setStatus(`Auto-aligned: ${j.anchors.length - manual.length} automatic anchors around your ${manual.length} (${autoNote(j.auto)}). Not saved yet.`);
 });

 const save = () => runBusy("Saving…", async () => {
@@ -357,7 +366,7 @@ function updateUi() {
  fitEl.classList.toggle("stale", stale);
  if (fit) {
    fitEl.textContent = `Fit${stale ? " (updating…)" : ""}: ${fitSummary(fit)}`;
-    fitEl.classList.toggle("bad", (fit.matchRate ?? 1) < 0.5 || fit.windows.some((w) => w.flagged));
+    fitEl.classList.toggle("bad", fit.windows.some((w) => w.flagged));
  }
  const man = anchors.filter((a) => a.kind === "manual").length;
  countsEl.textContent = `· anchors: ${man} yours, ${anchors.length - man} automatic`;
@@ -8,7 +8,7 @@ import type { Role } from "../core/types.ts";
 import { audioOgg, loadSong, readAlignment, readManifest, writeAlignment, type AlignmentFile, type LoadedSong } from "../ref/manifest.ts";
 import { trackKeys } from "./roles.ts";
 import { makeWarp, stretches, type Anchor } from "./warp.ts";
-import { autoAlign, fitFromOnsets, recordingOnsets } from "./auto.ts";
+import { autoAlign, fitFromCues, recordingCues, type AutoResult, type RecordingCues } from "./auto.ts";
 import { renderMidi } from "./render.ts";
 import { peaks } from "./peaks.ts";
 import { stretchInfo } from "./tempo.ts";
@@ -32,9 +32,9 @@ export interface AlignServer { url: string; origin: string; token: string; port:

 interface SongState extends LoadedSong {
  roles: Record<string, Role | null>;
-  onsets: number[];
+  cues: RecordingCues;
  peaks: { bucketSec: number; min: number[]; max: number[] };
-  auto?: { anchors: Anchor[]; fit: FitReport }; // first automatic pass, reused while no alignment.json exists
+  auto?: AutoResult; // first automatic pass, reused while no alignment.json exists
 }

 class HttpError extends Error { constructor(readonly status: number, msg: string) { super(msg); } }
@@ -98,7 +98,7 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
    const s = await loadSong(id, root);
    const keys = trackKeys(s.smf.tracks);
    const roles = Object.fromEntries(keys.map((k) => [k, s.entry.roles[k] ?? null]));
-    const onsets = recordingOnsets(s.audio);
+    const cues = recordingCues(s.audio);
    const peaksFile = Bun.file(join(root, id, "peaks.json"));
    const recWav = Bun.file(join(root, id, "recording.wav"));
    let pk: SongState["peaks"] | null = null;
@@ -114,7 +114,7 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
      await Bun.write(peaksFile, JSON.stringify({ version: PEAKS_VERSION, peaks: pk }));
    }
    log(`loaded ${id} (${s.audioDur.toFixed(1)} s) in ${((performance.now() - t0) / 1000).toFixed(1)} s`);
-    return { ...s, roles, onsets, peaks: pk };
+    return { ...s, roles, cues, peaks: pk };
  }

  function song(id: string): Promise<SongState> {
@@ -127,17 +127,19 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
    return p;
  }

-  const fitOf = (s: SongState, anchors: Anchor[]) => fitFromOnsets(s.onsets, s.smf, s.roles, anchors, s.audioDur);
+  const fitOf = (s: SongState, anchors: Anchor[]) => fitFromCues(s.cues, s.smf, s.roles, anchors);
  const stretchesOf = (s: SongState, anchors: Anchor[]) => stretchInfo(stretches(anchors, s.smf.durationSec), s.smf.tempoMap);
  const state = (s: SongState, anchors: Anchor[], fit = fitOf(s, anchors)) => ({ anchors, stretches: stretchesOf(s, anchors), fit });

  function runAuto(s: SongState, manual: Anchor[]) {
    const t0 = performance.now();
    const r = autoAlign(s.audio, s.smf, s.roles, manual, sf2);
-    log(`auto-aligned ${s.entry.id}: ${r.anchors.length} anchors (${manual.length} manual) in ${((performance.now() - t0) / 1000).toFixed(1)} s`);
+    log(`auto-aligned ${s.entry.id}: ${r.anchors.length} anchors (${manual.length} manual, ${r.chosen}) in ${((performance.now() - t0) / 1000).toFixed(1)} s`);
    return r;
  }

+  const autoInfo = (r: AutoResult) => ({ chosen: r.chosen, baseline: r.baseline, dtwPrecision: r.dtwPrecision });
+
  /** Saved alignment with validated anchors; an unreadable/invalid file yields `error` instead of failing the request. */
  async function savedAlignment(id: string): Promise<{ file: AlignmentFile | null; error?: string }> {
    try {
@@ -175,7 +177,7 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
        id, title: s.entry.title, audioDur: s.audioDur, midiDur: s.smf.durationSec,
        tracks: trackKeys(s.smf.tracks).map((name) => ({ name, role: s.roles[name] ?? null })),
        recordingPeaks: s.peaks, ...state(s, anchors, fit), midiTempo: s.smf.tempoMap,
-        source: saved ? "saved" : "auto", savedAt: saved?.savedAt ?? null,
+        source: saved ? "saved" : "auto", savedAt: saved?.savedAt ?? null, ...(saved ? {} : { auto: autoInfo(s.auto!) }),
        ...(error ? { warning: `${error}. Showing the automatic alignment instead; saving will overwrite the file.` } : {}),
      });
    }
@@ -208,7 +210,7 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
      const manual = parseAnchors(await readJson(req)).filter((a) => a.kind === "manual");
      const s = await song(id);
      const r = runAuto(s, manual);
-      return Response.json(state(s, r.anchors, r.fit));
+      return Response.json({ ...state(s, r.anchors, r.fit), auto: autoInfo(r) });
    }
    if (route === "POST reset") return Response.json(state(await song(id), []));
    if (route === "PUT alignment") {
@@ -23,7 +23,9 @@ function sortedCopy(anchors: Anchor[]): Anchor[] {

 const ratioOf = (a: Anchor, b: Anchor) => (b.audioSec - a.audioSec) / (b.midiSec - a.midiSec);

-// Slopes used beyond the outermost anchors: the nearest interior stretch, or 1 with fewer than two anchors.
+// Slopes used beyond the outermost anchors: the nearest interior stretch, or 1 with fewer than two anchors. So
+// before the first anchor the map extrapolates with the *first* stretch's slope (there is no implicit 0 -> 0 anchor:
+// a warp whose first anchor is (m0, a0) maps MIDI 0 to a0 − m0·slope₀), and after the last anchor with the last one's.
 function edgeRatios(A: Anchor[]): { first: number; last: number } {
  if (A.length < 2) return { first: 1, last: 1 };
  return { first: ratioOf(A[0]!, A[1]!), last: ratioOf(A[A.length - 2]!, A[A.length - 1]!) };
@@ -43,6 +45,11 @@ function piecewise(X: number[], Y: number[], x: number, first: number, last: num
  return Y[lo]! + ((x - X[lo]!) * (Y[hi]! - Y[lo]!)) / (X[hi]! - X[lo]!);
 }

+/**
+ * Monotonic piecewise-linear MIDI <-> recording map through the anchors. Outside the anchors it extrapolates linearly:
+ * before the first anchor with the first stretch's slope, after the last anchor with the last stretch's slope (slope
+ * 1 with a single anchor, identity with none).
+ */
 export function makeWarp(anchors: Anchor[]): Warp {
  const A = sortedCopy(anchors);
  const midi = A.map((a) => a.midiSec), audio = A.map((a) => a.audioSec);
@@ -7,12 +7,20 @@ import { matchNotes, type MatchResult } from "../eval/notes.ts";
 import { DRUM_CLASSES, ROLE_ORDER, drumClass, toClassPitch, type DrumClass } from "../eval/score.ts";
 import { readManifest, readAlignment, loadSong, trackKeys } from "../ref/manifest.ts";
 import { makeWarp, type Anchor } from "../align/warp.ts";
-import { autoAlign, alignmentFit } from "../align/auto.ts";
+import { autoAlign, autoSummary, constantOffsetBaseline, fitFromCues, recordingCues, type Baseline } from "../align/auto.ts";
 import { fitSummary, type FitReport } from "../align/fit.ts";

 export interface RealSongResult {
  id: string; title: string;
-  alignment: { source: "manual" | "auto"; manual: number; auto: number; savedAt: string | null };
+  /**
+   * source: "saved" = reference/<id>/alignment.json, "computed" = auto-aligned this run (--auto). chosen: how the auto
+   * alignment was made ("dtw" or the constant-offset "baseline"), null when unknown (saved from the page).
+   * baseline: the constant-offset baseline computed this run, for comparison.
+   */
+  alignment: {
+    source: "saved" | "computed"; manual: number; auto: number; savedAt: string | null;
+    chosen: "baseline" | "dtw" | null; baseline: Baseline | { offsetSec: number; precision: number | null }; dtwPrecision: number | null;
+  };
  fit: FitReport;
  perRole: Partial<Record<Role, MatchResult>>;
  /** F1 at the lenient +-100 ms tolerance, per role (null when the role has no notes on either side). */
@@ -37,19 +45,37 @@ export async function runReal(opts: { runId: string; auto: boolean; root?: strin
  });
  const sf2 = opts.auto ? new Uint8Array(await Bun.file(SF2).arrayBuffer()) : null;
  const results: RealSongResult[] = [];
-  const skipped: string[] = [];
+  const skipped: SkippedSong[] = [];
+  const skip = (id: string, reason: string) => { skipped.push({ id, reason }); console.log(`${id}: skipped (${reason})`); };
  for (const entry of songs) {
-    const saved = await readAlignment(entry.id, root);
-    if (!saved && !opts.auto) { skipped.push(entry.id); console.log(`${entry.id}: skipped (no alignment.json; pass --auto to compute one)`); continue; }
+    let saved: Awaited<ReturnType<typeof readAlignment>>;
+    try {
+      saved = await readAlignment(entry.id, root);
+      if (saved) makeWarp(saved.anchors); // validates the anchors
+    } catch (e) { skip(entry.id, (e as Error).message); continue; }
+    if (!saved && !opts.auto) { skip(entry.id, "no alignment.json; pass --auto to compute one"); continue; }
    const t0 = performance.now();
    const { smf, audio } = await loadSong(entry.id, root);
    const keys = trackKeys(smf.tracks);
    const roleOf = (i: number): Role | null => entry.roles[keys[i]!] ?? null;
+    const roles = Object.fromEntries(keys.map((k) => [k, entry.roles[k] ?? null]));

-    let anchors: Anchor[], savedAt: string | null, source: "manual" | "auto";
-    if (saved) { anchors = saved.anchors; savedAt = saved.savedAt ?? null; source = "manual"; }
-    else { anchors = autoAlign(audio, smf, entry.roles, [], sf2!).anchors; savedAt = null; source = "auto"; }
-    const fit = alignmentFit(audio, smf, entry.roles, anchors);
+    const cues = recordingCues(audio);
+    let anchors: Anchor[], alignment: RealSongResult["alignment"];
+    if (saved) {
+      anchors = saved.anchors;
+      alignment = {
+        source: "saved", manual: 0, auto: 0, savedAt: saved.savedAt ?? null,
+        chosen: saved.auto?.chosen ?? null, baseline: constantOffsetBaseline(cues, smf, roles), dtwPrecision: saved.auto?.dtwPrecision ?? null,
+      };
+    } else {
+      const r = autoAlign(audio, smf, roles, [], sf2!);
+      anchors = r.anchors;
+      alignment = { source: "computed", manual: 0, auto: 0, savedAt: null, chosen: r.chosen, baseline: r.baseline, dtwPrecision: r.dtwPrecision };
+    }
+    alignment.manual = anchors.filter((a) => a.kind === "manual").length;
+    alignment.auto = anchors.filter((a) => a.kind === "auto").length;
+    const fit = fitFromCues(cues, smf, roles, anchors);
    const warp = makeWarp(anchors);

    const res = await pipeline({ mix: audio }, { oracleStems: false, oracleTempo: false, oracleNotes: false });
@@ -77,7 +103,7 @@ export async function runReal(opts: { runId: string; auto: boolean; root?: strin
    const seconds = (performance.now() - t0) / 1000;
    const result: RealSongResult = {
      id: entry.id, title: entry.title,
-      alignment: { source, manual: anchors.filter((a) => a.kind === "manual").length, auto: anchors.filter((a) => a.kind === "auto").length, savedAt },
+      alignment,
      fit, perRole, lenient, drumsPerClass,
      excludedTracks: smf.tracks.map((t, i) => (roleOf(i) ? null : keys[i]!)).filter((k): k is string => k !== null),
      capo: entry.capo ?? {}, seconds,
@@ -100,16 +126,25 @@ const NA = "n/a";
 const fmt = (x: number | null | undefined) => (x == null ? NA : x.toFixed(2));
 const row = (cells: string[]) => `| ${cells.join(" | ")} |`;

-export function renderRealReport(runId: string, results: RealSongResult[], skipped: string[] = []): string {
+export interface SkippedSong { id: string; reason: string }
+
+export function renderRealReport(runId: string, results: RealSongResult[], skipped: SkippedSong[] = []): string {
  const o: string[] = [`# STSheets real-song benchmark: ${runId}`, ""];
  o.push("> Reference = the team's Guitar Pro-exported MIDI warped onto the recording by the saved alignment; scores are note-onset matches (pitch must equal; drums compared by class). " +
-    "The fit line reports the *consistency of the alignment* (warped MIDI onsets vs onset-flux peaks of the recording), not an independent validation of it: " +
-    "a poor alignment lowers every score below. Capo is informational and not applied.", "");
+    "The fit line reports the *consistency of the alignment*, not an independent validation of it: precision = share of the recording's onset-flux peaks with a warped MIDI onset within ±70 ms " +
+    "(flag < 60% per 30 s window), lift = precision minus its value with the MIDI shifted off-grid (flag < 0.25), a symbolic-chroma check for whole-beat/bar shifts, " +
+    "and \"MIDI onsets audible as attacks\" (information only; a full mix exposes far fewer attacks than the score has notes). " +
+    "A poor alignment lowers every score below. Capo is informational and not applied.", "");
  for (const r of results) {
    const a = r.alignment;
-    const src = a.source === "auto" ? `auto (${a.manual} manual / ${a.auto} auto anchors, computed this run)` : `saved (${a.manual} manual / ${a.auto} auto anchors, saved ${a.savedAt ?? "unknown"})`;
+    const counts = `${a.manual} manual / ${a.auto} auto anchors`;
+    const src = a.source === "computed" ? `computed this run (${counts})` : `saved ${a.savedAt ?? "(time unknown)"} (${counts})`;
+    const pc = (x: number | null) => (x === null ? NA : `${(x * 100).toFixed(1)}%`);
+    const off = `${a.baseline.offsetSec >= 0 ? "+" : "−"}${Math.abs(a.baseline.offsetSec).toFixed(2)} s`;
+    const method = a.chosen ? autoSummary({ chosen: a.chosen, baseline: a.baseline, dtwPrecision: a.dtwPrecision })
+      : `method unknown (saved from the page) · constant-offset baseline ${off}: precision ${pc(a.baseline.precision)}`;
    o.push(`## ${r.title} (${r.id})`, "");
-    o.push(`- Alignment: ${src}`, `- Consistency of the alignment: ${fitSummary(r.fit)}`);
+    o.push(`- Alignment: ${src}`, `- Method: ${method}`, `- Consistency of the alignment: ${fitSummary(r.fit)}`);
    const capos = Object.entries(r.capo);
    o.push(`- Capo: ${capos.length ? capos.map(([k, v]) => `${k}: ${v}`).join(", ") : "none listed"}`);
    o.push(`- Excluded tracks (no role): ${r.excludedTracks.length ? r.excludedTracks.join(", ") : "none"}`, `- Pipeline + scoring: ${r.seconds.toFixed(1)} s`, "");
@@ -128,6 +163,6 @@ export function renderRealReport(runId: string, results: RealSongResult[], skipp
      o.push("");
    }
  }
-  if (skipped.length) o.push(`Skipped (no alignment.json): ${skipped.join(", ")}`, "");
+  if (skipped.length) o.push(`Skipped: ${skipped.map((s) => `${s.id} (${s.reason})`).join("; ")}`, "");
  return o.join("\n");
 }
@@ -45,7 +45,9 @@ export async function addSong(e: Omit<SongEntry, "roles"> & { roles?: SongEntry[

 // ---- per-song caches and alignment file (reference/<id>/) ----

-export interface AlignmentFile { anchors: Anchor[]; savedAt: string; fit: FitReport | null }
+/** `auto`: how `bun run align auto` produced the anchors (absent for alignments saved from the page). */
+export interface AlignmentFile { anchors: Anchor[]; savedAt: string; fit: FitReport | null; auto?: AutoInfo }
+export interface AutoInfo { chosen: "baseline" | "dtw"; baseline: { offsetSec: number; precision: number | null }; dtwPrecision: number | null }

 const inflight = new Map<string, Promise<string>>();

@@ -1,5 +1,5 @@
 import { test, expect, beforeAll } from "bun:test";
-import { alignmentFit, autoAlign, bandedWithRetry, barTimes, fitFromOnsets, recordingOnsets } from "../../src/align/auto.ts";
+import { alignmentFit, autoAlign, bandedWithRetry, barTimes, beatTimes, constantOffsetBaseline, fitFromCues, meterGrid, recordingCues, rejectOffsetOutliers } from "../../src/align/auto.ts";
 import { makeWarp, type Anchor, type Warp } from "../../src/align/warp.ts";
 import type { RefTrack, Smf } from "../../src/align/smf.ts";
 import type { Audio, Role, TruthTrack } from "../../src/core/types.ts";
@@ -49,18 +49,22 @@ function recoveryErrors(smf: Smf, got: Anchor[], trueMidiToAudio: (m: number) =>
    .map((m) => Math.abs(w.toAudio(m) - trueMidiToAudio(m)));
 }

-test("recovers a known piecewise warp: median error < 40 ms, fit match rate > 0.7", () => {
+test("recovers a known piecewise warp: median error < 40 ms, fit precision ≥ 0.8, DTW beats the constant baseline", () => {
  const D = distortion(song.song.bpm);
  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, D.toAudio);
  const t0 = performance.now();
-  const { anchors, fit } = autoAlign(song.mix, smf, rolesOf(song.truth.tracks), [], SF2);
+  const { anchors, fit, baseline, chosen } = autoAlign(song.mix, smf, rolesOf(song.truth.tracks), [], SF2);
  const ms = performance.now() - t0;
  const err = recoveryErrors(smf, anchors, D.toMidi, durOf(song.mix));
-  debug(`synthetic warp: ${anchors.length} anchors, median err ${(median(err) * 1000).toFixed(1)} ms, p90 ${(p90(err) * 1000).toFixed(1)} ms, fit ${fit.matchRate?.toFixed(3)} / ${fit.medianResidualMs?.toFixed(1)} ms, ${ms.toFixed(0)} ms`);
+  debug(`synthetic warp: ${anchors.length} anchors, median err ${(median(err) * 1000).toFixed(1)} ms, p90 ${(p90(err) * 1000).toFixed(1)} ms, fit ${fit.precision?.toFixed(3)} lift ${fit.lift?.toFixed(3)} / ${fit.medianResidualMs?.toFixed(1)} ms, baseline ${baseline.offsetSec} s ${baseline.precision?.toFixed(3)}, ${ms.toFixed(0)} ms`);
+  expect(chosen).toBe("dtw");
  expect(anchors.every((a) => a.kind === "auto")).toBe(true);
  expect(median(err)).toBeLessThan(0.04);
  expect(p90(err)).toBeLessThan(0.08);
-  expect(fit.matchRate!).toBeGreaterThan(0.7);
+  expect(fit.precision!).toBeGreaterThanOrEqual(0.8);
+  expect(fit.precision!).toBeGreaterThanOrEqual(baseline.precision! + 0.02);
+  expect(fit.windows.some((w) => w.flagged)).toBe(false);
+  expect(fit.stretches!.total).toBe(anchors.length + 1);
  expect(fit.windows.length).toBe(2);
  expect(anchors.every((a) => a.audioSec > 0 || a.midiSec === 0)).toBe(true);
 }, BIG);
@@ -86,7 +90,7 @@ test("a silent intro in the recording does not drag the path", () => {
  const { anchors, fit } = autoAlign(mix, smf, rolesOf(song.truth.tracks), [], SF2);
  const err = recoveryErrors(smf, anchors, (m) => m + 4, durOf(mix));
  expect(median(err)).toBeLessThan(0.04);
-  expect(fit.matchRate!).toBeGreaterThan(0.7);
+  expect(fit.precision!).toBeGreaterThanOrEqual(0.8);
 }, BIG);

 test("5-minute input aligns in < 15 s", () => {
@@ -94,12 +98,18 @@ test("5-minute input aligns in < 15 s", () => {
  const mix: Audio = { sampleRate: song.mix.sampleRate, channels: song.mix.channels.map((c) => { const o = new Float32Array(c.length * reps); for (let r = 0; r < reps; r++) o.set(c, r * c.length); return o; }) };
  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, (t) => t + 0.3, reps, period);
  const t0 = performance.now();
-  const { anchors, fit } = autoAlign(mix, smf, rolesOf(song.truth.tracks), [], SF2);
+  const { anchors, fit, baseline, chosen } = autoAlign(mix, smf, rolesOf(song.truth.tracks), [], SF2);
  const sec = (performance.now() - t0) / 1000;
-  debug(`5-min (${durOf(mix).toFixed(0)} s) input: ${sec.toFixed(2)} s, ${anchors.length} anchors, fit ${fit.matchRate?.toFixed(3)}`);
+  debug(`5-min (${durOf(mix).toFixed(0)} s) input: ${sec.toFixed(2)} s, ${anchors.length} anchors, fit ${fit.precision?.toFixed(3)}, ${chosen}, baseline ${baseline.offsetSec} ${baseline.precision?.toFixed(3)}`);
  expect(durOf(mix)).toBeGreaterThan(290);
  expect(sec).toBeLessThan(15);
  expect(fit.windows.length).toBe(10);
+  // a constant offset: DTW cannot beat the baseline by 0.02, so the single-anchor baseline is returned
+  expect(baseline.offsetSec).toBeCloseTo(-0.3, 1);
+  expect(chosen).toBe("baseline");
+  expect(anchors).toHaveLength(1);
+  expect(anchors[0]!.kind).toBe("auto");
+  expect(anchors[0]!.audioSec - anchors[0]!.midiSec).toBeCloseTo(baseline.offsetSec, 6);
 }, BIG);

 test("barTimes follows tempo and time-signature changes", () => {
@@ -130,13 +140,13 @@ test("fit exposes a bad alignment instead of hiding it", () => {
  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, D.toAudio);
  const roles = rolesOf(song.truth.tracks);
  const bad = alignmentFit(song.mix, smf, roles, []); // identity: off by 0.4–0.8 s throughout
-  expect(bad.matchRate!).toBeLessThan(0.5);
+  expect(bad.precision!).toBeLessThan(0.6);
  // the −0.8 s section is exposed; the +0.4 s section is not, as 0.4 s is ≈ 40 ms from two 16ths at 68 bpm (grid alias)
  expect(bad.windows[1]!.flagged).toBe(true);
  const good = alignmentFit(song.mix, smf, roles, autoAlign(song.mix, smf, roles, [], SF2).anchors);
  expect(good.windows.some((w) => w.flagged)).toBe(false);
  // unmapped tracks are excluded: with no scored roles there is nothing to measure
-  expect(alignmentFit(song.mix, smf, Object.fromEntries(Object.keys(roles).map((k) => [k, null])), []).matchRate).toBeNull();
+  expect(alignmentFit(song.mix, smf, Object.fromEntries(Object.keys(roles).map((k) => [k, null])), []).precision).toBeNull();
 }, BIG);

 test("a deliberately wrong manual anchor (+0.3 s) is kept exactly and the result stays monotonic", () => {
@@ -158,23 +168,122 @@ test("when every fine band fails, the coarse-path fallback still yields auto anc
  const D = distortion(song.song.bpm);
  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, D.toAudio);
  const manual: Anchor = { midiSec: D.toAudio(30), audioSec: 30, kind: "manual" };
-  const { anchors, fit } = autoAlign(song.mix, smf, rolesOf(song.truth.tracks), [manual], SF2, { fineBandsSec: [] });
+  const { anchors, fit, chosen } = autoAlign(song.mix, smf, rolesOf(song.truth.tracks), [manual], SF2, { fineBandsSec: [] });
+  expect(chosen).toBe("dtw"); // manual anchors present: never substituted by the baseline
  const autos = anchors.filter((a) => a.kind === "auto");
  // both segments (before and after the manual anchor) get anchors
  expect(autos.filter((a) => a.audioSec < 30).length).toBeGreaterThan(3);
  expect(autos.filter((a) => a.audioSec > 30).length).toBeGreaterThan(3);
  expect(strictlyMonotone(anchors)).toBe(true);
  const err = recoveryErrors(smf, anchors, D.toMidi, durOf(song.mix));
-  debug(`coarse fallback: ${autos.length} autos, median err ${(median(err) * 1000).toFixed(1)} ms, p90 ${(p90(err) * 1000).toFixed(1)} ms, fit ${fit.matchRate?.toFixed(3)}`);
+  debug(`coarse fallback: ${autos.length} autos, median err ${(median(err) * 1000).toFixed(1)} ms, p90 ${(p90(err) * 1000).toFixed(1)} ms, fit ${fit.precision?.toFixed(3)}`);
  expect(median(err)).toBeLessThan(0.06); // 10 Hz resolution: coarser than the fine pass, still hand-correctable
  expect(p90(err)).toBeLessThan(0.12);
 }, BIG);

-test("fitFromOnsets with cached recordingOnsets equals alignmentFit", () => {
+test("fitFromCues with cached recordingCues equals alignmentFit", () => {
  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, (t) => t);
  const roles = rolesOf(song.truth.tracks);
  const anchors: Anchor[] = [{ midiSec: 1, audioSec: 1.05, kind: "manual" }, { midiSec: 20, audioSec: 20.1, kind: "manual" }];
-  const onsets = recordingOnsets(song.mix);
-  expect(onsets.length).toBeGreaterThan(10);
-  expect(fitFromOnsets(onsets, smf, roles, anchors, durOf(song.mix))).toEqual(alignmentFit(song.mix, smf, roles, anchors));
+  const cues = recordingCues(song.mix);
+  expect(cues.onsets.length).toBeGreaterThan(10);
+  expect(cues.chroma.length).toBeGreaterThan(cues.durSec * cues.chromaRate - 2);
+  expect(fitFromCues(cues, smf, roles, anchors)).toEqual(alignmentFit(song.mix, smf, roles, anchors));
 });
+
+// ---- ruling A7: calibrated fit, beat-shift check, constant-offset baseline ----
+
+test("fit precision: correct alignment ≥ 0.8, a 0.12 s off-grid shift < 0.5 (and flagged)", () => {
+  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, (t) => t);
+  const roles = rolesOf(song.truth.tracks);
+  const good = alignmentFit(song.mix, smf, roles, []);
+  const off = alignmentFit(song.mix, smf, roles, [{ midiSec: 10, audioSec: 10.12, kind: "manual" }]);
+  debug(`precision: correct ${good.precision?.toFixed(3)} (lift ${good.lift?.toFixed(3)}, audible ${good.audibleRate?.toFixed(3)}), +0.12 s ${off.precision?.toFixed(3)} (lift ${off.lift?.toFixed(3)})`);
+  expect(good.precision!).toBeGreaterThanOrEqual(0.8);
+  expect(good.lift!).toBeGreaterThanOrEqual(0.25);
+  expect(good.windows.some((w) => w.flagged)).toBe(false);
+  expect(off.precision!).toBeLessThan(0.5);
+  expect(off.windows.filter((w) => w.precision !== null).every((w) => w.flagged)).toBe(true);
+});
+
+test("beat-shift check flags an alignment that is one beat late, though onset precision cannot see it", () => {
+  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, (t) => t);
+  const roles = rolesOf(song.truth.tracks);
+  const beat = 60 / song.song.bpm;
+  const late = alignmentFit(song.mix, smf, roles, [{ midiSec: 10, audioSec: 10 + beat, kind: "manual" }]);
+  const good = alignmentFit(song.mix, smf, roles, []);
+  debug(`beat shift: late precision ${late.precision?.toFixed(3)}, windows ${JSON.stringify(late.windows.map((w) => [w.precision?.toFixed(2), w.lift?.toFixed(2), w.beatShift]))}`);
+  expect(good.windows.some((w) => w.beatShift)).toBe(false);
+  const shifted = late.windows.filter((w) => w.beatShift);
+  expect(shifted.length).toBeGreaterThan(0);
+  expect(shifted.every((w) => w.flagged && w.reasons.some((r) => r.startsWith("beat shift suspected: −")))).toBe(true);
+});
+
+test("constantOffsetBaseline grid-searches −2..+2 s for the offset with the best precision", () => {
+  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, (t) => t + 0.27);
+  const b = constantOffsetBaseline(recordingCues(song.mix), smf, rolesOf(song.truth.tracks));
+  expect(b.offsetSec).toBeCloseTo(-0.27, 1);
+  expect(Math.round(b.offsetSec * 100)).toBe(b.offsetSec * 100); // 10 ms grid
+  expect(b.precision!).toBeGreaterThanOrEqual(0.8);
+});
+
+// ---- ruling A8: aligner guards ----
+
+test("rejectOffsetOutliers drops bars > ¼ beat from the robust median of ±3 neighbours (slope-transported), iterating until stable", () => {
+  const bars = Array.from({ length: 20 }, (_, k) => ({ midiSec: k * 2, off: 0.1 + k * 0.004, beatSec: 0.5 }));
+  bars[5]!.off = 0.6; // a beat out
+  bars[6]!.off = 0.55; // its neighbour, also out
+  bars[15]!.off = -0.3;
+  const kept = rejectOffsetOutliers(bars).map((b) => b.midiSec / 2);
+  expect(kept).toEqual(Array.from({ length: 20 }, (_, k) => k).filter((k) => k !== 5 && k !== 6 && k !== 15));
+  // a smooth ramp (genuine tempo change) is kept whole
+  const ramp = Array.from({ length: 20 }, (_, k) => ({ midiSec: k * 2, off: k * 0.05, beatSec: 0.5 }));
+  expect(rejectOffsetOutliers(ramp)).toHaveLength(20);
+  // ... also a steep one (10% tempo difference: 0.2 s per 2 s bar), whose ends a plain one-sided median would erode
+  // bar by bar; neighbours are transported along their median (Theil–Sen) slope
+  const steep = Array.from({ length: 20 }, (_, k) => ({ midiSec: k * 2, off: k * 0.2, beatSec: 0.5 }));
+  expect(rejectOffsetOutliers(steep)).toHaveLength(20);
+  // a beat jump inside the steep ramp is still rejected
+  steep[10]!.off += 0.5;
+  expect(rejectOffsetOutliers(steep).map((b) => b.midiSec / 2)).toEqual(Array.from({ length: 20 }, (_, k) => k).filter((k) => k !== 10));
+});
+
+test("meterGrid: beats and bars from tempo/time signatures; the final bar is partial when the MIDI ends mid-bar", () => {
+  const smf: Smf = { durationSec: 9, tracks: [], tempoMap: [{ sec: 0, bpm: 120 }, { sec: 4, bpm: 60 }], timeSigs: [{ sec: 0, num: 4, den: 4 }, { sec: 4, num: 3, den: 4 }] };
+  const g = meterGrid(smf);
+  expect(g.bars).toEqual([0, 2, 4, 7]);
+  expect(g.beats).toEqual([0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9]);
+  expect(g.lastBarPartial).toBe(true); // bar 7..10 ends after 9
+  expect(meterGrid({ ...smf, durationSec: 10 }).lastBarPartial).toBe(false);
+  expect(beatTimes({ durationSec: 3, tracks: [], tempoMap: [{ sec: 0, bpm: 120 }], timeSigs: [{ sec: 0, num: 6, den: 8 }] })).toEqual([0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75]);
+});
+
+test("no auto anchor in the final partial bar", () => {
+  const D = distortion(song.song.bpm);
+  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, D.toAudio);
+  const g = meterGrid(smf);
+  const last = g.bars[g.bars.length - 1]!;
+  const cut: Smf = { ...smf, durationSec: last + 0.5 * (last - g.bars[g.bars.length - 2]!) }; // ends half-way into the last bar
+  expect(meterGrid(cut).lastBarPartial).toBe(true);
+  const { anchors } = autoAlign(song.mix, cut, rolesOf(song.truth.tracks), [], SF2);
+  expect(anchors.some((a) => a.midiSec >= last - 1e-9)).toBe(false);
+}, BIG);
+
+test("12 s of extra outro in the recording: open-end DTW, no ramp at the end", () => {
+  const D = distortion(song.song.bpm);
+  const smf = smfFrom(song.truth.tracks, song.song.tracks.map((t) => t.program), song.song.bpm, D.toAudio);
+  const sr = song.mix.sampleRate, extra = 12 * sr;
+  // the outro repeats the first 12 s of the song (material that DTW could be tempted to match)
+  const mix: Audio = { sampleRate: sr, channels: song.mix.channels.map((c) => { const o = new Float32Array(c.length + extra); o.set(c); o.set(c.subarray(0, extra), c.length); return o; }) };
+  const { anchors, chosen } = autoAlign(mix, smf, rolesOf(song.truth.tracks), [], SF2);
+  expect(chosen).toBe("dtw");
+  const autos = anchors.filter((a) => a.kind === "auto");
+  const tail = autos.slice(-4).map((a) => a.audioSec - D.toMidi(a.midiSec));
+  debug(`outro: last anchor offsets vs truth ${tail.map((e) => (e * 1000).toFixed(0)).join(", ")} ms`);
+  expect(tail.every((e) => Math.abs(e) < 0.06)).toBe(true);
+  const w = makeWarp(anchors);
+  const endErr = Math.abs(w.toAudio(smf.durationSec) - D.toMidi(smf.durationSec));
+  expect(endErr).toBeLessThan(0.1);
+  const err = recoveryErrors(smf, anchors, D.toMidi, durOf(song.mix));
+  expect(median(err)).toBeLessThan(0.04);
+}, BIG);
@@ -43,3 +43,69 @@ test("3000x3000 unbanded dtw finishes in < 5 s", () => {
  expect(p[p.length - 1]).toEqual([n - 1, n - 1]);
  expect(ms).toBeLessThan(5000);
 });
+
+// ---- slope-limited steps and open begin/end (ruling A8) ----
+const stepsOf = (p: [number, number][]) => p.slice(1).map(([i, j], k) => [i - p[k]![0], j - p[k]![1]] as const);
+/** No two consecutive non-diagonal steps in the same direction: local slope stays within [1/2, 2]. */
+function slopeLimited(p: [number, number][]): boolean {
+  const s = stepsOf(p);
+  return s.every(([di, dj], k) => {
+    if (!((di === 1 && dj === 1) || (di === 1 && dj === 0) || (di === 0 && dj === 1))) return false;
+    const prev = s[k - 1];
+    return !(k > 0 && di + dj === 1 && prev![0] === di && prev![1] === dj);
+  });
+}
+
+test("slope-limited steps: path never makes an abrupt jump even when the cost invites one", () => {
+  // the cost is 0 on a path that jumps 20 columns at i = 50; symmetric steps follow the jump, slope-limited ones cannot
+  const jumpCost = (i: number, j: number) => Math.min(1, Math.abs(j - (i < 50 ? i : i + 20)) * 0.2);
+  const sym = dtw(jumpCost, 100, 120);
+  expect(stepsOf(sym).some(([di, dj], k, s) => di === 0 && dj === 1 && s[k + 1]?.[0] === 0)).toBe(true); // a horizontal run
+  const p = dtw(jumpCost, 100, 120, undefined, { steps: "slope" });
+  expect(p[0]).toEqual([0, 0]);
+  expect(p[p.length - 1]).toEqual([99, 119]);
+  expect(slopeLimited(p)).toBe(true);
+});
+
+test("slope-limited steps follow a smooth diagonal-with-kink path exactly like the default (banded too)", () => {
+  const p = dtw(cost, N, M, undefined, { steps: "slope" });
+  expect(slopeLimited(p)).toBe(true);
+  for (const [i, j] of p) if (i < 45 || i > 60) expect(Math.abs(j - truthJ(i))).toBeLessThanOrEqual(1);
+  expect(dtw(cost, N, M, { center: truthJ, width: 15 }, { steps: "slope" })).toEqual(p);
+});
+
+test("slope-limited steps throw 'unreachable' when the aspect ratio exceeds 2", () => {
+  expect(() => dtw(cost, 10, 30, undefined, { steps: "slope" })).toThrow(/unreachable/);
+});
+
+test("open end: the path may stop before the recording ends (subsequence end)", () => {
+  // the query (rows) matches columns 0..99 diagonally; the last 40 columns are extra material
+  const c = (i: number, j: number) => (j < 100 ? Math.min(1, Math.abs(i - j) * 0.2) : 0.5);
+  const closed = dtw(c, 100, 140, undefined, { steps: "slope" });
+  expect(closed[closed.length - 1]).toEqual([99, 139]);
+  const open = dtw(c, 100, 140, undefined, { steps: "slope", openEnd: true });
+  expect(open[open.length - 1]).toEqual([99, 99]);
+  for (const [i, j] of open) expect(Math.abs(i - j)).toBeLessThanOrEqual(1);
+  // the query being longer than the target (its extra rows mismatch everything, costing more than an unconsumed row):
+  // the path may end on the last column
+  const longer = dtw((i, j) => (i < 60 ? Math.min(1, Math.abs(i - j) * 0.2) : 1.5), 100, 60, undefined, { steps: "slope", openEnd: true });
+  expect(longer[longer.length - 1]).toEqual([59, 59]);
+});
+
+test("open begin: the path may start after the beginning of the target", () => {
+  const c = (i: number, j: number) => (j >= 30 ? Math.min(1, Math.abs(i + 30 - j) * 0.2) : 0.5);
+  const p = dtw(c, 100, 130, undefined, { steps: "slope", openBegin: true });
+  expect(p[0]).toEqual([0, 30]);
+  expect(p[p.length - 1]).toEqual([99, 129]);
+  for (const [i, j] of p) expect(Math.abs(i + 30 - j)).toBeLessThanOrEqual(1);
+  // banded with a band that does not include column 0 at row 0
+  expect(dtw(c, 100, 130, { center: (i) => i + 30, width: 10 }, { steps: "slope", openBegin: true })).toEqual(p);
+});
+
+test("open begin + end: a trivial path in a zero-cost corner does not win (unconsumed rows cost 1 each)", () => {
+  // the last column is free everywhere (e.g. silence vs silence), the real match is the diagonal of the first 100 columns
+  const c = (i: number, j: number) => (j === 119 ? 0 : j < 100 ? Math.min(1, Math.abs(i - j) * 0.2) : 0.6);
+  const p = dtw(c, 100, 120, undefined, { steps: "slope", openBegin: true, openEnd: true });
+  expect(p.length).toBeGreaterThan(90);
+  expect(p[p.length - 1]).toEqual([99, 99]);
+});
@@ -55,3 +55,16 @@ test("pickOnsets enforces 50 ms min gap and ignores flat input", () => {
  expect(pickOnsets(o, 100)).toEqual([0.22, 0.6]);
  expect(pickOnsets(new Float32Array(100), 100)).toEqual([]);
 });
+
+test("pickOnsets refines peaks to sub-frame times by parabolic interpolation", () => {
+  const o = new Float32Array(100);
+  o[30] = 4; o[31] = 8; o[32] = 6; // peak leans right: δ = ½(4 − 6)/(4 − 16 + 6) = 1/6 frame
+  const got = pickOnsets(o, 50);
+  expect(got).toHaveLength(1);
+  expect(got[0]!).toBeCloseTo((31 + 1 / 6) / 50, 9);
+  // min-gap competition still compares the peak heights, not the interpolated times
+  const p = new Float32Array(100);
+  p[40] = 5; p[41] = 3; p[42] = 7; p[43] = 1;
+  expect(pickOnsets(p, 100)).toHaveLength(1);
+  expect(pickOnsets(p, 100)[0]!).toBeCloseTo((42 + 0.5 * (3 - 1) / (3 - 14 + 1)) / 100, 9);
+});
@@ -1,72 +1,106 @@
 import { test, expect } from "bun:test";
-import { fitQuality, fitSummary } from "../../src/align/fit.ts";
+import { fitQuality, fitSummary, FLAG_MIN_PRECISION, FLAG_MIN_LIFT, MIN_REC_ONSETS, SHIFT_MARGIN, type FitReport, type FitWindow } from "../../src/align/fit.ts";

-test("perfect fit: match rate 1, residual 0, one window per 30 s", () => {
-  const on = [1, 2, 3, 31, 32, 61];
-  const r = fitQuality(on, on, 75);
-  expect(r.matchRate).toBe(1);
-  expect(r.medianResidualMs).toBe(0);
-  expect(r.windows.map((w) => w.startSec)).toEqual([0, 30, 60]);
-  expect(r.windows.every((w) => !w.flagged && w.matchRate === 1)).toBe(true);
+const grid = (from: number, to: number, step: number) => { const o: number[] = []; for (let t = from; t < to - 1e-9; t += step) o.push(Math.round(t * 1000) / 1000); return o; };
+
+test("constants match ruling A7", () => {
+  expect(FLAG_MIN_PRECISION).toBe(0.6);
+  expect(FLAG_MIN_LIFT).toBe(0.25);
+  expect(MIN_REC_ONSETS).toBe(8);
+  expect(SHIFT_MARGIN).toBe(0.05);
 });

-test("match within ±70 ms only; residual is median |audio − midi| of matched onsets", () => {
-  const midi = [1, 2, 3, 4];
-  const audio = [1.01, 2.03, 3.069, 4.2]; // last is 200 ms away: unmatched
-  const r = fitQuality(audio, midi, 10);
-  expect(r.matchRate).toBe(0.75);
+test("precision = share of recording onsets with a MIDI onset within ±70 ms; audibleRate = the reverse (info)", () => {
+  // recording hears every 2nd MIDI onset (dense score, sparse attacks) plus 2 extra attacks out of the score
+  const midi = grid(0.5, 20.5, 0.25); // 80 onsets
+  const rec = [...midi.filter((_, k) => k % 2 === 0), 3.13, 7.63]; // 40 + 2
+  const r = fitQuality(rec, midi, 20.5);
+  expect(r.precision).toBeCloseTo(40 / 42, 6);
+  expect(r.audibleRate).toBeCloseTo(0.5, 6);
+  expect(r.medianResidualMs).toBe(0);
+  expect(r.windows).toHaveLength(1);
+  expect(r.windows[0]!.recOnsets).toBe(42);
+  // low audibleRate alone does not flag (that was the old, miscalibrated rule)
+  expect(r.windows[0]!.flagged).toBe(false);
+});
+
+test("lift: precision minus mean precision at off-grid shifts (+0.11, +0.23, +0.37 s)", () => {
+  const midi = grid(1, 29, 1); // one onset per second: shifts of 0.11–0.37 s never land
+  const r = fitQuality(midi, midi, 30);
+  expect(r.precision).toBe(1);
+  expect(r.lift).toBeCloseTo(1, 6);
+  expect(r.windows[0]!.lift).toBeCloseTo(1, 6);
+  // a dense MIDI grid (every 0.12 s) matches anything: precision 1 but no lift -> flagged
+  const dense = grid(0, 30, 0.12);
+  const rec = grid(0.5, 29.5, 0.5).map((t) => t + 0.031);
+  const d = fitQuality(rec, dense, 30);
+  expect(d.precision).toBe(1);
+  expect(d.lift!).toBeLessThan(FLAG_MIN_LIFT);
+  expect(d.windows[0]!.flagged).toBe(true);
+  expect(d.windows[0]!.reasons.join(" ")).toContain("lift");
+});
+
+test("window flagged when precision < 0.6; null and unflagged with < 8 recording onsets", () => {
+  const midi = grid(1, 89, 1);
+  const rec = [
+    ...grid(1, 11, 1), // window 0: 10 onsets, all on the score
+    ...grid(31.5, 41.5, 1), // window 1: 10 onsets, all half-way between score onsets
+    61, 62, 63, // window 2: only 3 onsets
+  ];
+  const r = fitQuality(rec, midi, 90);
+  expect(r.windows.map((w) => w.startSec)).toEqual([0, 30, 60]);
+  expect(r.windows[0]!.precision).toBe(1);
+  expect(r.windows[0]!.flagged).toBe(false);
+  expect(r.windows[1]!.precision).toBe(0);
+  expect(r.windows[1]!.flagged).toBe(true);
+  expect(r.windows[1]!.reasons[0]).toContain("precision");
+  const w2 = r.windows[2]!;
+  expect(w2.recOnsets).toBe(3);
+  expect(w2.precision).toBeNull();
+  expect(w2.lift).toBeNull();
+  expect(w2.flagged).toBe(false);
+});
+
+test("no MIDI onsets: overall nulls, nothing flagged", () => {
+  const r = fitQuality([1, 2], [], 10);
+  expect(r.precision).toBeNull();
+  expect(r.audibleRate).toBeNull();
+  expect(r.lift).toBeNull();
+  expect(r.windows.every((w) => !w.flagged)).toBe(true);
+});
+
+test("residual: median |recording − nearest MIDI| over matched recording onsets", () => {
+  const midi = grid(1, 11, 1);
+  const r = fitQuality(midi.map((t, k) => t + (k % 2 ? 0.02 : 0.04)), midi, 12);
  expect(r.medianResidualMs!).toBeCloseTo(30, 6);
 });

-test("nearest recording onset is used and may be shared by several MIDI onsets", () => {
-  const r = fitQuality([1.0], [0.98, 1.0, 1.02], 5);
-  expect(r.matchRate).toBe(1);
-  expect(r.medianResidualMs!).toBeCloseTo(20, 6);
+test("beat-shift check: flags a window where a shifted symbolic chroma beats the current one by the margin", () => {
+  const rate = 10, n = 600; // 60 s, two windows
+  const pc = (k: number) => { const c = new Float32Array(12); c[k % 12] = 1; return c; };
+  // chord changes every second (one "beat"); recording = sequence s(t); current MIDI = s shifted by one beat in window 1
+  const seq = (t: number) => pc(Math.floor(t / rate) * 5);
+  const recording = Array.from({ length: n }, (_, t) => seq(t));
+  const current = Array.from({ length: n }, (_, t) => (t < 300 ? seq(t) : seq(t - rate)));
+  const plus1 = Array.from({ length: n }, (_, t) => seq(t - rate)), minus1 = Array.from({ length: n }, (_, t) => (t < 300 ? seq(t - rate) : seq(t)));
+  const onsets = grid(0.5, 59.5, 0.5);
+  const r = fitQuality(onsets, onsets, 60, { shift: { rate, recording, current, shifts: [{ label: "+1 beat", chroma: plus1 }, { label: "−1 beat", chroma: minus1 }] } });
+  expect(r.windows[0]!.beatShift).toBeNull();
+  expect(r.windows[0]!.flagged).toBe(false);
+  expect(r.windows[1]!.beatShift?.label).toBe("−1 beat");
+  expect(r.windows[1]!.beatShift!.gain).toBeGreaterThan(SHIFT_MARGIN);
+  expect(r.windows[1]!.flagged).toBe(true);
+  expect(r.windows[1]!.reasons).toContain("beat shift suspected: −1 beat");
 });

-test("empty window gives nulls and is not flagged", () => {
-  const r = fitQuality([1, 2, 65], [1, 2, 65], 90);
-  expect(r.windows).toHaveLength(3);
-  const w = r.windows[1]!;
-  expect(w).toEqual({ startSec: 30, matchRate: null, medianResidualMs: null, flagged: false });
-});
-
-test("no MIDI onsets at all gives overall nulls", () => {
-  const r = fitQuality([1, 2], [], 10);
-  expect(r.matchRate).toBeNull();
-  expect(r.medianResidualMs).toBeNull();
-  expect(r.windows).toEqual([{ startSec: 0, matchRate: null, medianResidualMs: null, flagged: false }]);
-});
-
-test("window flagged when match rate < 0.5", () => {
-  const midi = [1, 2, 3, 4, 31, 32];
-  const audio = [1, 31, 32]; // window 0: 1/4 matched
-  const r = fitQuality(audio, midi, 60);
-  expect(r.windows[0]!.matchRate).toBe(0.25);
-  expect(r.windows[0]!.flagged).toBe(true);
-  expect(r.windows[1]!.flagged).toBe(false);
-  expect(r.matchRate).toBe(0.5);
-});
-
-test("window flagged when median residual > 60 ms even with full match", () => {
-  const midi = [1, 2, 3];
-  const r = fitQuality(midi.map((t) => t + 0.065), midi, 10);
-  expect(r.matchRate).toBe(1);
-  expect(r.windows[0]!.medianResidualMs!).toBeCloseTo(65, 6);
-  expect(r.windows[0]!.flagged).toBe(true);
-});
-
-test("unsorted and duplicate MIDI onsets: duplicates count once", () => {
-  const r = fitQuality([2, 1], [2, 1, 1, 1], 5);
-  expect(r.matchRate).toBe(1);
-  const s = fitQuality([1], [1, 1, 1, 3], 5);
-  expect(s.matchRate).toBe(0.5); // unique onsets {1, 3}
-});
-
-test("fitSummary: one line with match rate, median residual and flagged windows", () => {
-  const w = (startSec: number, flagged: boolean) => ({ startSec, matchRate: 0.9, medianResidualMs: 10, flagged });
-  expect(fitSummary({ matchRate: 0.8234, medianResidualMs: 14.24, windows: [w(0, false), w(30, true), w(60, false), w(90, true)] }))
-    .toBe("82.3% of MIDI onsets matched · median residual 14.2 ms · 2/4 windows flagged (0:30, 1:30)");
-  expect(fitSummary({ matchRate: 1, medianResidualMs: 3, windows: [w(0, false)] })).toBe("100.0% of MIDI onsets matched · median residual 3.0 ms · 0/1 windows flagged");
-  expect(fitSummary({ matchRate: null, medianResidualMs: null, windows: [] })).toBe("no scored MIDI onsets");
+test("fitSummary: precision, lift, audible rate, residual, flagged windows with reasons, stretch warnings", () => {
+  const w = (startSec: number, reasons: string[]): FitWindow => ({ startSec, recOnsets: 20, precision: 0.9, lift: 0.7, audibleRate: 0.4, medianResidualMs: 10, beatShift: null, flagged: reasons.length > 0, reasons });
+  const f: FitReport = {
+    precision: 0.8234, lift: 0.6512, audibleRate: 0.3811, medianResidualMs: 14.24,
+    windows: [w(0, []), w(30, ["beat shift suspected: +1 beat"]), w(60, []), w(90, ["precision 41%"])],
+    stretches: { total: 40, warned: 3, over15: 1 },
+  };
+  expect(fitSummary(f)).toBe("precision 82.3% (lift +0.65) · MIDI onsets audible as attacks 38.1% · median residual 14.2 ms · 2/4 windows flagged (0:30 beat shift suspected: +1 beat; 1:30 precision 41%) · stretches: 3/40 warned, 1 > 15%");
+  expect(fitSummary({ ...f, windows: [w(0, [])], stretches: undefined })).toBe("precision 82.3% (lift +0.65) · MIDI onsets audible as attacks 38.1% · median residual 14.2 ms · 0/1 windows flagged");
+  expect(fitSummary({ precision: null, lift: null, audibleRate: null, medianResidualMs: null, windows: [] })).toBe("no scored MIDI onsets");
 });
@@ -127,12 +127,18 @@ describe("API", () => {
    expect(Math.max(...first.recordingPeaks.max)).toBeGreaterThan(0.05);
    expect(first.source).toBe("auto");
    expect(first.savedAt).toBeNull();
-    expect(first.anchors.length).toBeGreaterThan(2);
+    // the fixture's MIDI is the recording's own score (identity alignment): the constant-offset baseline wins
+    expect(first.auto.chosen).toBe("baseline");
+    expect(first.anchors).toHaveLength(1);
+    expect(Math.abs(first.auto.baseline.offsetSec)).toBeLessThan(0.05);
    expect(first.anchors.every((a: Anchor) => a.kind === "auto")).toBe(true);
    expect(first.stretches.length).toBeGreaterThanOrEqual(first.anchors.length); // +1, minus an empty edge stretch
    expect(first.stretches[1].midiBpm).toBeGreaterThan(30);
    expect(first.stretches[1].bpm).toBeCloseTo(first.stretches[1].midiBpm / first.stretches[1].ratio, 6);
-    expect(first.fit.matchRate).toBeGreaterThan(0.5);
+    expect(first.fit.precision).toBeGreaterThan(0.6);
+    expect(first.fit.stretches.total).toBe(first.stretches.length);
+    expect(["dtw", "baseline"]).toContain(first.auto.chosen);
+    expect(typeof first.auto.baseline.offsetSec).toBe("number");
    expect(first.midiTempo[0].bpm).toBeGreaterThan(30);
    expect(await Bun.file(join(root, "fix", "recording.wav")).exists()).toBe(true);
    expect(await Bun.file(join(root, "fix", "peaks.json")).exists()).toBe(true);
@@ -164,7 +170,7 @@ describe("API", () => {
    const r = await api("/api/song/fix/fit", json({ anchors: [{ midiSec: 1, audioSec: 1.4, kind: "manual" }] }));
    const j = await r.json();
    expect(j.fit.windows.length).toBeGreaterThan(0);
-    expect(j.fit.matchRate).toBeLessThan(first.fit.matchRate);
+    expect(j.fit.precision).toBeLessThan(first.fit.precision);
    expect(j.stretches.length).toBe(2);
  });
  test("bad anchors are rejected with 400", async () => {
@@ -173,12 +179,13 @@ describe("API", () => {
    expect((await api("/api/song/fix/fit", { method: "POST", body: "{not json" })).status).toBe(400);
  });
  test("auto: keeps manual anchors and re-solves autos around them", async () => {
-    const manual: Anchor = { ...first.anchors[3], kind: "manual" };
-    const r = await api("/api/song/fix/auto", json({ anchors: [manual, ...first.anchors.slice(5)] }));
+    const manual: Anchor = { midiSec: 10, audioSec: 10, kind: "manual" };
+    const r = await api("/api/song/fix/auto", json({ anchors: [manual, { midiSec: 15, audioSec: 15.5, kind: "auto" }] }));
    const j = await r.json();
    expect(j.anchors.filter((a: Anchor) => a.kind === "manual")).toEqual([manual]);
    expect(j.anchors.filter((a: Anchor) => a.kind === "auto").length).toBeGreaterThan(2);
-    expect(j.fit.matchRate).toBeGreaterThan(0.5);
+    expect(j.fit.precision).toBeGreaterThan(0.6);
+    expect(j.auto.chosen).toBe("dtw"); // manual anchors present: never the baseline
    expect(j.stretches.length).toBeGreaterThanOrEqual(j.anchors.length);
  }, BIG);
  test("reset: raw MIDI (no anchors)", async () => {
@@ -217,6 +224,6 @@ describe("API", () => {
    const j = await g.json();
    expect(j.source).toBe("auto");
    expect(j.warning).toContain("alignment.json");
-    expect(j.anchors.length).toBeGreaterThan(2);
+    expect(j.anchors.length).toBeGreaterThanOrEqual(1);
  });
 });
@@ -230,3 +230,10 @@ describe("deleteAnchor", () => {
    expect(() => deleteAnchor([M(1, 1)], 1)).toThrow();
  });
 });
+
+test("extrapolation before the first anchor uses the first stretch's slope (no implicit 0 -> 0 anchor)", () => {
+  const w = makeWarp([{ midiSec: 10, audioSec: 12, kind: "auto" }, { midiSec: 20, audioSec: 23, kind: "auto" }]); // slope 1.1
+  expect(w.toAudio(0)).toBeCloseTo(12 - 10 * 1.1, 9);
+  expect(w.toMidi(12 - 10 * 1.1)).toBeCloseTo(0, 9);
+  expect(makeWarp([{ midiSec: 10, audioSec: 12, kind: "auto" }]).toAudio(0)).toBe(2); // single anchor: slope 1
+});
@@ -9,31 +9,46 @@ import { writeAlignment, type SongEntry } from "../../src/ref/manifest.ts";
 import { createPipeline } from "../../src/pipeline/run.ts";
 import { scoreSong } from "../../src/eval/score.ts";
 import type { Role } from "../../src/core/types.ts";
+import type { FitReport, FitWindow } from "../../src/align/fit.ts";

 const hasReference = await Bun.file("reference/songs.json").exists();
 const mr = (f1: number | null, tp = 8, fp = 2, fn = 2) => ({ tp, fp, fn, precision: f1, recall: f1, f1, octaveErrors: 0, matches: [] as [number, number][] });
-const fit = (rate: number) => ({ matchRate: rate, medianResidualMs: 12.3, windows: [{ startSec: 0, matchRate: rate, medianResidualMs: 12.3, flagged: false }, { startSec: 30, matchRate: 0.2, medianResidualMs: 80, flagged: true }] });
+const win = (startSec: number, precision: number, reasons: string[]): FitWindow => ({ startSec, recOnsets: 40, precision, lift: 0.6, audibleRate: 0.4, medianResidualMs: 12.3, beatShift: null, flagged: reasons.length > 0, reasons });
+const fit = (p: number): FitReport => ({
+  precision: p, lift: 0.62, audibleRate: 0.38, medianResidualMs: 12.3,
+  windows: [win(0, p, []), win(30, 0.5, ["precision 50%"])], stretches: { total: 41, warned: 4, over15: 2 },
+});
+const base = { offsetSec: 0.06, precision: 0.88 };
 const fixture = (): RealSongResult[] => [
  {
-    id: "a", title: "Song A", alignment: { source: "auto", manual: 0, auto: 40, savedAt: null }, fit: fit(0.8),
+    id: "a", title: "Song A", alignment: { source: "computed", manual: 0, auto: 40, savedAt: null, chosen: "dtw", baseline: base, dtwPrecision: 0.8 }, fit: fit(0.8),
    perRole: { bass: mr(0.5), guitar: mr(0.9), keys: mr(null, 0, 0, 0) }, lenient: { bass: 0.6, guitar: 0.95, keys: null },
    drumsPerClass: null, excludedTracks: ["Click", "Pad"], capo: { Lead: 2 }, seconds: 3,
  },
  {
-    id: "b", title: "Song B", alignment: { source: "manual", manual: 5, auto: 30, savedAt: "2026-10-29T10:00:00Z" }, fit: fit(0.6),
+    id: "b", title: "Song B", alignment: { source: "saved", manual: 0, auto: 1, savedAt: "2026-10-29T10:00:00Z", chosen: "baseline", baseline: base, dtwPrecision: 0.85 }, fit: fit(0.6),
    perRole: { guitar: mr(0.3) }, lenient: { guitar: 0.4 }, drumsPerClass: null, excludedTracks: [], capo: {}, seconds: 4,
  },
 ];

 describe("renderRealReport", () => {
-  const md = renderRealReport("fx", fixture());
-  test("guitar row precedes the other roles; auto source and fit shown", () => {
+  const md = renderRealReport("fx", fixture(), [{ id: "c", reason: "reference/c/alignment.json: not valid JSON" }, { id: "d", reason: "no alignment.json" }]);
+  test("guitar row precedes the other roles; source, method, baseline, fit and stretch warnings shown", () => {
    expect(md).toContain("Song A");
    expect(md).toContain("computed this run");
-    expect(md).toContain("2026-10-29T10:00:00Z");
+    expect(md).toContain("saved 2026-10-29T10:00:00Z");
    expect(md).toContain("0 manual / 40 auto");
-    expect(md).toContain("80.0% of MIDI onsets matched");
+    expect(md).not.toMatch(/manual \(/);
+    expect(md).toContain("chosen: DTW (precision 80.0%)");
+    expect(md).toContain("chosen: baseline (DTW precision 85.0% not better");
+    expect(md).toContain("constant-offset baseline +0.06 s: precision 88.0%");
+    expect(md).toContain("precision 80.0% (lift +0.62)");
+    expect(md).toContain("MIDI onsets audible as attacks 38.0%");
+    expect(md).toContain("1/2 windows flagged (0:30 precision 50%)");
+    expect(md).toContain("stretches: 4/41 warned, 2 > 15%");
    expect(md).toContain("consistency of the alignment");
+    expect(md).toContain("c (reference/c/alignment.json: not valid JSON)");
+    expect(md).toContain("d (no alignment.json)");
    const a = md.slice(md.indexOf("Song A"), md.indexOf("Song B"));
    expect(a.indexOf("| guitar")).toBeGreaterThan(-1);
    expect(a.indexOf("| guitar")).toBeLessThan(a.indexOf("| bass"));
@@ -59,15 +74,18 @@ describe("runReal", () => {
        id: "t1a-1", title: "Synthetic t1a/1", audio: join(dir, "mix.wav"), midi: join(ref, "t1a-1", "song.mid"),
        roles: Object.fromEntries(truth.tracks.map((t) => [t.name, t.role])), capo: {},
      };
-      await Bun.write(join(ref, "songs.json"), JSON.stringify([entry, { ...entry, id: "noalign", title: "No alignment" }]));
+      await Bun.write(join(ref, "songs.json"), JSON.stringify([entry, { ...entry, id: "noalign", title: "No alignment" }, { ...entry, id: "corrupt", title: "Corrupt alignment" }]));
+      await Bun.write(join(ref, "corrupt", "alignment.json"), "{ not json");
      const dur = 600;
      await writeAlignment("t1a-1", { anchors: [{ midiSec: 0, audioSec: 0, kind: "manual" }, { midiSec: dur, audioSec: dur, kind: "manual" }], savedAt: "2026-10-30T00:00:00Z", fit: null }, ref);

      const results = await runReal({ runId: "t", auto: false, root: ref, out: join(root, "runs"), resultsDir: join(root, "results") });
      expect(results.map((r) => r.id)).toEqual(["t1a-1"]);
      const r = results[0]!;
-      expect(r.alignment).toEqual({ source: "manual", manual: 2, auto: 0, savedAt: "2026-10-30T00:00:00Z" });
-      expect(r.fit.matchRate).not.toBeNull();
+      expect(r.alignment).toEqual({ source: "saved", manual: 2, auto: 0, savedAt: "2026-10-30T00:00:00Z", chosen: null, baseline: expect.any(Object), dtwPrecision: null });
+      expect(Math.abs(r.alignment.baseline.offsetSec)).toBeLessThan(0.05); // identity alignment
+      expect(r.fit.precision).not.toBeNull();
+      expect(r.fit.stretches).toBeDefined();

      const pipe = await createPipeline({
        demucs: new Uint8Array(await Bun.file("models/demucs/htdemucs_6s.onnx").arrayBuffer()),
@@ -81,7 +99,10 @@ describe("runReal", () => {
      for (const f of ["notes.json", "out.gp", "score.json"]) expect(await Bun.file(join(root, "runs", "real-t", "t1a-1", f)).exists()).toBe(true);
      const json = await Bun.file(join(root, "results", "real-t.json")).text();
      expect(json).not.toContain('"matches"');
-      expect(await Bun.file(join(root, "results", "real-t.md")).text()).toContain("Synthetic t1a/1");
+      const md = await Bun.file(join(root, "results", "real-t.md")).text();
+      expect(md).toContain("Synthetic t1a/1");
+      expect(md).toMatch(/corrupt \(.*alignment\.json.*not valid JSON/);
+      expect(md).toContain("noalign (no alignment.json; pass --auto to compute one)");
    } finally { await rm(root, { recursive: true, force: true }); }
  }, 600_000);