diff --git a/src/worker/protocol.ts b/src/worker/protocol.ts
index 688333a..7cdcd9b 100644
--- a/src/worker/protocol.ts
+++ b/src/worker/protocol.ts
@@ -435,6 +435,22 @@ export function applyLoadProgress(
 // Transcription progress accumulation (pure; drives the in-progress UI)
 // ---------------------------------------------------------------------------
 
+/**
+ * One generate pass's slice of the audio, for
+ * `TranscribeProgressTracker.forWindows` (variable-length chunks, D-WS3a).
+ */
+export interface ProgressWindow {
+  /** Window-relative seconds → absolute seconds on the file's timeline. */
+  toAbsolute(seconds: number): number;
+  /** Absolute end of the audio this window covers; timestamps clamp here. */
+  endSeconds: number;
+  /**
+   * Absolute progress once this window's pass finished: the next window's
+   * start, or the total for the last window.
+   */
+  nextStartSeconds: number;
+}
+
 /**
  * Maps the streamer's window-relative timestamps to absolute audio progress.
  *
@@ -446,25 +462,56 @@ export function applyLoadProgress(
  * ends. This tracker turns those into absolute seconds that are monotone
  * (windows overlap, so a later window's early timestamps can precede an
  * earlier window's late ones — regressions are swallowed) and clamped to
- * [0, totalSeconds].
+ * [0, totalSeconds]. The long-form pipeline (D-WS3a) uses `forWindows`
+ * instead: its windows are variable-length speech chunks with skipped
+ * silence between them.
  */
 export class TranscribeProgressTracker {
   private windowIndex = 0;
   private reported = 0;
+  private readonly window: (index: number) => ProgressWindow | undefined;
 
+  /**
+   * Fixed-stride windows, as the chunked ASR pipeline produces:
+   * `windowSeconds` long, advancing by `jumpSeconds` (window − 2·stride).
+   */
   constructor(
     private readonly totalSeconds: number,
-    private readonly windowSeconds: number,
-    private readonly jumpSeconds: number,
-  ) {}
+    windowSeconds: number,
+    jumpSeconds: number,
+    windows?: readonly ProgressWindow[],
+  ) {
+    this.window = windows
+      ? (index) => windows[index]
+      : (index) => ({
+          toAbsolute: (t) => index * jumpSeconds + Math.min(t, windowSeconds),
+          endSeconds: index * jumpSeconds + windowSeconds,
+          nextStartSeconds: (index + 1) * jumpSeconds,
+        });
+  }
+
+  /**
+   * Variable-length windows — the long-form pipeline's speech chunks
+   * (D-WS3a): each maps its own relative time onto the file's timeline, and
+   * skipped silence between chunks counts as processed once a chunk ends.
+   */
+  static forWindows(
+    totalSeconds: number,
+    windows: readonly ProgressWindow[],
+  ): TranscribeProgressTracker {
+    return new TranscribeProgressTracker(totalSeconds, 0, 0, windows);
+  }
 
   /**
    * Absolute progress for a window-relative timestamp, or null when it does
    * not advance past what was already reported.
    */
   timestamp(withinWindowSeconds: number): number | null {
-    const clamped = Math.min(Math.max(withinWindowSeconds, 0), this.windowSeconds);
-    return this.advance(this.windowIndex * this.jumpSeconds + clamped);
+    const window = this.window(this.windowIndex);
+    if (!window) return null;
+    return this.advance(
+      Math.min(window.toAbsolute(Math.max(withinWindowSeconds, 0)), window.endSeconds),
+    );
   }
 
   /**
@@ -472,8 +519,9 @@ export class TranscribeProgressTracker {
    * of the next window (the amount of audio no later window revisits).
    */
   finalizeWindow(): number | null {
+    const window = this.window(this.windowIndex);
     this.windowIndex += 1;
-    return this.advance(this.windowIndex * this.jumpSeconds);
+    return this.advance(window ? window.nextStartSeconds : this.totalSeconds);
   }
 
   private advance(processedSeconds: number): number | null {
diff --git a/tests/protocol.test.ts b/tests/protocol.test.ts
index 43bf74c..f5dde6e 100644
--- a/tests/protocol.test.ts
+++ b/tests/protocol.test.ts
@@ -274,6 +274,37 @@ describe("TranscribeProgressTracker", () => {
     expect(tracker.timestamp(8)).toBe(8);
     expect(tracker.finalizeWindow()).toBeNull(); // 20 clamps to 8, not an advance
   });
+
+  describe("forWindows (variable-length VAD chunks, D-WS3a)", () => {
+    // Two speech chunks with a skipped 15 s silence between them: [0–20 s]
+    // and [35–60 s] of a 70 s file.
+    const windows = [
+      { toAbsolute: (t: number) => t, endSeconds: 20, nextStartSeconds: 35 },
+      { toAbsolute: (t: number) => 35 + t, endSeconds: 60, nextStartSeconds: 70 },
+    ];
+
+    test("maps chunk-relative times through each window", () => {
+      const tracker = TranscribeProgressTracker.forWindows(70, windows);
+      expect(tracker.timestamp(5)).toBe(5);
+      // Finishing a chunk counts the skipped silence up to the next one.
+      expect(tracker.finalizeWindow()).toBe(35);
+      expect(tracker.timestamp(10)).toBe(45);
+      expect(tracker.timestamp(2)).toBeNull(); // 37 < 45: no regression
+      expect(tracker.finalizeWindow()).toBe(70);
+      expect(tracker.finalizeWindow()).toBeNull(); // past the last window
+    });
+
+    test("clamps a stray timestamp to the window's end", () => {
+      const tracker = TranscribeProgressTracker.forWindows(70, windows);
+      expect(tracker.timestamp(400)).toBe(20);
+    });
+
+    test("no windows (no speech) → nothing to report but the end", () => {
+      const tracker = TranscribeProgressTracker.forWindows(70, []);
+      expect(tracker.timestamp(3)).toBeNull();
+      expect(tracker.finalizeWindow()).toBe(70);
+    });
+  });
 });
 
 describe("applyLoadProgress", () => {