Reject task=translate on the server for models that cannot translate (D-044)

The UI already hid it; a direct API call to POST /api/transcribe or the live
WebSocket could still get the source language back labelled as a translation.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/WorkBreakdown.md

@@ -427,8 +427,9 @@ Found while building transcript translation (D-045), 2026-10-29:
       every row of a padded batch, so translation runs one sentence per call
       (~4× slower than batch 8 would be). Re-check after the 4.3 upgrade with
       `bun scripts/bench-translate.ts` ("output identical" across batch sizes).
-- [ ] The server API still accepts `task: "translate"` with large-v3-turbo
-      (only the UI hides it, D-044).
+- [x] The server API still accepts `task: "translate"` with large-v3-turbo
+      (only the UI hides it, D-044). — Fixed: `POST /api/transcribe` and the
+      live WebSocket answer 400 via `translateNotSupported` (src/server/transcribe.ts).
 
 Found in spec 001 workstream 6 (D-046), open:
 - [ ] Transformers.js 4.2 has no beam search: `num_beams > 1` in a model's

src/index.ts

@@ -7,7 +7,13 @@ import { ortAliasPlugin } from "../scripts/ortAlias.ts";
 import { injectPrecacheManifest } from "./pwa/precache.ts";
 import { auth } from "./server/auth.ts";
 import { AUTH_ENABLED } from "./server/flags.ts";
-import { MAX_UPLOAD_BYTES, parseLanguageAndTask, sidecar, transcribe } from "./server/transcribe.ts";
+import {
+  MAX_UPLOAD_BYTES,
+  parseLanguageAndTask,
+  sidecar,
+  transcribe,
+  translateNotSupported,
+} from "./server/transcribe.ts";
 import { liveWebSocket } from "./server/live.ts";
 import { getFasterWhisperModel } from "./lib/models.ts";
 
@@ -140,6 +146,8 @@ async function handleApiRequest(request: Request, server: any): Promise<Response
       // because this entry point is a WebSocket, not JSON over HTTP).
       const options = parseLanguageAndTask(url);
       if (typeof options === "string") return new Response(options, { status: 400 });
+      const translateError = translateNotSupported(modelId, options.task);
+      if (translateError) return new Response(translateError, { status: 400 });
       // Fail fast like the HTTP route (D-013 "degrade gracefully") instead of
       // opening a socket that can only report unavailability once the first
       // segment reaches the sidecar.

src/server/transcribe.ts

@@ -27,7 +27,7 @@ import { tmpdir } from "node:os";
 import { join } from "node:path";
 import type { TranscribeStreamLine } from "../lib/engine";
 import { HINT_MAX_CHARS, normalizeHint } from "../lib/hint";
-import { getFasterWhisperModel } from "../lib/models";
+import { getFasterWhisperModel, getModel } from "../lib/models";
 import type { TranscriptChunk, WhisperTask } from "../worker/protocol";
 import { getSessionUser as appGetSessionUser, type SessionUser } from "./auth.ts";
 
@@ -377,6 +377,17 @@ export function parseLanguageAndTask(
   return { language, task: rawTask, ...(prompt && { prompt }) };
 }
 
+/**
+ * Whisper's translate task only on models trained for it (D-044): the UI
+ * hides it elsewhere, and the server rejects it the same way so a direct
+ * API call can't get the source language back labelled as a translation.
+ * Returns the 400 error text, or undefined when the request is fine.
+ */
+export function translateNotSupported(modelId: string, task: WhisperTask): string | undefined {
+  if (task !== "translate" || getModel(modelId)?.canTranslate) return undefined;
+  return `model ${modelId} cannot translate; use task=transcribe`;
+}
+
 /** Parse + validate query params; a string is the 400 error text. */
 function parseParams(url: URL): TranscribeParams | string {
   const modelId = url.searchParams.get("model");
@@ -387,6 +398,8 @@ function parseParams(url: URL): TranscribeParams | string {
 
   const options = parseLanguageAndTask(url);
   if (typeof options === "string") return options;
+  const translateError = translateNotSupported(modelId, options.task);
+  if (translateError) return translateError;
 
   return { modelId, fasterWhisperModel, ...options };
 }

tests/transcribe.test.ts

@@ -6,6 +6,7 @@ import {
   createSidecar,
   createTranscribeHandlers,
   parseLanguageAndTask,
+  translateNotSupported,
   type Sidecar,
   type SidecarSegment,
   type SidecarTranscribeRequest,
@@ -262,6 +263,41 @@ describe("POST /api/transcribe", () => {
     }
   });
 
+  test("400 for translate on a model not trained for it; translate still works where it is (D-044)", async () => {
+    const sidecar = fakeSidecar();
+    const handlers = createTranscribeHandlers({
+      sidecar,
+      getSessionUser: sessionFromCookie,
+      tempDir: await makeTempDir(),
+    });
+    for (const model of [
+      "onnx-community/whisper-large-v3-turbo",
+      "onnx-community/whisper-small.en",
+    ]) {
+      const response = await handlers.post(
+        postRequest(`?model=${model}&task=translate`, pcmBody([0.1]), { cookie: "sid=ok" }),
+      );
+      expect(response.status).toBe(400);
+      expect(await response.text()).toContain("cannot translate");
+    }
+    expect(sidecar.requests).toHaveLength(0);
+
+    const ok = await handlers.post(
+      postRequest("?model=onnx-community/whisper-small&task=translate", pcmBody([0.1]), {
+        cookie: "sid=ok",
+      }),
+    );
+    expect(ok.status).toBe(200);
+  });
+
+  test("translateNotSupported: the shared check for the HTTP and live routes", () => {
+    expect(translateNotSupported("onnx-community/whisper-large-v3-turbo", "translate")).toContain(
+      "cannot translate",
+    );
+    expect(translateNotSupported("onnx-community/whisper-large-v3-turbo", "transcribe")).toBeUndefined();
+    expect(translateNotSupported("onnx-community/whisper-small", "translate")).toBeUndefined();
+  });
+
   test("503 with the reason when the sidecar is unavailable", async () => {
     const sidecar = fakeSidecar({ available: false, reason: "faster-whisper is not installed" });
     const handlers = createTranscribeHandlers({