Carry the vocabulary hint as prompt from the engines through the worker into every window

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/engine.test.ts

@@ -77,6 +77,17 @@ describe("BrowserEngine", () => {
     expect("modelId" in message).toBe(false);
   });
 
+  test("forwards the vocabulary hint as `prompt`, and omits an empty one (D-WS2a)", () => {
+    const worker = new FakeWorker();
+    const engine = new BrowserEngine(worker);
+    void engine.transcribe(new Float32Array(4), { prompt: "Maxwell\nVoid" });
+    void engine.transcribe(new Float32Array(4), { prompt: "" });
+    void engine.transcribe(new Float32Array(4));
+    expect(worker.posted[0]!.message.prompt).toBe("Maxwell\nVoid");
+    expect("prompt" in worker.posted[1]!.message).toBe(false);
+    expect("prompt" in worker.posted[2]!.message).toBe(false);
+  });
+
   test("asks the worker to stream only when a consumer listens", () => {
     const worker = new FakeWorker();
     const engine = new BrowserEngine(worker);
@@ -241,6 +252,23 @@ describe("ServerEngine", () => {
     expect(url).not.toContain("language");
   });
 
+  test("sends the vocabulary hint as the `prompt` query param, only when non-empty", async () => {
+    const urls: string[] = [];
+    const engine = new ServerEngine(async (input) => {
+      urls.push(input);
+      return ndjsonResponse([{ type: "result", text: "", chunks: [] }]);
+    });
+    await engine.transcribe(new Float32Array(4), {
+      modelId: "onnx-community/whisper-tiny",
+      prompt: "Århus, Voices of the Void",
+    });
+    await engine.transcribe(new Float32Array(4), { modelId: "onnx-community/whisper-tiny", prompt: "" });
+    expect(new URL(urls[0]!, "http://localhost").searchParams.get("prompt")).toBe(
+      "Århus, Voices of the Void",
+    );
+    expect(urls[1]).not.toContain("prompt");
+  });
+
   test("requires a modelId", () => {
     const engine = new ServerEngine(async () => ndjsonResponse([]));
     expect(engine.transcribe(new Float32Array(4))).rejects.toThrow(/model id/);