diff --git a/src/server/live.ts b/src/server/live.ts
index 25bf36f..61bfe5b 100644
--- a/src/server/live.ts
+++ b/src/server/live.ts
@@ -60,6 +60,8 @@ export function createLiveWebSocket(deps: LiveWebSocketDeps) {
                 audioPath,
                 language: ws.data.language,
                 task: ws.data.task,
+                // Short passes: sequential decode, no batching/VAD (D-WS5a).
+                mode: "live",
               },
               (segment) => {
                 text.push(segment.text);
diff --git a/src/server/transcribe.ts b/src/server/transcribe.ts
index aacf696..fba9192 100644
--- a/src/server/transcribe.ts
+++ b/src/server/transcribe.ts
@@ -316,6 +316,8 @@ export interface TranscribeDeps {
   maxUploadBytes?: number;
   /** Directory for uploaded-PCM temp files. Default: the OS temp dir. */
   tempDir?: string;
+  /** Blank-line heartbeat period on the ndjson stream (D-021). Default 5 s. */
+  heartbeatIntervalMs?: number;
 }
 
 function json(body: unknown, status: number): Response {
@@ -409,6 +411,7 @@ async function writeBodyToFile(
 export function createTranscribeHandlers(deps: TranscribeDeps): TranscribeHandlers {
   const maxUploadBytes = deps.maxUploadBytes ?? DEFAULT_MAX_UPLOAD_BYTES;
   const tempDir = deps.tempDir ?? tmpdir();
+  const heartbeatIntervalMs = deps.heartbeatIntervalMs ?? HEARTBEAT_INTERVAL_MS;
 
   return {
     async status(request) {
@@ -460,7 +463,7 @@ export function createTranscribeHandlers(deps: TranscribeDeps): TranscribeHandle
             } catch {
               clearInterval(heartbeat);
             }
-          }, HEARTBEAT_INTERVAL_MS);
+          }, heartbeatIntervalMs);
           const chunks: TranscriptChunk[] = [];
           try {
             await deps.sidecar.transcribe(
diff --git a/tests/fixtures/fake_sidecar.py b/tests/fixtures/fake_sidecar.py
index ac4f3e6..b69b8e2 100644
--- a/tests/fixtures/fake_sidecar.py
+++ b/tests/fixtures/fake_sidecar.py
@@ -5,7 +5,7 @@ Used by tests/transcribe.test.ts to exercise the real Bun-side supervisor
 requested model name:
 
   "ok"              two segments (the second reports the audio byte count,
-                    language and task for pass-through assertions), then done
+                    language, task and mode for pass-through assertions), then done
   "boom"            an error line
   "die"             exit(1) mid-request (crash simulation)
   "wait-for-cancel" one segment, then block until the next stdin line
@@ -53,6 +53,7 @@ for line in sys.stdin:
         emit({"type": "segment", "id": request_id, "start": 0.0, "end": 1.5,
               "text": " Hello"})
         emit({"type": "segment", "id": request_id, "start": 1.5, "end": 3.0,
-              "text": " world. [model=%s bytes=%d lang=%s task=%s]"
-              % (model, size, message.get("language"), message.get("task"))})
+              "text": " world. [model=%s bytes=%d lang=%s task=%s mode=%s]"
+              % (model, size, message.get("language"), message.get("task"),
+                 message.get("mode"))})
         emit({"type": "done", "id": request_id, "cancelled": False})
diff --git a/tests/server-live.test.ts b/tests/server-live.test.ts
index 3c41acc..da024e7 100644
--- a/tests/server-live.test.ts
+++ b/tests/server-live.test.ts
@@ -111,7 +111,7 @@ function sessionData(overrides: Partial<LiveSessionData> = {}): LiveSessionData
 }
 
 describe("createLiveWebSocket", () => {
-  test("a committed segment writes the exact PCM bytes and passes model/language/task through", async () => {
+  test("a committed segment writes the exact PCM bytes and passes model/language/task/mode through", async () => {
     const tempDir = await makeTempDir();
     const sidecar = fakeSidecar();
     const handler = createLiveWebSocket({ sidecar });
@@ -123,7 +123,8 @@ describe("createLiveWebSocket", () => {
 
     await waitFor(() => sidecar.requests.length > 0);
     expect(sidecar.requests[0]).toEqual(
-      expect.objectContaining({ model: "base", language: "da", task: "translate" }),
+      // Live passes are short: sequential decoding, no batching/VAD (D-WS5a).
+      expect.objectContaining({ model: "base", language: "da", task: "translate", mode: "live" }),
     );
     // The bytes written to the temp file are exactly the committed segment's
     // samples — not, e.g., a whole shared buffer (see the byteOffset/byteLength fix).
diff --git a/tests/transcribe.test.ts b/tests/transcribe.test.ts
index 25e92e1..eb7cdfb 100644
--- a/tests/transcribe.test.ts
+++ b/tests/transcribe.test.ts
@@ -52,6 +52,8 @@ function fakeSidecar(
     segments?: SidecarSegment[];
     error?: string;
     waitForAbort?: boolean;
+    /** Quiet time before the first segment (model load, VAD, first batch). */
+    delayMs?: number;
   } = {},
 ): FakeSidecar {
   const segments = behavior.segments ?? [
@@ -72,6 +74,7 @@ function fakeSidecar(
         new Uint8Array(await Bun.file(request.audioPath).arrayBuffer()),
       );
       if (behavior.error) throw new Error(behavior.error);
+      if (behavior.delayMs) await Bun.sleep(behavior.delayMs);
       onSegment(segments[0]!);
       if (behavior.waitForAbort) {
         await new Promise<void>((resolve) => {
@@ -323,6 +326,25 @@ describe("POST /api/transcribe", () => {
     expect(await readdir(tempDir)).toEqual([]);
   });
 
+  test("heartbeat blank lines keep a quiet stream alive before the first segment (D-021)", async () => {
+    // The batched pipeline (D-WS5a) runs VAD over the whole file and decodes a
+    // full batch before its first segment, so the quiet stretch is longer.
+    const handlers = createTranscribeHandlers({
+      sidecar: fakeSidecar({ delayMs: 200 }),
+      getSessionUser: sessionFromCookie,
+      tempDir: await makeTempDir(),
+      heartbeatIntervalMs: 20,
+    });
+    const response = await handlers.post(
+      postRequest("?model=onnx-community/whisper-base", pcmBody([0.1]), { cookie: "sid=ok" }),
+    );
+    const text = await response.text();
+    const beforeFirstSegment = text.slice(0, text.indexOf("{"));
+    expect(beforeFirstSegment.length).toBeGreaterThanOrEqual(2);
+    expect(beforeFirstSegment.trim()).toBe("");
+    expect(text).toContain('"type":"result"');
+  });
+
   test("sidecar errors travel in-stream after the 200 header", async () => {
     const tempDir = await makeTempDir();
     const handlers = createTranscribeHandlers({
@@ -385,8 +407,12 @@ describe("createSidecar (fake python sidecar)", () => {
     expect(outcome).toEqual({ cancelled: false });
     expect(segments).toHaveLength(2);
     expect(segments[0]).toEqual({ start: 0, end: 1.5, text: " Hello" });
-    // The fixture echoes what it received (audio bytes, language, task).
-    expect(segments[1]!.text).toContain("model=ok bytes=12 lang=da task=translate");
+    // The fixture echoes what it received (audio bytes, language, task, mode).
+    expect(segments[1]!.text).toContain("model=ok bytes=12 lang=da task=translate mode=file");
+
+    const live: SidecarSegment[] = [];
+    await sidecar.transcribe({ model: "ok", audioPath, mode: "live" }, (s) => live.push(s));
+    expect(live[1]!.text).toContain("mode=live");
   });
 
   test("sidecar 'error' lines reject that request only", async () => {