feat(cli): configurable transcribe timeout with duration-scaled default

Adds a `--timeout <ms>` CLI flag (and `HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS`
env var) plus a model-slowdown factor in the auto-scaled default so
`hyperframes transcribe` doesn't hard-fail with `spawnSync ETIMEDOUT`
on slow CPUs running heavier whisper models.

Field-signal ts=1784165471 (win32/arm64 emulating x64 on Snapdragon,
CLI 0.7.59) reported the failure on a 63s wav with `-m medium` at ~13x
realtime — the historical 10x-realtime scale (PR #2463) gave 10.5 min
while the machine needed ~13.7 min. Splitting audio and merging offsets
was the manual workaround.

- Add `--timeout <ms>` and `HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS` (min 5000).
  Explicit override bypasses auto-scaling; still capped at 12h.
- Add per-model slowdown factor (tiny 0.5, base 0.7, small 1, medium 2,
  large 4, large-v3-turbo 2). Multiplied into the 10s/audio-second
  baseline so medium/large get proportional headroom while `small.en`
  (the default) preserves the historical safety window.
- Wrap whisper's spawn error with a discoverability hint naming
  `--timeout`, the env var, and the effective timeout when the child
  was killed by SIGTERM/ETIMEDOUT (mirrors PR #2504 protocol-timeout).
- Docs: new `--timeout` row in `docs/packages/cli.mdx` Flags table.

Regression coverage in `packages/cli/src/whisper/transcribe.test.ts`
(56 tests) and `packages/cli/src/commands/transcribe.test.ts` (5 tests):
- Model factor per known name + case-insensitive + safe unknown fallback.
- 63s field-signal case on medium.en → 1_260_000ms (was 630_000ms).
- Explicit override honored below the auto floor + capped at 12h.
- Model factor ignored when overrideMs is set.
- SIGTERM/ETIMEDOUT detection + augmented message contract.
- CLI rejects below-minimum `--timeout` with error naming both the flag
  and the 5000ms floor.

— Via

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

docs/packages/cli.mdx

@@ -289,6 +289,7 @@ This is suppressed in CI environments, non-TTY shells, and when `HYPERFRAMES_NO_
     | `--to` | Export transcript sidecar format: `srt` or `vtt` |
     | `--output, -o` | Output path for exported SRT/VTT sidecar |
     | `--preserve-cues` | Keep each transcript entry as its own caption cue (skip word-level grouping) |
+    | `--timeout` | Whisper spawn timeout in ms. Overrides the duration+model auto-scaled default. Increase on slow CPUs (e.g. emulated arm64/x64) where medium/large models take many seconds per audio second. Minimum 5000. Env: `HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS`. |
     | `--json` | Output result as JSON |
 
     The command auto-detects the input type. Audio/video files are transcribed with whisper.cpp. Transcript files (`.json`, `.srt`, `.vtt`) are normalized and imported. Pass `--to srt` or `--to vtt` with a transcript input to write a caption sidecar instead.

packages/cli/src/commands/transcribe.test.ts

@@ -99,6 +99,32 @@ Render video. Built for agents.
     });
   });
 
+  it("rejects a below-minimum --timeout with a discoverable error", async () => {
+    const { dir, input } = dummyAudio();
+    dirs.push(dir);
+    const consoleLog = vi.mocked(console.log);
+    const exitSpy = vi.spyOn(process, "exit").mockImplementation(((code?: number) => {
+      throw new Error(`__exit_${code}__`);
+    }) as never);
+
+    // 100 is well below the 5000ms minimum — must fail loud instead of silently
+    // reverting to the auto-scaled default (the whole point of the flag is
+    // that the user explicitly asked for a specific value).
+    await expect(
+      transcribeCmd.run!({ args: { input, json: true, timeout: "100" } } as never),
+    ).rejects.toThrow("__exit_1__");
+
+    const log = consoleLog.mock.calls.at(-1)?.[0];
+    expect(typeof log).toBe("string");
+    if (typeof log !== "string") throw new Error("Expected JSON log output");
+    const parsed = JSON.parse(log);
+    expect(parsed.ok).toBe(false);
+    expect(parsed.error).toContain("--timeout");
+    expect(parsed.error).toContain("5000");
+
+    exitSpy.mockRestore();
+  });
+
   it("--preserve-cues keeps single-word cues separate when exporting from JSON", async () => {
     const dir = mkdtempSync(join(tmpdir(), "hf-transcribe-test-"));
     dirs.push(dir);

packages/cli/src/commands/transcribe.ts

@@ -23,6 +23,14 @@ import { resolve, join, extname, dirname } from "node:path";
 import * as clack from "@clack/prompts";
 import { c } from "../ui/colors.js";
 import { DEFAULT_MODEL, isWhisperUnavailable } from "../whisper/manager.js";
+
+// Minimum accepted value for `--timeout` / `HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS`.
+// Kept out of `whisper/transcribe.ts` (avoids a top-level import into this
+// command module) so the CLI test file can hoist its
+// `vi.mock("../whisper/transcribe.js")` factory without the mocked module
+// entering the sync-import graph. Below this floor the whisper spawn has no
+// realistic chance of completing even on the fastest hardware for the shortest clip.
+const CLI_TIMEOUT_MIN_MS = 5000;
 import { trackCommandFailure, trackTranscribeUnavailable } from "../telemetry/events.js";
 
 export default defineCommand({
@@ -85,6 +93,16 @@ export default defineCommand({
         "Treat captions as optional: if whisper-cpp is unavailable, skip and exit 0 instead of failing. For pipelines that continue without captions.",
       default: false,
     },
+    timeout: {
+      type: "string",
+      description:
+        "Whisper spawn timeout in ms. Overrides the duration+model auto-scaled " +
+        "default. Increase on slow CPUs (e.g. emulated arm64/x64, low-power " +
+        "laptops) where whisper.cpp takes many seconds per audio second on " +
+        "medium/large models. Applies to the whisper engine only; Parakeet has " +
+        "a separate fixed timeout. Minimum 5000 (5 s). " +
+        "Env: HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS.",
+    },
   },
   async run({ args }) {
     const inputPath = resolve(args.input);
@@ -120,16 +138,42 @@ export default defineCommand({
     }
 
     // ── Transcribe mode: run the ASR engine ──────────────────────────────
+    const timeoutMs = parseTimeoutMs(args.timeout, args.json);
+
     return transcribeAudio(inputPath, dir, {
       engine: args.engine,
       model: args.model,
       language: args.language,
       json: args.json,
       optional: args.optional,
+      timeoutMs,
     });
   },
 });
 
+/**
+ * Resolve the whisper timeout override from `--timeout <ms>` or the
+ * `HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS` env var. The flag wins over env; both
+ * paths share the same integer + minimum-5000ms validation so a bad value
+ * fails loud instead of silently reverting to the auto-scaled default.
+ * Returns `undefined` when neither source is set — the transcribe layer then
+ * derives the timeout from audio duration and model factor.
+ */
+function parseTimeoutMs(raw: string | undefined, json: boolean): number | undefined {
+  const source = raw ?? process.env["HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS"];
+  if (source == null || source === "") return undefined;
+
+  const parsed = Number.parseInt(source, 10);
+  if (!Number.isFinite(parsed) || parsed < CLI_TIMEOUT_MIN_MS) {
+    const origin = raw != null ? "--timeout" : "HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS";
+    failWith(
+      `Invalid ${origin}: "${source}". Must be an integer >= ${CLI_TIMEOUT_MIN_MS} (ms).`,
+      json,
+    );
+  }
+  return parsed;
+}
+
 function failWith(message: string, json: boolean): never {
   trackCommandFailure("transcribe", message);
   if (json) {
@@ -226,7 +270,14 @@ async function exportTranscript(
 async function transcribeAudio(
   inputPath: string,
   dir: string,
-  opts: { engine?: string; model?: string; language?: string; json?: boolean; optional?: boolean },
+  opts: {
+    engine?: string;
+    model?: string;
+    language?: string;
+    json?: boolean;
+    optional?: boolean;
+    timeoutMs?: number;
+  },
 ): Promise<void> {
   const { transcribe } = await import("../whisper/transcribe.js");
   const { loadTranscript, patchCaptionHtml, stripBeforeOnset } =
@@ -260,6 +311,7 @@ async function transcribeAudio(
           model,
           language: opts.language,
           onProgress: spin ? (msg) => spin.message(msg) : undefined,
+          timeoutMs: opts.timeoutMs,
         });
 
     let { words } = loadTranscript(result.transcriptPath);

packages/cli/src/whisper/transcribe.test.ts

@@ -1,8 +1,11 @@
 import { describe, expect, it, test } from "vitest";
 import {
   dtwPresetForModel,
+  isWhisperTimeoutError,
   resolveAudioPreparationTimeoutMs,
   resolveWhisperTimeoutMs,
+  whisperModelSlowdownFactor,
+  wrapWhisperTimeoutError,
 } from "./transcribe.js";
 
 describe("dtwPresetForModel", () => {
@@ -55,6 +58,179 @@ describe("resolveWhisperTimeoutMs", () => {
   });
 });
 
+describe("whisperModelSlowdownFactor", () => {
+  it.each([
+    ["tiny", 0.5],
+    ["tiny.en", 0.5],
+    ["base", 0.7],
+    ["base.en", 0.7],
+    ["small", 1],
+    ["small.en", 1],
+    ["medium", 2],
+    ["medium.en", 2],
+    ["large-v1", 4],
+    ["large-v2", 4],
+    ["large-v3", 4],
+    ["large-v3-turbo", 2],
+  ])("returns factor %s for known model %s", (model, expected) => {
+    expect(whisperModelSlowdownFactor(model)).toBe(expected);
+  });
+
+  it("is case-insensitive", () => {
+    expect(whisperModelSlowdownFactor("MEDIUM.EN")).toBe(2);
+    expect(whisperModelSlowdownFactor("Large-V3")).toBe(4);
+  });
+
+  it("falls back to the small.en baseline (1) for unknown model names", () => {
+    // Unknown model names must never shorten the safety window, so the default
+    // is the baseline factor rather than a smaller (tiny/base) value.
+    expect(whisperModelSlowdownFactor("my-custom-finetune")).toBe(1);
+  });
+});
+
+describe("resolveWhisperTimeoutMs with model factor", () => {
+  it("keeps small.en (default) at the historical baseline for 63s clips", () => {
+    // Regression guard: `small.en` (the CLI default model) must not lose
+    // headroom when the caller passes a model. 63s * 10s/s * 1 = 630_000ms.
+    expect(resolveWhisperTimeoutMs(63, { model: "small.en" })).toBe(630_000);
+  });
+
+  it("doubles the auto-scaled window for medium.en (field-signal ts=1784165471)", () => {
+    // 63s clip on medium.en: emulated arm64/x64 saw ~13x realtime — 63*13 =
+    // 819s needed. The 10x baseline (630_000ms = 10.5min) was insufficient;
+    // the 2x medium factor lifts it to 1_260_000ms (21min), covering the
+    // reported case with margin.
+    expect(resolveWhisperTimeoutMs(63, { model: "medium.en" })).toBe(1_260_000);
+  });
+
+  it("quadruples the auto-scaled window for the large family", () => {
+    expect(resolveWhisperTimeoutMs(60, { model: "large-v3" })).toBe(2_400_000);
+  });
+
+  it("still enforces the five-minute floor on tiny/base short clips", () => {
+    // 10s * 10 * 0.5 = 50_000ms — below the floor, so the floor wins.
+    expect(resolveWhisperTimeoutMs(10, { model: "tiny.en" })).toBe(300_000);
+  });
+
+  it("still enforces the twelve-hour cap on large-model marathon clips", () => {
+    // 4320s * 10 * 4 = 172_800_000ms — the cap wins.
+    expect(resolveWhisperTimeoutMs(4320, { model: "large-v3" })).toBe(43_200_000);
+  });
+
+  it("scales the null-duration fallback by the model factor", () => {
+    // When ffprobe can't read the WAV, we lose duration signal but still know
+    // the model. Scaling the floor keeps heavy models proportionally covered.
+    expect(resolveWhisperTimeoutMs(null, { model: "medium.en" })).toBe(600_000);
+    expect(resolveWhisperTimeoutMs(null, { model: "large-v3" })).toBe(1_200_000);
+    expect(resolveWhisperTimeoutMs(null, { model: "small.en" })).toBe(300_000);
+  });
+});
+
+describe("resolveWhisperTimeoutMs with overrideMs", () => {
+  it("respects the explicit override even below the auto-scaled floor", () => {
+    // A user who deliberately passed --timeout 30000 on a short clip meant 30s,
+    // not five minutes. The auto-scaled floor must NOT stomp on the explicit
+    // value — that would silently defeat the flag.
+    expect(resolveWhisperTimeoutMs(60, { overrideMs: 30_000 })).toBe(30_000);
+  });
+
+  it("caps the override at the twelve-hour ceiling", () => {
+    // A runaway value still can't hang the process forever.
+    expect(resolveWhisperTimeoutMs(60, { overrideMs: 999_999_999 })).toBe(43_200_000);
+  });
+
+  it("ignores the model factor when overrideMs is set", () => {
+    // Override wins outright — the model factor only affects the auto-scaled
+    // path. Otherwise `--timeout 60000` on `medium.en` would silently become
+    // 120_000ms and break the discoverability contract.
+    expect(resolveWhisperTimeoutMs(60, { overrideMs: 60_000, model: "medium.en" })).toBe(60_000);
+  });
+
+  it.each([0, -1, Number.NaN, Number.POSITIVE_INFINITY])(
+    "falls back to auto-scaling when overrideMs is invalid (%s)",
+    (invalid) => {
+      // Invalid override → auto-scaled default kicks in (10s * 10 = 100_000ms →
+      // floor 300_000ms). Prevents callers from accidentally disabling the
+      // timeout via a bad env var.
+      expect(resolveWhisperTimeoutMs(10, { overrideMs: invalid })).toBe(300_000);
+    },
+  );
+});
+
+describe("isWhisperTimeoutError", () => {
+  it("returns true for a Node SIGTERM child-timeout error", () => {
+    const err = Object.assign(new Error("Command failed"), { signal: "SIGTERM" });
+    expect(isWhisperTimeoutError(err)).toBe(true);
+  });
+
+  it("returns true for an ETIMEDOUT-coded error", () => {
+    const err = Object.assign(new Error("Command failed"), { code: "ETIMEDOUT" });
+    expect(isWhisperTimeoutError(err)).toBe(true);
+  });
+
+  it("returns false for a non-timeout child error", () => {
+    const err = Object.assign(new Error("Exit code 1"), { status: 1, signal: null });
+    expect(isWhisperTimeoutError(err)).toBe(false);
+  });
+
+  it("returns false for a plain string", () => {
+    expect(isWhisperTimeoutError("boom")).toBe(false);
+  });
+});
+
+describe("wrapWhisperTimeoutError", () => {
+  it("passes non-timeout errors through unchanged", () => {
+    const err = new Error("segfault");
+    const result = wrapWhisperTimeoutError(err, {
+      effectiveTimeoutMs: 600_000,
+      model: "small.en",
+      wasOverride: false,
+    });
+    expect(result).toBe(err);
+  });
+
+  it("wraps SIGTERM timeouts with a discoverable hint naming --timeout and the env var", () => {
+    const original = Object.assign(new Error("Command failed"), { signal: "SIGTERM" });
+    const wrapped = wrapWhisperTimeoutError(original, {
+      effectiveTimeoutMs: 1_260_000,
+      model: "medium.en",
+      wasOverride: false,
+    });
+    // Discoverability contract: message must name the flag, env var, effective
+    // timeout, and cite the model so slow-CPU reporters see the knob.
+    expect(wrapped).not.toBe(original);
+    expect(wrapped.message).toContain("--timeout");
+    expect(wrapped.message).toContain("HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS");
+    expect(wrapped.message).toContain("1260s");
+    expect(wrapped.message).toContain("medium.en");
+    expect((wrapped as { cause?: unknown }).cause).toBe(original);
+  });
+
+  it("labels the source as explicit when wasOverride is true", () => {
+    const original = Object.assign(new Error("Command failed"), { code: "ETIMEDOUT" });
+    const wrapped = wrapWhisperTimeoutError(original, {
+      effectiveTimeoutMs: 90_000,
+      model: "medium.en",
+      wasOverride: true,
+    });
+    expect(wrapped.message).toContain("explicit --timeout 90000ms");
+  });
+
+  it("coerces non-Error timeout-shaped values to Error without augmenting", () => {
+    // A non-Error thrown value can't carry `.signal`, so it flows through
+    // untouched but is still normalized to an Error instance for downstream
+    // handlers that use instanceof checks.
+    const wrapped = wrapWhisperTimeoutError("something bad", {
+      effectiveTimeoutMs: 600_000,
+      model: "small.en",
+      wasOverride: false,
+    });
+    expect(wrapped).toBeInstanceOf(Error);
+    expect(wrapped.message).toBe("something bad");
+    expect(wrapped.message).not.toContain("--timeout");
+  });
+});
+
 describe("resolveAudioPreparationTimeoutMs", () => {
   it.each([
     [10, 120_000],

packages/cli/src/whisper/transcribe.ts

@@ -46,19 +46,95 @@ const AUDIO_PREPARATION_TIMEOUT_PER_MEDIA_SECOND_MS = 500;
 const AUDIO_PREPARATION_TIMEOUT_CAP_MS = 21_600_000;
 
 /**
- * Give long recordings enough time to transcribe while retaining a bounded
+ * Model-specific slowdown factors relative to the `small.en` default. whisper.cpp's
- * failure window. Short recordings keep the historical five-minute timeout.
+ * per-token inference cost scales with model size — `medium` runs ~2x slower than
+ * `small`, and the `large` family ~4x slower — so the 10x-realtime baseline that
+ * comfortably covers `small.en` can still time out on `medium.en`/`large-v3` when
+ * the CPU itself is slow. Applying the factor keeps the historical safety window
+ * for the default model while giving heavier models the headroom they need on
+ * emulated arm64/x64 hardware (field-signal ts=1784165471: Snapdragon emulating
+ * x64 saw ~13x realtime on medium.en for a 63s clip).
+ *
+ * Values are conservative bounds, not tight upper bounds — the auto-scaled
+ * timeout is still capped at 12h and gated by an explicit `--timeout` override.
  */
-export function resolveWhisperTimeoutMs(durationSeconds: number | null): number {
+const WHISPER_MODEL_SLOWDOWN_FACTORS: Readonly<Record<string, number>> = {
+  tiny: 0.5,
+  "tiny.en": 0.5,
+  base: 0.7,
+  "base.en": 0.7,
+  small: 1,
+  "small.en": 1,
+  medium: 2,
+  "medium.en": 2,
+  "large-v1": 4,
+  "large-v2": 4,
+  "large-v3": 4,
+  "large-v3-turbo": 2,
+};
+
+// Unknown model names fall back to the `small.en` baseline so the returned
+// timeout never dips below the historical safe window for a novel/custom model.
+const DEFAULT_MODEL_SLOWDOWN_FACTOR = 1;
+
+/**
+ * Look up the auto-scale slowdown factor for a whisper model name. Case-
+ * insensitive. Unknown names fall back to the `small.en` baseline (factor 1)
+ * rather than a smaller factor so unknown models never accidentally shorten
+ * the safety window.
+ */
+export function whisperModelSlowdownFactor(model: string): number {
+  return WHISPER_MODEL_SLOWDOWN_FACTORS[model.toLowerCase()] ?? DEFAULT_MODEL_SLOWDOWN_FACTOR;
+}
+
+export interface ResolveWhisperTimeoutOptions {
+  /** Whisper model name (e.g. `small.en`, `medium.en`, `large-v3`). Selects the slowdown factor. */
+  model?: string;
+  /**
+   * Explicit override in milliseconds. Bypasses duration+model auto-scaling.
+   * Still clamped to the 12h cap so a runaway value can't hang the process
+   * indefinitely; validation of the lower bound is the caller's responsibility.
+   */
+  overrideMs?: number;
+}
+
+/**
+ * Give long recordings enough time to transcribe while retaining a bounded
+ * failure window. Short recordings keep the historical five-minute floor.
+ *
+ * Formula: `clamp(FLOOR, duration * PER_SECOND * modelFactor, CAP)`.
+ * An explicit `overrideMs` bypasses the formula entirely (still capped at 12h).
+ */
+export function resolveWhisperTimeoutMs(
+  durationSeconds: number | null,
+  options?: ResolveWhisperTimeoutOptions,
+): number {
+  // Explicit override wins — respect the caller's exact value (still capped at
+  // the 12h ceiling so a runaway value can't leave the process hung forever).
+  // We do NOT re-apply the floor here: a user who deliberately passed
+  // `--timeout 30000` on a 3s clip meant 30 seconds, not five minutes.
+  if (
+    options?.overrideMs != null &&
+    Number.isFinite(options.overrideMs) &&
+    options.overrideMs > 0
+  ) {
+    return Math.min(WHISPER_TIMEOUT_CAP_MS, options.overrideMs);
+  }
+
+  const factor = options?.model ? whisperModelSlowdownFactor(options.model) : 1;
+
   if (durationSeconds === null || !Number.isFinite(durationSeconds) || durationSeconds <= 0) {
-    return WHISPER_TIMEOUT_FLOOR_MS;
+    // Duration unknown: keep the historical five-minute floor for the default
+    // model, but scale it up for heavier models so `medium`/`large` still get
+    // a proportionate window when ffprobe can't read the WAV header.
+    return Math.min(WHISPER_TIMEOUT_CAP_MS, Math.ceil(WHISPER_TIMEOUT_FLOOR_MS * factor));
   }
 
   return Math.min(
     WHISPER_TIMEOUT_CAP_MS,
     Math.max(
       WHISPER_TIMEOUT_FLOOR_MS,
-      Math.ceil(durationSeconds * WHISPER_TIMEOUT_PER_AUDIO_SECOND_MS),
+      Math.ceil(durationSeconds * WHISPER_TIMEOUT_PER_AUDIO_SECOND_MS * factor),
     ),
   );
 }
@@ -185,6 +261,12 @@ export interface TranscribeOptions {
   model?: string;
   language?: string;
   onProgress?: (message: string) => void;
+  /**
+   * Explicit whisper spawn timeout in ms. Overrides the duration+model auto-
+   * scaled default. Callers that leave this undefined get the auto-scaled
+   * default derived from prepared WAV duration and the selected model.
+   */
+  timeoutMs?: number;
 }
 
 export interface TranscribeResult {
@@ -388,11 +470,26 @@ export async function transcribe(
   }
   whisperArgs.push(wavPath);
 
-  const whisperTimeoutMs = resolveWhisperTimeoutMs(getPreparedWavDurationSeconds(wavPath));
+  const whisperTimeoutMs = resolveWhisperTimeoutMs(getPreparedWavDurationSeconds(wavPath), {
+    model: effectiveModel,
+    overrideMs: options?.timeoutMs,
+  });
+  try {
     execFileSync(whisper.executablePath, whisperArgs, {
       stdio: "ignore",
       timeout: whisperTimeoutMs,
     });
+  } catch (err) {
+    // Surface the timeout knob when the child was killed by our own timeout —
+    // otherwise the reporter sees a bare ETIMEDOUT / SIGTERM with no hint that
+    // `--timeout` even exists. Non-timeout errors flow through unchanged so the
+    // existing stderr-tail handling in `transcribeAudio` still applies.
+    throw wrapWhisperTimeoutError(err, {
+      effectiveTimeoutMs: whisperTimeoutMs,
+      model: effectiveModel,
+      wasOverride: options?.timeoutMs != null,
+    });
+  }
 
   // 6. Read and validate output
   const transcriptPath = `${outputBase}.json`;
@@ -433,3 +530,51 @@ export async function transcribe(
     speechOnsetSeconds,
   };
 }
+
+// ---------------------------------------------------------------------------
+// Timeout error discoverability
+// ---------------------------------------------------------------------------
+
+// Node's `execFileSync` kills the child with SIGTERM when its `timeout` option
+// fires, so the resulting Error carries `signal === "SIGTERM"`. On some platforms
+// / Node versions `code === "ETIMEDOUT"` is also set. Match either signal so we
+// don't miss a timeout on a platform we haven't validated.
+export function isWhisperTimeoutError(err: unknown): boolean {
+  if (!(err instanceof Error)) return false;
+  const record = err as { signal?: unknown; code?: unknown };
+  return record.signal === "SIGTERM" || record.code === "ETIMEDOUT";
+}
+
+export interface WrapWhisperTimeoutOptions {
+  effectiveTimeoutMs: number;
+  model: string;
+  /** True when the timeout was set via `--timeout`; false when it was auto-scaled. */
+  wasOverride: boolean;
+}
+
+/**
+ * Wrap a whisper spawn error with a discoverability hint when the child was
+ * killed by our timeout. Names the effective timeout, the CLI flag, and the
+ * env var so slow-CPU users see the knob rather than a bare `ETIMEDOUT`.
+ * Non-timeout errors flow through unchanged (as `Error` for well-typed
+ * downstream handling).
+ */
+export function wrapWhisperTimeoutError(err: unknown, options: WrapWhisperTimeoutOptions): Error {
+  if (!isWhisperTimeoutError(err)) {
+    return err instanceof Error ? err : new Error(String(err));
+  }
+
+  const seconds = Math.round(options.effectiveTimeoutMs / 1000);
+  const source = options.wasOverride
+    ? `explicit --timeout ${options.effectiveTimeoutMs}ms`
+    : `auto-scaled default for model ${options.model}`;
+  const message =
+    `Whisper transcription exceeded ${seconds}s (${source}). ` +
+    `Raise --timeout <ms> or set HYPERFRAMES_TRANSCRIBE_TIMEOUT_MS. ` +
+    `Slow CPUs (e.g. emulated arm64/x64, low-power laptops) may need many ` +
+    `multiples of realtime on heavier models — medium.en can run ~10-15x ` +
+    `realtime on constrained hardware.`;
+  const wrapped = new Error(message);
+  (wrapped as { cause?: unknown }).cause = err;
+  return wrapped;
+}