fix(audio): refuse trim windows shorter than one codec frame (#679)

trim-audio stream-copies, and a window below one frame ships a container with zero audio frames as a 200 success. Floor the window at 0.1s in the schema, and refuse after endS clamps to the real duration if the window drops under the floor. Fixes #671.

apps/api/src/routes/tools/trim-audio.ts

@@ -5,12 +5,19 @@ import { audioContentType, formatFfmpegSeconds, runMediaTool } from "../../lib/m
 import { InputValidationError } from "../../modality/contract.js";
 import { createToolRoute } from "../tool-factory.js";
 
+// Stream copy cannot cut below one codec frame (mp3 ~26ms, flac up to ~93ms);
+// smaller windows produce a container with zero audio frames.
+const MIN_TRIM_WINDOW_S = 0.1;
+
 const settingsSchema = z
   .object({
     startS: z.number().finite().min(0).default(0),
     endS: z.number().finite().min(0.000001),
   })
-  .refine((s) => s.endS > s.startS, { message: "End must be after start" });
+  .refine((s) => s.endS > s.startS, { message: "End must be after start" })
+  .refine((s) => s.endS - s.startS >= MIN_TRIM_WINDOW_S, {
+    message: `Trim window must be at least ${MIN_TRIM_WINDOW_S} seconds`,
+  });
 
 export function registerTrimAudio(app: FastifyInstance) {
   createToolRoute(app, {
@@ -34,6 +41,11 @@ export function registerTrimAudio(app: FastifyInstance) {
           throw new InputValidationError("Start is beyond the end of the audio");
         }
         const endS = Math.min(settings.endS, info.durationS);
+        if (endS - settings.startS < MIN_TRIM_WINDOW_S) {
+          throw new InputValidationError(
+            `Trim window is shorter than ${MIN_TRIM_WINDOW_S} seconds after clamping to the audio duration`,
+          );
+        }
         // Fast seek with stream-copy for audio
         return [
           "-ss",

tests/integration/tools/audio/trim-audio.test.ts

@@ -67,4 +67,17 @@ describe.skipIf(!ffmpegAvailable())("trim-audio (requires ffmpeg)", () => {
     const res = await runTool({ startS: 0.5, endS: 0.2 });
     expect(res.statusCode).toBe(400);
   });
+
+  it("rejects a sub-frame trim window instead of returning an empty file", async () => {
+    const res = await runTool({ startS: 0, endS: 0.000001 });
+    expect(res.statusCode).toBe(400);
+  });
+
+  it("rejects when clamping endS to the duration leaves a sub-frame window", async () => {
+    // tiny.mp3 is ~1.0-1.045s depending on the ffprobe build; endS clamps to
+    // the duration, leaving well under 0.1s either way.
+    const res = await runTool({ startS: 0.98, endS: 9 });
+    expect(res.statusCode).toBe(422);
+    expect(JSON.parse(res.body).details).toMatch(/Trim window/);
+  });
 });

tests/unit/api/media-settings-schema.test.ts

@@ -41,6 +41,14 @@ describe("media time settings schemas", () => {
     expect(parses("trim-video", { startS: 0, endS: Number.MIN_VALUE })).toBe(false);
   });
 
+  it("rejects trim-audio windows shorter than the stream-copy floor", () => {
+    // Stream copy cannot cut below one codec frame (mp3 ~26ms, flac up to
+    // ~93ms), so windows under 0.1s ship a file with zero audio frames.
+    expect(parses("trim-audio", { startS: 0, endS: 0.000001 })).toBe(false);
+    expect(parses("trim-audio", { startS: 5, endS: 5.05 })).toBe(false);
+    expect(parses("trim-audio", { startS: 0, endS: 0.5 })).toBe(true);
+  });
+
   it("requires at least one representable fade duration", () => {
     expect(parses("fade-audio", { fadeInS: Number.MIN_VALUE, fadeOutS: 0 })).toBe(false);
     expect(parses("fade-audio", { fadeInS: 0, fadeOutS: 0.000001 })).toBe(true);