fix(video): parse multipart/form-data create body (OpenAI SDK 6.28+)

CHANGELOG.md

@@ -13,6 +13,10 @@
 
 - `POST /__aimock/reset` — now a deprecated alias for `/__aimock/reset/fixtures`; it still performs a full reset but emits a `Deprecation` response header and a `deprecated` field in the body. Use the explicit `/reset/fixtures` or `/reset/journal` routes instead.
 
+### Fixed
+
+- **Video** — `POST /v1/videos` (`videos.create`) now parses `multipart/form-data` bodies. The OpenAI SDK (>=6.28.0) sends video-create requests as multipart instead of JSON (even for File-less bodies), which previously returned a `400 invalid_json`. The handler reuses the existing transcription multipart field parser and preserves the JSON path for older SDKs.
+
 ## [1.28.0] - 2026-06-02
 
 ### Added

docs/video/index.html

@@ -68,7 +68,7 @@ <h2>Endpoints</h2>
             <tr>
               <td>POST</td>
               <td>/v1/videos</td>
-              <td>JSON (create video job)</td>
+              <td>JSON or multipart form-data (create video job)</td>
             </tr>
             <tr>
               <td>GET</td>

src/__tests__/multimedia.test.ts

@@ -319,6 +319,33 @@ describe("video generation", () => {
     expect(res.status).toBe(404);
     await mock.stop();
   });
+
+  test("video creation parses multipart/form-data body (OpenAI SDK 6.28.0+)", async () => {
+    const mock = new LLMock({ port: 0 });
+    mock.addFixture({
+      match: { userMessage: "a guitar", endpoint: "video" },
+      response: {
+        video: { id: "vid_mp", status: "completed", url: "https://example.com/video.mp4" },
+      },
+    });
+    await mock.start();
+
+    // The OpenAI SDK (>=6.28.0) sends videos.create as multipart/form-data.
+    // `fetch` sets the multipart Content-Type (with boundary) automatically.
+    const form = new FormData();
+    form.set("model", "sora-2");
+    form.set("prompt", "a guitar");
+    form.set("seconds", "8");
+    const create = await fetch(`${mock.url}/v1/videos`, {
+      method: "POST",
+      headers: { Authorization: "Bearer test" },
+      body: form,
+    });
+    const job = await create.json();
+    expect(job.id).toBe("vid_mp");
+    expect(job.status).toBe("completed");
+    await mock.stop();
+  });
 });
 
 describe("convenience methods", () => {

src/video.ts

@@ -16,6 +16,7 @@ import { writeErrorResponse } from "./sse-writer.js";
 import type { Journal } from "./journal.js";
 import { applyChaos } from "./chaos.js";
 import { proxyAndRecord } from "./recorder.js";
+import { extractBoundary, extractFormField } from "./transcription.js";
 
 interface VideoRequest {
   model?: string;
@@ -119,30 +120,55 @@ export async function handleVideoCreate(
   const path = req.url ?? "/v1/videos";
   const method = req.method ?? "POST";
 
+  const contentType = Array.isArray(req.headers["content-type"])
+    ? req.headers["content-type"][0]
+    : req.headers["content-type"];
+  const isMultipart = (contentType ?? "").toLowerCase().includes("multipart/form-data");
+
   let videoReq: VideoRequest;
-  try {
-    videoReq = JSON.parse(raw) as VideoRequest;
-  } catch (parseErr) {
-    const detail = parseErr instanceof Error ? parseErr.message : "unknown";
-    journal.add({
-      method,
-      path,
-      headers: flattenHeaders(req.headers),
-      body: null,
-      response: { status: 400, fixture: null },
-    });
-    writeErrorResponse(
-      res,
-      400,
-      JSON.stringify({
-        error: {
-          message: `Malformed JSON: ${detail}`,
-          type: "invalid_request_error",
-          code: "invalid_json",
-        },
-      }),
-    );
-    return;
+  if (isMultipart) {
+    // The OpenAI SDK (6.28.0+) sends POST /v1/videos as multipart/form-data;
+    // older SDKs sent JSON for a File-less body. Parse the form fields into the
+    // same shape the JSON path produces, reusing the transcription multipart
+    // helpers. Numeric fields (e.g. `seconds`) arrive as strings and are
+    // coerced to numbers to match the JSON body's types.
+    const boundary = extractBoundary(contentType);
+    const prompt = extractFormField(raw, "prompt", boundary);
+    const model = extractFormField(raw, "model", boundary);
+    const size = extractFormField(raw, "size", boundary);
+    const secondsRaw = extractFormField(raw, "seconds", boundary);
+    videoReq = { prompt: prompt ?? "" };
+    if (model !== undefined) videoReq.model = model;
+    if (size !== undefined) videoReq.size = size;
+    if (secondsRaw !== undefined) {
+      const secondsNum = Number(secondsRaw);
+      videoReq.seconds = Number.isNaN(secondsNum) ? secondsRaw : secondsNum;
+    }
+  } else {
+    try {
+      videoReq = JSON.parse(raw) as VideoRequest;
+    } catch (parseErr) {
+      const detail = parseErr instanceof Error ? parseErr.message : "unknown";
+      journal.add({
+        method,
+        path,
+        headers: flattenHeaders(req.headers),
+        body: null,
+        response: { status: 400, fixture: null },
+      });
+      writeErrorResponse(
+        res,
+        400,
+        JSON.stringify({
+          error: {
+            message: `Malformed JSON: ${detail}`,
+            type: "invalid_request_error",
+            code: "invalid_json",
+          },
+        }),
+      );
+      return;
+    }
   }
 
   if (!videoReq.prompt) {