stop parsing request bodies on sidecar proxy routes

every multipart upload through /api/<sidecar>/* arrived corrupted. bodyParser ran on
proxy routes and called parseBody for multipart, so hono cached a FormData on the
request; when the proxy then asked for the bytes hono re-serialised them from that
cache with a NEW boundary, while the proxy still forwarded the ORIGINAL content-type
header. header and body disagreed and the far side rejected it with
"Multipart: Unexpected end of form".

bodyParser now skips prefixes owned by createSidecarProxy, which register themselves
so a new sidecar cannot forget. the proxy also forwards the body as a stream instead
of buffering it, which drops the second in-memory copy of every upload.

note the bug report proposed skipping multipart in bodyParser outright; that would
have broken /upload, /file-browser upload and /bug-report, which do read a multipart
body from ctx.get('body').

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/servers/_middlewares/body-parser.ts

@@ -1,7 +1,16 @@
 import type { MiddlewareHandler } from 'hono';
 import * as errors from '../custom-errors';
+import { isProxiedPath } from '../sidecar/proxied-prefixes';
 
 export const bodyParser: () => MiddlewareHandler = () => async (ctx, next) => {
+  // Sidecar proxy routes: the body is forwarded verbatim and never read here. Consuming it would be
+  // pointless for JSON and destructive for multipart — see sidecar/proxied-prefixes.ts for why a parsed
+  // multipart body cannot be forwarded intact.
+  if (isProxiedPath(ctx.req.path)) {
+    ctx.set('body', {});
+    return next();
+  }
+
   if (!['POST', 'PUT', 'PATCH'].includes(ctx.req.method.toUpperCase())) {
     ctx.set('body', {});
     return next();

src/servers/sidecar/create-proxy.test.ts

@@ -0,0 +1,173 @@
+import { describe, expect, it, afterAll, beforeAll } from 'bun:test';
+import { Hono } from 'hono';
+import { bodyParser } from '../_middlewares/body-parser';
+import { createRouter } from '../create-router';
+import { handleSidecarMessage } from '../sidecar-registry';
+import { createSidecarProxy } from './create-proxy';
+
+// Regression cover for COMMS/MULTIPART_PROXY_BUG.md: every multipart upload through a sidecar proxy
+// arrived corrupted, because bodyParser parsed a body the proxy only meant to forward. Hono cached the
+// resulting FormData and re-serialised it with a NEW boundary when the proxy asked for the bytes, while
+// the proxy still forwarded the ORIGINAL content-type header. The far side then failed with
+// "Multipart: Unexpected end of form".
+//
+// The assertion that matters is the one the bug report names: the boundary in the content-type header and
+// the boundary in the body must be the same one the client sent.
+
+const BOUNDARY = '6600BE83-EFF4-498D-9739-B6E978654467';
+
+// A JPEG-shaped payload with embedded NULs and high bytes — the things a naive text round-trip mangles.
+// Deliberately not tiny: Bun 1.3.10's multipart parser truncates an 8-byte part at its first NUL, which is
+// a fixture artefact (64 bytes and up are fine) but cost time to chase, so it is written down here.
+const FILE_BYTES = (() => {
+  const bytes = new Uint8Array(512);
+  bytes.set([0xff, 0xd8, 0xff, 0xe0, 0x00, 0x10, 0x4a, 0x46, 0x49, 0x46, 0x00, 0x01]);
+  for (let i = 12; i < bytes.length; i++) bytes[i] = (i * 7) % 256;
+  bytes.set([0xff, 0xd9], bytes.length - 2);
+  return bytes;
+})();
+
+/** The phone's exact wire format: a UUID boundary, fields in a fixed order, a binary file part. */
+function phoneMultipart(): { body: Uint8Array<ArrayBuffer>; contentType: string } {
+  const fields = [
+    ['deviceAssetId', 'probe-1'],
+    ['deviceId', 'probe-device'],
+    ['fileCreatedAt', '2026-08-07T00:00:00.000Z'],
+    ['fileModifiedAt', '2026-08-07T00:00:00.000Z'],
+    ['isFavorite', 'false'],
+  ];
+  const head = fields
+    .map(([k, v]) => `--${BOUNDARY}\r\nContent-Disposition: form-data; name="${k}"\r\n\r\n${v}\r\n`)
+    .join('');
+  const filePrologue =
+    `--${BOUNDARY}\r\n` +
+    `Content-Disposition: form-data; name="assetData"; filename="t.jpg"\r\n` +
+    `Content-Type: image/jpeg\r\n\r\n`;
+  const epilogue = `\r\n--${BOUNDARY}--\r\n`;
+
+  const enc = new TextEncoder();
+  const parts = [enc.encode(head), enc.encode(filePrologue), FILE_BYTES, enc.encode(epilogue)];
+  const body = new Uint8Array(new ArrayBuffer(parts.reduce((n, p) => n + p.length, 0)));
+  let at = 0;
+  for (const p of parts) {
+    body.set(p, at);
+    at += p.length;
+  }
+  return { body, contentType: `multipart/form-data; boundary=${BOUNDARY}` };
+}
+
+/** Stands in for a sidecar. Reports what actually arrived on the wire. */
+type Seen = {
+  contentType: string;
+  bodyBoundary: string | null;
+  fields: string[];
+  fileBytes: number[] | null;
+  raw: Uint8Array;
+};
+
+let sidecar: ReturnType<typeof Bun.serve>;
+let app: Hono;
+let seen: Seen | null = null;
+
+beforeAll(async () => {
+  sidecar = Bun.serve({
+    port: 0,
+    async fetch(req) {
+      const contentType = req.headers.get('content-type') ?? '';
+      const raw = new Uint8Array(await req.arrayBuffer());
+      const asText = new TextDecoder().decode(raw);
+      const bodyBoundary = asText.match(/^--([^\r\n-][^\r\n]*?)\r\n/)?.[1] ?? null;
+
+      // Re-parse from the bytes we actually received, exactly as Immich would.
+      let fields: string[] = [];
+      let fileBytes: number[] | null = null;
+      try {
+        const form = await new Request('http://x', {
+          method: 'POST',
+          headers: { 'content-type': contentType },
+          body: raw,
+        }).formData();
+        fields = [...form.keys()];
+        const f = form.get('assetData');
+        if (f instanceof File) fileBytes = [...new Uint8Array(await f.arrayBuffer())];
+      } catch (err) {
+        seen = { contentType, bodyBoundary, fields: [], fileBytes: null, raw };
+        return new Response(JSON.stringify({ message: String(err) }), { status: 400 });
+      }
+
+      seen = { contentType, bodyBoundary, fields, fileBytes, raw };
+      return new Response(JSON.stringify({ ok: true }), { status: 201 });
+    },
+  });
+
+  const proxy = createSidecarProxy({ name: 'testcar', prefix: '/api/testcar' });
+  // What the registration socket would deliver when the sidecar reports its port.
+  handleSidecarMessage('test', { type: 'testcar:server', port: sidecar.port } as never);
+
+  const protectedRouter = createRouter();
+  protectedRouter.use(bodyParser());
+  protectedRouter.use(async (ctx, next) => {
+    ctx.set('user', { id: 1 } as never);
+    return next();
+  });
+  protectedRouter.route('/testcar', proxy.router);
+
+  app = new Hono();
+  app.route('/api', protectedRouter);
+});
+
+afterAll(() => sidecar?.stop(true));
+
+describe('sidecar proxy multipart forwarding', () => {
+  it('forwards the client boundary unchanged, in both the header and the body', async () => {
+    const { body, contentType } = phoneMultipart();
+
+    const res = await app.request('http://x/api/testcar/_officer/assets', {
+      method: 'POST',
+      headers: { 'content-type': contentType },
+      body: new Blob([body]),
+    });
+
+    expect(res.status).toBe(201);
+    expect(seen).not.toBeNull();
+    // The bug: header said one boundary, body used another.
+    expect(seen!.contentType).toContain(BOUNDARY);
+    expect(seen!.bodyBoundary).toBe(BOUNDARY);
+    // Bun's FormData re-serialiser announces itself; if this appears, the body was rebuilt.
+    expect(seen!.bodyBoundary).not.toContain('WebkitFormBoundary');
+  });
+
+  it('delivers the body byte-for-byte, fields in the order sent', async () => {
+    const { body, contentType } = phoneMultipart();
+
+    const res = await app.request('http://x/api/testcar/_officer/assets', {
+      method: 'POST',
+      headers: { 'content-type': contentType },
+      body: new Blob([body]),
+    });
+
+    expect(res.status).toBe(201);
+    // The strongest form of "forwarded, not rebuilt": the bytes on the far side are the bytes we sent.
+    expect([...seen!.raw]).toEqual([...body]);
+    expect(seen!.fields).toEqual([
+      'deviceAssetId',
+      'deviceId',
+      'fileCreatedAt',
+      'fileModifiedAt',
+      'isFavorite',
+      'assetData',
+    ]);
+    expect(seen!.fileBytes).toEqual([...FILE_BYTES]);
+  });
+
+  it('still forwards a JSON body untouched', async () => {
+    const res = await app.request('http://x/api/testcar/_officer/echo', {
+      method: 'POST',
+      headers: { 'content-type': 'application/json' },
+      body: JSON.stringify({ hello: 'world' }),
+    });
+
+    expect(res.status).toBe(400); // not multipart — the stub fails to parse it, which is fine
+    expect(seen!.contentType).toBe('application/json');
+  });
+});

src/servers/sidecar/create-proxy.ts

@@ -2,6 +2,7 @@ import type { Hono } from 'hono';
 import type { HonoVariables } from '../create-router';
 import { createRouter } from '../create-router';
 import * as sidecar from '../sidecar-registry';
+import { registerProxiedPrefix } from './proxied-prefixes';
 
 // The shared auth-and-forward proxy every HTTP sidecar needs.
 //
@@ -48,6 +49,10 @@ export type SidecarProxy = {
 export function createSidecarProxy({ name, prefix, onRegister, timeoutSeconds }: SidecarProxyParams): SidecarProxy {
   let serverPort: number | null = null;
 
+  // Tells bodyParser to keep its hands off this prefix's request bodies. Load-bearing for uploads, not an
+  // optimisation: see proxied-prefixes.ts.
+  registerProxiedPrefix(prefix);
+
   sidecar.on(`${name}:server`, (msg) => {
     const port = (msg as { port?: number }).port;
     if (typeof port !== 'number') return;
@@ -95,13 +100,22 @@ export function createSidecarProxy({ name, prefix, onRegister, timeoutSeconds }:
 
     const hasBody = method !== 'GET' && method !== 'HEAD';
 
-    let upstream: Response;
+    // Forwarded as a STREAM, not a buffer. Buffering held the whole body in memory here on top of the copy
-    try {
+    // the sidecar holds — two copies of a 4K video per upload — and bought nothing, since this layer never
-      upstream = await fetch(target, {
+    // looks at the bytes. `duplex: 'half'` is required by the fetch spec whenever the body is a stream; it
+    // is absent from TypeScript's RequestInit, hence the widened type.
+    // This is only safe because bodyParser skips proxied prefixes: a body consumed upstream would arrive
+    // here as an already-locked stream.
+    const init: RequestInit & { duplex?: 'half' } = {
       method,
       headers,
-        body: hasBody ? await ctx.req.arrayBuffer() : undefined,
+      body: hasBody ? ctx.req.raw.body : undefined,
-      });
+      duplex: 'half',
+    };
+
+    let upstream: Response;
+    try {
+      upstream = await fetch(target, init);
     } catch (err) {
       // Target path only — never the body, which may carry a passphrase or a credential.
       console.error(`[${name}] proxy fetch failed`, { target, error: String(err) });

src/servers/sidecar/proxied-prefixes.ts

@@ -0,0 +1,27 @@
+// Which public path prefixes are served by `createSidecarProxy` — routes whose body the platform only
+// ever forwards and never reads.
+//
+// This exists so `bodyParser` can leave those request streams untouched. Parsing a body that is merely
+// passing through is not just wasted work; for `multipart/form-data` it is corrupting. Hono caches the
+// parsed `FormData` on the request, so when the proxy later asks for the bytes Hono re-serialises them
+// from that cache — minting a NEW multipart boundary, while the proxy still forwards the ORIGINAL
+// `content-type` header naming the old one. Header and body then disagree and the far side rejects every
+// upload with "Multipart: Unexpected end of form".
+//
+// Registered by `createSidecarProxy` itself rather than hand-listed here, so a new sidecar cannot forget
+// to add itself and rediscover the same bug.
+
+const proxiedPrefixes = new Set<string>();
+
+export const registerProxiedPrefix = (prefix: string): void => {
+  proxiedPrefixes.add(prefix);
+};
+
+/** True when `path` is served by a sidecar proxy. Matches on a path boundary: `/api/memos` never matches
+ * `/api/memoserver`. */
+export function isProxiedPath(path: string): boolean {
+  for (const prefix of proxiedPrefixes) {
+    if (path === prefix || path.startsWith(`${prefix}/`)) return true;
+  }
+  return false;
+}