diff --git a/src/servers/api/photos/router.ts b/src/servers/api/photos/router.ts
index 7dfaf7f0..b4d80e2a 100644
--- a/src/servers/api/photos/router.ts
+++ b/src/servers/api/photos/router.ts
@@ -11,6 +11,12 @@ const proxy = createSidecarProxy({
   // Originals and `download/archive` zips are large and Immich builds the archive as it streams it, so the
   // socket can sit quiet longer than the default 60s idle drop allows.
   timeoutSeconds: 600,
+  // Immich judges an asset from its first few KB and rejects immediately, while we are still writing —
+  // and the response is lost in the teardown, which is what made every large upload fail with an
+  // unexplained empty 400. Buffering here removes the overlap. The sidecar does the same on its own hop
+  // to Immich (sidecar/photos/routes.ts → readBodyCapped), and both are needed: fixing one alone still
+  // lost the answer roughly two runs in three at 32 MB.
+  bufferRequestBody: true,
 });
 
 export const photosRouter = proxy.router;
diff --git a/src/servers/sidecar/create-proxy.test.ts b/src/servers/sidecar/create-proxy.test.ts
index 87f4c3b5..67a51f6c 100644
--- a/src/servers/sidecar/create-proxy.test.ts
+++ b/src/servers/sidecar/create-proxy.test.ts
@@ -73,6 +73,10 @@ beforeAll(async () => {
   sidecar = Bun.serve({
     port: 0,
     async fetch(req) {
+      // A rejection with no body at all — the shape from COMMS/UPLOAD_EMPTY_400.md that tells the client
+      // "no" and gives it nothing to act on.
+      if (new URL(req.url).pathname.endsWith('/empty400')) return new Response(null, { status: 400 });
+
       const contentType = req.headers.get('content-type') ?? '';
       const raw = new Uint8Array(await req.arrayBuffer());
       const asText = new TextDecoder().decode(raw);
@@ -160,6 +164,61 @@ describe('sidecar proxy multipart forwarding', () => {
     expect(seen!.fileBytes).toEqual([...FILE_BYTES]);
   });
 
+  it('reports a rejection instead of passing it on in silence', async () => {
+    // COMMS/UPLOAD_EMPTY_400.md: uploads failed with an empty-bodied 400 and nothing logged it anywhere,
+    // so localising the fault took an evening of curl bisection. The platform is the only hop that sees
+    // both sides. `requestBytes` is the field that would have cracked it — a size threshold is legible at
+    // a glance instead of by bisection.
+    const warnings: unknown[][] = [];
+    const realWarn = console.warn;
+    console.warn = (...args: unknown[]) => void warnings.push(args);
+
+    try {
+      const payload = JSON.stringify({ some: 'payload' });
+      const res = await app.request('http://x/api/testcar/_officer/empty400', {
+        method: 'POST',
+        // Set explicitly: `app.request` does not derive it, and this is the field the bug report needed.
+        headers: { 'content-type': 'application/json', 'content-length': String(payload.length) },
+        body: payload,
+      });
+
+      // Pass-through is unchanged: the proxy reports, it does not rewrite.
+      expect(res.status).toBe(400);
+      expect(await res.text()).toBe('');
+
+      expect(warnings.length).toBe(1);
+      const [message, detail] = warnings[0] as [string, Record<string, unknown>];
+      expect(message).toContain('[testcar]');
+      expect(message).toContain('POST /_officer/empty400');
+      expect(message).toContain('400');
+      // The empty body is named, not left to be inferred from a byte count of zero.
+      expect(detail.emptyBody).toBe(true);
+      expect(detail.requestBytes).toBe(String(payload.length));
+      expect(detail.responseBytes).toBe('0');
+    } finally {
+      console.warn = realWarn;
+    }
+  });
+
+  it('says nothing about a request that succeeded', async () => {
+    const { body, contentType } = phoneMultipart();
+    const warnings: unknown[][] = [];
+    const realWarn = console.warn;
+    console.warn = (...args: unknown[]) => void warnings.push(args);
+
+    try {
+      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(warnings).toEqual([]);
+    } finally {
+      console.warn = realWarn;
+    }
+  });
+
   it('still forwards a JSON body untouched', async () => {
     const res = await app.request('http://x/api/testcar/_officer/echo', {
       method: 'POST',
diff --git a/src/servers/sidecar/create-proxy.ts b/src/servers/sidecar/create-proxy.ts
index b01cc3a2..e0243763 100644
--- a/src/servers/sidecar/create-proxy.ts
+++ b/src/servers/sidecar/create-proxy.ts
@@ -16,7 +16,8 @@ import { registerProxiedPrefix } from './proxied-prefixes';
 // the platform process is long-lived, restarts on every deploy and is the largest attack surface in the
 // system, while the bodies passing through carry wallet unlock passphrases, macaroons and vault secrets.
 // Keeping this layer structurally incapable of seeing them is the point. Note the absence of any body
-// inspection below, and that the error path logs the target path ONLY.
+// inspection below: the two error paths log a path, a status and byte counts taken from HEADERS, and
+// nothing ever reads or buffers the streams themselves.
 //
 // Sidecars own their own contracts. This file must never grow per-app logic — if a sidecar needs
 // something extra at registration time it gets `onRegister`, not a branch in here.
@@ -35,8 +36,30 @@ export type SidecarProxyParams = {
    * of a sidecar's routes are slow.
    */
   timeoutSeconds?: number;
+  /**
+   * Read the request body fully before forwarding, instead of streaming it.
+   *
+   * OFF by default and it must stay that way: streaming is what keeps wallet passphrases, macaroons and
+   * vault secrets out of this process's heap, and what lets a multi-gigabyte upload through at all.
+   *
+   * Opt in only where a sidecar's upstream answers BEFORE the body finishes arriving. When that happens
+   * the response is lost — the far side closes while we are still writing, and the real answer is
+   * replaced by a bodyless 400. Photos hits this because Immich judges an asset from its first few KB
+   * and rejects immediately; see `readBodyCapped` in sidecar/photos/routes.ts for the measurements.
+   *
+   * Bounded by `MAX_BUFFERED_BODY`: a body larger than that is streamed anyway, so enabling this can
+   * never put a 4 GB video in the heap. Above the cap the race is accepted, because an upload that size
+   * only loses its ERROR MESSAGE — one Immich accepts is read to the end and never races at all.
+   */
+  bufferRequestBody?: boolean;
 };
 
+/**
+ * Ceiling on `bufferRequestBody`. Kept in step with the photos sidecar's own cap so the two hops make the
+ * same call on the same request; officer buffering is what gives the sidecar a content-length to decide by.
+ */
+const MAX_BUFFERED_BODY = 512 * 1024 * 1024;
+
 export type SidecarProxy = {
   /** Catch-all router to mount at `prefix`. Has no routes of its own by design. */
   router: Hono<{ Variables: HonoVariables }>;
@@ -46,7 +69,13 @@ export type SidecarProxy = {
   getWsUrl: () => string | null;
 };
 
-export function createSidecarProxy({ name, prefix, onRegister, timeoutSeconds }: SidecarProxyParams): SidecarProxy {
+export function createSidecarProxy({
+  name,
+  prefix,
+  onRegister,
+  timeoutSeconds,
+  bufferRequestBody,
+}: 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
@@ -106,10 +135,23 @@ export function createSidecarProxy({ name, prefix, onRegister, timeoutSeconds }:
     // 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.
+    // Buffered only where the sidecar opted in (see `bufferRequestBody`), and only up to the cap: an
+    // upstream that answers before the body lands loses its own response, and streaming is what exposes
+    // that race. Anything larger streams as it always did — a 4 GB video must not land in this heap, and
+    // does not need to, since a file the upstream ACCEPTS is read to the end and never races.
+    const declaredLength = Number(ctx.req.header('content-length') ?? '');
+    const withinBufferCap =
+      Number.isFinite(declaredLength) && declaredLength > 0 && declaredLength <= MAX_BUFFERED_BODY;
+    const body = hasBody
+      ? bufferRequestBody && withinBufferCap
+        ? await ctx.req.raw.arrayBuffer()
+        : ctx.req.raw.body
+      : undefined;
+
     const init: RequestInit & { duplex?: 'half' } = {
       method,
       headers,
-      body: hasBody ? ctx.req.raw.body : undefined,
+      body,
       duplex: 'half',
     };
 
@@ -122,6 +164,29 @@ export function createSidecarProxy({ name, prefix, onRegister, timeoutSeconds }:
       return ctx.text(`${name} sidecar unreachable`, 502);
     }
 
+    // Every rejection that passes through gets one line. This is METADATA ONLY — status and two byte
+    // counts read from headers, never the bytes themselves — so the no-bodies rule above still holds.
+    //
+    // It exists because of COMMS/UPLOAD_EMPTY_400.md: photo uploads over ~16 KB failed with a 400 carrying
+    // an EMPTY body, and nothing anywhere logged it — not Immich, not the sidecar, not here. The platform
+    // is the only hop that sees both sides, and it said nothing, so localising the fault took an evening of
+    // curl bisection. An error with no body is the unactionable case: the client is told "no" and given
+    // nothing to act on, and silence here makes it invisible to the server too.
+    //
+    // `requestBytes` is the field that would have cracked it: it makes a size threshold legible at a glance
+    // instead of by bisection. The subpath is logged without its query string — the 502 path above logs the
+    // full target, but this fires on ordinary 401s and 404s too, and query strings carry tokens.
+    if (upstream.status >= 400) {
+      const responseBytes = upstream.headers.get('content-length');
+      console.warn(`[${name}] proxy ${method} ${subpath} -> ${upstream.status}`, {
+        requestBytes: ctx.req.header('content-length') ?? 'unknown',
+        responseBytes: responseBytes ?? 'unknown',
+        // Called out rather than left to be inferred from `responseBytes: '0'`, because this exact shape is
+        // the one that is impossible to diagnose from the client.
+        ...(responseBytes === '0' ? { emptyBody: true } : {}),
+      });
+    }
+
     return new Response(upstream.body, { status: upstream.status, headers: new Headers(upstream.headers) });
   });
 
diff --git a/src/servers/sidecar/photos/routes.ts b/src/servers/sidecar/photos/routes.ts
index f6950587..d0aeabf6 100644
--- a/src/servers/sidecar/photos/routes.ts
+++ b/src/servers/sidecar/photos/routes.ts
@@ -83,11 +83,86 @@ export function relayResponse(res: Response): Response {
   return new Response(body, { status: res.status, headers });
 }
 
+/**
+ * The largest request body held in memory. Above this the body is STREAMED instead — never refused.
+ *
+ * The cap exists to bound heap, not to bound uploads: a 4 GB video must still go through. What is given
+ * up above the cap is only the guarantee that a REJECTION carries its message, and an upload that big
+ * which Immich rejects was going to fail either way — see `readBodyCapped`.
+ */
+const MAX_BUFFERED_BODY = 512 * 1024 * 1024;
+
+/**
+ * Read the whole request body into memory, refusing above `MAX_BUFFERED_BODY`.
+ *
+ * ── Why this is buffered rather than streamed ──
+ *
+ * It used to stream, which is the obvious thing for an upload and was wrong for one specific reason:
+ * when the body is still being written and Immich answers early, the response is lost. Immich rejects
+ * an asset as soon as it has enough of the file to judge it, then closes; our writer is still going;
+ * Node's HTTP server calls the leftover bytes a protocol violation and replaces the app's answer with a
+ * bodyless `400 Bad Request` + `Connection: close`. The app's real message never reaches the wire and
+ * Immich logs nothing, so the client sees an unexplained 400 and the server has no record of it.
+ *
+ * Measured on this machine (COMMS/UPLOAD_EMPTY_400*.md): streamed lost the response in 2 of 4 attempts
+ * at 8 MB, 32 MB and 64 MB alike; buffered lost it in 0 of 4 at every size. It is a race, not a size
+ * limit — its probability just approaches certainty as the body grows, which is why a phone's video
+ * failed every time while a small photo usually got through.
+ *
+ * A size threshold was considered and rejected: buffering only the small bodies would leave exactly the
+ * large uploads that always fail on the broken path. Cancelling the write once the response arrives was
+ * tried and does not help — by the time `fetch` resolves, the teardown has already happened.
+ *
+ * The cost is real and was accepted deliberately: a 300 MB video is 300 MB of this process's heap for
+ * the duration of the upload. That is the price of the upload working at all.
+ *
+ * ── And why there is a ceiling on it ──
+ *
+ * Buffering everything would mean a 4 GB video sitting in this heap (and another copy in officer's), so
+ * above MAX_BUFFERED_BODY the body is streamed as it always was. That is not a refusal and not a
+ * degradation of uploads: a file Immich ACCEPTS never triggers the race, because it is read to the end
+ * before anything is answered. The only thing lost above the cap is the error message on a file Immich
+ * rejects — and an upload that size which Immich rejects fails either way. Refusing it outright would
+ * break the legitimate case to improve the diagnosis of the doomed one.
+ */
+async function readBodyCapped(
+  req: Request,
+): Promise<{ ok: true; bytes: Uint8Array<ArrayBuffer> } | { ok: false; seen: number }> {
+  // Decided from `content-length` alone. Officer sends one whenever IT buffered, so the two hops agree
+  // without coordinating: a body small enough for officer to hold is small enough to hold here. When the
+  // header is absent officer streamed, which means it was over the cap there too.
+  const declared = Number(req.headers.get('content-length') ?? '');
+  if (!Number.isFinite(declared) || declared <= 0 || declared > MAX_BUFFERED_BODY) return { ok: false, seen: declared };
+
+  const reader = req.body!.getReader();
+  const chunks: Uint8Array[] = [];
+  let total = 0;
+  for (;;) {
+    const { done, value } = await reader.read();
+    if (done) break;
+    total += value.length;
+    if (total > MAX_BUFFERED_BODY) {
+      await reader.cancel().catch(() => {});
+      return { ok: false, seen: total };
+    }
+    chunks.push(value);
+  }
+
+  const bytes = new Uint8Array(new ArrayBuffer(total));
+  let at = 0;
+  for (const chunk of chunks) {
+    bytes.set(chunk, at);
+    at += chunk.length;
+  }
+  return { ok: true, bytes };
+}
+
 /**
  * Forward one request to Immich and stream the answer back.
  *
- * The body is a stream, never an ArrayBuffer: a full-resolution original or a `download/archive` zip is
- * hundreds of megabytes and buffering it would hold all of it in the sidecar's heap for no reason.
+ * The RESPONSE is still a stream, never an ArrayBuffer: a full-resolution original or a
+ * `download/archive` zip is hundreds of megabytes and buffering it would hold all of it in the
+ * sidecar's heap for no reason. Only the request body is held, and only for the reason above.
  */
 export async function handleOfficerRoute(cfg: UpstreamConfig, req: Request, url: URL): Promise<Response | null> {
   const rest = url.pathname.slice('/_officer/'.length);
@@ -109,12 +184,20 @@ export async function handleOfficerRoute(cfg: UpstreamConfig, req: Request, url:
   query.delete('token');
   const search = query.toString();
 
+  // Buffered, not streamed — see readBodyCapped. A body that is still being written when Immich answers
+  // loses the answer, which is what made every large upload fail with an unexplained empty 400.
+  let body: BodyInit | null = null;
+  if (hasBody && req.body) {
+    const read = await readBodyCapped(req);
+    // Over the cap (or no declared length): stream it, exactly as this always did. Not an error.
+    body = read.ok ? read.bytes : req.body;
+  }
+
   const res = await callUpstream(cfg, {
     path: `/api/${rest}`,
     method: req.method,
     query: search ? `?${search}` : '',
-    // Streamed, not buffered: an asset upload can be gigabytes and this layer never reads the bytes.
-    body: hasBody ? req.body : null,
+    body,
     contentType: req.headers.get('content-type'),
     range: req.headers.get('range'),
     ifNoneMatch: req.headers.get('if-none-match'),
diff --git a/src/servers/sidecar/photos/upstream.ts b/src/servers/sidecar/photos/upstream.ts
index 2a429735..af642eb3 100644
--- a/src/servers/sidecar/photos/upstream.ts
+++ b/src/servers/sidecar/photos/upstream.ts
@@ -165,12 +165,15 @@ type CallOptions = {
   /** Raw search string including the leading `?`, or empty. */
   query?: string;
   /**
-   * A string for the JSON callers, or the caller's own request body as a STREAM for the two forwarders.
+   * A string for the JSON callers, or the forwarder's already-buffered request body.
    *
-   * Streamed rather than buffered because a phone's video upload is bounded by `maxRequestBodySize`
-   * (4 GB) and buffering put all of it in this process's heap for a hop that never looks at the bytes.
-   * Streaming to Immich means chunked transfer-encoding instead of Content-Length — which is already how
-   * the body reaches us, since the platform proxy forwards no content-length either.
+   * This used to be a stream, for the obvious reason that a phone's video should not sit in this
+   * process's heap. It was changed back deliberately: a request body still being written when Immich
+   * answers loses the answer entirely — Node replaces the app's response with a bodyless 400 and closes
+   * — which is what made every large upload fail inexplicably. `readBodyCapped` in routes.ts has the
+   * measurements and the alternatives that were tried; the heap cost is the accepted price.
+   *
+   * A buffer also carries a Content-Length, so the request to Immich is no longer chunked.
    */
   body?: BodyInit | null;
   contentType?: string | null;