diff --git a/backend/__tests__/services/chunkedUploadStreaming.test.js b/backend/__tests__/services/chunkedUploadStreaming.test.js
index 38fccb51..ad3fb40d 100644
--- a/backend/__tests__/services/chunkedUploadStreaming.test.js
+++ b/backend/__tests__/services/chunkedUploadStreaming.test.js
@@ -222,6 +222,33 @@ describe('chunked upload streams the body under a cap (#1403)', () => {
     });
   });
 
+  // These were plain Errors, so the routes answered 500 for what are plainly
+  // client mistakes — a backend fault in monitoring, and an invitation to
+  // retry something that can never succeed.
+  describe('client-caused states carry their own status code', () => {
+    it('404s an unknown upload id rather than 500', async () => {
+      await expect(chunkedUpload.uploadChunk('does-not-exist', 0, Buffer.alloc(10)))
+        .rejects.toMatchObject({ statusCode: 404 });
+    });
+
+    // 409 (wrong status) and 410 (expired) share uploadStateError with the two
+    // covered here. Reaching them from the public surface needs either a clock
+    // or a setter the service does not expose, and a test that pretends to
+    // exercise them while actually hitting the 404 path is worse than none.
+
+    it('404s completing an unknown upload rather than 500', async () => {
+      await expect(chunkedUpload.completeUpload('does-not-exist'))
+        .rejects.toMatchObject({ statusCode: 404 });
+    });
+
+    it('400s completing an upload that is missing chunks', async () => {
+      const { uploadId } = await init();
+      await chunkedUpload.uploadChunk(uploadId, 0, Buffer.alloc(10));
+      await expect(chunkedUpload.completeUpload(uploadId))
+        .rejects.toMatchObject({ statusCode: 400 });
+    });
+  });
+
   describe('the happy path still works', () => {
     it('writes a streamed chunk and reports progress', async () => {
       const { uploadId } = await init();
diff --git a/backend/src/routes/adminPhotos.js b/backend/src/routes/adminPhotos.js
index 09f46c81..a9557b5d 100644
--- a/backend/src/routes/adminPhotos.js
+++ b/backend/src/routes/adminPhotos.js
@@ -1714,7 +1714,10 @@ router.post('/:eventId/chunked-upload/:uploadId/chunk/:chunkIndex', adminAuth, r
 
     res.json(result);
   } catch (error) {
-    if (error.statusCode === 413 || error.statusCode === 400) {
+    // Client-caused states (unknown/finished/expired upload, bad index, too
+    // large) carry their own status. Only a genuinely unexpected error should
+    // reach the 500 below and the error log with it.
+    if (error.statusCode) {
       return res.status(error.statusCode).json({ error: error.message });
     }
     logger.error('Error uploading chunk:', error);
@@ -1764,8 +1767,10 @@ router.post('/:eventId/chunked-upload/:uploadId/complete', adminAuth, requirePer
       photos: uploadedPhotos
     });
   } catch (error) {
-    if (error.statusCode === 413) {
-      return res.status(413).json({ error: error.message });
+    // Same rule as the chunk route: a tagged status is a client-caused state
+    // (unknown/expired upload, missing chunks), not a server fault.
+    if (error.statusCode) {
+      return res.status(error.statusCode).json({ error: error.message });
     }
     logger.error('Error completing chunked upload:', error);
     res.status(500).json({ error: error.message || 'Failed to complete upload' });
diff --git a/backend/src/services/chunkedUploadService.js b/backend/src/services/chunkedUploadService.js
index 6b11b4f3..3ca14df1 100644
--- a/backend/src/services/chunkedUploadService.js
+++ b/backend/src/services/chunkedUploadService.js
@@ -41,6 +41,17 @@ function overAllowanceError() {
   return Object.assign(new Error('CHUNK_OVER_ALLOWANCE'), { overAllowance: true });
 }
 
+// A client-supplied upload id that is unknown, finished or expired is the
+// client's mistake, not the server's. These used to be plain Errors, so the
+// routes answered 500 — which reads as a backend fault in monitoring and
+// invites the client to retry something that will never succeed.
+function uploadStateError(message, statusCode) {
+  const err = new Error(message);
+  err.code = 'UPLOAD_STATE';
+  err.statusCode = statusCode;
+  return err;
+}
+
 function invalidChunkError(message) {
   const err = new Error(message);
   err.code = 'INVALID_CHUNK';
@@ -209,17 +220,17 @@ async function uploadChunk(uploadId, chunkIndex, source, { declaredBytes } = {})
   const uploadMeta = activeUploads.get(uploadId);
 
   if (!uploadMeta) {
-    throw new Error('Upload not found or expired');
+    throw uploadStateError('Upload not found or expired', 404);
   }
 
   if (uploadMeta.status !== 'in_progress') {
-    throw new Error(`Upload is ${uploadMeta.status}`);
+    throw uploadStateError(`Upload is ${uploadMeta.status}`, 409);
   }
 
   // Check expiration
   if (Date.now() > uploadMeta.expiresAt) {
     await abortUpload(uploadId);
-    throw new Error('Upload expired');
+    throw uploadStateError('Upload expired', 410);
   }
 
   // Only the announced chunk indices are valid — anything else would merge
@@ -320,12 +331,13 @@ async function completeUpload(uploadId) {
   const uploadMeta = activeUploads.get(uploadId);
 
   if (!uploadMeta) {
-    throw new Error('Upload not found or expired');
+    throw uploadStateError('Upload not found or expired', 404);
   }
 
   // Verify all chunks received
   if (uploadMeta.receivedChunks.size !== uploadMeta.expectedChunks) {
-    throw new Error(`Missing chunks: received ${uploadMeta.receivedChunks.size} of ${uploadMeta.expectedChunks}`);
+    throw uploadStateError(
+      `Missing chunks: received ${uploadMeta.receivedChunks.size} of ${uploadMeta.expectedChunks}`, 400);
   }
 
   uploadMeta.status = 'merging';