diff --git a/backend/__tests__/services/chunkedUploadStreaming.test.js b/backend/__tests__/services/chunkedUploadStreaming.test.js
index 8ba4ac24..38fccb51 100644
--- a/backend/__tests__/services/chunkedUploadStreaming.test.js
+++ b/backend/__tests__/services/chunkedUploadStreaming.test.js
@@ -192,6 +192,24 @@ describe('chunked upload streams the body under a cap (#1403)', () => {
         .rejects.toMatchObject({ code: 'ENOENT' });
     });
 
+    it('counts chunks that landed while another was still streaming', async () => {
+      const { uploadId } = await init({ totalChunks: 3 });
+
+      // Start a slow 0.75MB chunk. Its allowance is computed now, when nothing
+      // else is banked.
+      const slow = new Readable({ read() {} });
+      const slowDone = chunkedUpload.uploadChunk(uploadId, 0, slow);
+
+      // A second 0.75MB chunk completes in the meantime.
+      await chunkedUpload.uploadChunk(uploadId, 1, Buffer.alloc(0.75 * MB));
+
+      // Finishing the first must not publish: 1.5MB against a 1MB cap.
+      slow.push(Buffer.alloc(0.75 * MB));
+      slow.push(null);
+      await expect(slowDone).rejects.toMatchObject({ code: 'FILE_TOO_LARGE', statusCode: 413 });
+      expect(chunkedUpload.getUploadStatus(uploadId)).toBeNull();
+    });
+
     it('does not destroy the request stream when it trips the cap', async () => {
       const { uploadId } = await init();
       const source = countingSource(8 * MB);
diff --git a/backend/src/services/chunkedUploadService.js b/backend/src/services/chunkedUploadService.js
index 5e25e606..6b11b4f3 100644
--- a/backend/src/services/chunkedUploadService.js
+++ b/backend/src/services/chunkedUploadService.js
@@ -266,6 +266,18 @@ async function uploadChunk(uploadId, chunkIndex, source, { declaredBytes } = {})
     const partPath = `${chunkPath}.${crypto.randomBytes(6).toString('hex')}.part`;
     try {
       chunkLength = await writeChunkStream(source, partPath, allowance);
+      // Re-check the aggregate before publishing. `allowance` was computed
+      // before the body arrived, so a chunk that completed while this one was
+      // still streaming is not counted in it — two overlapping 0.75MB chunks
+      // under a 1MB cap would otherwise both be accepted. The buffered version
+      // got this right for free by checking after the read; streaming has to
+      // ask again.
+      const bankedNow = totalReceivedBytes(uploadMeta) - (uploadMeta.chunkSizes.get(chunkIndex) || 0);
+      if (bankedNow + chunkLength > uploadMeta.maxFileSizeBytes) {
+        await fs.rm(partPath, { force: true }).catch(() => {});
+        await abortUpload(uploadId);
+        throw fileTooLargeError(uploadMeta.maxFileSizeBytes);
+      }
       await fs.rename(partPath, chunkPath);
     } catch (err) {
       if (err.overAllowance) {