diff --git a/apps/api/src/server.ts b/apps/api/src/server.ts
index 7b18549..ad6b56a 100644
--- a/apps/api/src/server.ts
+++ b/apps/api/src/server.ts
@@ -180,7 +180,7 @@ export async function createServer() {
     const upload = await runtime.store.getUpload(uploadId);
     if (!upload || upload.objectKey !== key) return reply.code(404).send({ error: "UPLOAD_NOT_FOUND" });
     const body = request.body as NodeJS.ReadableStream;
-    const receivedSize = await runtime.storage.writeUploadFromStream(key, body);
+    const receivedSize = await runtime.storage.writeUploadFromStream(key, body, upload.expectedSize);
     if (receivedSize !== upload.expectedSize) return reply.code(400).send({ error: "UPLOAD_SIZE_MISMATCH", expectedSize: upload.expectedSize, receivedSize });
     await runtime.store.updateUpload(upload.id, { receivedSize, status: "uploaded" });
     return { ok: true, receivedSize };
diff --git a/apps/api/src/storage.ts b/apps/api/src/storage.ts
index 8048f1d..28a7ed7 100644
--- a/apps/api/src/storage.ts
+++ b/apps/api/src/storage.ts
@@ -27,7 +27,7 @@ export type UploadPlan =
 export interface StorageAdapter {
   createUploadPlan(key: string, contentType: string, sizeBytes: number): Promise<UploadPlan>;
   completeMultipartUpload(key: string, uploadId: string | undefined, parts: Array<{ partNumber: number; etag: string }> | undefined): Promise<void>;
-  writeUploadFromStream(key: string, stream: NodeJS.ReadableStream): Promise<number>;
+  writeUploadFromStream(key: string, stream: NodeJS.ReadableStream, sizeBytes?: number): Promise<number>;
   writeObject(key: string, body: Buffer, contentType: string): Promise<void>;
   writeStream(key: string, transform: NodeJS.ReadWriteStream, contentType: string, sizeBytes: number, source: NodeJS.ReadableStream): Promise<void>;
   downloadToFile(key: string, filePath: string): Promise<void>;
@@ -66,7 +66,8 @@ export class LocalStorage implements StorageAdapter {
     return;
   }
 
-  async writeUploadFromStream(key: string, stream: NodeJS.ReadableStream) {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars -- sizeBytes is only needed by S3.
+  async writeUploadFromStream(key: string, stream: NodeJS.ReadableStream, sizeBytes?: number) {
     const filePath = this.pathFor(key);
     await mkdir(dirname(filePath), { recursive: true });
     await pipeline(stream, createWriteStream(filePath, { flags: "w" }));
@@ -158,8 +159,11 @@ export class S3Storage implements StorageAdapter {
     await this.client.send(new CompleteMultipartUploadCommand({ Bucket: this.appConfig.S3_BUCKET, Key: key, UploadId: uploadId, MultipartUpload: { Parts: parts.sort((a, b) => a.partNumber - b.partNumber).map((part) => ({ PartNumber: part.partNumber, ETag: part.etag })) } }));
   }
 
-  async writeUploadFromStream(key: string, stream: NodeJS.ReadableStream) {
-    await this.client.send(new PutObjectCommand({ Bucket: this.appConfig.S3_BUCKET, Key: key, Body: stream as any }));
+  async writeUploadFromStream(key: string, stream: NodeJS.ReadableStream, sizeBytes?: number) {
+    // ContentLength must be provided for streaming bodies, otherwise the SDK's
+    // flexible-checksums middleware sends `x-amz-decoded-content-length:
+    // undefined` and the upload fails with ERR_HTTP_INVALID_HEADER_VALUE.
+    await this.client.send(new PutObjectCommand({ Bucket: this.appConfig.S3_BUCKET, Key: key, Body: stream as any, ContentLength: sizeBytes }));
     const metadata = await this.headObject(key);
     return metadata?.sizeBytes || 0;
   }
@@ -169,7 +173,7 @@ export class S3Storage implements StorageAdapter {
   }
 
   async writeStream(key: string, transform: NodeJS.ReadWriteStream, contentType: string, sizeBytes: number, source: NodeJS.ReadableStream) {
-    const upload = this.client.send(new PutObjectCommand({ Bucket: this.appConfig.S3_BUCKET, Key: key, Body: transform as any, ContentType: contentType, ContentLength: sizeBytes || undefined }));
+    const upload = this.client.send(new PutObjectCommand({ Bucket: this.appConfig.S3_BUCKET, Key: key, Body: transform as any, ContentType: contentType, ContentLength: sizeBytes }));
     await pipeline(source, transform);
     await upload;
   }