diff --git a/server/src/__tests__/storage-s3-multipart.test.ts b/server/src/__tests__/storage-s3-multipart.test.ts
index 0074dd88f22..f7b0690f2d0 100644
--- a/server/src/__tests__/storage-s3-multipart.test.ts
+++ b/server/src/__tests__/storage-s3-multipart.test.ts
@@ -4,7 +4,10 @@ import { createHash } from "node:crypto";
 import { createServer, type ServerResponse } from "node:http";
 import type { AddressInfo } from "node:net";
 import { describe, expect, it, vi } from "vitest";
-import { putS3Multipart } from "../storage/s3-multipart.js";
+import { FILE_PART_CONCURRENCY, putS3Multipart, putS3MultipartFile } from "../storage/s3-multipart.js";
+import fs from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
 import { createS3StorageProvider } from "../storage/s3-provider.js";
 import { createStorageService } from "../storage/service.js";
 import { STORAGE_FAILURE_CLEANUP_MS } from "../storage/failure-cleanup.js";
@@ -419,3 +422,157 @@ describe("streamed S3 multipart uploads", () => {
     }
   });
 });
+
+describe("file-backed S3 multipart uploads", () => {
+  const partBytes = 8 * 1024 * 1024;
+  async function sourceFile(parts: number, tail = 0) {
+    const dir = await fs.mkdtemp(path.join(os.tmpdir(), "pc-s3-file-part-"));
+    const file = path.join(dir, "source.bin");
+    const handle = await fs.open(file, "wx", 0o600);
+    try {
+      for (let index = 0; index < parts; index++) await handle.write(Buffer.alloc(partBytes, index + 1));
+      if (tail) await handle.write(Buffer.alloc(tail, 0xee));
+    } finally { await handle.close(); }
+    return { dir, file, size: parts * partBytes + tail };
+  }
+  const md5 = (bytes: Buffer) => createHash("md5").update(bytes).digest("base64");
+  async function drain(body: unknown) {
+    const chunks: Buffer[] = [];
+    for await (const chunk of body as Readable) chunks.push(Buffer.from(chunk));
+    return Buffer.concat(chunks);
+  }
+
+  it("uploads ranges in parallel and sends each part's Content-MD5", async () => {
+    const source = await sourceFile(10, 4096);
+    const client = new S3Client({ region: "us-east-1" });
+    let active = 0, peak = 0, release!: () => void;
+    const gate = new Promise<void>(resolve => { release = resolve; });
+    const seen = new Map<number, { length: number; md5: string; declared: string | undefined }>();
+    const send = vi.spyOn(client, "send").mockImplementation(async (command: any) => {
+      if (command instanceof CreateMultipartUploadCommand) return { UploadId: "upload" };
+      if (command instanceof UploadPartCommand) {
+        active++; peak = Math.max(peak, active);
+        if (active === FILE_PART_CONCURRENCY) release();
+        await gate;
+        const bytes = await drain(command.input.Body);
+        expect(bytes.length).toBe(command.input.ContentLength);
+        seen.set(command.input.PartNumber!, { length: bytes.length, md5: md5(bytes), declared: command.input.ContentMD5 });
+        active--;
+        return { ETag: `part-${command.input.PartNumber}` };
+      }
+      expect(command).toBeInstanceOf(CompleteMultipartUploadCommand);
+      expect(command.input.MultipartUpload.Parts.map((part: { PartNumber: number }) => part.PartNumber)).toEqual([...Array(11).keys()].map(n => n + 1));
+      return {};
+    });
+    const body = Readable.from([Buffer.from("unused stream body")]);
+    try {
+      const receipt = await putS3MultipartFile(client, "bucket", "key", { objectKey: "key", contentType: "application/octet-stream",
+        contentLength: source.size, body, sourceFile: source.file });
+      expect(receipt).toEqual({ integrity: "server_verified_md5" });
+      expect(peak).toBe(FILE_PART_CONCURRENCY);
+      expect(body.destroyed).toBe(true);
+      expect(seen.size).toBe(11);
+      for (const [number, part] of seen) {
+        expect(part.declared).toBe(part.md5);
+        expect(part.length).toBe(number === 11 ? 4096 : partBytes);
+      }
+      expect(seen.get(3)!.md5).toBe(md5(Buffer.alloc(partBytes, 3)));
+    } finally { send.mockRestore(); client.destroy(); await fs.rm(source.dir, { recursive: true, force: true }); }
+  });
+
+  it("resends a failed part from the file instead of failing the upload", async () => {
+    const source = await sourceFile(3);
+    const client = new S3Client({ region: "us-east-1" });
+    const attempts = new Map<number, number>();
+    const send = vi.spyOn(client, "send").mockImplementation(async (command: any) => {
+      if (command instanceof CreateMultipartUploadCommand) return { UploadId: "upload" };
+      if (command instanceof UploadPartCommand) {
+        const number = command.input.PartNumber!;
+        attempts.set(number, (attempts.get(number) ?? 0) + 1);
+        const bytes = await drain(command.input.Body);
+        if (number === 2 && attempts.get(number) === 1) throw new Error("An error was encountered in a non-retryable streaming request.");
+        expect(md5(bytes)).toBe(command.input.ContentMD5);
+        return { ETag: `part-${number}` };
+      }
+      if (command instanceof AbortMultipartUploadCommand) throw new Error("must not abort a recovered upload");
+      return {};
+    });
+    try {
+      await expect(putS3MultipartFile(client, "bucket", "key", { objectKey: "key", contentType: "application/octet-stream",
+        contentLength: source.size, body: Buffer.alloc(0), sourceFile: source.file })).resolves.toEqual({ integrity: "server_verified_md5" });
+      expect(attempts.get(2)).toBe(2);
+      expect(send.mock.calls.some(([command]) => command instanceof CompleteMultipartUploadCommand)).toBe(true);
+    } finally { send.mockRestore(); client.destroy(); await fs.rm(source.dir, { recursive: true, force: true }); }
+  });
+
+  it("aborts the upload only after every in-flight part settles when a part keeps failing", async () => {
+    const source = await sourceFile(4);
+    const client = new S3Client({ region: "us-east-1" });
+    let inFlight = 0, abortedWhileInFlight = false;
+    const send = vi.spyOn(client, "send").mockImplementation(async (command: any, options?: any) => {
+      if (command instanceof CreateMultipartUploadCommand) return { UploadId: "upload" };
+      if (command instanceof UploadPartCommand) {
+        inFlight++;
+        try {
+          if (command.input.PartNumber === 1) throw new Error("storage refused part");
+          await new Promise<void>((_resolve, reject) => options?.abortSignal?.addEventListener("abort", () => reject(options.abortSignal!.reason), { once: true }));
+          return { ETag: "never" };
+        } finally { inFlight--; }
+      }
+      if (command instanceof AbortMultipartUploadCommand) { abortedWhileInFlight = inFlight > 0; return {}; }
+      throw new Error("must not complete a failed upload");
+    });
+    try {
+      await expect(putS3MultipartFile(client, "bucket", "key", { objectKey: "key", contentType: "application/octet-stream",
+        contentLength: source.size, body: Buffer.alloc(0), sourceFile: source.file })).rejects.toThrow("storage refused part");
+      const partOneAttempts = send.mock.calls.filter(([command]) => command instanceof UploadPartCommand && command.input.PartNumber === 1).length;
+      expect(partOneAttempts).toBe(3);
+      expect(send.mock.calls.filter(([command]) => command instanceof AbortMultipartUploadCommand)).toHaveLength(1);
+      expect(abortedWhileInFlight).toBe(false);
+    } finally { send.mockRestore(); client.destroy(); await fs.rm(source.dir, { recursive: true, force: true }); }
+  });
+
+  it("refuses a source file whose size differs from the declared length before starting an upload", async () => {
+    const source = await sourceFile(3);
+    const client = new S3Client({ region: "us-east-1" });
+    const send = vi.spyOn(client, "send");
+    try {
+      await expect(putS3MultipartFile(client, "bucket", "key", { objectKey: "key", contentType: "application/octet-stream",
+        contentLength: source.size + 1, body: Buffer.alloc(0), sourceFile: source.file })).rejects.toThrow("does not match its declared size");
+      expect(send).not.toHaveBeenCalled();
+    } finally { send.mockRestore(); client.destroy(); await fs.rm(source.dir, { recursive: true, force: true }); }
+  });
+
+  it("sends a small file source as one PUT with Content-MD5 through the real S3 HTTP client", async () => {
+    const dir = await fs.mkdtemp(path.join(os.tmpdir(), "pc-s3-small-file-"));
+    const file = path.join(dir, "small.bin"), bytes = Buffer.from("verified snapshot bytes\n".repeat(1000));
+    await fs.writeFile(file, bytes, { mode: 0o600 });
+    const requests: Array<{ method: string; md5: string | undefined; body: Buffer }> = [];
+    const server = createServer((request, response) => {
+      const chunks: Buffer[] = [];
+      request.on("data", chunk => chunks.push(chunk));
+      request.on("end", () => {
+        requests.push({ method: request.method!, md5: request.headers["content-md5"] as string | undefined, body: Buffer.concat(chunks) });
+        response.writeHead(200, { ETag: '"small"' }).end();
+      });
+    });
+    await new Promise<void>((resolve, reject) => { server.once("error", reject); server.listen(0, "127.0.0.1", resolve); });
+    vi.stubEnv("AWS_ACCESS_KEY_ID", "fixture"); vi.stubEnv("AWS_SECRET_ACCESS_KEY", "fixture");
+    const service = createStorageService(createS3StorageProvider({ bucket: "bucket", region: "us-east-1", forcePathStyle: true,
+      endpoint: `http://127.0.0.1:${(server.address() as AddressInfo).port}` }));
+    try {
+      const result = await service.putFile({ companyId: "company", objectKey: "company/workspace-snapshots/small.pcws", namespace: "workspace-snapshots",
+        originalFilename: null, contentType: "application/octet-stream", body: Readable.from([bytes]), byteSize: bytes.length,
+        sha256: createHash("sha256").update(bytes).digest("hex"), sourceFile: file });
+      expect(result.integrity).toBe("server_verified_md5");
+      expect(requests).toHaveLength(1);
+      expect(requests[0]).toMatchObject({ method: "PUT", md5: md5(bytes) });
+      expect(requests[0]!.body.equals(bytes)).toBe(true);
+    } finally {
+      vi.unstubAllEnvs();
+      server.closeAllConnections();
+      await new Promise<void>(resolve => server.close(() => resolve()));
+      await fs.rm(dir, { recursive: true, force: true });
+    }
+  });
+});
diff --git a/server/src/services/remote-workspace-snapshot-storage.test.ts b/server/src/services/remote-workspace-snapshot-storage.test.ts
index 93d9419a88a..eb5507fe761 100644
--- a/server/src/services/remote-workspace-snapshot-storage.test.ts
+++ b/server/src/services/remote-workspace-snapshot-storage.test.ts
@@ -19,6 +19,17 @@ import { localEncryptedProvider } from "../secrets/local-encrypted-provider.js";
 import { captureRemoteWorkspaceDirectory, restoreRemoteWorkspaceArchive } from "./remote-workspace-snapshot-archive.js";
 import { beginWorkspaceSnapshot, getWorkspaceSnapshot, loadWorkspaceSnapshotArchive, recoverWorkspaceSnapshotArchive, uploadWorkspaceSnapshot } from "./remote-workspace-snapshot-storage.js";
 
+// Pass-through, so one test can report a changed archive between the two
+// identity reads that bracket encryption.
+const lstatOverride = vi.hoisted(() => ({ current: null as null | ((file: string, real: (file: string) => Promise<import("node:fs").Stats>) => Promise<import("node:fs").Stats>) }));
+vi.mock("node:fs/promises", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs/promises")>();
+  const lstat = ((file: Parameters<typeof actual.lstat>[0], ...rest: unknown[]) => lstatOverride.current
+    ? lstatOverride.current(String(file), target => actual.lstat(target))
+    : (actual.lstat as (...args: unknown[]) => unknown)(file, ...rest)) as typeof actual.lstat;
+  return { ...actual, lstat, default: { ...actual, lstat } };
+});
+
 const externalDatabaseUrl = process.env.PAPERCLIP_TEST_DATABASE_URL;
 if (externalDatabaseUrl && !/^remote_workspace_[a-z_]*test$/.test(new URL(externalDatabaseUrl).pathname.slice(1))) {
   throw new Error("Workspace storage tests require a disposable remote_workspace_*test database");
@@ -136,6 +147,51 @@ suite("remote workspace snapshot storage", () => {
     expect(await fs.lstat(verifiedArchivePath).catch(() => null)).toBeNull();
     expect((await fs.readdir(root)).some(name => name.startsWith(".workspace-upload-readback-"))).toBe(false);
   });
+  it("confirms only the stored size when the backend verified every uploaded part", async () => {
+    const f = await fixture(randomBytes(2 * 1024 * 1024 + 91)); const put = storage.putFile.bind(storage);
+    let sourceFile: string | undefined;
+    storage.putFile = async input => {
+      sourceFile = "sourceFile" in input ? input.sourceFile : undefined;
+      return { ...await put(input), integrity: "server_verified_md5" };
+    };
+    const get = vi.spyOn(storage, "getObject"), head = vi.spyOn(storage, "headObject");
+    const ref = await uploadWorkspaceSnapshot(f.upload);
+    expect(sourceFile).toMatch(/archive\.encrypted$/);
+    expect(get).not.toHaveBeenCalled();
+    expect(head).toHaveBeenCalledWith(f.companyId, ref.objectKey, expect.anything());
+    const restoredArchive = path.join(root, "restored-verified.pcws");
+    await loadWorkspaceSnapshotArchive({ storage, reference: ref, targetPath: restoredArchive });
+    expect((await fs.readFile(restoredArchive)).equals(await fs.readFile(f.archivePath))).toBe(true);
+  });
+  it("still reads back a server-verified upload when the caller keeps the read-back file", async () => {
+    const f = await fixture(); const put = storage.putFile.bind(storage);
+    storage.putFile = async input => ({ ...await put(input), integrity: "server_verified_md5" });
+    const get = vi.spyOn(storage, "getObject"), verifiedArchivePath = path.join(root, "kept.pcws");
+    await uploadWorkspaceSnapshot({ ...f.upload, verifiedArchivePath });
+    expect(get).toHaveBeenCalled();
+    expect((await fs.readFile(verifiedArchivePath)).equals(await fs.readFile(f.archivePath))).toBe(true);
+  });
+  it("rejects a server-verified upload whose stored size differs", async () => {
+    const f = await fixture(); const put = storage.putFile.bind(storage), head = storage.headObject.bind(storage);
+    storage.putFile = async input => ({ ...await put(input), integrity: "server_verified_md5" });
+    storage.headObject = async (...args) => { const result = await head(...args); return { ...result, contentLength: (result.contentLength ?? 0) + 1 }; };
+    await expect(uploadWorkspaceSnapshot(f.upload)).rejects.toThrow("object_size_mismatch");
+    const [row] = await db.select().from(executionWorkspaceSnapshots).where(eq(executionWorkspaceSnapshots.id, f.row.id));
+    expect(row?.status).toBe("pending");
+  });
+  it("rejects an archive that changes while it is encrypted, before any upload", async () => {
+    const f = await fixture(); const put = vi.spyOn(storage, "putFile");
+    let reads = 0;
+    lstatOverride.current = async (file, real) => {
+      const stats = await real(file);
+      if (file !== f.archivePath || ++reads < 2) return stats;
+      return Object.assign(Object.create(Object.getPrototypeOf(stats)), stats, { mtimeMs: stats.mtimeMs + 1 });
+    };
+    try { await expect(uploadWorkspaceSnapshot(f.upload)).rejects.toThrow("archive_changed"); }
+    finally { lstatOverride.current = null; }
+    expect(reads).toBe(2);
+    expect(put).not.toHaveBeenCalled();
+  });
   it("reconciles an upload whose reply was lost after object persistence, without the old scratch file", async () => {
     const f = await fixture(); const put = storage.putFile.bind(storage); let loseReply = true;
     storage.putFile = async input => { const result = await put(input); if (loseReply) { loseReply = false; throw new Error("lost upload reply"); } return result; };
diff --git a/server/src/services/remote-workspace-snapshot-storage.ts b/server/src/services/remote-workspace-snapshot-storage.ts
index 4c520c90c8d..a9ad38d0208 100644
--- a/server/src/services/remote-workspace-snapshot-storage.ts
+++ b/server/src/services/remote-workspace-snapshot-storage.ts
@@ -59,6 +59,10 @@ async function digestFile(file: string, signal?: AbortSignal) {
   for await (const part of createReadStream(file, { signal, highWaterMark: WORKSPACE_ARCHIVE_IO_BUFFER_BYTES })) { digest.update(part); byteSize += part.length; }
   return { sha256: digest.digest("hex"), byteSize };
 }
+async function verifyStoredSize(storage: StorageService, ref: WorkspaceSnapshotRef, signal?: AbortSignal) {
+  const head = await storage.headObject(ref.companyId, ref.objectKey, { signal });
+  if (!head.exists || head.contentLength !== ref.byteSize) failure("object_size_mismatch");
+}
 async function verifyObject(storage: StorageService, ref: WorkspaceSnapshotRef, signal?: AbortSignal) {
   const object = await storage.getObject(ref.companyId, ref.objectKey, { signal });
   const digest = createHash("sha256"); let size = 0;
@@ -135,7 +139,7 @@ type WorkspaceSnapshotUpload = {
 /** Upload attempts are journaled before remote I/O. The caller owns publication. */
 export async function uploadWorkspaceSnapshot(input: WorkspaceSnapshotUpload): Promise<WorkspaceSnapshotRef> {
   if (input.verifiedArchivePath === undefined)
-    return uploadVerifiedWorkspaceSnapshot(input, ref => verifyObject(input.storage, ref, input.signal));
+    return uploadVerifiedWorkspaceSnapshot(input, ref => verifyObject(input.storage, ref, input.signal), { acceptServerVerifiedUpload: true });
   input.signal?.throwIfAborted();
   const target = input.verifiedArchivePath, parent = path.dirname(target);
   if (!path.isAbsolute(target) || path.normalize(target) !== target) failure("readback_parent_invalid");
@@ -160,7 +164,8 @@ export async function uploadWorkspaceSnapshot(input: WorkspaceSnapshotUpload): P
     verified.push({ ref: structuredClone(ref), file });
   };
   try {
-    const ref = await uploadVerifiedWorkspaceSnapshot(input, verify);
+    // This caller keeps the read-back file, so every upload is read back.
+    const ref = await uploadVerifiedWorkspaceSnapshot(input, verify, { acceptServerVerifiedUpload: false });
     await verify(ref);
     const file = verified.find(saved => isDeepStrictEqual(saved.ref, ref))!.file;
     const wrapped = await snapshotKey(ref), handle = await open(file, constants.O_RDONLY | constants.O_NOFOLLOW | constants.O_NONBLOCK);
@@ -183,7 +188,8 @@ export async function uploadWorkspaceSnapshot(input: WorkspaceSnapshotUpload): P
   } finally { await rm(scratch, { recursive: true, force: true }); }
 }
 
-async function uploadVerifiedWorkspaceSnapshot(input: WorkspaceSnapshotUpload, verify: (ref: WorkspaceSnapshotRef) => Promise<void>): Promise<WorkspaceSnapshotRef> {
+async function uploadVerifiedWorkspaceSnapshot(input: WorkspaceSnapshotUpload, verify: (ref: WorkspaceSnapshotRef) => Promise<void>,
+  options: { acceptServerVerifiedUpload: boolean }): Promise<WorkspaceSnapshotRef> {
   const [row] = await input.db.select().from(executionWorkspaceSnapshots).where(scope(input));
   if (!row) failure("missing");
   if (input.unpublishedRecapture && row!.sourceOperationId !== input.unpublishedRecapture.sourceOperationId) failure("operation_binding_changed");
@@ -235,17 +241,23 @@ async function uploadVerifiedWorkspaceSnapshot(input: WorkspaceSnapshotUpload, v
   try {
     const encrypted = path.join(scratch, "archive.encrypted"), dataKey = randomBytes(32), iv = randomBytes(12);
     const cipher = createCipheriv("aes-256-gcm", dataKey, iv); cipher.setAAD(aad(identity));
-    const plaintext = await digestFile(input.archivePath, input.signal);
-    const { material } = await localEncryptedProvider.createVersion({ value: JSON.stringify({ schema: KEY_SCHEMA,
-      ...identity, dataKey: dataKey.toString("base64"), plaintextSha256: plaintext.sha256, plaintextBytes: plaintext.byteSize }) });
     const header = await open(encrypted, "wx", 0o600);
     try { await header.writeFile(Buffer.concat([MAGIC, iv])); } finally { await header.close(); }
+    // One read of the archive both digests and encrypts it. An unchanged file
+    // identity across that read replaces the separate digest pass.
+    const before = await lstat(input.archivePath);
     const incomingDigest = createHash("sha256"); let incomingBytes = 0;
     const observed = new Transform({ transform(bytes, _encoding, callback) { incomingDigest.update(bytes); incomingBytes += bytes.length; callback(null, bytes); } });
     await pipeline(createReadStream(input.archivePath, { signal: input.signal, highWaterMark: WORKSPACE_ARCHIVE_IO_BUFFER_BYTES }), observed, cipher, createWriteStream(encrypted, { flags: "a", mode: 0o600, highWaterMark: WORKSPACE_ARCHIVE_IO_BUFFER_BYTES }), { signal: input.signal });
-    if (incomingBytes !== plaintext.byteSize || incomingDigest.digest("hex") !== plaintext.sha256) failure("archive_changed");
+    const after = await lstat(input.archivePath);
+    if (!before.isFile() || before.isSymbolicLink() || incomingBytes !== before.size || after.dev !== before.dev || after.ino !== before.ino
+      || after.size !== before.size || after.mtimeMs !== before.mtimeMs || after.ctimeMs !== before.ctimeMs) failure("archive_changed");
+    const plaintext = { sha256: incomingDigest.digest("hex"), byteSize: incomingBytes };
     const footer = await open(encrypted, "a"); try { await footer.writeFile(cipher.getAuthTag()); } finally { await footer.close(); }
+    const { material } = await localEncryptedProvider.createVersion({ value: JSON.stringify({ schema: KEY_SCHEMA,
+      ...identity, dataKey: dataKey.toString("base64"), plaintextSha256: plaintext.sha256, plaintextBytes: plaintext.byteSize }) });
     dataKey.fill(0);
+    // Digest the encrypted file as stored: upload parts are read from these bytes.
     const content = await digestFile(encrypted, input.signal);
     const attempt: UploadAttempt = { objectKey: `${identity.companyId}/workspace-snapshots/${identity.workspaceId}/${identity.id}/${randomUUID()}.pcws`,
       ...content, key: material, manifestDigest: input.manifestDigest };
@@ -275,10 +287,15 @@ async function uploadVerifiedWorkspaceSnapshot(input: WorkspaceSnapshotUpload, v
     }
     // The object key is unique per attempt, so a late losing uploader cannot
     // overwrite an object already used by a committed generation.
-    await input.storage.putFile({ companyId: identity.companyId, namespace: "workspace-snapshots", objectKey: attempt.objectKey,
+    const stored = await input.storage.putFile({ companyId: identity.companyId, namespace: "workspace-snapshots", objectKey: attempt.objectKey,
       originalFilename: null, contentType: "application/octet-stream", body: createReadStream(encrypted, { highWaterMark: WORKSPACE_ARCHIVE_IO_BUFFER_BYTES }),
-      byteSize: attempt.byteSize, sha256: attempt.sha256, signal: input.signal });
-    await verify({ schema: SCHEMA, ...identity, ...attempt } as WorkspaceSnapshotRef);
+      byteSize: attempt.byteSize, sha256: attempt.sha256, sourceFile: encrypted, signal: input.signal });
+    const uploaded = { schema: SCHEMA, ...identity, ...attempt } as WorkspaceSnapshotRef;
+    // The backend checked every part of this upload against digests of these
+    // exact bytes, so only its size needs confirming. Any other provider, and
+    // a caller that needs the bytes back, still reads the object back.
+    if (options.acceptServerVerifiedUpload && stored.integrity === "server_verified_md5") await verifyStoredSize(input.storage, uploaded, input.signal);
+    else await verify(uploaded);
     if (recapture) {
       const recovered = await availableAttempt(attempts);
       if (recovered) return publish(recovered);
diff --git a/server/src/storage/s3-multipart.ts b/server/src/storage/s3-multipart.ts
index d6803f0a06b..0c187233eae 100644
--- a/server/src/storage/s3-multipart.ts
+++ b/server/src/storage/s3-multipart.ts
@@ -1,7 +1,11 @@
 import { AbortMultipartUploadCommand, CompleteMultipartUploadCommand, CreateMultipartUploadCommand, UploadPartCommand, type S3Client } from "@aws-sdk/client-s3";
+import { createHash } from "node:crypto";
 import { once } from "node:events";
+import { createReadStream } from "node:fs";
+import { stat } from "node:fs/promises";
 import { PassThrough, Readable } from "node:stream";
-import type { PutObjectInput } from "./types.js";
+import { setTimeout as delay } from "node:timers/promises";
+import type { PutObjectInput, PutObjectReceipt } from "./types.js";
 import { withS3FailureDiagnostics } from "./s3-failure.js";
 import { cleanupStorageFailure } from "./failure-cleanup.js";
 
@@ -122,3 +126,88 @@ export async function putS3Multipart(client: S3Client, bucket: string, key: stri
     await cleanupStorageFailure(async () => iterator.return?.());
   }
 }
+
+export const FILE_PART_CONCURRENCY = 8;
+const FILE_PART_ATTEMPTS = 3;
+const FILE_READ_BYTES = 1024 * 1024;
+
+/** Base64 MD5 of an inclusive byte range, as the Content-MD5 header expects. */
+export async function fileRangeMd5(file: string, start: number, end: number, signal?: AbortSignal) {
+  const digest = createHash("md5");
+  for await (const chunk of createReadStream(file, { start, end, signal, highWaterMark: FILE_READ_BYTES })) digest.update(chunk);
+  return digest.digest("base64");
+}
+
+export async function assertSourceFileSize(file: string, contentLength: number) {
+  const stats = await stat(file);
+  if (!stats.isFile() || stats.size !== contentLength) throw new Error("S3 upload source file does not match its declared size");
+}
+
+/** A file source is read per part instead of through one stream. Parts upload
+ * in parallel without the shared memory window, each carries Content-MD5 so
+ * the backend rejects bytes it did not receive intact, and a failed part is
+ * resent from disk instead of failing the whole upload. */
+export async function putS3MultipartFile(
+  client: S3Client, bucket: string, key: string, input: PutObjectInput & { sourceFile: string },
+): Promise<PutObjectReceipt> {
+  const file = input.sourceFile;
+  const controller = new AbortController();
+  const onAbort = () => controller.abort(input.signal!.reason);
+  input.signal?.addEventListener("abort", onAbort, { once: true });
+  if (input.signal?.aborted) onAbort();
+  // This path never reads the stream body; release its file descriptor now.
+  if (input.body instanceof Readable) input.body.destroy();
+  let uploadId: string | undefined;
+  try {
+    await assertSourceFileSize(file, input.contentLength);
+    controller.signal.throwIfAborted();
+    const partSize = Math.max(bufferedPartSize, Math.ceil(input.contentLength / 10_000));
+    const count = Math.ceil(input.contentLength / partSize);
+    uploadId = (await withS3FailureDiagnostics("create_multipart_upload", () => client.send(new CreateMultipartUploadCommand({ Bucket: bucket, Key: key, ContentType: input.contentType }), { abortSignal: controller.signal }))).UploadId;
+    if (!uploadId) throw new Error("S3 did not return a multipart upload id");
+    const id = uploadId;
+    const parts: { ETag: string; PartNumber: number }[] = new Array(count);
+    const sendPart = async (index: number) => {
+      const start = index * partSize, length = Math.min(partSize, input.contentLength - start), end = start + length - 1;
+      const contentMd5 = await fileRangeMd5(file, start, end, controller.signal);
+      for (let attempt = 1; ; attempt++) {
+        controller.signal.throwIfAborted();
+        const body = createReadStream(file, { start, end, highWaterMark: 64 * 1024 });
+        try {
+          const result = await withS3FailureDiagnostics("upload_part", () => client.send(new UploadPartCommand({
+            Bucket: bucket, Key: key, UploadId: id, PartNumber: index + 1, Body: body, ContentLength: length, ContentMD5: contentMd5,
+          }), { abortSignal: controller.signal }));
+          if (!result.ETag) throw new Error("S3 upload part is incomplete");
+          parts[index] = { ETag: result.ETag, PartNumber: index + 1 };
+          return;
+        } catch (error) {
+          // A part read from disk can be resent; the stream path cannot.
+          if (controller.signal.aborted || attempt >= FILE_PART_ATTEMPTS) throw error;
+          await delay(attempt * 500, undefined, { signal: controller.signal });
+        } finally { body.destroy(); }
+      }
+    };
+    let next = 0;
+    const worker = async () => {
+      while (next < count) {
+        const index = next++;
+        try { await sendPart(index); }
+        catch (error) { if (!controller.signal.aborted) controller.abort(error); throw error; }
+      }
+    };
+    // Wait for every sibling to settle before the multipart upload is aborted.
+    const settled = await Promise.allSettled(Array.from({ length: Math.min(FILE_PART_CONCURRENCY, count) }, worker));
+    const rejected = settled.find((entry): entry is PromiseRejectedResult => entry.status === "rejected");
+    if (rejected) throw controller.signal.aborted && controller.signal.reason !== undefined ? controller.signal.reason : rejected.reason;
+    controller.signal.throwIfAborted();
+    await withS3FailureDiagnostics("complete_multipart_upload", () => client.send(new CompleteMultipartUploadCommand({ Bucket: bucket, Key: key, UploadId: id, MultipartUpload: { Parts: parts } }), { abortSignal: controller.signal }));
+    uploadId = undefined;
+    return { integrity: "server_verified_md5" };
+  } catch (error) {
+    const failedUpload = uploadId;
+    if (failedUpload) await cleanupStorageFailure(signal => client.send(new AbortMultipartUploadCommand({ Bucket: bucket, Key: key, UploadId: failedUpload }), { abortSignal: signal }));
+    throw error;
+  } finally {
+    input.signal?.removeEventListener("abort", onAbort);
+  }
+}
diff --git a/server/src/storage/s3-provider.ts b/server/src/storage/s3-provider.ts
index 6edd3e4e152..a87d279982c 100644
--- a/server/src/storage/s3-provider.ts
+++ b/server/src/storage/s3-provider.ts
@@ -7,7 +7,8 @@ import {
   ListObjectsV2Command,
   ListObjectVersionsCommand,
 } from "@aws-sdk/client-s3";
-import { putS3Multipart } from "./s3-multipart.js";
+import { assertSourceFileSize, fileRangeMd5, putS3Multipart, putS3MultipartFile } from "./s3-multipart.js";
+import { createReadStream } from "node:fs";
 import { recordS3BodyFailure, withS3FailureDiagnostics } from "./s3-failure.js";
 import { addAbortSignal, Readable } from "node:stream";
 import { retryS3RequestTimeouts } from "./s3-request-timeout-retry.js";
@@ -81,6 +82,16 @@ export function createS3StorageProvider(config: S3ProviderConfig): StorageProvid
     forcePathStyle: Boolean(config.forcePathStyle),
   });
   retryS3RequestTimeouts(client);
+  // File sources carry Content-MD5 as their integrity check. Send plain bodies
+  // so the backend verifies that digest directly; the default client wraps
+  // bodies in aws-chunked framing with its own trailing checksum.
+  const fileClient = new S3Client({
+    region,
+    endpoint: config.endpoint,
+    forcePathStyle: Boolean(config.forcePathStyle),
+    requestChecksumCalculation: "WHEN_REQUIRED",
+  });
+  retryS3RequestTimeouts(fileClient);
 
   async function listExactVersions(key: string, signal?: AbortSignal) {
     const versions = new Set<string>(), cursors = new Set<string>();
@@ -140,6 +151,24 @@ export function createS3StorageProvider(config: S3ProviderConfig): StorageProvid
 
     async putObject(input) {
       const key = buildKey(prefix, input.objectKey);
+      const sourceFile = input.sourceFile;
+      if (sourceFile && input.contentLength > 16 * 1024 * 1024) {
+        return putS3MultipartFile(fileClient, bucket, key, { ...input, sourceFile });
+      }
+      if (sourceFile) {
+        // Content-MD5 makes the backend reject any byte it did not receive intact.
+        if (input.body instanceof Readable) input.body.destroy();
+        await assertSourceFileSize(sourceFile, input.contentLength);
+        const contentMd5 = await fileRangeMd5(sourceFile, 0, input.contentLength - 1, input.signal);
+        const body = createReadStream(sourceFile);
+        try {
+          await withS3FailureDiagnostics("put_object", () => fileClient.send(new PutObjectCommand({
+            Bucket: bucket, Key: key, Body: body, ContentType: input.contentType,
+            ContentLength: input.contentLength, ContentMD5: contentMd5,
+          }), { abortSignal: input.signal }));
+        } finally { body.destroy(); }
+        return { integrity: "server_verified_md5" } as const;
+      }
       if (input.contentLength > 16 * 1024 * 1024) {
         await putS3Multipart(client, bucket, key, input);
         return;
diff --git a/server/src/storage/service.ts b/server/src/storage/service.ts
index 69023bc5bb2..b742bef8d66 100644
--- a/server/src/storage/service.ts
+++ b/server/src/storage/service.ts
@@ -1,7 +1,7 @@
 import { createHash, randomUUID } from "node:crypto";
 import path from "node:path";
 import { Readable } from "node:stream";
-import type { StorageService, StorageProvider, PutFileInput, PutFileResult } from "./types.js";
+import type { StorageService, StorageProvider, PutFileInput, PutFileResult, PutObjectReceipt } from "./types.js";
 import { badRequest, forbidden, unprocessable } from "../errors.js";
 import { cleanupStorageFailure } from "./failure-cleanup.js";
 
@@ -114,13 +114,15 @@ export function createStorageService(provider: StorageProvider): StorageService
       ensureCompanyPrefix(input.companyId, objectKey);
       const byteSize = "byteSize" in input ? input.byteSize : input.body.length;
       const contentType = input.contentType.trim().toLowerCase();
+      let receipt: void | PutObjectReceipt;
       try {
-        await provider.putObject({
+        receipt = await provider.putObject({
           objectKey,
           body: input.body,
           contentType,
           contentLength: byteSize,
           signal: input.signal,
+          ...("sourceFile" in input && input.sourceFile ? { sourceFile: input.sourceFile } : {}),
         });
       } catch (error) {
         await cleanupStorageFailure(signal => provider.deleteObject({ objectKey, signal }));
@@ -136,6 +138,7 @@ export function createStorageService(provider: StorageProvider): StorageService
         byteSize,
         sha256: "sha256" in input ? input.sha256 : hashBuffer(input.body),
         originalFilename: input.originalFilename,
+        ...(receipt ? { integrity: receipt.integrity } : {}),
       };
     },
 
diff --git a/server/src/storage/types.ts b/server/src/storage/types.ts
index 6ab5a725664..11cb9449f41 100644
--- a/server/src/storage/types.ts
+++ b/server/src/storage/types.ts
@@ -9,6 +9,21 @@ export interface PutObjectInput {
   body: Buffer | Readable;
   contentType: string;
   contentLength: number;
+  /**
+   * Absolute path of a private file whose bytes are exactly `body`. A provider
+   * may read ranges of it instead of the stream, so parts can upload in
+   * parallel and be resent after a transient failure.
+   */
+  sourceFile?: string;
+}
+
+/**
+ * The backend verified every uploaded byte against a digest the client sent
+ * with it (Content-MD5 on each request), so a read-back is not needed to
+ * prove the stored content.
+ */
+export interface PutObjectReceipt {
+  integrity: "server_verified_md5";
 }
 
 export interface GetObjectInput {
@@ -46,7 +61,7 @@ export interface HeadObjectResult {
 
 export interface StorageProvider {
   id: StorageProviderId;
-  putObject(input: PutObjectInput): Promise<void>;
+  putObject(input: PutObjectInput): Promise<void | PutObjectReceipt>;
   getObject(input: GetObjectInput): Promise<GetObjectResult>;
   headObject(input: GetObjectInput): Promise<HeadObjectResult>;
   deleteObject(input: DeleteObjectInput): Promise<void>;
@@ -70,7 +85,7 @@ export type PutFileInput = {
   namespace: string;
   originalFilename: string | null;
   contentType: string;
-} & ({ body: Buffer } | { body: Readable; byteSize: number; sha256: string });
+} & ({ body: Buffer } | { body: Readable; byteSize: number; sha256: string; sourceFile?: string });
 
 export interface PutFileResult {
   provider: StorageProviderId;
@@ -79,6 +94,8 @@ export interface PutFileResult {
   byteSize: number;
   sha256: string;
   originalFilename: string | null;
+  /** Present only when the backend verified every byte on receipt. */
+  integrity?: PutObjectReceipt["integrity"];
 }
 
 export interface StorageService {