diff --git a/docs/spec.md b/docs/spec.md
index a769930..0a0522c 100644
--- a/docs/spec.md
+++ b/docs/spec.md
@@ -472,6 +472,8 @@ const meta = results.records[0]?.content as AttachmentContent | undefined;
 
 - `Stack.putAttachment(data, mimeType, filename?)` — owner-level upload. Creates an `_attachment@1` record with no `entityId`. No grant check.
 - `ScopedStack.putAttachment(data, mimeType, filename?)` — entity-scoped upload. Requires a `create` grant on `_attachment@1`. The created record's `entityId` is set to the uploading entity.
+- `Stack.putAttachmentBytes(data)` — owner-level, bytes only. Stores the file and returns its `fileId` without creating an `_attachment@1` record. No grant check.
+- `ScopedStack.putAttachmentBytes(data)` — entity-scoped, bytes only. Gated identically to `ScopedStack.putAttachment()` (a `create` grant on `_attachment@1`): a bytes upload is only meaningful as a precursor to metadata creation, so the two share one authorization check. Anonymous requesters are always denied.
 
 - `Stack.getAttachment(fileId)` — no permission check; always succeeds if the bytes exist.
 - `ScopedStack.getAttachment(fileId)` — accessible if the requester is the owner, can read any record that references the file, or uploaded the file themselves and it hasn't been associated with a record yet. Throws `StackPermissionError` otherwise.
@@ -755,6 +757,8 @@ Authorization: Bearer <token>
 <binary data>
 ```
 
+`POST /attachments` requires the same authorization as creating an `_attachment@1` record: `401` for anonymous/missing tokens, `403` if the requester lacks a `create` grant on `_attachment@1`. A bytes upload is only meaningful as a precursor to metadata creation, so the two share one authorization requirement.
+
 Returns `413 Request Entity Too Large` if the payload exceeds the server's configured limit (default 50 MB, controlled by `MAX_ATTACHMENT_BYTES`).
 
 The SDK's `Stack.putAttachment()` and `ScopedStack.putAttachment()` perform both steps automatically. Direct HTTP callers must create the `_attachment@1` record separately if metadata is needed.
diff --git a/packages/core/src/stack.ts b/packages/core/src/stack.ts
index 72afe2b..c67dabe 100644
--- a/packages/core/src/stack.ts
+++ b/packages/core/src/stack.ts
@@ -132,6 +132,7 @@ export interface StackClient {
   getVersion(id: string, version: number): Promise<RecordVersion | null>;
   restoreVersion(id: string, version: number): Promise<StackRecord>;
   getAttachment(fileId: string): Promise<Uint8Array>;
+  putAttachmentBytes(data: Uint8Array): Promise<string>;
   putAttachment(data: Uint8Array, mimeType: string, filename?: string): Promise<string>;
   deleteAttachment(fileId: string): Promise<void>;
 }
@@ -986,11 +987,13 @@ export class ScopedStack implements StackClient {
   }
 
   /**
-   * Store bytes and create an _attachment@1 metadata record owned by the
-   * requester. Requires a `create` grant on `_attachment@1`.
-   * Anonymous requesters are always denied.
+   * Store raw bytes only — no _attachment@1 record. Gated identically to
+   * putAttachment(): a bytes upload is only meaningful as a precursor to
+   * metadata creation, so the two share one authorization check.
+   * Requires a `create` grant on `_attachment@1`. Anonymous requesters are
+   * always denied.
    */
-  async putAttachment(data: Uint8Array, mimeType: string, filename?: string): Promise<string> {
+  async putAttachmentBytes(data: Uint8Array): Promise<string> {
     const requester = this.requesterEntityId;
     if (!requester) {
       throw new StackPermissionError('Anonymous requesters cannot upload attachments');
@@ -998,7 +1001,18 @@ export class ScopedStack implements StackClient {
     if (!(await this.checkCreateGrant(`${SYSTEM_TYPES.ATTACHMENT}@1`))) {
       throw new StackPermissionError(`No create grant for type "${SYSTEM_TYPES.ATTACHMENT}@1"`);
     }
-    const fileId = await this.stack.putAttachmentBytes(data);
+    return this.stack.putAttachmentBytes(data);
+  }
+
+  /**
+   * Store bytes and create an _attachment@1 metadata record owned by the
+   * requester. Requires a `create` grant on `_attachment@1`.
+   * Anonymous requesters are always denied.
+   */
+  async putAttachment(data: Uint8Array, mimeType: string, filename?: string): Promise<string> {
+    const fileId = await this.putAttachmentBytes(data);
+    // putAttachmentBytes() above throws if requesterEntityId is null.
+    const requester = this.requesterEntityId as string;
     await this.stack.create(
       `${SYSTEM_TYPES.ATTACHMENT}@1`,
       {
diff --git a/packages/core/tests/scoped-stack.test.ts b/packages/core/tests/scoped-stack.test.ts
index 363e7dc..3468f12 100644
--- a/packages/core/tests/scoped-stack.test.ts
+++ b/packages/core/tests/scoped-stack.test.ts
@@ -570,3 +570,55 @@ describe('ScopedStack.putAttachment', () => {
     expect(typeof fileId).toBe('string');
   });
 });
+
+// -------------------------------------------------------
+// ScopedStack.putAttachmentBytes — bytes-only upload, gated like putAttachment
+// -------------------------------------------------------
+
+describe('ScopedStack.putAttachmentBytes', () => {
+  const data = new Uint8Array([1, 2, 3]);
+
+  test('owner can always upload without a grant', async () => {
+    const fileId = await stack.asEntity(OWNER).putAttachmentBytes(data);
+    expect(typeof fileId).toBe('string');
+  });
+
+  test('anonymous requester cannot upload', async () => {
+    await expect(stack.asEntity(null).putAttachmentBytes(data)).rejects.toThrow(
+      StackPermissionError,
+    );
+  });
+
+  test('authenticated entity without a grant cannot upload', async () => {
+    await expect(stack.asEntity(MEMBER).putAttachmentBytes(data)).rejects.toThrow(
+      StackPermissionError,
+    );
+  });
+
+  test('entity with create grant on _attachment@1 can upload', async () => {
+    await stack.grant(MEMBER, [{ actions: ['create'], typeId: '_attachment@1' }]);
+    const fileId = await stack.asEntity(MEMBER).putAttachmentBytes(data);
+    expect(typeof fileId).toBe('string');
+  });
+
+  test('default grant allows any authenticated entity to upload', async () => {
+    await stack.grant(null, [{ actions: ['create'], typeId: '_attachment@1' }]);
+    const fileId = await stack.asEntity(STRANGER).putAttachmentBytes(data);
+    expect(typeof fileId).toBe('string');
+  });
+
+  test('does not create an _attachment@1 record', async () => {
+    await stack.grant(MEMBER, [{ actions: ['create'], typeId: '_attachment@1' }]);
+    await stack.asEntity(MEMBER).putAttachmentBytes(data);
+    const result = await stack.query({ filter: { typeId: '_attachment@1' } });
+    expect(result.records).toHaveLength(0);
+  });
+
+  test('putAttachment() still succeeds and creates its metadata record via the shared gate', async () => {
+    await stack.grant(MEMBER, [{ actions: ['create'], typeId: '_attachment@1' }]);
+    const fileId = await stack.asEntity(MEMBER).putAttachment(data, 'image/png', 'photo.png');
+    const result = await stack.query({ filter: { typeId: '_attachment@1', entityId: MEMBER } });
+    expect(result.records).toHaveLength(1);
+    expect(result.records[0].content).toMatchObject({ fileId, mimeType: 'image/png' });
+  });
+});