diff --git a/docs/systems/uploads.md b/docs/systems/uploads.md
index e0f7e244..750126cd 100644
--- a/docs/systems/uploads.md
+++ b/docs/systems/uploads.md
@@ -79,6 +79,7 @@ Stats: `getStorageStats()` exposes `staleTusSessions` + `staleTusSize` for the a
 ### Security
 
 - JWT verified on every tus request (PRE_CREATE, PRE_PATCH, finalize, HEAD, DELETE).
+- **Federated uploads use a per-origin JWT.** When the target space is hosted on a remote instance, the client must send that instance's scoped token (resolved via `getTokenForOrigin(origin)` in `crossStoreResolvers.ts`), not the home-instance token — otherwise the remote rejects the request as it can't verify the home signature or resolve the userId.
 - PRE_PATCH ownership check: `metadata.userId === req.user.id`. Required to prevent in-flight upload hijack between session creation and finalize.
 - Size validated against `instance_settings.maxUploadSizeBytes` at PRE_CREATE; tus's own `maxSize` is set as defense-in-depth.
 - Original filename round-trips through tus metadata (base64-encoded per spec); the on-disk filename uses snowflake + sanitized extension only.
@@ -466,7 +467,7 @@ Three stores with strict separation of concerns:
 
 | Store | Source file | Ownership |
 |-------|-------------|-----------|
-| `transferStore` | `packages/web/src/stores/transferStore.ts` | Every byte transfer (uploads + downloads). Source of truth for the global tray. Persists transfer metadata via `transferStore@v1`. |
+| `transferStore` | `packages/web/src/stores/transferStore.ts` | Every byte transfer (uploads + downloads). Source of truth for the global tray. Persists transfer metadata via `transferStore@v1`. Tus uploads route the bearer token through `getTokenForOrigin(origin)` so federated uploads use the per-instance JWT, not the home-instance one. |
 | `composerStore` | `packages/web/src/stores/composerStore.ts` | Per-channel staged transfer IDs + draft text + replyTo. Replaces `MessageInput` component-local state. Persists via `composerStore@v1`. |
 | `pendingMessageStore` | `packages/web/src/stores/pendingMessageStore.ts` | Composed-but-not-yet-sent attachment-bearing bubbles, keyed by `clientId`. Persists via `pendingMessageStore@v1`. Text-only messages are out of scope -- they keep the existing `chatStore.sendMessage` `temp_*` optimistic path. |
 
diff --git a/packages/web/src/stores/instanceStore.ts b/packages/web/src/stores/instanceStore.ts
index ae2c3e8d..78264fcd 100644
--- a/packages/web/src/stores/instanceStore.ts
+++ b/packages/web/src/stores/instanceStore.ts
@@ -6,6 +6,7 @@ import {
   setApiForOriginResolver,
   setUserIdForOriginResolver,
   setOriginFromHostnameResolver,
+  setTokenForOriginResolver,
 } from '../utils/crossStoreResolvers';
 import { useSpaceStore } from './spaceStore';
 import { connectInstance, disconnectInstance as disconnectWs, disconnectAllRemote } from '../hooks/useWebSocket';
@@ -1112,6 +1113,18 @@ setUserIdForOriginResolver((origin: string): string | undefined => {
   return instance?.user.id;
 });
 
+// ─── Token resolution (federation) ────────────────────────────────────────────
+// Maps an origin to the JWT for that instance. Used by transferStore for tus
+// uploads and any other path that constructs raw HTTP requests to a federated
+// instance and needs to pass an Authorization header. Empty origin falls back
+// to the home-instance token from authStore.
+
+setTokenForOriginResolver((origin: string): string | null => {
+  if (!origin) return useAuthStore.getState().token;
+  const instance = useInstanceStore.getState().instances.find(i => i.origin === origin);
+  return instance?.token ?? null;
+});
+
 // ─── Electron: push connected-instance origins to main process ───────────────
 // Enables the main process to intercept invite URLs that point to instances
 // we're already signed into. Uses the basic subscribe(listener) form (no
diff --git a/packages/web/src/stores/spaceStore.ts b/packages/web/src/stores/spaceStore.ts
index 6d953950..580e959f 100644
--- a/packages/web/src/stores/spaceStore.ts
+++ b/packages/web/src/stores/spaceStore.ts
@@ -976,6 +976,8 @@ export {
   setOriginFromHostnameResolver,
   setUserIdForOriginResolver,
   setMyUserIdForOrigin,
+  setTokenForOriginResolver,
+  getTokenForOrigin,
 } from '../utils/crossStoreResolvers';
 
 /**
diff --git a/packages/web/src/stores/transferStore.ts b/packages/web/src/stores/transferStore.ts
index 9e75b028..fae1ef36 100644
--- a/packages/web/src/stores/transferStore.ts
+++ b/packages/web/src/stores/transferStore.ts
@@ -3,6 +3,7 @@ import { persist, type PersistStorage, type StorageValue } from 'zustand/middlew
 import { Upload, type UploadOptions } from 'tus-js-client';
 import type { Attachment } from '@backspace/shared';
 import { useAuthStore } from './authStore';
+import { getTokenForOrigin } from '../utils/crossStoreResolvers';
 
 export type TransferType = 'upload' | 'download';
 export type TransferState =
@@ -229,7 +230,7 @@ export const useTransferStore = create<TransferStore>()(
       },
 
       startUpload: async (file, opts) => {
-        const token = useAuthStore.getState().token;
+        const token = getTokenForOrigin(opts.origin ?? '');
         const user = useAuthStore.getState().user;
         if (!token) throw new Error('Cannot start upload — not authenticated');
 
@@ -317,7 +318,7 @@ export const useTransferStore = create<TransferStore>()(
           // No live instance, but server-side .tus state still exists (e.g., this
           // transfer failed mid-flight or was paused with a stored URL). Send DELETE
           // directly so the partial bytes don't sit on disk until the janitor sweeps.
-          const token = useAuthStore.getState().token;
+          const token = getTokenForOrigin(t.origin ?? '');
           if (token) {
             const fullUrl = t.tusUploadUrl.startsWith('http')
               ? t.tusUploadUrl
@@ -349,9 +350,9 @@ export const useTransferStore = create<TransferStore>()(
         const t = get().get(id);
         if (!t || t.type !== 'upload') return;
 
-        const token = useAuthStore.getState().token;
+        const token = getTokenForOrigin(t.origin ?? '');
         if (!token) {
-          get().setError(id, { message: 'Not authenticated', permanent: true });
+          get().setError(id, { message: 'Not authenticated for this instance', permanent: true });
           return;
         }
 
diff --git a/packages/web/src/stores/transferStore.upload.test.ts b/packages/web/src/stores/transferStore.upload.test.ts
index c1d68d07..e2f1e2d1 100644
--- a/packages/web/src/stores/transferStore.upload.test.ts
+++ b/packages/web/src/stores/transferStore.upload.test.ts
@@ -4,6 +4,14 @@ import 'fake-indexeddb/auto';
 // We mock tus-js-client to drive lifecycle synchronously without real network.
 const startMock = vi.fn();
 const abortMock = vi.fn().mockResolvedValue(undefined);
+// Captures every set of UploadOptions handed to `new Upload(...)`. Tests can
+// inspect `lastUploadOpts()` to assert on endpoint + Authorization header,
+// which is the only way we verify per-origin token routing without hitting
+// the network.
+const constructedOpts: any[] = [];
+function lastUploadOpts(): any {
+  return constructedOpts[constructedOpts.length - 1];
+}
 
 vi.mock('tus-js-client', () => {
   class MockUpload {
@@ -11,6 +19,7 @@ vi.mock('tus-js-client', () => {
     public url: string | null = null;
     constructor(_file: File, opts: any) {
       this.opts = opts;
+      constructedOpts.push(opts);
     }
     start() {
       startMock();
@@ -59,13 +68,27 @@ vi.mock('./authStore', () => ({
 }));
 
 import { useTransferStore } from './transferStore';
+import { setTokenForOriginResolver } from '../utils/crossStoreResolvers';
+import { useAuthStore as authStoreMod } from './authStore';
+
+// Default resolver mirrors prod wiring: empty origin → home authStore token,
+// any other origin → null (no federated instance is wired up in tests unless
+// the test explicitly registers a different resolver).
+function installDefaultTokenResolver(): void {
+  setTokenForOriginResolver((origin: string): string | null => {
+    if (origin) return null;
+    return authStoreMod.getState().token ?? null;
+  });
+}
 
 describe('transferStore.startUpload', () => {
   beforeEach(() => {
     useTransferStore.setState({ transfers: new Map(), hasInMemoryFile: new Set() });
     startMock.mockClear();
     abortMock.mockClear();
+    constructedOpts.length = 0;
     if (typeof localStorage !== 'undefined') localStorage.clear();
+    installDefaultTokenResolver();
   });
 
   it('drives a transfer through tus → completed with attachmentId + attachmentFilename', async () => {
@@ -114,6 +137,47 @@ describe('transferStore.startUpload', () => {
     useTransferStore.getState().pauseUpload(id);
     expect(useTransferStore.getState().get(id)!.state).toBe('paused');
   });
+
+  it('startUpload uses the per-origin token + endpoint for federated uploads', async () => {
+    // Register a resolver that returns a federated token for a specific origin
+    // and the home token (from the mocked authStore) for the empty origin.
+    setTokenForOriginResolver((origin: string): string | null => {
+      if (origin === 'https://remote.example.com') return 'federated-token-XYZ';
+      if (!origin) return authStoreMod.getState().token ?? null;
+      return null;
+    });
+    const file = new File([new Uint8Array(100)], 'a.png', { type: 'image/png' });
+    const id = await useTransferStore.getState().startUpload(file, {
+      tray: true,
+      origin: 'https://remote.example.com',
+    });
+    const t = useTransferStore.getState().get(id);
+    expect(t).toBeDefined();
+    // The synchronous mock drives the full lifecycle, so we end completed.
+    expect(t!.state).toBe('completed');
+    // The transfer carries the remote origin so resume/abort route correctly.
+    expect(t!.origin).toBe('https://remote.example.com');
+    // The tus client was constructed with the federated bearer + remote endpoint —
+    // proves we did NOT fall back to the home authStore token.
+    const opts = lastUploadOpts();
+    expect(opts.endpoint).toBe('https://remote.example.com/api/files/');
+    expect(opts.headers.Authorization).toBe('Bearer federated-token-XYZ');
+  });
+
+  it('startUpload throws when no resolver is registered for the federated origin', async () => {
+    // Resolver returns null for unknown origin → no fallback to home token.
+    setTokenForOriginResolver((origin: string): string | null => {
+      if (!origin) return authStoreMod.getState().token ?? null;
+      return null;
+    });
+    const file = new File([new Uint8Array(10)], 'a.png', { type: 'image/png' });
+    await expect(
+      useTransferStore.getState().startUpload(file, {
+        tray: true,
+        origin: 'https://unknown.example.com',
+      }),
+    ).rejects.toThrow(/not authenticated/i);
+  });
 });
 
 describe('transferStore.resumeUpload', () => {
@@ -121,6 +185,7 @@ describe('transferStore.resumeUpload', () => {
     useTransferStore.setState({ transfers: new Map(), hasInMemoryFile: new Set() });
     startMock.mockClear();
     abortMock.mockClear();
+    installDefaultTokenResolver();
   });
 
   it('marks failed when transfer has no available blob to resume from', async () => {
@@ -177,6 +242,7 @@ describe('transferStore.hasInMemoryFile', () => {
     startMock.mockClear();
     abortMock.mockClear();
     if (typeof localStorage !== 'undefined') localStorage.clear();
+    installDefaultTokenResolver();
   });
 
   it('hasInMemoryFile tracks startUpload + remove lifecycle', async () => {
diff --git a/packages/web/src/utils/crossStoreResolvers.ts b/packages/web/src/utils/crossStoreResolvers.ts
index 8b385b26..3cdbb3ab 100644
--- a/packages/web/src/utils/crossStoreResolvers.ts
+++ b/packages/web/src/utils/crossStoreResolvers.ts
@@ -97,3 +97,32 @@ export function getCachedUserIdForOrigin(origin: string): string | undefined {
 export function clearMyUserIdCache(): void {
   _myUserIdByOrigin.clear();
 }
+
+// ─── Token resolution (federation) ────────────────────────────────────────────
+// Registered by instanceStore on import; maps an origin to the local user's
+// JWT for that instance. Used by transferStore (tus uploads) and any other
+// path that constructs raw HTTP requests to a federated instance and needs to
+// pass an Authorization header.
+
+let _getTokenForOrigin: ((origin: string) => string | null) | null = null;
+
+export function setTokenForOriginResolver(
+  resolver: (origin: string) => string | null,
+): void {
+  _getTokenForOrigin = resolver;
+}
+
+/**
+ * Returns the JWT to use when calling APIs on the given origin.
+ * - Empty origin → home-instance token from authStore.
+ * - Connected remote → that instance's scoped token.
+ * - Unknown / not-yet-connected → null.
+ *
+ * Note: this module imports nothing from `./stores/*` to avoid TDZ cycles.
+ * The home-instance fallback is supplied by the caller via the resolver itself
+ * (instanceStore registers a resolver that knows how to read authStore for `''`).
+ */
+export function getTokenForOrigin(origin: string): string | null {
+  if (!_getTokenForOrigin) return null;
+  return _getTokenForOrigin(origin);
+}