diff --git a/src/gateway/handlers/agent.ts b/src/gateway/handlers/agent.ts
index 4255a1b..4f08138 100644
--- a/src/gateway/handlers/agent.ts
+++ b/src/gateway/handlers/agent.ts
@@ -1,7 +1,8 @@
-import type { GatewayRequest, OutboundMessage } from '../protocol.js';
+import type { GatewayRequest, GatewayAttachment, OutboundMessage } from '../protocol.js';
 import type { SendFn } from '../router.js';
 import { makeEvent, makeError, ErrorCode } from '../protocol.js';
 import type { SessionBridge } from '../session-bridge.js';
+import type { Attachment } from '../../channels/types.js';
 
 export interface AgentHandlerDeps {
   sessionBridge: SessionBridge;
@@ -10,7 +11,7 @@ export interface AgentHandlerDeps {
 export function createAgentHandlers(deps: AgentHandlerDeps) {
   return {
     'agent.send': async (request: GatewayRequest, send: SendFn): Promise<OutboundMessage | void> => {
-      const params = request.params as { message?: string; connectionId?: string } | undefined;
+      const params = request.params as { message?: string; connectionId?: string; attachments?: GatewayAttachment[] } | undefined;
       if (!params?.message) {
         return makeError(request.id, ErrorCode.InvalidRequest, 'message is required');
       }
@@ -48,7 +49,15 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
       });
 
       try {
-        const response = await agent.process(params.message);
+        // Convert gateway attachments to channel attachments
+        const attachments: Attachment[] | undefined = params.attachments?.map(a => ({
+          mimeType: a.mimeType,
+          data: a.data,
+          url: a.url,
+          filename: a.filename,
+        }));
+
+        const response = await agent.process(params.message, attachments);
         send(makeEvent(request.id, 'done', { content: response }));
       } catch (err) {
         const message = err instanceof Error ? err.message : 'Unknown error';
diff --git a/src/gateway/handlers/handlers.test.ts b/src/gateway/handlers/handlers.test.ts
index 4c4eac3..a2ed374 100644
--- a/src/gateway/handlers/handlers.test.ts
+++ b/src/gateway/handlers/handlers.test.ts
@@ -206,13 +206,51 @@ describe('agent handlers', () => {
 
     await handlers['agent.send'](req, send);
 
-    expect(mockAgent.process).toHaveBeenCalledWith('hello');
+    expect(mockAgent.process).toHaveBeenCalledWith('hello', undefined);
     expect(sent).toHaveLength(1);
     const doneEvent = sent[0] as GatewayEvent;
     expect(doneEvent.event).toBe('done');
     expect((doneEvent.data as any).content).toBe('response text');
   });
 
+  it('agent.send passes attachments to agent.process', async () => {
+    const attachments = [
+      { mimeType: 'image/png', data: 'iVBOR...', filename: 'screenshot.png' },
+      { mimeType: 'application/pdf', url: 'https://example.com/doc.pdf' },
+    ];
+    const req: GatewayRequest = {
+      id: 10,
+      method: 'agent.send',
+      params: { message: 'describe this', connectionId: 'conn-1', attachments },
+    };
+    const sent: OutboundMessage[] = [];
+    const send = vi.fn((msg: OutboundMessage) => sent.push(msg));
+
+    await handlers['agent.send'](req, send);
+
+    expect(mockAgent.process).toHaveBeenCalledWith('describe this', [
+      { mimeType: 'image/png', data: 'iVBOR...', url: undefined, filename: 'screenshot.png' },
+      { mimeType: 'application/pdf', data: undefined, url: 'https://example.com/doc.pdf', filename: undefined },
+    ]);
+    const doneEvent = sent[0] as GatewayEvent;
+    expect(doneEvent.event).toBe('done');
+  });
+
+  it('agent.send works with empty attachments array', async () => {
+    const req: GatewayRequest = {
+      id: 11,
+      method: 'agent.send',
+      params: { message: 'hi', connectionId: 'conn-1', attachments: [] },
+    };
+    const sent: OutboundMessage[] = [];
+    const send = vi.fn((msg: OutboundMessage) => sent.push(msg));
+
+    await handlers['agent.send'](req, send);
+
+    expect(mockAgent.process).toHaveBeenCalledWith('hi', []);
+    expect(sent).toHaveLength(1);
+  });
+
   it('agent.send requires message', async () => {
     const req: GatewayRequest = { id: 2, method: 'agent.send', params: { connectionId: 'conn-1' } };
     const send = vi.fn();
diff --git a/src/gateway/index.ts b/src/gateway/index.ts
index 2236b25..1a0235c 100644
--- a/src/gateway/index.ts
+++ b/src/gateway/index.ts
@@ -20,11 +20,13 @@ export type {
   GatewayResponse,
   GatewayError,
   GatewayEvent,
+  GatewayAttachment,
   OutboundMessage,
   EventType,
   ContentEventData,
   ToolStartEventData,
   ToolEndEventData,
+  AttachmentEventData,
   DoneEventData,
   ErrorEventData,
 } from './protocol.js';
diff --git a/src/gateway/protocol.test.ts b/src/gateway/protocol.test.ts
index 4206d04..6ba4198 100644
--- a/src/gateway/protocol.test.ts
+++ b/src/gateway/protocol.test.ts
@@ -86,5 +86,23 @@ describe('protocol', () => {
         data: { text: 'hello' },
       });
     });
+
+    it('creates an attachment event message', () => {
+      const data = { mimeType: 'image/png', data: 'iVBOR...', filename: 'screenshot.png' };
+      expect(makeEvent(1, 'attachment', data)).toEqual({
+        id: 1,
+        event: 'attachment',
+        data,
+      });
+    });
+
+    it('creates an attachment event with url', () => {
+      const data = { mimeType: 'application/pdf', url: 'https://example.com/doc.pdf', filename: 'doc.pdf' };
+      expect(makeEvent(2, 'attachment', data)).toEqual({
+        id: 2,
+        event: 'attachment',
+        data,
+      });
+    });
   });
 });
diff --git a/src/gateway/protocol.ts b/src/gateway/protocol.ts
index 40374fa..15c38b6 100644
--- a/src/gateway/protocol.ts
+++ b/src/gateway/protocol.ts
@@ -29,12 +29,27 @@ export interface GatewayEvent {
   data: unknown;
 }
 
+// ── Attachment data for gateway protocol messages ───────────────
+
+/** Attachment data sent in agent.send params or emitted as events. */
+export interface GatewayAttachment {
+  /** MIME type (e.g. "image/jpeg", "audio/ogg") */
+  mimeType: string;
+  /** Base64-encoded data */
+  data?: string;
+  /** URL to the resource */
+  url?: string;
+  /** Filename hint */
+  filename?: string;
+}
+
 // ── Event types emitted during agent.send ──────────────────────
 
 export type EventType =
   | 'content'
   | 'tool_start'
   | 'tool_end'
+  | 'attachment'
   | 'done'
   | 'error';
 
@@ -56,6 +71,13 @@ export interface ToolEndEventData {
   };
 }
 
+export interface AttachmentEventData {
+  mimeType: string;
+  data?: string;
+  url?: string;
+  filename?: string;
+}
+
 export interface DoneEventData {
   content: string;
 }