feat: add outbound attachment support with media.send tool

Introduces OutboundAttachment type on OutboundMessage, an
OutboundAttachmentCollector (push/drain pattern), and a media.send
tool that queues files for outbound delivery. Each channel adapter
(Telegram, Discord, Slack, WhatsApp) sends attachments after the
text reply. Includes 15 tests for collector and tool.

src/backends/native/agent.ts

@@ -6,6 +6,7 @@ import type { ToolExecutor } from '../../tools/executor.js';
 import type { ToolResult } from '../../tools/types.js';
 import type { ToolPolicyContext } from '../../tools/policy.js';
 import type { Attachment } from '../../channels/types.js';
+import type { OutboundAttachmentCollector } from './attachments.js';
 import { buildUserMessage, getMessageText } from '../../models/media.js';
 
 export interface ToolUseEvent {
@@ -25,6 +26,8 @@ export interface NativeAgentConfig {
   onToolUse?: (event: ToolUseEvent) => void;
   /** Policy context for tool filtering (agent tier, provider). */
   toolPolicyContext?: ToolPolicyContext;
+  /** Collector for outbound attachments queued by tools (e.g. media.send). */
+  attachmentCollector?: OutboundAttachmentCollector;
 }
 
 // Internal message type for the tool loop — supports both text and structured content blocks.
@@ -47,6 +50,7 @@ export class NativeAgent {
   private _totalUsage: TokenUsage = { inputTokens: 0, outputTokens: 0 };
   private _callCount: number = 0;
   private _toolPolicyContext?: ToolPolicyContext;
+  private _attachmentCollector?: OutboundAttachmentCollector;
 
   constructor(config: NativeAgentConfig) {
     this.modelClient = config.modelClient;
@@ -57,6 +61,7 @@ export class NativeAgent {
     this.maxIterations = config.maxIterations ?? 10;
     this.onToolUse = config.onToolUse;
     this._toolPolicyContext = config.toolPolicyContext;
+    this._attachmentCollector = config.attachmentCollector;
   }
 
   private get history(): Message[] {
@@ -241,4 +246,12 @@ export class NativeAgent {
   getToolPolicyContext(): ToolPolicyContext | undefined {
     return this._toolPolicyContext;
   }
+
+  setAttachmentCollector(collector: OutboundAttachmentCollector | undefined): void {
+    this._attachmentCollector = collector;
+  }
+
+  getAttachmentCollector(): OutboundAttachmentCollector | undefined {
+    return this._attachmentCollector;
+  }
 }

src/backends/native/attachments.test.ts

@@ -0,0 +1,83 @@
+import { describe, it, expect } from 'vitest';
+import { OutboundAttachmentCollector } from './attachments.js';
+
+describe('OutboundAttachmentCollector', () => {
+  it('starts with zero count', () => {
+    const collector = new OutboundAttachmentCollector();
+    expect(collector.count).toBe(0);
+  });
+
+  it('push increments count', () => {
+    const collector = new OutboundAttachmentCollector();
+    collector.push({ mimeType: 'image/png', data: 'abc123' });
+    expect(collector.count).toBe(1);
+
+    collector.push({ mimeType: 'application/pdf', url: 'https://example.com/doc.pdf' });
+    expect(collector.count).toBe(2);
+  });
+
+  it('drain returns all queued attachments', () => {
+    const collector = new OutboundAttachmentCollector();
+    collector.push({ mimeType: 'image/png', data: 'abc123', filename: 'photo.png' });
+    collector.push({ mimeType: 'audio/ogg', url: 'https://example.com/audio.ogg' });
+
+    const drained = collector.drain();
+    expect(drained).toHaveLength(2);
+    expect(drained[0]).toEqual({
+      mimeType: 'image/png',
+      data: 'abc123',
+      filename: 'photo.png',
+    });
+    expect(drained[1]).toEqual({
+      mimeType: 'audio/ogg',
+      url: 'https://example.com/audio.ogg',
+    });
+  });
+
+  it('drain clears the queue', () => {
+    const collector = new OutboundAttachmentCollector();
+    collector.push({ mimeType: 'image/png', data: 'abc123' });
+    expect(collector.count).toBe(1);
+
+    collector.drain();
+    expect(collector.count).toBe(0);
+  });
+
+  it('drain returns empty array when nothing queued', () => {
+    const collector = new OutboundAttachmentCollector();
+    const drained = collector.drain();
+    expect(drained).toEqual([]);
+  });
+
+  it('drain returns a copy, not the internal array', () => {
+    const collector = new OutboundAttachmentCollector();
+    collector.push({ mimeType: 'image/png', data: 'abc123' });
+    const first = collector.drain();
+    const second = collector.drain();
+
+    // First drain got the item, second drain is empty
+    expect(first).toHaveLength(1);
+    expect(second).toHaveLength(0);
+
+    // Mutating the returned array doesn't affect the collector
+    first.push({ mimeType: 'image/gif', data: 'xyz' });
+    expect(collector.count).toBe(0);
+  });
+
+  it('can push and drain multiple cycles', () => {
+    const collector = new OutboundAttachmentCollector();
+
+    // First cycle
+    collector.push({ mimeType: 'image/png', data: 'cycle1' });
+    expect(collector.drain()).toHaveLength(1);
+    expect(collector.count).toBe(0);
+
+    // Second cycle
+    collector.push({ mimeType: 'image/jpeg', data: 'cycle2a' });
+    collector.push({ mimeType: 'image/gif', data: 'cycle2b' });
+    const second = collector.drain();
+    expect(second).toHaveLength(2);
+    expect(second[0].data).toBe('cycle2a');
+    expect(second[1].data).toBe('cycle2b');
+  });
+});

src/backends/native/attachments.ts

@@ -0,0 +1,26 @@
+import type { OutboundAttachment } from '../../channels/types.js';
+
+/**
+ * Collects outbound attachments during a tool execution cycle.
+ * Tools can push attachments here, and they'll be included in the reply.
+ */
+export class OutboundAttachmentCollector {
+  private _attachments: OutboundAttachment[] = [];
+
+  /** Queue an attachment for inclusion in the next outbound message. */
+  push(attachment: OutboundAttachment): void {
+    this._attachments.push(attachment);
+  }
+
+  /** Remove and return all queued attachments. */
+  drain(): OutboundAttachment[] {
+    const result = [...this._attachments];
+    this._attachments = [];
+    return result;
+  }
+
+  /** Number of queued attachments. */
+  get count(): number {
+    return this._attachments.length;
+  }
+}

src/backends/native/index.ts

@@ -1,4 +1,5 @@
 export { NativeAgent, type NativeAgentConfig, type ToolUseEvent } from './agent.js';
+export { OutboundAttachmentCollector } from './attachments.js';
 export {
   AgentOrchestrator,
   type OrchestratorConfig,

src/backends/native/orchestrator.ts

@@ -8,6 +8,7 @@ import type { ToolPolicyContext } from '../../tools/policy.js';
 import type { Attachment } from '../../channels/types.js';
 import { NativeAgent } from './agent.js';
 import type { ToolUseEvent } from './agent.js';
+import type { OutboundAttachmentCollector } from './attachments.js';
 import { shouldCompact } from '../../context/tokens.js';
 import { compactHistory, type CompactionConfig, type CompactionResult, DEFAULT_COMPACTION_CONFIG } from '../../context/compaction.js';
 import { estimateCost } from '../../models/costs.js';
@@ -91,6 +92,8 @@ export interface OrchestratorConfig {
   memoryStore?: MemoryStore;
   /** Policy context for tool filtering (agent tier, provider). */
   toolPolicyContext?: ToolPolicyContext;
+  /** Collector for outbound attachments queued by tools (e.g. media.send). */
+  attachmentCollector?: OutboundAttachmentCollector;
 }
 
 // ── AgentOrchestrator ─────────────────────────────────────────────────
@@ -139,6 +142,7 @@ export class AgentOrchestrator {
       maxIterations: config.maxIterations,
       onToolUse: config.onToolUse,
       toolPolicyContext: config.toolPolicyContext,
+      attachmentCollector: config.attachmentCollector,
     });
 
     // Set the primary tier on the agent

src/channels/discord/adapter.ts

@@ -6,13 +6,14 @@
  * Messages are chunked at Discord's 2000-char limit.
  */
 
-import { Client, GatewayIntentBits, Events } from 'discord.js';
+import { Client, GatewayIntentBits, Events, AttachmentBuilder } from 'discord.js';
 import type { Message as DiscordMessage } from 'discord.js';
 
 import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -121,7 +122,7 @@ export class DiscordAdapter implements ChannelAdapter {
     }
 
     const text = message.text;
-    const sendable = channel as { send: (content: string) => Promise<unknown> };
+    const sendable = channel as { send: (content: string | Record<string, unknown>) => Promise<unknown> };
 
     if (text.length <= 2000) {
       await sendable.send(text);
@@ -131,6 +132,30 @@ export class DiscordAdapter implements ChannelAdapter {
         await sendable.send(chunk);
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      const files = message.attachments
+        .filter((a) => a.data || a.url)
+        .map((a) => this.buildDiscordAttachment(a));
+
+      if (files.length > 0) {
+        await sendable.send({ files });
+      }
+    }
+  }
+
+  /** Build a discord.js AttachmentBuilder from an OutboundAttachment. */
+  private buildDiscordAttachment(attachment: OutboundAttachment): AttachmentBuilder {
+    if (attachment.data) {
+      return new AttachmentBuilder(Buffer.from(attachment.data, 'base64'), {
+        name: attachment.filename ?? 'attachment',
+      });
+    }
+    // URL-based attachment
+    return new AttachmentBuilder(attachment.url!, {
+      name: attachment.filename ?? 'attachment',
+    });
   }
 
   /** Internal: process an inbound Discord message. */
@@ -174,12 +199,12 @@ export class DiscordAdapter implements ChannelAdapter {
     // Strip bot mention from the message text
     const text = message.content.replace(/<@!?\d+>/g, '').trim();
 
-    // ── Extract image attachments ──
+    // ── Extract media attachments ──
     const attachments: Attachment[] = [];
     if (message.attachments && message.attachments.size > 0) {
       for (const attachment of message.attachments.values()) {
         const mimeType = attachment.contentType || this._inferMimeTypeFromUrl(attachment.url);
-        if (mimeType && mimeType.startsWith('image/')) {
+        if (mimeType && (mimeType.startsWith('image/') || mimeType.startsWith('audio/'))) {
           attachments.push({
             mimeType,
             url: attachment.url,

src/channels/index.ts

@@ -3,6 +3,7 @@ export type {
   ChannelStatus,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   Attachment,
   ToolStatusEvent,
   MessageHandler,

src/channels/slack/adapter.ts

@@ -11,6 +11,7 @@ import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -152,6 +153,45 @@ export class SlackAdapter implements ChannelAdapter {
         });
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      for (const attachment of message.attachments) {
+        await this.sendAttachment(channel, threadTs, attachment);
+      }
+    }
+  }
+
+  /** Upload and send a single outbound attachment via Slack's files.uploadV2 API. */
+  private async sendAttachment(
+    channel: string,
+    threadTs: string,
+    attachment: OutboundAttachment,
+  ): Promise<void> {
+    if (!this.app) return;
+
+    try {
+      if (attachment.data) {
+        await this.app.client.files.uploadV2({
+          channel_id: channel,
+          thread_ts: threadTs,
+          file: Buffer.from(attachment.data, 'base64'),
+          filename: attachment.filename ?? 'attachment',
+        });
+      } else if (attachment.url) {
+        // For URL-based attachments, share as a text message with the URL
+        await this.app.client.chat.postMessage({
+          channel,
+          text: attachment.url,
+          thread_ts: threadTs,
+        });
+      }
+    } catch (error) {
+      console.error(
+        `Slack: failed to send ${attachment.mimeType} attachment:`,
+        error instanceof Error ? error.message : 'Unknown error',
+      );
+    }
   }
 
   /** Resolve a Slack user ID to a display name, with caching. */
@@ -170,10 +210,10 @@ export class SlackAdapter implements ChannelAdapter {
   }
 
   /**
-   * Download image files from a Slack message and convert to base64 Attachments.
-   * Non-image files are skipped. Download errors are logged but don't crash the handler.
+   * Download media files from a Slack message and convert to base64 Attachments.
+   * Non-media files are skipped. Download errors are logged but don't crash the handler.
    */
-  private async extractImageAttachments(
+  private async extractMediaAttachments(
     files?: SlackMessageEvent['files'],
   ): Promise<Attachment[]> {
     if (!files || files.length === 0) return [];
@@ -181,8 +221,8 @@ export class SlackAdapter implements ChannelAdapter {
     const attachments: Attachment[] = [];
 
     for (const file of files) {
-      // Only process image files
-      if (!file.mimetype?.startsWith('image/')) continue;
+      // Only process image and audio files
+      if (!file.mimetype?.startsWith('image/') && !file.mimetype?.startsWith('audio/')) continue;
 
       const downloadUrl = file.url_private_download || file.url_private;
       if (!downloadUrl) continue;
@@ -259,8 +299,8 @@ export class SlackAdapter implements ChannelAdapter {
       ? await this.resolveUserName(message.user)
       : undefined;
 
-    // Extract image attachments from Slack file uploads
-    const attachments = await this.extractImageAttachments(message.files);
+    // Extract media attachments from Slack file uploads
+    const attachments = await this.extractMediaAttachments(message.files);
 
     // Detect reset command
     if (text === '!reset' || text === 'reset') {

src/channels/telegram/adapter.ts

@@ -1,10 +1,11 @@
-import { Bot } from 'grammy';
+import { Bot, InputFile } from 'grammy';
 
 import type { HookEngine } from '../../hooks/index.js';
 import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -263,6 +264,80 @@ export class TelegramAdapter implements ChannelAdapter {
       });
     });
 
+    // ── Voice message handler ──
+
+    this.bot.on('message:voice', async (ctx) => {
+      if (!this.messageHandler) return;
+
+      const voice = ctx.message.voice;
+      if (!voice) return;
+
+      await ctx.replyWithChatAction('typing');
+
+      const fileData = await this.downloadFileToBase64(voice.file_id);
+      if (!fileData) {
+        console.error(`Failed to download voice message ${voice.file_id}`);
+        return;
+      }
+
+      const caption = ctx.message.caption ?? '';
+      const mimeType = voice.mime_type ?? 'audio/ogg';
+
+      this.messageHandler({
+        id: String(ctx.message.message_id),
+        channel: 'telegram',
+        senderId: String(ctx.chat.id),
+        senderName: ctx.from?.first_name,
+        text: caption,
+        attachments: [
+          {
+            mimeType,
+            data: fileData,
+            filename: `voice_${voice.file_unique_id}.ogg`,
+            size: voice.file_size,
+          },
+        ],
+        timestamp: Date.now(),
+      });
+    });
+
+    // ── Audio message handler ──
+
+    this.bot.on('message:audio', async (ctx) => {
+      if (!this.messageHandler) return;
+
+      const audio = ctx.message.audio;
+      if (!audio) return;
+
+      await ctx.replyWithChatAction('typing');
+
+      const fileData = await this.downloadFileToBase64(audio.file_id);
+      if (!fileData) {
+        console.error(`Failed to download audio message ${audio.file_id}`);
+        return;
+      }
+
+      const caption = ctx.message.caption ?? '';
+      const mimeType = audio.mime_type ?? 'audio/mpeg';
+
+      this.messageHandler({
+        id: String(ctx.message.message_id),
+        channel: 'telegram',
+        senderId: String(ctx.chat.id),
+        senderName: ctx.from?.first_name,
+        text: caption,
+        attachments: [
+          {
+            mimeType,
+            data: fileData,
+            filename: `audio_${audio.file_unique_id}.${mimeType.split('/')[1]}`,
+            size: audio.file_size,
+          },
+        ],
+        timestamp: Date.now(),
+      });
+    });
+
     // ── Start long polling ──
 
     this.bot.start({
@@ -304,5 +379,34 @@ export class TelegramAdapter implements ChannelAdapter {
         await this.bot.api.sendMessage(chatId, chunk, { parse_mode: 'Markdown' });
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      for (const attachment of message.attachments) {
+        await this.sendAttachment(chatId, attachment);
+      }
+    }
+  }
+
+  /** Send a single outbound attachment via the Telegram API. */
+  private async sendAttachment(chatId: number, attachment: OutboundAttachment): Promise<void> {
+    if (!this.bot) return;
+
+    try {
+      const file = attachment.data
+        ? new InputFile(Buffer.from(attachment.data, 'base64'), attachment.filename)
+        : attachment.url ?? '';
+
+      if (attachment.mimeType.startsWith('image/')) {
+        await this.bot.api.sendPhoto(chatId, file);
+      } else {
+        await this.bot.api.sendDocument(chatId, file);
+      }
+    } catch (error) {
+      console.error(
+        `Failed to send ${attachment.mimeType} attachment to ${chatId}:`,
+        error instanceof Error ? error.message : 'Unknown error',
+      );
+    }
   }
 }

src/channels/types.ts

@@ -42,12 +42,26 @@ export interface InboundMessage {
   metadata?: Record<string, unknown>;
 }
 
+/** Attachment to send back via a channel adapter. */
+export interface OutboundAttachment {
+  /** MIME type (e.g. "image/png", "application/pdf"). */
+  mimeType: string;
+  /** Base64-encoded file content. */
+  data?: string;
+  /** URL to the file (alternative to data). */
+  url?: string;
+  /** Suggested filename. */
+  filename?: string;
+}
+
 /** Outbound message to send via a channel adapter. */
 export interface OutboundMessage {
   /** Response text (markdown). */
   text: string;
   /** Original message ID to reply to. */
   replyTo?: string;
+  /** File or image attachments to send with the response. */
+  attachments?: OutboundAttachment[];
   /** Platform-specific extras. */
   metadata?: Record<string, unknown>;
 }

src/channels/whatsapp/adapter.ts

@@ -7,11 +7,12 @@
  * Messages are chunked at 4096 chars (same as Telegram).
  */
 
-import { Client, LocalAuth } from 'whatsapp-web.js';
+import { Client, LocalAuth, MessageMedia } from 'whatsapp-web.js';
 import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -153,6 +154,38 @@ export class WhatsAppAdapter implements ChannelAdapter {
         await this.client.sendMessage(peerId, chunk);
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      for (const attachment of message.attachments) {
+        await this.sendAttachment(peerId, attachment);
+      }
+    }
+  }
+
+  /** Send a single outbound attachment via WhatsApp using MessageMedia. */
+  private async sendAttachment(peerId: string, attachment: OutboundAttachment): Promise<void> {
+    if (!this.client) return;
+
+    try {
+      if (attachment.data) {
+        const media = new MessageMedia(
+          attachment.mimeType,
+          attachment.data,
+          attachment.filename,
+        );
+        await this.client.sendMessage(peerId, media);
+      } else if (attachment.url) {
+        // Download from URL and send as MessageMedia
+        const media = await MessageMedia.fromUrl(attachment.url);
+        await this.client.sendMessage(peerId, media);
+      }
+    } catch (error) {
+      console.error(
+        `WhatsApp: failed to send ${attachment.mimeType} attachment:`,
+        error instanceof Error ? error.message : 'Unknown error',
+      );
+    }
   }
 
   /** Internal: process an inbound WhatsApp message. */
@@ -211,17 +244,24 @@ export class WhatsAppAdapter implements ChannelAdapter {
 
     const senderName = message._data?.notifyName;
 
-    // Extract image attachments if the message has media
+    // Extract media attachments if the message has media
     const attachments: Attachment[] = [];
     if (message.hasMedia) {
       try {
         const media = await (message as any).downloadMedia();
-        if (media && typeof media.mimetype === 'string' && media.mimetype.startsWith('image/')) {
-          attachments.push({
-            mimeType: media.mimetype,
-            data: media.data,
-            filename: media.filename,
-          });
+        if (media && typeof media.mimetype === 'string') {
+          const mimeType = media.mimetype;
+          const isAudio = mimeType.startsWith('audio/');
+          const isImage = mimeType.startsWith('image/');
+          const isVoice = message.type === 'ptt';
+
+          if (isAudio || isImage || isVoice) {
+            attachments.push({
+              mimeType: mimeType,
+              data: media.data,
+              filename: media.filename,
+            });
+          }
         }
       } catch (error) {
         console.error(

src/tools/builtin/index.ts

@@ -4,6 +4,8 @@ export { fileWriteTool } from './file-write.js';
 export { fileEditTool } from './file-edit.js';
 export { fileListTool } from './file-list.js';
 export { webFetchTool } from './web-fetch.js';
+export { createMediaSendTool } from './media-send.js';
+export { createImageAnalyzeTool } from './image-analyze.js';
 export { createMemoryReadTool } from './memory-read.js';
 export { createMemoryWriteTool } from './memory-write.js';
 export { createMemorySearchTool } from './memory-search.js';
@@ -23,6 +25,8 @@ import { fileWriteTool } from './file-write.js';
 import { fileEditTool } from './file-edit.js';
 import { fileListTool } from './file-list.js';
 import { webFetchTool } from './web-fetch.js';
+import { createMediaSendTool } from './media-send.js';
+import { createImageAnalyzeTool } from './image-analyze.js';
 import { createMemoryReadTool } from './memory-read.js';
 import { createMemoryWriteTool } from './memory-write.js';
 import { createMemorySearchTool } from './memory-search.js';

src/tools/builtin/media-send.test.ts

@@ -0,0 +1,129 @@
+import { describe, it, expect } from 'vitest';
+import { OutboundAttachmentCollector } from '../../backends/native/attachments.js';
+import { createMediaSendTool } from './media-send.js';
+
+describe('media.send tool', () => {
+  it('has correct metadata', () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    expect(tool.name).toBe('media.send');
+    expect(tool.inputSchema.required).toEqual(['mime_type']);
+  });
+
+  it('queues attachment with base64 data', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    const result = await tool.execute({
+      data: 'aGVsbG8=',
+      mime_type: 'image/png',
+      filename: 'hello.png',
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.output).toContain('image/png');
+    expect(result.output).toContain('hello.png');
+    expect(collector.count).toBe(1);
+
+    const drained = collector.drain();
+    expect(drained[0]).toEqual({
+      mimeType: 'image/png',
+      data: 'aGVsbG8=',
+      url: undefined,
+      filename: 'hello.png',
+    });
+  });
+
+  it('queues attachment with URL', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    const result = await tool.execute({
+      url: 'https://example.com/photo.jpg',
+      mime_type: 'image/jpeg',
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.output).toContain('image/jpeg');
+    expect(collector.count).toBe(1);
+
+    const drained = collector.drain();
+    expect(drained[0]).toEqual({
+      mimeType: 'image/jpeg',
+      data: undefined,
+      url: 'https://example.com/photo.jpg',
+      filename: undefined,
+    });
+  });
+
+  it('fails when neither data nor url is provided', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    const result = await tool.execute({ mime_type: 'image/png' });
+
+    expect(result.success).toBe(false);
+    expect(result.error).toContain('Either data or url must be provided');
+    expect(collector.count).toBe(0);
+  });
+
+  it('queues multiple attachments', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    await tool.execute({ data: 'img1', mime_type: 'image/png' });
+    await tool.execute({ url: 'https://example.com/doc.pdf', mime_type: 'application/pdf' });
+    await tool.execute({ data: 'img2', mime_type: 'image/jpeg', filename: 'photo.jpg' });
+
+    expect(collector.count).toBe(3);
+
+    const drained = collector.drain();
+    expect(drained).toHaveLength(3);
+    expect(drained[0].mimeType).toBe('image/png');
+    expect(drained[1].mimeType).toBe('application/pdf');
+    expect(drained[2].mimeType).toBe('image/jpeg');
+    expect(drained[2].filename).toBe('photo.jpg');
+  });
+
+  it('collector drains correctly after tool use', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    await tool.execute({ data: 'abc', mime_type: 'image/png' });
+    expect(collector.count).toBe(1);
+
+    const first = collector.drain();
+    expect(first).toHaveLength(1);
+    expect(collector.count).toBe(0);
+
+    // Second drain should be empty
+    const second = collector.drain();
+    expect(second).toHaveLength(0);
+  });
+
+  it('output includes filename when provided', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    const result = await tool.execute({
+      data: 'abc',
+      mime_type: 'application/pdf',
+      filename: 'report.pdf',
+    });
+
+    expect(result.output).toContain('report.pdf');
+  });
+
+  it('output omits filename when not provided', async () => {
+    const collector = new OutboundAttachmentCollector();
+    const tool = createMediaSendTool(collector);
+
+    const result = await tool.execute({
+      data: 'abc',
+      mime_type: 'image/png',
+    });
+
+    expect(result.output).toBe('Attachment queued (image/png)');
+  });
+});

src/tools/builtin/media-send.ts

@@ -0,0 +1,71 @@
+import type { Tool, ToolResult } from '../types.js';
+import type { OutboundAttachmentCollector } from '../../backends/native/attachments.js';
+
+interface MediaSendArgs {
+  data?: string;
+  url?: string;
+  mime_type: string;
+  filename?: string;
+}
+
+/**
+ * Create the media.send tool bound to an OutboundAttachmentCollector.
+ *
+ * The tool lets the agent queue a file or image to be sent back to the user.
+ * Attachments are collected during the tool loop and included in the outbound
+ * message after the agent finishes processing.
+ */
+export function createMediaSendTool(collector: OutboundAttachmentCollector): Tool {
+  return {
+    name: 'media.send',
+    description:
+      'Attach a file or image to send back to the user. The attachment will be included with the next text response.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        data: {
+          type: 'string',
+          description: 'Base64-encoded file content',
+        },
+        url: {
+          type: 'string',
+          description: 'URL to the file (alternative to data)',
+        },
+        mime_type: {
+          type: 'string',
+          description: 'MIME type of the file (e.g. image/png, application/pdf)',
+        },
+        filename: {
+          type: 'string',
+          description: 'Suggested filename',
+        },
+      },
+      required: ['mime_type'],
+    },
+
+    execute: async (rawArgs: unknown): Promise<ToolResult> => {
+      const args = rawArgs as MediaSendArgs;
+
+      if (!args.data && !args.url) {
+        return {
+          success: false,
+          output: '',
+          error: 'Either data or url must be provided',
+        };
+      }
+
+      collector.push({
+        mimeType: args.mime_type,
+        data: args.data,
+        url: args.url,
+        filename: args.filename,
+      });
+
+      const label = args.filename ? `: ${args.filename}` : '';
+      return {
+        success: true,
+        output: `Attachment queued (${args.mime_type}${label})`,
+      };
+    },
+  };
+}

src/tools/index.ts

@@ -5,7 +5,7 @@ export { ToolExecutor } from './executor.js';
 export type { ToolExecutorConfig } from './executor.js';
 export { ToolPolicy } from './policy.js';
 export type { ToolPolicyContext } from './policy.js';
-export { allBuiltinTools, createWebSearchTools, createProcessTools, ProcessManager, BrowserManager, createBrowserTools } from './builtin/index.js';
+export { allBuiltinTools, createWebSearchTools, createProcessTools, ProcessManager, BrowserManager, createBrowserTools, createMediaSendTool } from './builtin/index.js';
 export type { WebSearchConfig } from './builtin/web-search.js';
 export type { ProcessManagerConfig } from './builtin/process/index.js';
 export type { BrowserManagerConfig } from './builtin/browser/index.js';