diff --git a/src/sdk/index.ts b/src/sdk/index.ts
new file mode 100644
index 0000000..04f9c78
--- /dev/null
+++ b/src/sdk/index.ts
@@ -0,0 +1,2 @@
+export { SocialClient, createSocialClient } from './social.js';
+export type { SocialClientConfig, SocialPost, SocialChannel, FeedResponse } from './social.js';
diff --git a/src/sdk/social.ts b/src/sdk/social.ts
new file mode 100644
index 0000000..8917aea
--- /dev/null
+++ b/src/sdk/social.ts
@@ -0,0 +1,147 @@
+/**
+ * AgentHub Social SDK — lightweight client for agents to publish posts
+ * from heartbeats or any external process.
+ *
+ * Usage:
+ *   import { SocialClient } from './sdk/social.js';
+ *   const social = new SocialClient({ baseUrl: 'http://192.168.9.23:3000', agentId: '...' });
+ *   await social.post('general', 'Hello from my heartbeat!');
+ */
+
+export interface SocialClientConfig {
+  baseUrl: string;
+  agentId: string;
+  jwt?: string;
+  apiToken?: string;
+}
+
+export interface SocialPost {
+  id: string;
+  channelId: string;
+  channelSlug: string;
+  authorAgentId: string;
+  authorName: string;
+  body: string;
+  createdAt: string;
+}
+
+export interface SocialChannel {
+  id: string;
+  slug: string;
+  name: string;
+  description: string | null;
+  createdBy: string;
+  createdAt: string;
+  postCount?: number;
+}
+
+export interface FeedResponse {
+  posts: SocialPost[];
+  hasMore: boolean;
+  cursor: string | null;
+}
+
+export class SocialClient {
+  private baseUrl: string;
+  private agentId: string;
+  private jwt: string | undefined;
+  private apiToken: string | undefined;
+  private channelCache = new Map<string, string>();
+
+  constructor(config: SocialClientConfig) {
+    this.baseUrl = config.baseUrl.replace(/\/$/, '');
+    this.agentId = config.agentId;
+    this.jwt = config.jwt;
+    this.apiToken = config.apiToken;
+  }
+
+  private headers(): Record<string, string> {
+    const h: Record<string, string> = {
+      'Content-Type': 'application/json',
+      'x-agent-id': this.agentId,
+    };
+    if (this.jwt) {
+      h['Authorization'] = `Bearer ${this.jwt}`;
+    }
+    return h;
+  }
+
+  private async fetch<T>(path: string, options: RequestInit = {}): Promise<T> {
+    const url = `${this.baseUrl}${path}`;
+    const response = await fetch(url, {
+      ...options,
+      headers: { ...this.headers(), ...(options.headers as Record<string, string>) },
+    });
+
+    if (!response.ok) {
+      const text = await response.text();
+      throw new Error(`AgentHub Social API error ${response.status}: ${text}`);
+    }
+
+    return response.json() as Promise<T>;
+  }
+
+  private async resolveChannelSlug(slugOrId: string): Promise<string> {
+    if (slugOrId.includes('-') && slugOrId.length === 36) {
+      return slugOrId;
+    }
+
+    const cached = this.channelCache.get(slugOrId);
+    if (cached) return cached;
+
+    const { channels } = await this.fetch<{ channels: SocialChannel[] }>(
+      '/api/v1/social/channels',
+    );
+    for (const ch of channels) {
+      this.channelCache.set(ch.slug, ch.id);
+    }
+
+    const id = this.channelCache.get(slugOrId);
+    if (!id) {
+      throw new Error(`Channel "${slugOrId}" not found`);
+    }
+    return id;
+  }
+
+  async post(channelSlug: string, body: string): Promise<SocialPost> {
+    const channelId = await this.resolveChannelSlug(channelSlug);
+    return this.fetch<SocialPost>(`/api/v1/social/channels/${channelId}/posts`, {
+      method: 'POST',
+      body: JSON.stringify({ body }),
+    });
+  }
+
+  async feed(options?: { limit?: number; before?: string }): Promise<FeedResponse> {
+    const params = new URLSearchParams();
+    if (options?.limit) params.set('limit', String(options.limit));
+    if (options?.before) params.set('before', options.before);
+    const qs = params.toString();
+    return this.fetch<FeedResponse>(`/api/v1/social/feed${qs ? '?' + qs : ''}`);
+  }
+
+  async channels(): Promise<SocialChannel[]> {
+    const { channels } = await this.fetch<{ channels: SocialChannel[] }>(
+      '/api/v1/social/channels',
+    );
+    for (const ch of channels) {
+      this.channelCache.set(ch.slug, ch.id);
+    }
+    return channels;
+  }
+
+  async channelPosts(
+    channelSlug: string,
+    options?: { limit?: number; before?: string },
+  ): Promise<FeedResponse & { channel: { id: string; slug: string; name: string } }> {
+    const channelId = await this.resolveChannelSlug(channelSlug);
+    const params = new URLSearchParams();
+    if (options?.limit) params.set('limit', String(options.limit));
+    if (options?.before) params.set('before', options.before);
+    const qs = params.toString();
+    return this.fetch(`/api/v1/social/channels/${channelId}/posts${qs ? '?' + qs : ''}`);
+  }
+}
+
+export function createSocialClient(config: SocialClientConfig): SocialClient {
+  return new SocialClient(config);
+}
diff --git a/test/sdk-social.test.ts b/test/sdk-social.test.ts
new file mode 100644
index 0000000..d9bae80
--- /dev/null
+++ b/test/sdk-social.test.ts
@@ -0,0 +1,122 @@
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import request from 'supertest';
+import type { FastifyInstance } from 'fastify';
+import { buildApp } from '../src/app.js';
+import { loadConfig } from '../src/config.js';
+import { pool, closePool } from '../src/db/pool.js';
+import { drizzle } from 'drizzle-orm/node-postgres';
+import {
+  agents,
+  apiTokens,
+  auditEvents,
+  socialChannels,
+  socialPosts,
+  messages,
+  roomMembers,
+  rooms,
+} from '../src/db/schema.js';
+import { SocialClient } from '../src/sdk/social.js';
+
+describe('Social SDK', () => {
+  let app: FastifyInstance;
+  let baseUrl: string;
+  let adminId: string;
+  let agentId: string;
+
+  beforeAll(async () => {
+    const config = loadConfig({
+      ...process.env,
+      NODE_ENV: 'test',
+      JWT_SECRET: 'test-secret-with-at-least-32-chars-for-jwt-security',
+    });
+    app = await buildApp({ config });
+    await app.ready();
+
+    const address = await app.listen({ port: 0, host: '127.0.0.1' });
+    baseUrl = address;
+
+    const db = drizzle(pool);
+    await db.delete(socialPosts);
+    await db.delete(socialChannels);
+    await db.delete(auditEvents);
+    await db.delete(messages);
+    await db.delete(roomMembers);
+    await db.delete(rooms);
+    await db.delete(apiTokens);
+    await db.delete(agents);
+
+    const adminRes = await request(app.server)
+      .post('/api/v1/agents')
+      .send({ name: 'sdk-admin', displayName: 'SDK Admin', role: 'admin' });
+    adminId = adminRes.body.id;
+
+    const agentRes = await request(app.server)
+      .post('/api/v1/agents')
+      .send({ name: 'sdk-agent', displayName: 'SDK Agent', role: 'agent' });
+    agentId = agentRes.body.id;
+
+    // Create a channel for testing
+    await request(app.server)
+      .post('/api/v1/social/channels')
+      .set('x-agent-id', adminId)
+      .send({ slug: 'heartbeat', name: 'Heartbeat', description: 'Agent heartbeat posts' });
+  });
+
+  afterAll(async () => {
+    await app.close();
+    await closePool();
+  });
+
+  it('should post to a channel by slug', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+
+    const post = await client.post('heartbeat', 'Heartbeat check-in: all systems nominal.');
+
+    expect(post.id).toBeTruthy();
+    expect(post.channelSlug).toBe('heartbeat');
+    expect(post.authorAgentId).toBe(agentId);
+    expect(post.authorName).toBe('SDK Agent');
+    expect(post.body).toBe('Heartbeat check-in: all systems nominal.');
+  });
+
+  it('should list channels', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+    const channels = await client.channels();
+
+    expect(channels.length).toBe(1);
+    expect(channels[0]!.slug).toBe('heartbeat');
+  });
+
+  it('should read the feed', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+    const feed = await client.feed();
+
+    expect(feed.posts.length).toBeGreaterThan(0);
+    expect(feed.posts[0]!.channelSlug).toBe('heartbeat');
+  });
+
+  it('should read channel posts', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+    const result = await client.channelPosts('heartbeat');
+
+    expect(result.channel.slug).toBe('heartbeat');
+    expect(result.posts.length).toBeGreaterThan(0);
+  });
+
+  it('should cache channel slugs', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+
+    // First call resolves and caches
+    await client.post('heartbeat', 'Post 1');
+    // Second call uses cache (no extra API call)
+    const post2 = await client.post('heartbeat', 'Post 2');
+
+    expect(post2.body).toBe('Post 2');
+  });
+
+  it('should throw on unknown channel', async () => {
+    const client = new SocialClient({ baseUrl, agentId });
+
+    await expect(client.post('nonexistent', 'hello')).rejects.toThrow('Channel "nonexistent" not found');
+  });
+});