diff --git a/lib/api/middleware/mcp_auth.js b/lib/api/middleware/mcp_auth.js
new file mode 100644
index 0000000..2e82fe8
--- /dev/null
+++ b/lib/api/middleware/mcp_auth.js
@@ -0,0 +1,44 @@
+// Auth gate for /mcp. External agents must present a Void agent bearer token
+// bound to a Space (scopes.space_id). Owner / CF-Access identities are NOT
+// accepted here — external agents never inherit owner powers. CF Access service
+// tokens are enforced at the edge (Cloudflare policy on mcp.void.hynesy.com).
+import * as agents from '../../db/repos/agents.js';
+
+// Minimal fixed-window in-memory rate limit per token (defense-in-depth; the
+// real gate is CF Access + bearer). Window is 60s.
+const WINDOW_MS = 60_000;
+const hits = new Map(); // token -> { count, resetAt }
+export function _resetMcpRate() { hits.clear(); }
+function rateLimited(token) {
+  const limit = Number(process.env.MCP_RATE_LIMIT || 120);
+  const now = Date.now();
+  let h = hits.get(token);
+  if (!h || now > h.resetAt) { h = { count: 0, resetAt: now + WINDOW_MS }; hits.set(token, h); }
+  h.count += 1;
+  return h.count > limit;
+}
+
+export async function mcpAuth(req, res, next) {
+  const auth = req.headers.authorization || '';
+  const [scheme, token] = auth.split(' ');
+  if (scheme !== 'Bearer' || !token) {
+    return res.status(401).json({ error: { code: 'unauthorized', message: 'missing bearer token' } });
+  }
+  if (process.env.OWNER_TOKEN && token === process.env.OWNER_TOKEN) {
+    return res.status(401).json({ error: { code: 'agent_required', message: 'owner token not valid for MCP' } });
+  }
+  if (rateLimited(token)) {
+    return res.status(429).json({ error: { code: 'rate_limited', message: 'too many requests' } });
+  }
+  let agent;
+  try { agent = await agents.verifyToken(token); }
+  catch (e) { return next(e); }
+  if (!agent) {
+    return res.status(401).json({ error: { code: 'unauthorized', message: 'invalid token' } });
+  }
+  if (!(agent.scopes && agent.scopes.space_id)) {
+    return res.status(403).json({ error: { code: 'no_space_scope', message: 'agent has no space scope' } });
+  }
+  req.mcpAgent = agent;
+  next();
+}
diff --git a/tests/mcp/mcp_auth.test.js b/tests/mcp/mcp_auth.test.js
new file mode 100644
index 0000000..96a3f67
--- /dev/null
+++ b/tests/mcp/mcp_auth.test.js
@@ -0,0 +1,62 @@
+import { describe, it, expect, beforeAll, vi } from 'vitest';
+import { resetDb } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+import { pool } from '../../lib/db/pool.js';
+import * as agentsRepo from '../../lib/db/repos/agents.js';
+import { mcpAuth, _resetMcpRate } from '../../lib/api/middleware/mcp_auth.js';
+
+const owner = { kind: 'user', id: null };
+function mockRes() {
+  return { statusCode: 200, body: null,
+    status(c) { this.statusCode = c; return this; },
+    json(b) { this.body = b; return this; } };
+}
+async function run(headers) {
+  const req = { headers };
+  const res = mockRes();
+  const next = vi.fn();
+  await mcpAuth(req, res, next);
+  return { req, res, next };
+}
+
+let scopedToken, unscopedToken;
+beforeAll(async () => {
+  await resetDb(); await migrateUp();
+  process.env.OWNER_TOKEN = 'test-token';
+  const { rows: [{ id: spaceId }] } = await pool.query(`INSERT INTO spaces(slug,name) VALUES('s','S') RETURNING id`);
+  const scoped = await agentsRepo.create({ slug: `s-${Date.now()}`, name: 'S', kind: 'claude', model: 'sonnet',
+    capabilities: { read: true, suggest: true }, scopes: { space_id: spaceId } }, owner);
+  ({ token: scopedToken } = await agentsRepo.createToken(scoped.id, 'mcp'));
+  const unscoped = await agentsRepo.create({ slug: `u-${Date.now()}`, name: 'U', kind: 'claude', model: 'sonnet',
+    capabilities: { read: true, suggest: true }, scopes: {} }, owner);
+  ({ token: unscopedToken } = await agentsRepo.createToken(unscoped.id, 'mcp'));
+});
+
+describe('mcpAuth', () => {
+  it('missing bearer → 401', async () => {
+    const { res, next } = await run({}); expect(res.statusCode).toBe(401); expect(next).not.toHaveBeenCalled();
+  });
+  it('owner token → 401 agent_required', async () => {
+    const { res } = await run({ authorization: 'Bearer test-token' });
+    expect(res.statusCode).toBe(401); expect(res.body.error.code).toBe('agent_required');
+  });
+  it('invalid token → 401', async () => {
+    const { res } = await run({ authorization: 'Bearer vk_nope.nope' }); expect(res.statusCode).toBe(401);
+  });
+  it('agent without space scope → 403 no_space_scope', async () => {
+    const { res } = await run({ authorization: `Bearer ${unscopedToken}` });
+    expect(res.statusCode).toBe(403); expect(res.body.error.code).toBe('no_space_scope');
+  });
+  it('valid scoped agent → next() + req.mcpAgent', async () => {
+    const { req, next } = await run({ authorization: `Bearer ${scopedToken}` });
+    expect(next).toHaveBeenCalled(); expect(req.mcpAgent.scopes.space_id).toBeTruthy();
+  });
+  it('rate limit → 429 past the cap', async () => {
+    _resetMcpRate(); process.env.MCP_RATE_LIMIT = '2';
+    await run({ authorization: `Bearer ${scopedToken}` });
+    await run({ authorization: `Bearer ${scopedToken}` });
+    const { res } = await run({ authorization: `Bearer ${scopedToken}` });
+    expect(res.statusCode).toBe(429);
+    delete process.env.MCP_RATE_LIMIT;
+  });
+});