diff --git a/lib/db/migrations/011_yerin.sql b/lib/db/migrations/011_yerin.sql
new file mode 100644
index 0000000..e1b1fac
--- /dev/null
+++ b/lib/db/migrations/011_yerin.sql
@@ -0,0 +1,10 @@
+-- Seed Yerin, the security agent. READ-ONLY by capability (no suggest/write):
+-- she investigates and reports; remediation goes through you. Runs on the same
+-- claude CLI subscription path as Dross (model NULL = server default); switch to
+-- a local Ollama model later by setting agents.model. See docs/yerin-security-agent.md.
+INSERT INTO agents (slug, name, kind, model, capabilities)
+VALUES (
+  'yerin', 'Yerin', 'claude', NULL,
+  '{"read":true,"suggest":false,"write":false}'::jsonb
+)
+ON CONFLICT (slug) DO NOTHING;
diff --git a/lib/mcp/companion-stdio.js b/lib/mcp/companion-stdio.js
index dc4d0d0..f2c036a 100644
--- a/lib/mcp/companion-stdio.js
+++ b/lib/mcp/companion-stdio.js
@@ -20,8 +20,16 @@ import {
   CallToolRequestSchema
 } from '@modelcontextprotocol/sdk/types.js';
 import { companionRegistry } from '../ai/agent/tools/index.js';
+import { securityRegistry } from '../ai/agent/tools/security/index.js';
 import { buildCtxFromEnv } from './context.js';
 
+// Which toolset this stdio server exposes, selected at launch via
+// VOID_TOOL_REGISTRY (default: Dross's companion tools; 'security' = Yerin's).
+const REGISTRIES = { companion: companionRegistry, security: securityRegistry };
+function resolveRegistry(env = process.env) {
+  return REGISTRIES[env.VOID_TOOL_REGISTRY] || companionRegistry;
+}
+
 // ---------------------------------------------------------------------------
 // Transport-free helpers (exported for testing)
 // ---------------------------------------------------------------------------
@@ -30,8 +38,8 @@ import { buildCtxFromEnv } from './context.js';
  * Returns the list of MCP tool descriptors (no handler) from the companion registry.
  * @returns {{ name: string, description: string, input_schema: object }[]}
  */
-export function listMcpTools() {
-  return companionRegistry.listTools().map(({ name, description, input_schema }) => ({
+export function listMcpTools(env = process.env) {
+  return resolveRegistry(env).listTools().map(({ name, description, input_schema }) => ({
     name,
     description,
     input_schema
@@ -45,8 +53,8 @@ export function listMcpTools() {
  * @param {object} ctx  – tool context {agent, space_id, view, actor}
  * @returns {Promise<object>} raw handler result
  */
-export async function callMcpTool(name, args, ctx) {
-  const tool = companionRegistry.getTool(name);
+export async function callMcpTool(name, args, ctx, env = process.env) {
+  const tool = resolveRegistry(env).getTool(name);
   if (!tool) {
     throw new Error(`Unknown tool: ${name}`);
   }
diff --git a/tests/mcp/registry_select.test.js b/tests/mcp/registry_select.test.js
new file mode 100644
index 0000000..74c7ab0
--- /dev/null
+++ b/tests/mcp/registry_select.test.js
@@ -0,0 +1,40 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { pool } from '../../lib/db/pool.js';
+import { resetDb } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+import { listMcpTools, callMcpTool } from '../../lib/mcp/companion-stdio.js';
+
+// The stdio MCP server must be able to expose Yerin's securityRegistry instead
+// of Dross's companionRegistry, selected by VOID_TOOL_REGISTRY at launch.
+
+describe('MCP registry selection', () => {
+  it('defaults to the companion registry when VOID_TOOL_REGISTRY is unset', () => {
+    const names = listMcpTools({}).map(t => t.name).sort();
+    expect(names).toEqual(['context', 'propose_change', 'read', 'search']);
+  });
+
+  it('selects the security registry when VOID_TOOL_REGISTRY=security', () => {
+    const names = listMcpTools({ VOID_TOOL_REGISTRY: 'security' }).map(t => t.name).sort();
+    expect(names).toContain('audit_log');
+    expect(names).toContain('agent_inventory');
+    expect(names).not.toContain('propose_change'); // Yerin cannot propose mutations
+  });
+
+  it('callMcpTool routes to the selected registry', async () => {
+    await resetDb(); await migrateUp();
+    const out = await callMcpTool('agent_inventory', {}, {}, { VOID_TOOL_REGISTRY: 'security' });
+    expect(Array.isArray(out.agents)).toBe(true);
+    // an unknown tool for the default registry must not be reachable in security mode
+    await expect(callMcpTool('propose_change', {}, {}, { VOID_TOOL_REGISTRY: 'security' }))
+      .rejects.toThrow(/unknown tool/i);
+  });
+});
+
+describe('migration 011 seeds Yerin', () => {
+  it('creates a read-only yerin agent', async () => {
+    await resetDb(); await migrateUp();
+    const { rows: [y] } = await pool.query(`SELECT * FROM agents WHERE slug='yerin'`);
+    expect(y).toBeTruthy();
+    expect(y.capabilities).toEqual({ read: true, suggest: false, write: false });
+  });
+});