diff --git a/lib/db/migrations/007_companion.sql b/lib/db/migrations/007_companion.sql
new file mode 100644
index 0000000..7720720
--- /dev/null
+++ b/lib/db/migrations/007_companion.sql
@@ -0,0 +1,14 @@
+-- lib/db/migrations/007_companion.sql
+-- Plan 5: per-Space ambient companion conversation + default companion agent.
+
+ALTER TABLE conversations
+  ADD COLUMN space_id uuid REFERENCES spaces(id) ON DELETE CASCADE;
+
+CREATE INDEX idx_conversations_space ON conversations(space_id, started_at DESC);
+
+INSERT INTO agents (slug, name, kind, model, capabilities)
+VALUES (
+  'companion', 'Companion', 'claude', NULL,
+  '{"read":true,"suggest":true,"write":false}'::jsonb
+)
+ON CONFLICT (slug) DO NOTHING;
diff --git a/tests/db/migration_007.test.js b/tests/db/migration_007.test.js
new file mode 100644
index 0000000..5e752c9
--- /dev/null
+++ b/tests/db/migration_007.test.js
@@ -0,0 +1,23 @@
+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';
+
+beforeAll(async () => { await resetDb(); await migrateUp(); });
+
+describe('migration 007', () => {
+  it('adds conversations.space_id column', async () => {
+    const { rows } = await pool.query(
+      `SELECT column_name FROM information_schema.columns
+       WHERE table_name='conversations' AND column_name='space_id'`
+    );
+    expect(rows).toHaveLength(1);
+  });
+
+  it('seeds a default companion agent', async () => {
+    const { rows } = await pool.query(`SELECT slug, kind, capabilities FROM agents WHERE slug='companion'`);
+    expect(rows).toHaveLength(1);
+    expect(rows[0].kind).toBe('claude');
+    expect(rows[0].capabilities).toMatchObject({ read: true, suggest: true, write: false });
+  });
+});