diff --git a/v2/src/lib/types/ids.test.ts b/v2/src/lib/types/ids.test.ts
new file mode 100644
index 0000000..43798b3
--- /dev/null
+++ b/v2/src/lib/types/ids.test.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect } from 'vitest';
+import { SessionId, ProjectId, type SessionId as SessionIdType, type ProjectId as ProjectIdType } from './ids';
+
+describe('branded types', () => {
+  describe('SessionId', () => {
+    it('creates a SessionId from a string', () => {
+      const id = SessionId('sess-abc-123');
+      expect(id).toBe('sess-abc-123');
+    });
+
+    it('is usable as a string (template literal)', () => {
+      const id = SessionId('sess-1');
+      expect(`session: ${id}`).toBe('session: sess-1');
+    });
+
+    it('is usable as a Map key', () => {
+      const map = new Map<SessionIdType, number>();
+      const id = SessionId('sess-1');
+      map.set(id, 42);
+      expect(map.get(id)).toBe(42);
+    });
+
+    it('equality works between two SessionIds with same value', () => {
+      const a = SessionId('sess-1');
+      const b = SessionId('sess-1');
+      expect(a === b).toBe(true);
+    });
+  });
+
+  describe('ProjectId', () => {
+    it('creates a ProjectId from a string', () => {
+      const id = ProjectId('proj-xyz');
+      expect(id).toBe('proj-xyz');
+    });
+
+    it('is usable as a Map key', () => {
+      const map = new Map<ProjectIdType, string>();
+      const id = ProjectId('proj-1');
+      map.set(id, 'test-project');
+      expect(map.get(id)).toBe('test-project');
+    });
+  });
+
+  describe('type safety (compile-time)', () => {
+    it('both types are strings at runtime', () => {
+      const sid = SessionId('s1');
+      const pid = ProjectId('p1');
+      expect(typeof sid).toBe('string');
+      expect(typeof pid).toBe('string');
+    });
+  });
+});
diff --git a/v2/src/lib/types/ids.ts b/v2/src/lib/types/ids.ts
new file mode 100644
index 0000000..1a51aca
--- /dev/null
+++ b/v2/src/lib/types/ids.ts
@@ -0,0 +1,18 @@
+// Branded types for domain identifiers — prevents accidental swapping of sessionId/projectId
+// These are compile-time only; at runtime they are plain strings.
+
+/** Unique identifier for an agent session */
+export type SessionId = string & { readonly __brand: 'SessionId' };
+
+/** Unique identifier for a project */
+export type ProjectId = string & { readonly __brand: 'ProjectId' };
+
+/** Create a SessionId from a raw string */
+export function SessionId(value: string): SessionId {
+  return value as SessionId;
+}
+
+/** Create a ProjectId from a raw string */
+export function ProjectId(value: string): ProjectId {
+  return value as ProjectId;
+}