diff --git a/packages/iios-messaging-ui/src/types.test.ts b/packages/iios-messaging-ui/src/types.test.ts
new file mode 100644
index 0000000..da334a2
--- /dev/null
+++ b/packages/iios-messaging-ui/src/types.test.ts
@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { isOwnMessage } from './types';
+import type { Message } from './types';
+
+const base: Message = {
+  id: 'm1',
+  actorId: 'actor_a',
+  text: 'hello',
+  at: '2026-07-17T10:00:00.000Z',
+};
+
+describe('isOwnMessage', () => {
+  it('is true when the message actor matches the current actor', () => {
+    expect(isOwnMessage(base, 'actor_a')).toBe(true);
+  });
+
+  it('is false when the actors differ', () => {
+    expect(isOwnMessage(base, 'actor_b')).toBe(false);
+  });
+
+  it('is false when the current actor is unknown', () => {
+    expect(isOwnMessage(base, null)).toBe(false);
+  });
+
+  it('is false when the message has no actor', () => {
+    expect(isOwnMessage({ ...base, actorId: null }, 'actor_a')).toBe(false);
+  });
+});
diff --git a/packages/iios-messaging-ui/src/types.ts b/packages/iios-messaging-ui/src/types.ts
new file mode 100644
index 0000000..ae99f9b
--- /dev/null
+++ b/packages/iios-messaging-ui/src/types.ts
@@ -0,0 +1,64 @@
+// Domain types for the messaging UI. Zero imports on purpose: this file must never
+// reach for a transport package. Adapters map their own DTOs onto these.
+
+export type Membership = 'dm' | 'group';
+
+export interface Person {
+  id: string;
+  name: string;
+  kind: 'staff' | 'customer';
+}
+
+export interface Conversation {
+  threadId: string;
+  title: string;
+  subject: string | null;
+  membership: Membership | null;
+  participants: string[];
+  unread: number;
+  lastMessage?: string;
+  lastAt?: string;
+}
+
+export interface Reaction {
+  emoji: string;
+  count: number;
+  mine: boolean;
+}
+
+export interface Attachment {
+  url: string;
+  mime: string;
+  name: string;
+}
+
+export interface Message {
+  id: string;
+  actorId: string | null;
+  text: string;
+  at: string;
+  parentInteractionId?: string | null;
+  reactions?: Reaction[];
+  attachment?: Attachment;
+  /** True only when the transport is optimistic-local and not yet acknowledged. */
+  pending?: boolean;
+}
+
+export interface SendOpts {
+  parentInteractionId?: string;
+  attachment?: Attachment;
+}
+
+export type MessageEvent =
+  | { kind: 'message'; message: Message }
+  | { kind: 'typing'; userId: string }
+  | { kind: 'receipt'; messageId: string; actorId: string }
+  | { kind: 'reaction'; messageId: string; reactions: Reaction[] };
+
+export type Unsubscribe = () => void;
+
+/** Ownership is a pure function of explicit identity — never inferred from history.
+ *  See the spec: inferring it is the bug this SDK exists partly to kill. */
+export function isOwnMessage(message: Message, currentActorId: string | null): boolean {
+  return currentActorId !== null && message.actorId !== null && message.actorId === currentActorId;
+}