diff --git a/packages/iios-messaging-ui/src/components/conversation-list.tsx b/packages/iios-messaging-ui/src/components/conversation-list.tsx
new file mode 100644
index 0000000..14e75fb
--- /dev/null
+++ b/packages/iios-messaging-ui/src/components/conversation-list.tsx
@@ -0,0 +1,52 @@
+import type { Conversation } from '../types';
+
+/** Initials for the avatar chip — first letters of the first two words. */
+function initials(title: string): string {
+  const parts = title.trim().split(/\s+/).filter(Boolean);
+  const chars = (parts[0]?.[0] ?? '') + (parts[1]?.[0] ?? '');
+  return (chars || '?').toUpperCase();
+}
+
+/**
+ * Presentational conversation list. Owns no data fetching — the parent passes the
+ * conversations (from useConversations) so a host can also drive it from its own store.
+ */
+export function ConversationList({
+  conversations,
+  selectedId,
+  onSelect,
+}: {
+  conversations: Conversation[];
+  selectedId?: string | null;
+  onSelect: (threadId: string) => void;
+}) {
+  if (conversations.length === 0) {
+    return <div className="miu-empty">No conversations yet.</div>;
+  }
+  return (
+    <ul className="miu-convlist" role="list">
+      {conversations.map((c) => (
+        <li key={c.threadId}>
+          <button
+            type="button"
+            className={`miu-convrow${c.threadId === selectedId ? ' is-active' : ''}`}
+            onClick={() => onSelect(c.threadId)}
+          >
+            <span className="miu-avatar" aria-hidden="true">
+              {initials(c.title)}
+            </span>
+            <span className="miu-convrow-main">
+              <span className="miu-convrow-title">{c.title}</span>
+              {c.lastMessage ? <span className="miu-convrow-preview">{c.lastMessage}</span> : null}
+            </span>
+            {c.unread > 0 ? (
+              <span className="miu-badge" aria-label={`${c.unread} unread`}>
+                {c.unread}
+              </span>
+            ) : null}
+          </button>
+        </li>
+      ))}
+    </ul>
+  );
+}
diff --git a/packages/iios-messaging-ui/src/components/messenger.test.tsx b/packages/iios-messaging-ui/src/components/messenger.test.tsx
new file mode 100644
index 0000000..1a78968
--- /dev/null
+++ b/packages/iios-messaging-ui/src/components/messenger.test.tsx
@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import { MessagingProvider } from '../provider';
+import { Messenger } from './messenger';
+import { MockAdapter } from '../adapters/mock';
+
+function mount() {
+  return render(
+    <MessagingProvider adapter={new MockAdapter()}>
+      <Messenger />
+    </MessagingProvider>,
+  );
+}
+
+describe('<Messenger /> (rendered UI over an adapter)', () => {
+  it('renders the conversation list and auto-selects the first thread', async () => {
+    mount();
+    // Seeded group conversation title from the mock.
+    expect(await screen.findByText('Storm response — East side')).toBeTruthy();
+    // Auto-selected thread shows its seeded message.
+    expect(await screen.findByText('Crew is rolling out at 7.')).toBeTruthy();
+  });
+
+  it('sends a message through the adapter and shows it in the thread', async () => {
+    mount();
+    await screen.findByText('Crew is rolling out at 7.');
+
+    const input = screen.getByLabelText('Message') as HTMLInputElement;
+    fireEvent.change(input, { target: { value: 'on our way' } });
+    fireEvent.click(screen.getByText('Send'));
+
+    await waitFor(() => expect(screen.getByText('on our way')).toBeTruthy());
+    // Composer cleared after send.
+    expect(input.value).toBe('');
+  });
+
+  it('switches threads when another conversation is clicked', async () => {
+    mount();
+    // Click the DM (its title is the other participant's name from the mock directory).
+    fireEvent.click(await screen.findByText('Sofia Ramirez'));
+    expect(await screen.findByText('Can you review the Henderson estimate?')).toBeTruthy();
+  });
+});
diff --git a/packages/iios-messaging-ui/src/components/messenger.tsx b/packages/iios-messaging-ui/src/components/messenger.tsx
new file mode 100644
index 0000000..da4cc7e
--- /dev/null
+++ b/packages/iios-messaging-ui/src/components/messenger.tsx
@@ -0,0 +1,32 @@
+import { useEffect, useState } from 'react';
+import { useConversations } from '../hooks/use-conversations';
+import { ConversationList } from './conversation-list';
+import { Thread } from './thread';
+
+/**
+ * The drop-in messenger: conversation list + open thread. Owns only selection state;
+ * all data flows through the injected adapter via the hooks. Auto-selects the first
+ * conversation so the pane is never empty on load.
+ */
+export function Messenger() {
+  const { conversations, loading, error } = useConversations();
+  const [selected, setSelected] = useState<string | null>(null);
+
+  useEffect(() => {
+    if (selected && conversations.some((c) => c.threadId === selected)) return;
+    setSelected(conversations[0]?.threadId ?? null);
+  }, [conversations, selected]);
+
+  return (
+    <div className="miu-messenger">
+      <aside className="miu-sidebar">
+        {loading && conversations.length === 0 ? <div className="miu-empty">Loading…</div> : null}
+        {error ? <div className="miu-empty miu-error">{error}</div> : null}
+        <ConversationList conversations={conversations} selectedId={selected} onSelect={setSelected} />
+      </aside>
+      <section className="miu-main">
+        <Thread threadId={selected} />
+      </section>
+    </div>
+  );
+}
diff --git a/packages/iios-messaging-ui/src/components/thread.tsx b/packages/iios-messaging-ui/src/components/thread.tsx
new file mode 100644
index 0000000..0f44569
--- /dev/null
+++ b/packages/iios-messaging-ui/src/components/thread.tsx
@@ -0,0 +1,75 @@
+import { useState, type FormEvent } from 'react';
+import { useMessages } from '../hooks/use-messages';
+
+/**
+ * One conversation: message list + composer, driven entirely by useMessages (which
+ * handles history, live subscribe, optimistic send, typing, and seen state). Renders
+ * nothing transport-specific — swap the adapter and this is unchanged.
+ */
+export function Thread({ threadId }: { threadId: string | null }) {
+  const { messages, loading, error, send, typingUserIds, seenIds, sendTyping } = useMessages(threadId);
+  const [draft, setDraft] = useState('');
+  const [sending, setSending] = useState(false);
+
+  async function submit(e: FormEvent): Promise<void> {
+    e.preventDefault();
+    const text = draft.trim();
+    if (!text || sending) return;
+    setDraft('');
+    setSending(true);
+    try {
+      await send(text);
+    } catch {
+      // The error is surfaced via the hook's `error`; keep the composer usable.
+    } finally {
+      setSending(false);
+    }
+  }
+
+  if (!threadId) {
+    return <div className="miu-empty miu-thread-empty">Select a conversation.</div>;
+  }
+
+  return (
+    <div className="miu-thread">
+      <div className="miu-messages">
+        {loading && messages.length === 0 ? <div className="miu-empty">Loading…</div> : null}
+        {error ? <div className="miu-empty miu-error">{error}</div> : null}
+        {messages.map((m) => (
+          <div key={m.id} className={`miu-msg${m.mine ? ' is-mine' : ''}${m.pending ? ' is-pending' : ''}`}>
+            <div className="miu-bubble">{m.text}</div>
+            {m.reactions && m.reactions.length > 0 ? (
+              <div className="miu-reactions">
+                {m.reactions.map((r) => (
+                  <span key={r.emoji} className={`miu-reaction${r.mine ? ' is-mine' : ''}`}>
+                    {r.emoji} {r.count}
+                  </span>
+                ))}
+              </div>
+            ) : null}
+            {m.mine && seenIds.has(m.id) ? <span className="miu-seen">Seen</span> : null}
+          </div>
+        ))}
+        {typingUserIds.length > 0 ? (
+          <div className="miu-typing">{typingUserIds.length === 1 ? 'typing…' : 'several people are typing…'}</div>
+        ) : null}
+      </div>
+
+      <form className="miu-composer" onSubmit={submit}>
+        <input
+          className="miu-input"
+          value={draft}
+          placeholder="Type a message…"
+          aria-label="Message"
+          onChange={(e) => {
+            setDraft(e.target.value);
+            sendTyping();
+          }}
+        />
+        <button type="submit" className="miu-send" disabled={!draft.trim() || sending}>
+          Send
+        </button>
+      </form>
+    </div>
+  );
+}
diff --git a/packages/iios-messaging-ui/src/index.ts b/packages/iios-messaging-ui/src/index.ts
index 802d60d..7558faa 100644
--- a/packages/iios-messaging-ui/src/index.ts
+++ b/packages/iios-messaging-ui/src/index.ts
@@ -8,6 +8,11 @@ export { useConversations } from './hooks/use-conversations';
 export { useMessages } from './hooks/use-messages';
 export { isOwnMessage } from './types';
 
+// Rendered UI. Pair with the './styles.css' export (or override the --miu-* tokens).
+export { Messenger } from './components/messenger';
+export { ConversationList } from './components/conversation-list';
+export { Thread } from './components/thread';
+
 export type { MessagingAdapter } from './adapter';
 export type { ConversationsState } from './hooks/use-conversations';
 export type { MessagesState, UiMessage } from './hooks/use-messages';
diff --git a/packages/iios-messaging-ui/src/styles.css b/packages/iios-messaging-ui/src/styles.css
new file mode 100644
index 0000000..43efa1f
--- /dev/null
+++ b/packages/iios-messaging-ui/src/styles.css
@@ -0,0 +1,223 @@
+/* Default theme for @insignia/iios-messaging-ui. Everything is driven by CSS variables
+   scoped to .miu-messenger, so a host restyles by overriding the tokens — no component edits. */
+.miu-messenger {
+  --miu-bg: #0e0e13;
+  --miu-panel: #16161d;
+  --miu-panel-2: #1d1d26;
+  --miu-border: #262631;
+  --miu-text: #e9e9ef;
+  --miu-muted: #9a9aa7;
+  --miu-accent: #fda913;
+  --miu-accent-text: #1a1206;
+  --miu-radius: 12px;
+
+  display: flex;
+  height: 100%;
+  min-height: 0;
+  color: var(--miu-text);
+  background: var(--miu-bg);
+  font-family: system-ui, -apple-system, "Segoe UI", sans-serif;
+  font-size: 14px;
+}
+
+.miu-sidebar {
+  width: 300px;
+  flex-shrink: 0;
+  border-right: 1px solid var(--miu-border);
+  overflow-y: auto;
+  background: var(--miu-panel);
+}
+
+.miu-main {
+  flex: 1;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+/* conversation list */
+.miu-convlist {
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+.miu-convrow {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  width: 100%;
+  padding: 11px 14px;
+  border: none;
+  border-bottom: 1px solid var(--miu-border);
+  background: transparent;
+  color: inherit;
+  text-align: left;
+  cursor: pointer;
+  font: inherit;
+}
+.miu-convrow:hover {
+  background: var(--miu-panel-2);
+}
+.miu-convrow.is-active {
+  background: var(--miu-panel-2);
+}
+.miu-avatar {
+  width: 34px;
+  height: 34px;
+  flex-shrink: 0;
+  border-radius: 50%;
+  display: grid;
+  place-items: center;
+  font-size: 12px;
+  font-weight: 700;
+  color: var(--miu-accent-text);
+  background: var(--miu-accent);
+}
+.miu-convrow-main {
+  flex: 1;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+.miu-convrow-title {
+  font-weight: 600;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+.miu-convrow-preview {
+  color: var(--miu-muted);
+  font-size: 12.5px;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+.miu-badge {
+  flex-shrink: 0;
+  min-width: 18px;
+  height: 18px;
+  padding: 0 5px;
+  border-radius: 999px;
+  display: grid;
+  place-items: center;
+  font-size: 11px;
+  font-weight: 700;
+  color: var(--miu-accent-text);
+  background: var(--miu-accent);
+}
+
+/* thread */
+.miu-thread {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+  min-height: 0;
+}
+.miu-messages {
+  flex: 1;
+  min-height: 0;
+  overflow-y: auto;
+  padding: 16px;
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+.miu-msg {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  max-width: 78%;
+}
+.miu-msg.is-mine {
+  align-self: flex-end;
+  align-items: flex-end;
+}
+.miu-bubble {
+  padding: 8px 12px;
+  border-radius: 14px;
+  border: 1px solid var(--miu-border);
+  background: var(--miu-panel);
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+.miu-msg.is-mine .miu-bubble {
+  border: none;
+  color: var(--miu-accent-text);
+  background: var(--miu-accent);
+}
+.miu-msg.is-pending {
+  opacity: 0.6;
+}
+.miu-reactions {
+  display: flex;
+  gap: 4px;
+  margin-top: 3px;
+}
+.miu-reaction {
+  font-size: 12px;
+  padding: 1px 7px;
+  border-radius: 999px;
+  border: 1px solid var(--miu-border);
+  background: var(--miu-panel-2);
+}
+.miu-reaction.is-mine {
+  border-color: var(--miu-accent);
+}
+.miu-seen {
+  font-size: 11px;
+  color: var(--miu-muted);
+  margin-top: 2px;
+}
+.miu-typing {
+  font-size: 12.5px;
+  color: var(--miu-muted);
+  font-style: italic;
+}
+
+/* composer */
+.miu-composer {
+  display: flex;
+  gap: 8px;
+  padding: 12px;
+  border-top: 1px solid var(--miu-border);
+}
+.miu-input {
+  flex: 1;
+  padding: 9px 12px;
+  border-radius: 10px;
+  border: 1px solid var(--miu-border);
+  background: var(--miu-panel);
+  color: var(--miu-text);
+  font: inherit;
+  outline: none;
+}
+.miu-input:focus {
+  border-color: var(--miu-accent);
+}
+.miu-send {
+  padding: 0 16px;
+  border-radius: 10px;
+  border: none;
+  font-weight: 700;
+  cursor: pointer;
+  color: var(--miu-accent-text);
+  background: var(--miu-accent);
+}
+.miu-send:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+/* misc */
+.miu-empty {
+  padding: 24px;
+  color: var(--miu-muted);
+  text-align: center;
+}
+.miu-thread-empty {
+  margin: auto;
+}
+.miu-error {
+  color: #f0563f;
+}
diff --git a/packages/iios-messaging-ui/tsup.config.ts b/packages/iios-messaging-ui/tsup.config.ts
index aea0692..3920383 100644
--- a/packages/iios-messaging-ui/tsup.config.ts
+++ b/packages/iios-messaging-ui/tsup.config.ts
@@ -6,9 +6,10 @@ export default defineConfig({
   // entry, never reachable from `index`, because it imports vitest, and bundling
   // that into the main barrel would drag a test runner into every consumer's
   // production build.
-  entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts'],
+  entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts', 'src/styles.css'],
   format: ['esm'],
-  dts: true,
+  // Types only for the TS entries — styles.css has no .d.ts (and tsc chokes on a .css root file).
+  dts: { entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts'] },
   clean: true,
   external: ['react', 'react-dom', 'vitest', '@insignia/iios-kernel-client'],
 });