diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index b6fe325..e8b8b66 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -51,6 +51,7 @@ import { Toaster, toast } from './components/ui/sonner';
 import { getStateKey } from './utils/conversationState';
 import { appendRawPacketUnique } from './utils/rawPacketIdentity';
 import { messageContainsMention } from './utils/messageParser';
+import { mergeContactIntoList } from './utils/contactMerge';
 import { cn } from '@/lib/utils';
 import type { Contact, Conversation, HealthStatus, Message, MessagePath, RawPacket } from './types';
 
@@ -265,21 +266,7 @@ export function App() {
         }
       },
       onContact: (contact: Contact) => {
-        setContacts((prev) => {
-          const idx = prev.findIndex((c) => c.public_key === contact.public_key);
-          if (idx >= 0) {
-            const existing = prev[idx];
-            const merged = { ...existing, ...contact };
-            const unchanged = (Object.keys(merged) as (keyof Contact)[]).every(
-              (k) => existing[k] === merged[k]
-            );
-            if (unchanged) return prev;
-            const updated = [...prev];
-            updated[idx] = merged;
-            return updated;
-          }
-          return [...prev, contact as Contact];
-        });
+        setContacts((prev) => mergeContactIntoList(prev, contact));
       },
       onRawPacket: (packet: RawPacket) => {
         setRawPackets((prev) => appendRawPacketUnique(prev, packet, MAX_RAW_PACKETS));
diff --git a/frontend/src/test/integration.test.ts b/frontend/src/test/integration.test.ts
index 9e616b4..a958ec7 100644
--- a/frontend/src/test/integration.test.ts
+++ b/frontend/src/test/integration.test.ts
@@ -12,8 +12,9 @@ import { describe, it, expect, beforeEach } from 'vitest';
 import fixtures from './fixtures/websocket_events.json';
 import { getMessageContentKey } from '../hooks/useConversationMessages';
 import { getStateKey } from '../utils/conversationState';
+import { mergeContactIntoList } from '../utils/contactMerge';
 import * as messageCache from '../messageCache';
-import type { Message } from '../types';
+import type { Contact, Message } from '../types';
 
 /**
  * Minimal state for testing message dedup and unread logic.
@@ -263,3 +264,175 @@ describe('Integration: State Key Contract', () => {
     expect(stateKey).toBe(`contact-${publicKey}`);
   });
 });
+
+// --- Contact merge tests (imports real mergeContactIntoList) ---
+
+function makeContact(overrides: Partial<Contact> = {}): Contact {
+  return {
+    public_key: 'abc123',
+    name: 'TestNode',
+    type: 1,
+    flags: 0,
+    last_path: null,
+    last_path_len: 0,
+    last_advert: null,
+    lat: null,
+    lon: null,
+    last_seen: null,
+    on_radio: true,
+    last_contacted: null,
+    last_read_at: null,
+    ...overrides,
+  };
+}
+
+describe('Integration: Contact Merge', () => {
+  it('appends new contact to list', () => {
+    const existing = [makeContact({ public_key: 'aaa', name: 'Alpha' })];
+    const incoming = makeContact({ public_key: 'bbb', name: 'Beta' });
+
+    const result = mergeContactIntoList(existing, incoming);
+
+    expect(result).toHaveLength(2);
+    expect(result[1].name).toBe('Beta');
+  });
+
+  it('merges existing contact (updates name, preserves other fields)', () => {
+    const existing = [makeContact({ public_key: 'aaa', name: 'Alpha', lat: 47.0 })];
+    const incoming = makeContact({ public_key: 'aaa', name: 'Alpha-Updated' });
+
+    const result = mergeContactIntoList(existing, incoming);
+
+    expect(result).toHaveLength(1);
+    expect(result[0].name).toBe('Alpha-Updated');
+    // Spread semantics: incoming lat (null) overwrites existing lat
+    expect(result[0].public_key).toBe('aaa');
+  });
+
+  it('returns same array reference when contact is unchanged', () => {
+    const contact = makeContact({ public_key: 'aaa', name: 'Alpha' });
+    const existing = [contact];
+    // Incoming with identical values
+    const incoming = makeContact({ public_key: 'aaa', name: 'Alpha' });
+
+    const result = mergeContactIntoList(existing, incoming);
+
+    expect(result).toBe(existing); // referential equality
+  });
+
+  it('partial update merges without clobbering unrelated fields', () => {
+    const existing = [makeContact({ public_key: 'aaa', name: 'Alpha', lat: 47.0, lon: -122.0 })];
+    // Incoming update only changes lat
+    const incoming = makeContact({ public_key: 'aaa', name: 'Alpha', lat: 48.0, lon: -122.0 });
+
+    const result = mergeContactIntoList(existing, incoming);
+
+    expect(result[0].lat).toBe(48.0);
+    expect(result[0].lon).toBe(-122.0);
+    expect(result[0].name).toBe('Alpha');
+  });
+});
+
+// --- ACK + messageCache propagation tests ---
+
+describe('Integration: ACK + messageCache propagation', () => {
+  beforeEach(() => {
+    messageCache.clear();
+  });
+
+  it('updateAck updates acked count on cached message', () => {
+    const msg: Message = {
+      id: 100,
+      type: 'PRIV',
+      conversation_key: 'pk_abc',
+      text: 'Hello',
+      sender_timestamp: 1700000000,
+      received_at: 1700000000,
+      paths: null,
+      txt_type: 0,
+      signature: null,
+      outgoing: true,
+      acked: 0,
+    };
+    messageCache.addMessage('pk_abc', msg, 'key-100');
+
+    messageCache.updateAck(100, 1);
+
+    const entry = messageCache.get('pk_abc');
+    expect(entry).toBeDefined();
+    expect(entry!.messages[0].acked).toBe(1);
+  });
+
+  it('updateAck updates paths when longer', () => {
+    const msg: Message = {
+      id: 101,
+      type: 'PRIV',
+      conversation_key: 'pk_abc',
+      text: 'Test',
+      sender_timestamp: 1700000001,
+      received_at: 1700000001,
+      paths: [{ path: 'aa', received_at: 1700000001 }],
+      txt_type: 0,
+      signature: null,
+      outgoing: true,
+      acked: 1,
+    };
+    messageCache.addMessage('pk_abc', msg, 'key-101');
+
+    const longerPaths = [
+      { path: 'aa', received_at: 1700000001 },
+      { path: 'bb', received_at: 1700000002 },
+    ];
+    messageCache.updateAck(101, 2, longerPaths);
+
+    const entry = messageCache.get('pk_abc');
+    expect(entry!.messages[0].paths).toHaveLength(2);
+    expect(entry!.messages[0].acked).toBe(2);
+  });
+
+  it('preserves higher existing ack count (max semantics)', () => {
+    const msg: Message = {
+      id: 102,
+      type: 'PRIV',
+      conversation_key: 'pk_abc',
+      text: 'Max test',
+      sender_timestamp: 1700000002,
+      received_at: 1700000002,
+      paths: null,
+      txt_type: 0,
+      signature: null,
+      outgoing: true,
+      acked: 5,
+    };
+    messageCache.addMessage('pk_abc', msg, 'key-102');
+
+    // Try to update with a lower ack count
+    messageCache.updateAck(102, 3);
+
+    const entry = messageCache.get('pk_abc');
+    expect(entry!.messages[0].acked).toBe(5); // max(5, 3) = 5
+  });
+
+  it('is a no-op for unknown message ID', () => {
+    const msg: Message = {
+      id: 103,
+      type: 'PRIV',
+      conversation_key: 'pk_abc',
+      text: 'Existing',
+      sender_timestamp: 1700000003,
+      received_at: 1700000003,
+      paths: null,
+      txt_type: 0,
+      signature: null,
+      outgoing: true,
+      acked: 0,
+    };
+    messageCache.addMessage('pk_abc', msg, 'key-103');
+
+    // Update a non-existent message ID — should not throw or modify anything
+    messageCache.updateAck(999, 1);
+
+    const entry = messageCache.get('pk_abc');
+    expect(entry!.messages[0].acked).toBe(0); // unchanged
+  });
+});
diff --git a/frontend/src/test/useWebSocket.dispatch.test.ts b/frontend/src/test/useWebSocket.dispatch.test.ts
new file mode 100644
index 0000000..88d2208
--- /dev/null
+++ b/frontend/src/test/useWebSocket.dispatch.test.ts
@@ -0,0 +1,198 @@
+/**
+ * Integration tests for useWebSocket onmessage dispatch.
+ *
+ * Verifies that the switch statement in useWebSocket.ts:91-134 routes
+ * incoming JSON messages to the correct handler callbacks with the
+ * correct data shapes.
+ */
+
+import { act, renderHook } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { useWebSocket } from '../useWebSocket';
+import fixtures from './fixtures/websocket_events.json';
+
+class MockWebSocket {
+  static CONNECTING = 0;
+  static OPEN = 1;
+  static CLOSING = 2;
+  static CLOSED = 3;
+  static instances: MockWebSocket[] = [];
+
+  url: string;
+  readyState = MockWebSocket.OPEN;
+  onopen: (() => void) | null = null;
+  onclose: (() => void) | null = null;
+  onerror: ((error: unknown) => void) | null = null;
+  onmessage: ((event: { data: string }) => void) | null = null;
+
+  constructor(url: string) {
+    this.url = url;
+    MockWebSocket.instances.push(this);
+  }
+
+  close(): void {
+    this.readyState = MockWebSocket.CLOSED;
+    this.onclose?.();
+  }
+
+  send(): void {}
+}
+
+const originalWebSocket = globalThis.WebSocket;
+
+/** Send a JSON message through the most recent MockWebSocket instance. */
+function fireMessage(data: unknown): void {
+  const ws = MockWebSocket.instances[MockWebSocket.instances.length - 1];
+  act(() => {
+    ws.onmessage?.({ data: JSON.stringify(data) });
+  });
+}
+
+describe('useWebSocket dispatch', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    MockWebSocket.instances = [];
+    globalThis.WebSocket = MockWebSocket as unknown as typeof WebSocket;
+  });
+
+  afterEach(() => {
+    globalThis.WebSocket = originalWebSocket;
+    vi.useRealTimers();
+  });
+
+  it('routes health message to onHealth', () => {
+    const onHealth = vi.fn();
+    renderHook(() => useWebSocket({ onHealth }));
+
+    const healthData = {
+      status: 'ok',
+      radio_connected: true,
+      connection_info: 'TCP: 1.2.3.4:4000',
+      database_size_mb: 1.5,
+      oldest_undecrypted_timestamp: null,
+    };
+    fireMessage({ type: 'health', data: healthData });
+
+    expect(onHealth).toHaveBeenCalledOnce();
+    expect(onHealth).toHaveBeenCalledWith(healthData);
+  });
+
+  it('routes message event to onMessage with correct Message shape', () => {
+    const onMessage = vi.fn();
+    renderHook(() => useWebSocket({ onMessage }));
+
+    const { type, data } = fixtures.channel_message.expected_ws_event;
+    fireMessage({ type, data });
+
+    expect(onMessage).toHaveBeenCalledOnce();
+    expect(onMessage).toHaveBeenCalledWith(data);
+  });
+
+  it('routes contact event to onContact with correct Contact shape', () => {
+    const onContact = vi.fn();
+    renderHook(() => useWebSocket({ onContact }));
+
+    const { type, data } = fixtures.advertisement_with_gps.expected_ws_event;
+    fireMessage({ type, data });
+
+    expect(onContact).toHaveBeenCalledOnce();
+    expect(onContact).toHaveBeenCalledWith(data);
+    expect(onContact.mock.calls[0][0]).toHaveProperty('public_key');
+    expect(onContact.mock.calls[0][0]).toHaveProperty('name');
+  });
+
+  it('routes message_acked to onMessageAcked with (messageId, ackCount, paths)', () => {
+    const onMessageAcked = vi.fn();
+    renderHook(() => useWebSocket({ onMessageAcked }));
+
+    const { type, data } = fixtures.message_acked.expected_ws_event;
+    fireMessage({ type, data });
+
+    expect(onMessageAcked).toHaveBeenCalledOnce();
+    expect(onMessageAcked).toHaveBeenCalledWith(42, 1, undefined);
+  });
+
+  it('routes message_acked with paths', () => {
+    const onMessageAcked = vi.fn();
+    renderHook(() => useWebSocket({ onMessageAcked }));
+
+    const paths = [{ path: 'aabb', received_at: 1700000000 }];
+    fireMessage({ type: 'message_acked', data: { message_id: 7, ack_count: 2, paths } });
+
+    expect(onMessageAcked).toHaveBeenCalledWith(7, 2, paths);
+  });
+
+  it('routes error event to onError', () => {
+    const onError = vi.fn();
+    renderHook(() => useWebSocket({ onError }));
+
+    const errorData = { message: 'Send failed', details: 'Radio busy' };
+    fireMessage({ type: 'error', data: errorData });
+
+    expect(onError).toHaveBeenCalledOnce();
+    expect(onError).toHaveBeenCalledWith(errorData);
+  });
+
+  it('routes success event to onSuccess', () => {
+    const onSuccess = vi.fn();
+    renderHook(() => useWebSocket({ onSuccess }));
+
+    const successData = { message: 'Message sent' };
+    fireMessage({ type: 'success', data: successData });
+
+    expect(onSuccess).toHaveBeenCalledOnce();
+    expect(onSuccess).toHaveBeenCalledWith(successData);
+  });
+
+  it('pong message calls no handlers', () => {
+    const handlers = {
+      onHealth: vi.fn(),
+      onMessage: vi.fn(),
+      onContact: vi.fn(),
+      onMessageAcked: vi.fn(),
+      onError: vi.fn(),
+      onSuccess: vi.fn(),
+    };
+    renderHook(() => useWebSocket(handlers));
+
+    fireMessage({ type: 'pong', data: null });
+
+    Object.values(handlers).forEach((fn) => expect(fn).not.toHaveBeenCalled());
+  });
+
+  it('unknown message type calls no handlers', () => {
+    const handlers = {
+      onHealth: vi.fn(),
+      onMessage: vi.fn(),
+      onContact: vi.fn(),
+      onMessageAcked: vi.fn(),
+      onError: vi.fn(),
+      onSuccess: vi.fn(),
+    };
+    renderHook(() => useWebSocket(handlers));
+
+    fireMessage({ type: 'something_unexpected', data: {} });
+
+    Object.values(handlers).forEach((fn) => expect(fn).not.toHaveBeenCalled());
+  });
+
+  it('malformed JSON calls no handlers (catch branch)', () => {
+    const handlers = {
+      onHealth: vi.fn(),
+      onMessage: vi.fn(),
+      onContact: vi.fn(),
+      onMessageAcked: vi.fn(),
+      onError: vi.fn(),
+      onSuccess: vi.fn(),
+    };
+    renderHook(() => useWebSocket(handlers));
+
+    const ws = MockWebSocket.instances[MockWebSocket.instances.length - 1];
+    act(() => {
+      ws.onmessage?.({ data: 'not valid json{{{' });
+    });
+
+    Object.values(handlers).forEach((fn) => expect(fn).not.toHaveBeenCalled());
+  });
+});
diff --git a/frontend/src/utils/contactMerge.ts b/frontend/src/utils/contactMerge.ts
new file mode 100644
index 0000000..dc9130d
--- /dev/null
+++ b/frontend/src/utils/contactMerge.ts
@@ -0,0 +1,25 @@
+import type { Contact } from '../types';
+
+/**
+ * Merge an incoming contact into an existing list.
+ *
+ * - If the contact exists (matched by public_key), merge fields and return
+ *   a new array only if something changed (preserves referential equality
+ *   for React when nothing changed).
+ * - If the contact is new, append it.
+ */
+export function mergeContactIntoList(contacts: Contact[], incoming: Contact): Contact[] {
+  const idx = contacts.findIndex((c) => c.public_key === incoming.public_key);
+  if (idx >= 0) {
+    const existing = contacts[idx];
+    const merged = { ...existing, ...incoming };
+    const unchanged = (Object.keys(merged) as (keyof Contact)[]).every(
+      (k) => existing[k] === merged[k]
+    );
+    if (unchanged) return contacts;
+    const updated = [...contacts];
+    updated[idx] = merged;
+    return updated;
+  }
+  return [...contacts, incoming];
+}