Add missing tests and address AGENTS.md gaps

app/AGENTS.md

@@ -203,13 +203,37 @@ Run backend tests:
 PYTHONPATH=. uv run pytest tests/ -v
 ```
 
-High-signal suites:
-- `tests/test_packet_pipeline.py`
-- `tests/test_event_handlers.py`
-- `tests/test_send_messages.py`
-- `tests/test_radio.py`
-- `tests/test_api.py`
-- `tests/test_migrations.py`
+Test suites:
+
+```text
+tests/
+├── conftest.py                 # Shared fixtures
+├── test_api.py                 # REST endpoint integration tests
+├── test_bot.py                 # Bot execution and sandboxing
+├── test_config.py              # Configuration validation
+├── test_contacts_router.py     # Contacts router endpoints
+├── test_decoder.py             # Packet parsing/decryption
+├── test_echo_dedup.py          # Echo/repeat deduplication (incl. concurrent)
+├── test_event_handlers.py      # ACK tracking, event registration, cleanup
+├── test_frontend_static.py     # Frontend static file serving
+├── test_key_normalization.py   # Public key normalization
+├── test_keystore.py            # Ephemeral keystore
+├── test_message_pagination.py  # Cursor-based message pagination
+├── test_message_prefix_claim.py # Message prefix claim logic
+├── test_migrations.py          # Schema migration system
+├── test_packet_pipeline.py     # End-to-end packet processing
+├── test_radio.py               # RadioManager, serial detection
+├── test_radio_operation.py     # radio_operation() context manager
+├── test_radio_router.py        # Radio router endpoints
+├── test_radio_sync.py          # Polling, sync, advertisement
+├── test_repeater_routes.py     # Repeater command/telemetry/trace
+├── test_repository.py          # Data access layer
+├── test_send_messages.py       # Outgoing messages, bot triggers, concurrent sends
+├── test_settings_router.py     # Settings endpoints, advert validation
+├── test_statistics.py          # Statistics aggregation
+├── test_websocket.py           # WS manager broadcast/cleanup
+└── test_websocket_route.py     # WS endpoint lifecycle
+```
 
 ## Editing Checklist
 

frontend/AGENTS.md

@@ -18,33 +18,47 @@ Keep it aligned with `frontend/src` source code.
 
 ```text
 frontend/src/
+├── main.tsx                # React entry point (StrictMode, root render)
 ├── App.tsx                 # App shell and orchestration
 ├── api.ts                  # Typed REST client
 ├── types.ts                # Shared TS contracts
 ├── useWebSocket.ts         # WS lifecycle + event dispatch
 ├── messageCache.ts         # Conversation-scoped cache
+├── prefetch.ts             # Consumes prefetched API promises started in index.html
 ├── index.css               # Global styles/utilities
 ├── styles.css              # Additional global app styles
+├── lib/
+│   └── utils.ts            # cn() — clsx + tailwind-merge helper
 ├── hooks/
-│   ├── useConversationMessages.ts
-│   ├── useUnreadCounts.ts
-│   ├── useRepeaterMode.ts
-│   └── useAirtimeTracking.ts
+│   ├── index.ts            # Central re-export of all hooks
+│   ├── useConversationMessages.ts  # Fetch, pagination, dedup, ACK buffering
+│   ├── useUnreadCounts.ts          # Unread counters, mentions, recent-sort timestamps
+│   ├── useRepeaterMode.ts          # Repeater login/command workflow
+│   ├── useAirtimeTracking.ts       # Repeater airtime stats polling
+│   ├── useRadioControl.ts          # Radio health/config state, reconnection
+│   ├── useAppSettings.ts           # Settings, favorites, preferences migration
+│   ├── useConversationRouter.ts    # URL hash → active conversation routing
+│   └── useContactsAndChannels.ts   # Contact/channel loading, creation, deletion
 ├── utils/
-│   ├── urlHash.ts
-│   ├── conversationState.ts
-│   ├── favorites.ts
-│   ├── messageParser.ts
-│   ├── pathUtils.ts
-│   ├── pubkey.ts
-│   └── contactAvatar.ts
+│   ├── urlHash.ts              # Hash parsing and encoding
+│   ├── conversationState.ts    # State keys, in-memory + localStorage helpers
+│   ├── favorites.ts            # LocalStorage migration for favorites
+│   ├── messageParser.ts        # Message text → rendered segments
+│   ├── pathUtils.ts            # Distance/validation helpers for paths + map
+│   ├── pubkey.ts               # getContactDisplayName (12-char prefix fallback)
+│   ├── contactAvatar.ts        # Avatar color derivation from public key
+│   ├── rawPacketIdentity.ts    # observation_id vs id dedup helpers
+│   ├── visualizerUtils.ts      # 3D visualizer node types, colors, particles
+│   └── lastViewedConversation.ts   # localStorage for last-viewed conversation
 ├── components/
 │   ├── StatusBar.tsx
 │   ├── Sidebar.tsx
+│   ├── ChatHeader.tsx          # Conversation header (trace, favorite, delete)
 │   ├── MessageList.tsx
 │   ├── MessageInput.tsx
 │   ├── NewMessageModal.tsx
 │   ├── SettingsModal.tsx
+│   ├── settingsConstants.ts    # Settings section ordering and labels
 │   ├── RawPacketList.tsx
 │   ├── MapView.tsx
 │   ├── VisualizerView.tsx
@@ -53,8 +67,12 @@ frontend/src/
 │   ├── CrackerPanel.tsx
 │   ├── BotCodeEditor.tsx
 │   ├── ContactAvatar.tsx
-│   └── ui/
+│   └── ui/                     # shadcn/ui primitives
+├── types/
+│   └── d3-force-3d.d.ts       # Type declarations for d3-force-3d
 └── test/
+    ├── setup.ts
+    ├── fixtures/websocket_events.json
     ├── api.test.ts
     ├── appFavorites.test.tsx
     ├── appStartupHash.test.tsx
@@ -64,26 +82,32 @@ frontend/src/
     ├── messageParser.test.ts
     ├── pathUtils.test.ts
     ├── radioPresets.test.ts
+    ├── rawPacketIdentity.test.ts
     ├── repeaterMode.test.ts
     ├── settingsModal.test.tsx
+    ├── sidebar.test.tsx
     ├── unreadCounts.test.ts
     ├── urlHash.test.ts
     ├── useConversationMessages.test.ts
+    ├── useConversationMessages.race.test.ts
     ├── useRepeaterMode.test.ts
     ├── useWebSocket.lifecycle.test.ts
-    ├── websocket.test.ts
-    └── setup.ts
+    └── websocket.test.ts
 ```
 
 ## Architecture Notes
 
 ### State ownership
 
-`App.tsx` orchestrates high-level state (health, config, contacts/channels, active conversation, UI flags).
-Specialized logic is delegated to hooks:
+`App.tsx` orchestrates high-level state and delegates to hooks:
+- `useRadioControl`: radio health/config state, reconnect/reboot polling
+- `useAppSettings`: settings CRUD, favorites, preferences migration
+- `useContactsAndChannels`: contact/channel lists, creation, deletion
+- `useConversationRouter`: URL hash → active conversation routing
 - `useConversationMessages`: fetch, pagination, dedup/update helpers
 - `useUnreadCounts`: unread counters, mention tracking, recent-sort timestamps
 - `useRepeaterMode`: repeater login/command workflow
+- `useAirtimeTracking`: repeater airtime stats polling
 
 ### Initial load + realtime
 

frontend/src/hooks/useConversationMessages.ts

@@ -319,6 +319,10 @@ export function useConversationMessages(
     fetchingConversationIdRef.current = newId;
     prevConversationIdRef.current = newId;
 
+    // Reset loadingOlder — the previous conversation's in-flight older-message
+    // fetch is irrelevant now (its stale-check will discard the response).
+    setLoadingOlder(false);
+
     // Clear state for new conversation
     if (!activeConversation || activeConversation.type === 'raw') {
       setMessages([]);

frontend/src/test/useConversationMessages.race.test.ts

@@ -1,5 +1,5 @@
 import { act, renderHook, waitFor } from '@testing-library/react';
-import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { beforeEach, describe, expect, it, vi, type Mock } from 'vitest';
 
 import * as messageCache from '../messageCache';
 import { useConversationMessages } from '../hooks/useConversationMessages';
@@ -124,3 +124,98 @@ describe('useConversationMessages ACK ordering', () => {
     expect(result.current.messages[0].paths).toEqual(highAckPaths);
   });
 });
+
+describe('useConversationMessages conversation switch', () => {
+  beforeEach(() => {
+    mockGetMessages.mockReset();
+    messageCache.clear();
+  });
+
+  it('resets loadingOlder when switching conversations mid-fetch', async () => {
+    const convA: Conversation = { type: 'contact', id: 'conv_a', name: 'Contact A' };
+    const convB: Conversation = { type: 'contact', id: 'conv_b', name: 'Contact B' };
+
+    // Conv A initial fetch: return 200 messages so hasOlderMessages = true
+    const fullPage = Array.from({ length: 200 }, (_, i) =>
+      createMessage({
+        id: i + 1,
+        conversation_key: 'conv_a',
+        text: `msg-${i}`,
+        sender_timestamp: 1700000000 + i,
+        received_at: 1700000000 + i,
+      })
+    );
+    mockGetMessages.mockResolvedValueOnce(fullPage);
+
+    const { result, rerender } = renderHook(
+      ({ conv }: { conv: Conversation }) => useConversationMessages(conv),
+      { initialProps: { conv: convA } }
+    );
+
+    await waitFor(() => expect(result.current.messagesLoading).toBe(false));
+    expect(result.current.hasOlderMessages).toBe(true);
+    expect(result.current.messages).toHaveLength(200);
+
+    // Start fetching older messages — use a deferred promise so it stays in-flight
+    const olderDeferred = createDeferred<Message[]>();
+    mockGetMessages.mockReturnValueOnce(olderDeferred.promise);
+
+    act(() => {
+      result.current.fetchOlderMessages();
+    });
+
+    expect(result.current.loadingOlder).toBe(true);
+
+    // Switch to conv B while older-messages fetch is still pending
+    mockGetMessages.mockResolvedValueOnce([createMessage({ id: 999, conversation_key: 'conv_b' })]);
+    rerender({ conv: convB });
+
+    // loadingOlder must reset immediately — no phantom spinner in conv B
+    await waitFor(() => expect(result.current.loadingOlder).toBe(false));
+    await waitFor(() => expect(result.current.messagesLoading).toBe(false));
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].conversation_key).toBe('conv_b');
+
+    // Resolve the stale older-messages fetch — should not affect conv B's state
+    olderDeferred.resolve([
+      createMessage({ id: 500, conversation_key: 'conv_a', text: 'stale-old' }),
+    ]);
+
+    // Give the stale response time to be processed (it should be discarded)
+    await new Promise((r) => setTimeout(r, 50));
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].conversation_key).toBe('conv_b');
+  });
+
+  it('aborts in-flight fetch when switching conversations', async () => {
+    const convA: Conversation = { type: 'contact', id: 'conv_a', name: 'Contact A' };
+    const convB: Conversation = { type: 'contact', id: 'conv_b', name: 'Contact B' };
+
+    // Conv A: never resolves (simulates slow network)
+    mockGetMessages.mockReturnValueOnce(new Promise(() => {}));
+
+    const { result, rerender } = renderHook(
+      ({ conv }: { conv: Conversation }) => useConversationMessages(conv),
+      { initialProps: { conv: convA } }
+    );
+
+    // Should be loading
+    expect(result.current.messagesLoading).toBe(true);
+
+    // Verify the API was called with an AbortSignal
+    const firstCallSignal = (mockGetMessages as Mock).mock.calls[0]?.[1];
+    expect(firstCallSignal).toBeInstanceOf(AbortSignal);
+
+    // Switch to conv B
+    mockGetMessages.mockResolvedValueOnce([createMessage({ id: 1, conversation_key: 'conv_b' })]);
+    rerender({ conv: convB });
+
+    // The signal from conv A's fetch should have been aborted
+    expect(firstCallSignal.aborted).toBe(true);
+
+    // Conv B should load normally
+    await waitFor(() => expect(result.current.messagesLoading).toBe(false));
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].conversation_key).toBe('conv_b');
+  });
+});

tests/e2e/specs/messaging.spec.ts

@@ -64,8 +64,22 @@ test.describe('Channel messaging in #flightless', () => {
     const messageContainer = messageEl.locator(
       'xpath=ancestor::div[contains(@class,"break-words")][1]'
     );
-    const resendButton = messageContainer.getByTitle('Resend message');
-    await expect(resendButton).toBeVisible({ timeout: 15_000 });
+    // Resend actions now live in the outgoing message status/path modal.
+    // Open it from either pending status (?) or echo-path indicator (✓...).
+    const statusOrPathTrigger = messageContainer.locator(
+      '[title="Message status"], [title="View echo paths"]'
+    );
+    await expect(statusOrPathTrigger.first()).toBeVisible({ timeout: 15_000 });
+    await statusOrPathTrigger.first().click();
+
+    const modal = page.getByRole('dialog');
+    await expect(modal).toBeVisible({ timeout: 10_000 });
+
+    // Byte-perfect resend option (within 30s) includes this helper text.
+    const resendButton = modal.getByRole('button', {
+      name: /Only repeated by new repeaters/i,
+    });
+    await expect(resendButton).toBeVisible({ timeout: 10_000 });
 
     const resendResponsePromise = page.waitForResponse(
       (response) =>

tests/e2e/specs/radio-settings.spec.ts

@@ -2,53 +2,54 @@ import { test, expect } from '@playwright/test';
 import { getRadioConfig, updateRadioConfig } from '../helpers/api';
 
 test.describe('Radio settings', () => {
-  let originalName: string;
-
-  test.beforeAll(async () => {
-    const config = await getRadioConfig();
-    originalName = config.name;
-  });
-
-  test.afterAll(async () => {
-    // Restore original name via API
-    try {
-      await updateRadioConfig({ name: originalName });
-    } catch {
-      console.warn('Failed to restore radio name — manual intervention may be needed');
-    }
-  });
-
   test('change radio name via settings UI and verify persistence', async ({ page }) => {
-    // Radio names are limited to 8 characters
-    const testName = 'E2Etest1';
+    const originalConfig = await getRadioConfig();
+    const originalName = originalConfig.name;
 
-    await page.goto('/');
-    await expect(page.getByText('Connected')).toBeVisible();
+    // Radio names are limited to 8 characters.
+    // Use a randomized name per run to avoid collisions with stale state.
+    const randomSuffix = Math.floor(Math.random() * 10000)
+      .toString()
+      .padStart(4, '0');
+    const testName = `E2E${randomSuffix}`; // 7 chars
 
-    // --- Step 1: Change the name via settings UI ---
-    await page.getByText('Settings').click();
-    await page.getByRole('button', { name: /Identity/i }).click();
+    try {
+      await page.goto('/');
+      await expect(page.getByText('Connected')).toBeVisible();
 
-    const nameInput = page.locator('#name');
-    await nameInput.clear();
-    await nameInput.fill(testName);
+      // --- Step 1: Change the name via settings UI ---
+      await page.getByText('Settings').click();
+      await page.getByRole('button', { name: /Identity/i }).click();
 
-    await page.getByRole('button', { name: 'Save Identity Settings' }).click();
-    await expect(page.getByText('Identity settings saved')).toBeVisible({ timeout: 10_000 });
+      const nameInput = page.locator('#name');
+      await nameInput.clear();
+      await nameInput.fill(testName);
 
-    // Exit settings page mode
-    await page.getByRole('button', { name: /Back to Chat/i }).click();
+      await page.getByRole('button', { name: 'Save Identity Settings' }).click();
+      await expect(page.getByText('Identity settings saved')).toBeVisible({ timeout: 10_000 });
 
-    // --- Step 2: Verify via API (now returns fresh data after send_appstart fix) ---
-    const config = await getRadioConfig();
-    expect(config.name).toBe(testName);
+      // Exit settings page mode
+      await page.getByRole('button', { name: /Back to Chat/i }).click();
 
-    // --- Step 3: Verify persistence across page reload ---
-    await page.reload();
-    await expect(page.getByText('Connected')).toBeVisible({ timeout: 15_000 });
+      // --- Step 2: Verify via API (now returns fresh data after send_appstart fix) ---
+      const config = await getRadioConfig();
+      expect(config.name).toBe(testName);
+
+      // --- Step 3: Verify persistence across page reload ---
+      await page.reload();
+      await expect(page.getByText('Connected')).toBeVisible({ timeout: 15_000 });
+
+      await page.getByText('Settings').click();
+      await page.getByRole('button', { name: /Identity/i }).click();
+      await expect(page.locator('#name')).toHaveValue(testName, { timeout: 10_000 });
+    } finally {
+      // Always restore original name, even when assertions fail.
+      try {
+        await updateRadioConfig({ name: originalName });
+      } catch {
+        console.warn('Failed to restore radio name — manual intervention may be needed');
+      }
+    }
 
-    await page.getByText('Settings').click();
-    await page.getByRole('button', { name: /Identity/i }).click();
-    await expect(page.locator('#name')).toHaveValue(testName, { timeout: 10_000 });
   });
 });

tests/test_echo_dedup.py

@@ -6,6 +6,7 @@ messages, accumulate multi-path routing info, and ensure the dual DM processing
 paths (packet_processor + event_handler fallback) don't double-store messages.
 """
 
+import asyncio
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -689,3 +690,108 @@ class TestDirectMessageDirectionDetection:
 
         # Not our message - should return None without attempting decryption
         assert result is None
+
+
+class TestConcurrentDMDedup:
+    """Test that concurrent DM processing deduplicates via atomic INSERT OR IGNORE.
+
+    On a mesh network, the same DM packet can arrive via two RF paths nearly
+    simultaneously, causing two concurrent calls to create_dm_message_from_decrypted.
+    SQLite's INSERT OR IGNORE ensures only one message is stored.
+    """
+
+    @pytest.mark.asyncio
+    async def test_concurrent_identical_dms_only_store_once(self, test_db, captured_broadcasts):
+        """Two concurrent create_dm_message_from_decrypted calls with identical content
+        should result in exactly one stored message."""
+        from app.packet_processor import create_dm_message_from_decrypted
+
+        pkt1, _ = await RawPacketRepository.create(b"concurrent_dm_1", SENDER_TIMESTAMP)
+        pkt2, _ = await RawPacketRepository.create(b"concurrent_dm_2", SENDER_TIMESTAMP + 1)
+
+        decrypted = DecryptedDirectMessage(
+            timestamp=SENDER_TIMESTAMP,
+            flags=0,
+            message="Concurrent dedup test",
+            dest_hash="fa",
+            src_hash="a1",
+        )
+
+        broadcasts, mock_broadcast = captured_broadcasts
+
+        with patch("app.packet_processor.broadcast_event", mock_broadcast):
+            results = await asyncio.gather(
+                create_dm_message_from_decrypted(
+                    packet_id=pkt1,
+                    decrypted=decrypted,
+                    their_public_key=CONTACT_PUB,
+                    our_public_key=OUR_PUB,
+                    received_at=SENDER_TIMESTAMP,
+                    path="aa",
+                    outgoing=False,
+                ),
+                create_dm_message_from_decrypted(
+                    packet_id=pkt2,
+                    decrypted=decrypted,
+                    their_public_key=CONTACT_PUB,
+                    our_public_key=OUR_PUB,
+                    received_at=SENDER_TIMESTAMP + 1,
+                    path="bbcc",
+                    outgoing=False,
+                ),
+            )
+
+        # Exactly one should create, the other should return None (duplicate)
+        created = [r for r in results if r is not None]
+        duplicates = [r for r in results if r is None]
+        assert len(created) == 1
+        assert len(duplicates) == 1
+
+        # Only one message in DB
+        messages = await MessageRepository.get_all(
+            msg_type="PRIV", conversation_key=CONTACT_PUB.lower(), limit=10
+        )
+        assert len(messages) == 1
+
+    @pytest.mark.asyncio
+    async def test_concurrent_channel_echoes_only_store_once(self, test_db, captured_broadcasts):
+        """Two concurrent create_message_from_decrypted calls with identical content
+        should result in exactly one stored message."""
+        from app.packet_processor import create_message_from_decrypted
+
+        pkt1, _ = await RawPacketRepository.create(b"concurrent_chan_1", SENDER_TIMESTAMP)
+        pkt2, _ = await RawPacketRepository.create(b"concurrent_chan_2", SENDER_TIMESTAMP + 1)
+
+        broadcasts, mock_broadcast = captured_broadcasts
+
+        with patch("app.packet_processor.broadcast_event", mock_broadcast):
+            results = await asyncio.gather(
+                create_message_from_decrypted(
+                    packet_id=pkt1,
+                    channel_key=CHANNEL_KEY,
+                    sender="Alice",
+                    message_text="Concurrent channel test",
+                    timestamp=SENDER_TIMESTAMP,
+                    received_at=SENDER_TIMESTAMP,
+                    path="aa",
+                ),
+                create_message_from_decrypted(
+                    packet_id=pkt2,
+                    channel_key=CHANNEL_KEY,
+                    sender="Alice",
+                    message_text="Concurrent channel test",
+                    timestamp=SENDER_TIMESTAMP,
+                    received_at=SENDER_TIMESTAMP + 1,
+                    path="bbcc",
+                ),
+            )
+
+        created = [r for r in results if r is not None]
+        duplicates = [r for r in results if r is None]
+        assert len(created) == 1
+        assert len(duplicates) == 1
+
+        messages = await MessageRepository.get_all(
+            msg_type="CHAN", conversation_key=CHANNEL_KEY, limit=10
+        )
+        assert len(messages) == 1

tests/test_event_handlers.py

@@ -96,6 +96,36 @@ class TestAckTracking:
 
         assert "recent" in _pending_acks
 
+    def test_cleanup_handles_many_expired_acks_without_growth(self):
+        """Many tracked ACKs that all expire should all be cleaned up,
+        preventing unbounded memory growth when no ACKs ever arrive."""
+        now = time.time()
+        for i in range(100):
+            _pending_acks[f"ack_{i}"] = (i, now - 300, 5000)  # All expired (300s ago, 5s timeout)
+
+        assert len(_pending_acks) == 100
+
+        cleanup_expired_acks()
+
+        assert len(_pending_acks) == 0
+
+    def test_cleanup_preserves_valid_acks_among_expired(self):
+        """Cleanup removes only expired ACKs, preserving valid ones."""
+        now = time.time()
+        # 50 expired
+        for i in range(50):
+            _pending_acks[f"expired_{i}"] = (i, now - 300, 5000)
+        # 50 valid
+        for i in range(50):
+            _pending_acks[f"valid_{i}"] = (100 + i, now, 60000)
+
+        assert len(_pending_acks) == 100
+
+        cleanup_expired_acks()
+
+        assert len(_pending_acks) == 50
+        assert all(k.startswith("valid_") for k in _pending_acks)
+
 
 class TestAckEventHandler:
     """Test the on_ack event handler."""

tests/test_send_messages.py

@@ -588,3 +588,102 @@ class TestResendChannelMessage:
 
         assert exc_info.value.status_code == 400
         assert "expired" in exc_info.value.detail.lower()
+
+
+class TestConcurrentChannelSends:
+    """Test that concurrent channel sends are serialized by the radio operation lock.
+
+    The send_channel_message endpoint uses set_channel (slot 0) then send_chan_msg.
+    Concurrent sends must be serialized so two messages don't clobber the same
+    temporary radio slot.
+    """
+
+    @pytest.mark.asyncio
+    async def test_concurrent_sends_to_different_channels_both_succeed(self, test_db):
+        """Two concurrent send_channel_message calls to different channels
+        should both succeed — the radio_operation lock serializes them."""
+        mc = _make_mc(name="TestNode")
+        chan_key_a = "aa" * 16
+        chan_key_b = "bb" * 16
+        await ChannelRepository.upsert(key=chan_key_a, name="#alpha")
+        await ChannelRepository.upsert(key=chan_key_b, name="#bravo")
+
+        with (
+            patch("app.routers.messages.require_connected", return_value=mc),
+            patch.object(radio_manager, "_meshcore", mc),
+            patch("app.routers.messages.broadcast_event"),
+        ):
+            results = await asyncio.gather(
+                send_channel_message(
+                    SendChannelMessageRequest(channel_key=chan_key_a, text="Hello alpha")
+                ),
+                send_channel_message(
+                    SendChannelMessageRequest(channel_key=chan_key_b, text="Hello bravo")
+                ),
+            )
+
+        # Both should have returned Message objects with distinct IDs
+        assert results[0].id != results[1].id
+        assert results[0].conversation_key == chan_key_a.upper()
+        assert results[1].conversation_key == chan_key_b.upper()
+
+        # set_channel should have been called twice (once per send, serialized)
+        assert mc.commands.set_channel.await_count == 2
+
+        # send_chan_msg should have been called twice
+        assert mc.commands.send_chan_msg.await_count == 2
+
+        # Both messages should be in DB
+        msgs_a = await MessageRepository.get_all(
+            msg_type="CHAN", conversation_key=chan_key_a.upper(), limit=10
+        )
+        msgs_b = await MessageRepository.get_all(
+            msg_type="CHAN", conversation_key=chan_key_b.upper(), limit=10
+        )
+        assert len(msgs_a) == 1
+        assert len(msgs_b) == 1
+
+    @pytest.mark.asyncio
+    async def test_concurrent_sends_to_same_channel_both_succeed(self, test_db):
+        """Two concurrent sends to the same channel should both succeed
+        with distinct timestamps (serialized, no slot clobber)."""
+        mc = _make_mc(name="TestNode")
+        chan_key = "cc" * 16
+        await ChannelRepository.upsert(key=chan_key, name="#charlie")
+
+        call_count = 0
+
+        # Mock time to return incrementing seconds so the two messages
+        # get distinct sender_timestamps (avoiding same-second collision).
+        original_time = time.time
+
+        def advancing_time():
+            nonlocal call_count
+            call_count += 1
+            return original_time() + call_count
+
+        with (
+            patch("app.routers.messages.require_connected", return_value=mc),
+            patch.object(radio_manager, "_meshcore", mc),
+            patch("app.routers.messages.broadcast_event"),
+            patch("app.routers.messages.time") as mock_time,
+        ):
+            mock_time.time = advancing_time
+            results = await asyncio.gather(
+                send_channel_message(
+                    SendChannelMessageRequest(channel_key=chan_key, text="Message one")
+                ),
+                send_channel_message(
+                    SendChannelMessageRequest(channel_key=chan_key, text="Message two")
+                ),
+            )
+
+        assert results[0].id != results[1].id
+        texts = {results[0].text, results[1].text}
+        assert "TestNode: Message one" in texts
+        assert "TestNode: Message two" in texts
+
+        msgs = await MessageRepository.get_all(
+            msg_type="CHAN", conversation_key=chan_key.upper(), limit=10
+        )
+        assert len(msgs) == 2