diff --git a/tests/test_telegram.py b/tests/test_telegram.py
index 17ec801..264625b 100644
--- a/tests/test_telegram.py
+++ b/tests/test_telegram.py
@@ -261,6 +261,71 @@ async def test_aggregator_buffer_cleared_after_flush():
         _cleanup(path)
 
 
+# ---------------------------------------------------------------------------
+# BATON-007: 400 "chat not found" handling
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_send_message_400_chat_not_found_does_not_raise():
+    """400 'chat not found' must not raise an exception (service stays alive)."""
+    with respx.mock(assert_all_called=False) as mock:
+        mock.post(SEND_URL).mock(
+            return_value=httpx.Response(
+                400,
+                json={"ok": False, "error_code": 400, "description": "Bad Request: chat not found"},
+            )
+        )
+        # Must not raise — service must stay alive even with wrong CHAT_ID
+        await send_message("test")
+
+
+@pytest.mark.asyncio
+async def test_send_message_400_chat_not_found_logs_error(caplog):
+    """400 response from Telegram must be logged as ERROR with the status code."""
+    import logging
+
+    with respx.mock(assert_all_called=False) as mock:
+        mock.post(SEND_URL).mock(
+            return_value=httpx.Response(
+                400,
+                json={"ok": False, "error_code": 400, "description": "Bad Request: chat not found"},
+            )
+        )
+        with caplog.at_level(logging.ERROR, logger="backend.telegram"):
+            await send_message("test chat not found")
+
+    assert any("400" in record.message for record in caplog.records), (
+        "Expected ERROR log containing '400' but got: " + str([r.message for r in caplog.records])
+    )
+
+
+@pytest.mark.asyncio
+async def test_send_message_400_breaks_after_first_attempt():
+    """On 400, send_message breaks immediately (no retry loop) — only one HTTP call made."""
+    with respx.mock(assert_all_called=False) as mock:
+        route = mock.post(SEND_URL).mock(
+            return_value=httpx.Response(
+                400,
+                json={"ok": False, "error_code": 400, "description": "Bad Request: chat not found"},
+            )
+        )
+        await send_message("test no retry on 400")
+
+    assert route.call_count == 1, f"Expected 1 call on 400, got {route.call_count}"
+
+
+@pytest.mark.asyncio
+async def test_send_message_all_5xx_retries_exhausted_does_not_raise():
+    """When all 3 attempts fail with 5xx, send_message logs error but does NOT raise."""
+    with respx.mock(assert_all_called=False) as mock:
+        mock.post(SEND_URL).mock(
+            return_value=httpx.Response(500, text="Internal Server Error")
+        )
+        with patch("backend.telegram.asyncio.sleep", new_callable=AsyncMock):
+            # Must not raise — message is dropped, service stays alive
+            await send_message("test all retries exhausted")
+
+
 @pytest.mark.asyncio
 async def test_aggregator_unknown_user_shows_uuid_prefix():
     """If user_name is None, the message shows first 8 chars of uuid."""