diff --git a/nanobot/agent/tools/session_messages.py b/nanobot/agent/tools/session_messages.py
index 4be28f663..bb84b9333 100644
--- a/nanobot/agent/tools/session_messages.py
+++ b/nanobot/agent/tools/session_messages.py
@@ -13,6 +13,8 @@ from dataclasses import dataclass
 from typing import Any, Protocol
 from uuid import uuid4
 
+from loguru import logger
+
 from nanobot.agent.tools.base import Tool, ToolResult, tool_parameters
 from nanobot.agent.tools.context import RequestContext, ToolContext, current_request_context
 from nanobot.agent.tools.schema import (
@@ -325,11 +327,20 @@ class SendSessionMessageTool(Tool):
         def expire() -> None:
             task = asyncio.create_task(self._expire_pending_reply(key, pending))
             self._expiry_tasks.add(task)
-            task.add_done_callback(self._expiry_tasks.discard)
+            task.add_done_callback(self._on_expiry_task_done)
 
         schedule = self._schedule_later or asyncio.get_running_loop().call_later
         pending.timer = schedule(float(timeout_seconds), expire)
 
+    def _on_expiry_task_done(self, task: asyncio.Task[None]) -> None:
+        self._expiry_tasks.discard(task)
+        if task.cancelled():
+            return
+        try:
+            task.result()
+        except Exception:
+            logger.exception("Session reply timeout delivery failed")
+
     async def _expire_pending_reply(
         self,
         key: tuple[str, str],
diff --git a/tests/tools/test_session_messages_tool.py b/tests/tools/test_session_messages_tool.py
index 0397b8fc6..2e223c478 100644
--- a/tests/tools/test_session_messages_tool.py
+++ b/tests/tools/test_session_messages_tool.py
@@ -2,6 +2,7 @@ import asyncio
 import json
 from pathlib import Path
 from typing import Callable
+from unittest.mock import patch
 
 import pytest
 
@@ -277,6 +278,45 @@ async def test_reply_timeout_injects_a_user_input_back_into_the_source(
     assert timeout.content == f"No reply from @{target.name} after 5 seconds."
 
 
+@pytest.mark.asyncio
+async def test_reply_timeout_observes_background_delivery_failure(
+    tmp_path: Path,
+) -> None:
+    sessions = SessionManager(tmp_path)
+    _persist(sessions, "websocket:source", "websocket:target")
+    bus = MessageBus()
+    scheduler = _Scheduler()
+    tool = SendSessionMessageTool(
+        sessions=sessions,
+        bus=bus,
+        schedule_later=scheduler,
+    )
+    target = _handle(sessions, "websocket:target")
+
+    await tool.enqueue(
+        source_session_key="websocket:source",
+        target_handle=target.name,
+        content="Question",
+        expect_reply=True,
+        reply_timeout_seconds=5,
+    )
+    await bus.consume_inbound()
+
+    async def fail_publish(_message) -> None:
+        raise RuntimeError("queue unavailable")
+
+    bus.publish_inbound = fail_publish
+    with patch("nanobot.agent.tools.session_messages.logger") as logger:
+        scheduler.calls[0][1].fire()
+        await asyncio.sleep(0)
+        await asyncio.sleep(0)
+
+    assert tool._expiry_tasks == set()
+    logger.exception.assert_called_once_with(
+        "Session reply timeout delivery failed",
+    )
+
+
 @pytest.mark.asyncio
 async def test_reverse_message_cancels_the_pending_reply_timeout(
     tmp_path: Path,