diff --git a/nanobot/agent/subagent.py b/nanobot/agent/subagent.py
index 1b88ede11..e71eb4834 100644
--- a/nanobot/agent/subagent.py
+++ b/nanobot/agent/subagent.py
@@ -99,7 +99,6 @@ class SubagentManager:
         self._running_tasks: dict[str, asyncio.Task[None]] = {}
         self._task_statuses: dict[str, SubagentStatus] = {}
         self._session_tasks: dict[str, set[str]] = {}  # session_key -> {task_id, ...}
-        self._tools_cache: ToolRegistry | None = None
 
     def _subagent_tools_config(self) -> ToolsConfig:
         """Build a ToolsConfig scoped for subagent use."""
@@ -110,9 +109,7 @@ class SubagentManager:
         )
 
     def _build_tools(self) -> ToolRegistry:
-        """Build the subagent tool registry via ToolLoader (cached)."""
-        if self._tools_cache is not None:
-            return self._tools_cache
+        """Build an isolated subagent tool registry via ToolLoader."""
         registry = ToolRegistry()
         ctx = ToolContext(
             config=self._subagent_tools_config(),
@@ -120,7 +117,6 @@ class SubagentManager:
             file_state_store=FileStates(),
         )
         ToolLoader().load(ctx, registry, scope="subagent")
-        self._tools_cache = registry
         return registry
 
     def set_provider(self, provider: LLMProvider, model: str) -> None:
diff --git a/nanobot/agent/tools/message.py b/nanobot/agent/tools/message.py
index fb36d330d..339f9bdcf 100644
--- a/nanobot/agent/tools/message.py
+++ b/nanobot/agent/tools/message.py
@@ -79,8 +79,7 @@ class MessageTool(Tool, ContextAware):
         self._default_channel.set(ctx.channel)
         self._default_chat_id.set(ctx.chat_id)
         self._default_message_id.set(ctx.message_id)
-        if ctx.metadata:
-            self._default_metadata.set(ctx.metadata)
+        self._default_metadata.set(dict(ctx.metadata or {}))
 
     def set_send_callback(self, callback: Callable[[OutboundMessage], Awaitable[None]]) -> None:
         """Set the callback for sending messages."""
diff --git a/tests/agent/test_subagent.py b/tests/agent/test_subagent.py
index 72a0f458d..ef6940a7c 100644
--- a/tests/agent/test_subagent.py
+++ b/tests/agent/test_subagent.py
@@ -28,3 +28,27 @@ async def test_subagent_uses_tool_loader():
     assert tools.has("glob")
     assert not tools.has("message")
     assert not tools.has("spawn")
+
+
+@pytest.mark.asyncio
+async def test_subagent_build_tools_isolates_file_read_state(tmp_path):
+    """Each spawned subagent needs a fresh file-state cache."""
+    (tmp_path / "note.txt").write_text("hello\n", encoding="utf-8")
+    provider = MagicMock(spec=LLMProvider)
+    provider.get_default_model.return_value = "test"
+    sm = SubagentManager(
+        provider=provider,
+        workspace=tmp_path,
+        bus=MessageBus(),
+        model="test",
+        max_tool_result_chars=16_000,
+    )
+
+    first_read = sm._build_tools().get("read_file")
+    second_read = sm._build_tools().get("read_file")
+
+    assert first_read is not second_read
+    assert (await first_read.execute(path="note.txt")).startswith("1| hello")
+    second_result = await second_read.execute(path="note.txt")
+    assert second_result.startswith("1| hello")
+    assert "File unchanged" not in second_result
diff --git a/tests/tools/test_message_tool.py b/tests/tools/test_message_tool.py
index d32b07778..d4439422a 100644
--- a/tests/tools/test_message_tool.py
+++ b/tests/tools/test_message_tool.py
@@ -91,6 +91,29 @@ async def test_message_tool_inherits_metadata_for_same_target() -> None:
     assert sent[0].metadata == slack_meta
 
 
+@pytest.mark.asyncio
+async def test_message_tool_clears_metadata_when_context_has_none() -> None:
+    sent: list[OutboundMessage] = []
+
+    async def _send(msg: OutboundMessage) -> None:
+        sent.append(msg)
+
+    tool = MessageTool(send_callback=_send)
+    from nanobot.agent.tools.context import RequestContext
+    tool.set_context(
+        RequestContext(
+            channel="slack",
+            chat_id="C123",
+            metadata={"slack": {"thread_ts": "111.222", "channel_type": "channel"}},
+        ),
+    )
+    tool.set_context(RequestContext(channel="slack", chat_id="C123", metadata={}))
+
+    await tool.execute(content="plain reply")
+
+    assert sent[0].metadata == {}
+
+
 @pytest.mark.asyncio
 async def test_message_tool_does_not_inherit_metadata_for_cross_target() -> None:
     sent: list[OutboundMessage] = []