diff --git a/tests/tools/test_mcp_tool.py b/tests/tools/test_mcp_tool.py
index 3212a350c37..b9a3cfcf8d9 100644
--- a/tests/tools/test_mcp_tool.py
+++ b/tests/tools/test_mcp_tool.py
@@ -1462,6 +1462,27 @@ class TestHTTPConfig:
 
         asyncio.run(_test())
 
+    def test_stdio_unavailable_raises_importerror_not_nameerror(self):
+        """Regression test for #30904.
+
+        When the mcp SDK isn't installed, ``_run_stdio`` previously leaked a
+        bare ``NameError: name 'StdioServerParameters' is not defined``. The
+        gate now raises a clear ``ImportError`` with install instructions,
+        mirroring ``_run_http``'s behaviour when the HTTP transport is
+        unavailable.
+        """
+        from tools.mcp_tool import MCPServerTask
+
+        server = MCPServerTask("local")
+        config = {"command": "python3", "args": ["/tmp/echo.py"]}
+
+        async def _test():
+            with patch("tools.mcp_tool._MCP_AVAILABLE", False):
+                with pytest.raises(ImportError, match=r"mcp.*SDK"):
+                    await server._run_stdio(config)
+
+        asyncio.run(_test())
+
     def test_http_seeds_initial_protocol_header(self):
         from tools.mcp_tool import LATEST_PROTOCOL_VERSION, MCPServerTask
 
diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index e50efc05a0c..75c1c5e8633 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -1255,6 +1255,15 @@ class MCPServerTask:
 
     async def _run_stdio(self, config: dict):
         """Run the server using stdio transport."""
+        if not _MCP_AVAILABLE:
+            raise ImportError(
+                f"MCP server '{self.name}' requires the 'mcp' Python SDK, but "
+                "it is not installed. Install with:\n"
+                "  pip install 'hermes-agent[mcp]'\n"
+                "or (full install):\n"
+                "  pip install 'hermes-agent[all]'"
+            )
+
         command = config.get("command")
         args = config.get("args", [])
         user_env = config.get("env")