Update stateless stdio support with example test script and test fixes

Author: efj-amzn

src/mcp/server/fastmcp/server.py

@@ -600,7 +600,7 @@ async def run_stdio_async(self) -> None:
                 read_stream,
                 write_stream,
                 self._mcp_server.create_initialization_options(),
-                stateless=self.settings.stateless_stdio
+                stateless=self.settings.stateless_stdio,
             )
 
     async def run_sse_async(self, mount_path: str | None = None) -> None:

test_stateless.py

@@ -0,0 +1,16 @@
+from mcp.server.fastmcp import FastMCP
+
+# Create FastMCP server
+mcp = FastMCP("StatelessTest")
+
+
+# Register a simple echo tool
+@mcp.tool()
+def echo(message: str) -> str:
+    """Echo a message back to the client."""
+    return f"Echo: {message}"
+
+
+if __name__ == "__main__":
+    # Run in STDIO mode
+    mcp.run()

tests/server/test_session.py

@@ -295,7 +295,8 @@ async def test_server_session_stateless_mode():
                 await server_session._received_request(responder)
             except RuntimeError as e:
                 if "initialization" in str(e).lower():
-                    pytest.fail(f"Unexpected initialization error in stateless mode: {e}")
+                    msg = f"Unexpected initialization error in stateless mode: {e}"
+                    pytest.fail(msg)
     finally:
         # Clean up the streams to prevent ResourceWarning
         await server_to_client_send.aclose()

tests/server/test_stdio.py

@@ -79,7 +79,7 @@ async def test_stateless_stdio():
     """Test that stateless stdio mode allows tool calls without initialization."""
     with tempfile.TemporaryDirectory() as temp_dir:
         server_path = Path(temp_dir) / "server.py"
-        
+
         with open(server_path, "w") as f:
             f.write("""
 from mcp.server.fastmcp import FastMCP