From 7613daadc65c7f64ba6d70a4e333c5d98c0253b4 Mon Sep 17 00:00:00 2001 From: Marek Fiala Date: Wed, 19 Aug 2026 08:56:11 +0200 Subject: [PATCH] fix(tools): send mcp-server startup diagnostics to stderr idf.py mcp-server printed startup, shutdown, and error messages on stdout. On the stdio MCP transport, stdout is the JSON-RPC channel, so those non-JSON lines can confuse or break strict clients. Route them to stderr like the other diagnostics in mcp_ext.py. Co-authored-by: Cursor --- tools/idf_py_actions/mcp_ext.py | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/tools/idf_py_actions/mcp_ext.py b/tools/idf_py_actions/mcp_ext.py index 51e9e307943..16588f2e16f 100644 --- a/tools/idf_py_actions/mcp_ext.py +++ b/tools/idf_py_actions/mcp_ext.py @@ -437,15 +437,16 @@ def action_extensions(base_actions: dict, project_path: str) -> dict: except Exception as e: return f'Error getting devices: {str(e)}' - # Start the MCP server - print('MCP Server running on stdio...') + # Start the MCP server. Diagnostics should go to stderr — stdout is reserved + # for the JSON-RPC transport and any non-JSON bytes can confuse MCP clients. + print('MCP Server running on stdio...', file=sys.stderr) try: mcp.run() except KeyboardInterrupt: - print('\nMCP Server stopped.') + print('\nMCP Server stopped.', file=sys.stderr) except Exception as e: - print(f'MCP Server error: {e}') + print(f'MCP Server error: {e}', file=sys.stderr) # Return the action extension return {