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Fix MCP server: print to stderr, not stdout

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Startup banner was printing to stdout which corrupts the
JSON-RPC stdio transport used by MCP clients.
This commit is contained in:
Ryan Malloy 2026-01-30 11:03:48 -07:00
parent fc12d6a2d7
commit a20689c463
1 changed files with 7 additions and 5 deletions
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12
src/cp210x_mcp/server.py
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@ -265,23 +265,25 @@ def lock_device(device_index: int = 0, confirm: bool = False) -> dict:
def main(): def main():
"""Entry point for the MCP server.""" """Entry point for the MCP server."""
import sys
try: try:
from importlib.metadata import version from importlib.metadata import version
package_version = version("cp210x-mcp") package_version = version("cp210x-mcp")
except Exception: except Exception:
package_version = "0.1.0" package_version = "0.1.0"
print(f"🔌 CP210x MCP Server v{package_version}") # Print to stderr only — stdout is reserved for JSON-RPC (MCP stdio transport)
print(f"CP210x MCP Server v{package_version}", file=sys.stderr)
try: try:
lib = get_lib() lib = get_lib()
num_devices = lib.get_num_devices() num_devices = lib.get_num_devices()
print(f" Found {num_devices} CP210x device(s)") print(f" Found {num_devices} CP210x device(s)", file=sys.stderr)
except FileNotFoundError as e: except FileNotFoundError as e:
print(f"⚠️ Library not found: {e}") print(f" Library not found: {e}", file=sys.stderr)
print(" Server will start but tools will fail until library is installed.")
except Exception as e: except Exception as e:
print(f"⚠️ Error initializing: {e}") print(f" Error initializing: {e}", file=sys.stderr)
mcp.run() mcp.run()