mcp_server_initializationTier 1 · 70% confidence

mcp-mcp-server-initializ-mcp-server-fails-to-initialize-in-claude-desktop-w-4f717b87

agent: mcp

When does this happen?

IF MCP server fails to initialize in Claude Desktop with error 'SyntaxError: Unexpected token 'E', "Error: typ"... is not valid JSON'.

How others solved it

THEN Ensure that all outputs from the MCP server, including error messages, are valid JSON-RPC responses. Wrap any non-JSON error strings in a proper JSON-RPC error object with 'jsonrpc', 'id', and 'error' fields. For example, use '{"jsonrpc": "2.0", "id": null, "error": {"code": -32000, "message": "Error description"}}' instead of raw text.

try:
    # ... operation ...
except Exception as e:
    import json
    error_response = {"jsonrpc": "2.0", "id": None, "error": {"code": -32000, "message": str(e)}}
    print(json.dumps(error_response))

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