Severity: High Affects: MCP test modal, MCP execution modal
Some community/marketplace MCP servers print human-readable messages (e.g. 🚀 Hypertool MCP is ready!) to stdout on startup instead of reserving stdout exclusively for JSON-RPC messages. This violates the MCP stdio protocol specification.
Symptoms:
- Test modal hangs or shows "process closed stdout unexpectedly"
- MCP client receives non-JSON output and cannot parse the initialize response
Known affected servers:
@toolprint/hypertool-mcp— prints🚀 Hypertool MCP is ready!to stdout
Root cause: The MCP stdio transport requires that stdout is used only for JSON-RPC messages. Any other output (logging, greeting banners, debug info) must go to stderr. This is a bug in the MCP server implementations, not in this application.
Workaround: Use well-tested MCP servers that follow the protocol correctly, such as the official @modelcontextprotocol/server-everything reference server.
Potential improvement: Detect non-JSON output on stdout and surface a clearer error message to the user explaining the server is misbehaving.