MCP stdio Server Crashing on console.log / Unexpected Token

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🎯 Target Goal & Friction

Goal: Prevent stdout pollution in Model Context Protocol stdio servers

Error Signature: JSON-RPC parse error on MCP stdio stream: Unexpected token in JSON at position 0

⚡ Direct Teleport Deep Link (0ms Navigation)

https://modelcontextprotocol.io/docs/tools/debugging →

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Verified Code / Configuration Diff

@@ src/index.ts @@
- console.log("Server running..."); // Corrupts stdio JSON-RPC stream!
+ console.error("Server running on stderr..."); // Always log to stderr in MCP servers

1-Line Instant CLI Fix

npx @modelcontextprotocol/inspector

Root Cause & Explanation

In MCP stdio transport, `stdout` is reserved strictly for JSON-RPC messages. Calling `console.log()` outputs raw strings to stdout and crashes the client parser.