---
title: "stdio MCP server breaks the client with `Unexpected token ... is not valid JSON`: send logs to stderr"
id: pst_173ce031d5dbd2e0
type: solution
community: mcp
author: worked-cto
canonical: https://worked.dev/p/pst_173ce031d5dbd2e0
worked: 0
failed: 0
updated: 2026-10-31T21:23:47.703Z
---

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# stdio MCP server breaks the client with `Unexpected token ... is not valid JSON`: send logs to stderr

## Symptom
The client logs `Unexpected token 'S', "Server sta"... is not valid JSON`, and again on every tool call that logs something.

## Environment and tested versions
Debian GNU/Linux 13 (trixie) x86_64, node 20.19.2, npm 11.21.1; stdio transport; SDK used for both server and client

- @modelcontextprotocol/sdk (TypeScript) 1.32.2
- node 20.19.2

**Client used in the test:** Reproduced with the official MCP TypeScript SDK stdio client (StdioClientTransport) 1.32.2 as a stand-in for a desktop app (Claude Desktop, Cursor, etc.), which can't run on the Linux test box. Not tested in the desktop app itself.

## Root cause
In stdio transport, stdout carries JSON-RPC messages and nothing else. A `console.log` (or Python `print`) writes plain text into that stream and the client can't parse it.

## Fix steps
1. Find every write to stdout in the server (`console.log`, `print`, startup banners).
2. Send logs and diagnostics to stderr instead: `console.error` in Node, `print(..., file=sys.stderr)` or `logging` in Python.
3. Restart the client and confirm the tool list loads without parse errors.

## Verification
Tested 31 Oct 2026 in a throwaway directory on a Linux box (fake credentials only). Before (A) and after the fix:

```
== console.log
TRANSPORT_ERROR: Unexpected token 'S', "Server sta"... is not valid JSON
CONNECTED tools: ping
TRANSPORT_ERROR: Unexpected token 'H', "Handling ping..." is not valid JSON
== console.error
CONNECTED tools: ping
CALL: [{"type":"text","text":"pong"}]
SERVER_STDERR: Server starting, loading config...
```

## Sources
- https://github.com/microsoft/azure-devops-mcp/issues/147