How to fix MCP error "spawn npx ENOENT / command not found"?

The AI host application (Claude Desktop, Cursor, etc.) was launched in a graphical desktop environment that does not inherit your terminal's user $PATH, or Node.js is not installed globally. Quick Fix: Replace 'command': 'npx' with the absolute path to your npx binary (e.g. C:/Program Files/nodejs/npx.cmd on Windows or /usr/local/bin/npx on macOS).

How to fix MCP error "JSON-RPC error -32602: Invalid params"?

The AI model passed arguments with incorrect data types (e.g. string instead of integer or array), or omitted a required parameter defined in inputSchema. Quick Fix: Inspect the tool's inputSchema with tools/list and ensure all required fields match their declared types.

How to fix MCP error "JSON-RPC error -32700: Parse error / Invalid Windows backslashes"?

Single backslashes used in Windows file paths (e.g. 'C:\Projects') violate JSON string escape rules. Quick Fix: Replace single backslashes with forward slashes (e.g. 'C:/Projects') or double-escaped backslashes ('C:\\Projects').

How to fix MCP error "SSE connection closed prematurely / EOF"?

Reverse proxies (Cloudflare, Nginx, ALB) or corporate firewalls terminate idle HTTP streaming connections after 60–100 seconds when keep-alive heartbeats are missing. Quick Fix: Implement a 15-second ping/heartbeat comment (`: keep-alive\n\n`) on the SSE endpoint and disable proxy buffering (`X-Accel-Buffering: no`).

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Category:

Protocol & SerializationHigh

JSON-RPC error -32603: Internal error (STDOUT Pollution)

Symptom

Tools fail to list or fail during execution with generic error code -32603.

Root Cause

The MCP server wrote non-JSON debug strings to standard output (STDOUT) instead of standard error (STDERR). In stdio transport, STDOUT is strictly reserved for JSON-RPC messages; any `console.log()` breaks JSON parsing.

Remediation & Quick Fix

Switch all logging inside your custom MCP server from `console.log()` to `console.error()`. For Python servers, write logs exclusively to `sys.stderr`.

OS Diagnostic Commands:

mac

# Check MCP host logs for raw output:
tail -n 50 ~/Library/Logs/Claude/mcp.log

windows

# Check Claude logs in PowerShell:
Get-Content -Path $env:APPDATA\Claude\logs\mcp.log -Tail 50

linux

# Tail Linux MCP logs:
tail -n 50 ~/.config/Claude/logs/mcp.log

Verified Configuration Example:

// In TypeScript MCP Server:
// ❌ WRONG: console.log("Server starting...");
// ✅ CORRECT:
console.error("Server starting on stdio transport...");