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 -32603: Internal error (STDOUT Pollution)"?

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. 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`.

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:

Environment & RuntimeCritical

spawn npx ENOENT / command not found

Symptom

AI client crashes or logs 'Error: spawn npx ENOENT' immediately when trying to launch a configured MCP server.

Root Cause

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.

Remediation & 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).

OS Diagnostic Commands:

mac

# Find your exact npx binary location:
which npx
# Example output: /usr/local/bin/npx or /opt/homebrew/bin/npx

windows

# Find your exact npx.cmd binary location in PowerShell:
where.exe npx.cmd
# Example output: C:\Program Files\nodejs\npx.cmd

linux

# Find npx binary path:
which npx
# Example output: /usr/bin/npx

Verified Configuration Example:

{
  "mcpServers": {
    "filesystem": {
      "command": "C:/Program Files/nodejs/npx.cmd",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "C:/Users/name/sandbox"]
    }
  }
}