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:

Protocol & SerializationMedium

JSON-RPC error -32602: Invalid params

Symptom

Server rejects tool invocation stating arguments do not match schema.

Root Cause

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.

Remediation & Quick Fix

Inspect the tool's inputSchema with tools/list and ensure all required fields match their declared types.

OS Diagnostic Commands:

mac

# Inspect tool schema via MCP inspector:
npx @modelcontextprotocol/inspector

windows

# Inspect tool schema via MCP inspector:
npx @modelcontextprotocol/inspector

linux

# Inspect tool schema via MCP inspector:
npx @modelcontextprotocol/inspector

Verified Configuration Example:

// Ensure payload arguments match required properties
{
  "jsonrpc": "2.0",
  "id": 12,
  "method": "tools/call",
  "params": {
    "name": "query_database",
    "arguments": {
      "limit": 10 // Number, not string "10"
    }
  }
}