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:

Network & TransportHigh

SSE connection closed prematurely / EOF

Symptom

Remote SSE MCP servers disconnect intermittently or fail to keep long-running tool calls open.

Root Cause

Reverse proxies (Cloudflare, Nginx, ALB) or corporate firewalls terminate idle HTTP streaming connections after 60–100 seconds when keep-alive heartbeats are missing.

Remediation & 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`).

OS Diagnostic Commands:

mac

# Test SSE stream headers with curl:
curl -N -H "Accept: text/event-stream" https://your-server.com/sse

windows

# Test SSE stream endpoint:
curl.exe -N -H "Accept: text/event-stream" https://your-server.com/sse

linux

# Test SSE stream endpoint:
curl -N -H "Accept: text/event-stream" https://your-server.com/sse

Verified Configuration Example:

// Nginx Proxy Configuration:
location /sse {
    proxy_pass ;
    proxy_set_header Connection '';
    proxy_http_version 1.1;
    chunked_transfer_encoding off;
    proxy_buffering off;
    proxy_cache off;
}