firecrawl-mcp: Unexpected token 'D', "[DEBUG] 202"... is not valid JSON

Fix Firecrawl MCP Unexpected token 'D', "[DEBUG] 202"... is not valid JSON

TL;DR

Stop the server from writing log lines to stdout: switch its logging to stderr or turn debug logging off. The [DEBUG] lines pollute the same pipe the MCP protocol uses, so the client chokes trying to parse a log line as JSON.

The exact error in the Claude Desktop log:

[error] Unexpected token 'D', "[DEBUG] 202"... is not valid JSON

Steps

1. Confirm stdout pollution in the logs

Open the Claude Desktop MCP log for firecrawl and look for log lines interleaved with protocol messages, like:

[mcp-server-firecrawl] [info] Server started and connected successfully
[mcp-server-firecrawl] [error] Unexpected token 'D', "[DEBUG] 202"... is not valid JSON

Success check: the error mentions [DEBUG] and the quoted fragment starts with [DEBUG] 202 (a timestamp). That confirms log output, not protocol output.

2. Redirect logging to stderr or disable debug logging

In your claude_desktop_config.json, set the server's log level so debug output does not go to stdout:

{
  "mcpServers": {
    "firecrawl": {
      "command": "npx",
      "args": ["-y", "firecrawl-mcp"],
      "env": {
        "FIRECRAWL_API_KEY": "[your Firecrawl API key]",
        "LOG_LEVEL": "error"
      }
    }
  }
}

Success check: no more [DEBUG] lines in the log after a restart.

3. Restart and verify the handshake

Fully quit Claude Desktop (kill leftover processes) and reopen it. Check the log for:

[mcp-server-firecrawl] [info] Server started and connected successfully

with no [error] Unexpected token after it. Success check: firecrawl_scrape and firecrawl_search appear in the tool list.

When this applies

• The firecrawl MCP server starts (you see the "connected successfully" line) but immediately errors with Unexpected token 'D' quoting a [DEBUG] fragment.
• Affects Claude Desktop and any stdio client; the JSON-RPC transport requires stdout to carry only protocol messages.

When it does not apply

• You see FIRECRAWL_API_KEY missing or 401 errors. That is an auth problem; set the key in env.
• The scrape/search tools run but return bad data. That is an API-side issue, not the transport.

Tool compatibility

• firecrawl-mcp npm package
• Claude Desktop (any recent version), also Cursor and other stdio MCP clients
• Node.js 18+

Why it happens

MCP over stdio uses stdout as the JSON-RPC message channel. Any server process that prints logs to stdout breaks the framing: the client reads a log line, calls JSON.parse on it, and dies with Unexpected token. Debug-level logging is the usual culprit because it is the noisiest and includes timestamps, which is why the error quotes [DEBUG] 202....

Edge cases

• dotenv banner lines ([dotenv@...]) cause the sibling error Unexpected token 'd'; the same fix (keep stdout clean) applies.
• If the env var for the log level is ignored by your package version, wrap the command in a small script that redirects stdout: run the server with its stdout filtered so only lines starting with { pass through.
• On Windows, leftover Claude.exe processes hold the old config; use Task Manager to end them all before retesting.