Debug MCP Server

Use the official MCP Inspector first. It supports stdio, SSE, and Streamable HTTP, and has both a browser UI and a scriptable --cli mode.

Docs:

• https://modelcontextprotocol.io/docs/tools/inspector
• https://github.com/modelcontextprotocol/inspector

CLI Smoke Tests#

List tools on a local stdio server:

npx -y @modelcontextprotocol/inspector --cli node build/index.js --method tools/list

List tools on a remote Streamable HTTP server:

npx -y @modelcontextprotocol/inspector --cli https://example.com/mcp \
  --transport http \
  --method tools/list

Pass headers without committing secrets:

doppler run --project os --config preview_2 -- sh -lc '
  npx -y @modelcontextprotocol/inspector --cli https://mcp.cloudflare.com/mcp \
    --transport http \
    --method tools/list \
    --header "Authorization: Bearer $CLOUDFLARE_API_TOKEN"
'

Call a tool:

npx -y @modelcontextprotocol/inspector --cli node build/index.js \
  --method tools/call \
  --tool-name mytool \
  --tool-arg key=value \
  --tool-arg 'options={"format":"json"}'

UI Workflow#

Run:

npx -y @modelcontextprotocol/inspector

Open the printed localhost URL with its proxy token. Do not disable proxy auth. Use the UI to inspect initialization, auth, tools/resources/prompts, schemas, request history, errors, and notifications.

Diagnosis Loop#

1. Prove transport: tools/list before calling a tool.
2. Prove auth separately: repeat without auth, then with the exact header.
3. Capture the raw Inspector output and exit code.
4. If tools/list works but a tool fails, call the smallest failing tool input.
5. Compare Inspector behavior with Codex/Claude config only after the server itself works.

Cloudflare General MCP#

Use the general Cloudflare API MCP server at https://mcp.cloudflare.com/mcp. Avoid configuring a separate product-specific observability endpoint.

Cloudflare's execute tool accepts account_id as a tool argument. Do not append it to the MCP URL. If a bearer token is rejected before initialization because it has access to multiple accounts, use normal MCP OAuth login or a narrower token.