Node.js Backend Debugging Skill

Debug Node.js backend processes using non-blocking tracepoints, logpoints, exceptionpoints, and JavaScript execution. Use when the user needs to debug API servers, workers, or backend scripts.

When to Use

This skill activates when:

• User asks to debug Node.js backend or API
• User wants to trace function calls in a running server
• User needs to inspect variables or state in a Node process
• User mentions backend errors, API bugs, or server-side issues
• User wants to run JavaScript in a connected Node process

Prerequisites

• Use MCP with PLATFORM=node or run node-devtools-mcp separately
• Connect to the target process before other debug commands (debug_connect)

Capabilities

Connection

• Connect by PID (debug_connect with pid)
• Connect by process name (debug_connect with processName)
• Connect by Docker container (debug_connect with containerName or containerId)
• Connect by inspector port or WebSocket URL

For Docker: use host.docker.internal or container name when MCP runs in a container. Mount /var/run/docker.sock in the MCP container.

Non-Blocking Debugging

• Tracepoints (debug_put-tracepoint): Capture call stack and local variables at code locations
• Logpoints (debug_put-logpoint): Evaluate and log expressions
• Exceptionpoints (debug_put-exceptionpoint): Capture on uncaught or all exceptions
• Watch expressions (debug_add-watch): Evaluate at every tracepoint hit

Snapshot Retrieval

• debug_get-probe-snapshots (types: tracepoint, logpoint, exceptionpoint; returns tracepointSnapshots, logpointSnapshots, exceptionpointSnapshots)
• debug_clear-probe-snapshots — clear when done to free memory

Console & Source Maps

• Console logs (debug_get-logs): Captures console.log, error, warn from the Node process
• Source maps (debug_resolve-source-location): Resolve bundled code to original source

JavaScript Execution

• Use execute on the Node platform for batch tool calls (callTool only; no page). See [commands/node/execute.md](../../commands/node/execute.md).

Debugging Workflow

1. Connect: debug_connect with pid, processName, or containerName
2. Set probes: Tracepoints on route handlers, exceptionpoints for errors
3. Trigger: Make API requests or trigger the code path
4. Collect: debug_get-probe-snapshots (types: tracepoint, exceptionpoint)
5. Investigate: Use execute on Node platform for dynamic inspection (callTool; no page binding)
6. Cleanup: debugclear-probes, debugdisconnect

Best Practices

• Use urlPattern matching script paths (e.g. routes/api.ts, server.js)
• Start with exceptionpoints to catch errors first
• Use logpoints for lightweight monitoring
• Poll snapshots with fromSequence for incremental retrieval
• Clear probes when done to avoid overhead