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pernosco-mcp

by jnjaeschke
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pernosco-mcp

MCP server that connects AI coding agents (Claude Code, etc.) to Pernosco debugging sessions. Query execution traces, inspect variables, navigate call stacks, and trace value histories — all through natural language.

How It Works

Claude Code  <--stdio-->  pernosco-mcp daemon  <--WebSocket-->  Firefox extension  <--window.client-->  Pernosco

A background daemon coordinates between any number of Claude Code instances and Pernosco browser tabs. The Firefox extension bridges into Pernosco's internal window.client API.

Related MCP server: gdb and rr Debugging

Installation

1. Install the npm package

npm install -g pernosco-mcp

This installs the MCP server and registers the native messaging host for Firefox.

2. Install the Firefox extension

Install from Firefox Add-ons.

3. Add to Claude Code

claude mcp add pernosco -- pernosco-mcp

Usage

  1. Open a Pernosco trace in Firefox

  2. In Claude Code: "Connect to my Pernosco trace at https://pernos.co/debug/abc123/index.html"

  3. Debug:

    • "Find all calls to nsDocShell::LoadURI and show me the URI argument"

    • "Go to result 3 and show me the call stack"

    • "What was written to this address throughout the trace?"

Tools

Tool

Description

session_connect

Connect to a trace by URL or trace ID

session_list

List open Pernosco tabs

session_status

Current position (event, source, line)

session_disconnect

Disconnect from session

find_executions

Find all calls to a function, optionally print expressions at each

find_breakpoint_hits

All hits of a source line

stack

Call stack at current position

evaluate

Evaluate a C++ expression

goto

Navigate to a query result or focus position

search

Search for symbols, functions, types

watchpoint_history

Write history for a memory address

watch_variable

Trace writes to a C++ variable (evaluate + watchpoint)

stdout_stderr

stdout/stderr output with event IDs

source_read

Read source code from the trace

dynamic_annotations

Which lines executed, with counts

step_to_next_hit

Step forward to next hit of current line

step_to_prev_hit

Step backward to previous hit

task_tree

Process/thread hierarchy

current_tasks

Active threads at current moment

notebook_read

Pernosco notebook annotations

Development

Building from source

git clone https://github.com/jnjaeschke/pernosco-mcp.git
cd pernosco-mcp
npm install
npm run build:all

Load the extension manually via about:debugging > "This Firefox" > "Load Temporary Add-on" > select extension/manifest.json.

Scripts

npm run dev              # Watch mode (TypeScript)
npm test                 # Run tests
npm run build            # Build server
npm run build:extension  # Package extension .xpi
npm run build:all        # Build everything

Architecture

  • Daemon (src/daemon.ts) — WebSocket server on random localhost port. Spawned on-demand, exits after 10 min idle.

  • Shim (src/shim.ts) — stdio-to-WebSocket bridge, one per Claude Code instance.

  • Extension (extension/) — Content script injected into pernos.co pages, queries window.client API.

  • PML (src/pml.ts) — Converts Pernosco's markup to text for LLM consumption.

License

MIT

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maintenance

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Maintenance

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4dRelease cycle
4Releases (12mo)
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