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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.

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

–Maintainers
–Response time
4dRelease cycle
4Releases (12mo)

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