Pernosco

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