karellen-rr-mcp: Instructions file for Claude Code

CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project

karellen-rr-mcp is an MCP server that enables LLM clients to use rr for reverse debugging. It records a failing program with rr, then replays it with full forward and reverse debugging via GDB/MI (using pygdbmi), so the LLM can inspect program state without modifying source code.

Build & Test

This is a PyBuilder project. The build tool is pyb.

pyb                          # default: analyze + publish (lint, test, coverage, package)
pyb run_unit_tests           # run all unit tests only
pyb analyze                  # lint (flake8) + test + coverage

Run a single test module by overriding the unittest_module_glob property:

pyb run_unit_tests -P unittest_module_glob=mi_commands_tests

Tests live in src/unittest/python/*_tests.py and use unittest with unittest.mock.

Lint

Flake8 with: max_line_length=130, extend_ignore=E303,E402. Applies to source, tests, and scripts. Flake8 violations break the build.

Architecture

Two-process model per debugging session:

1. rr gdbserver (rr replay -s <port>) -- managed by rr_manager.ReplayServer
2. GDB/MI via pygdbmi GdbController -- managed by gdb_session.GdbSession

Module responsibilities:

• server.py -- FastMCP server definition, all @mcp.tool() endpoints, module-level singleton session state (_replay_server, _gdb_session), cleanup/signal handling, and main() entry point. The _tag_errors decorator converts all exceptions to ToolError with prefixed messages (gdb:, rr:, internal:).
• rr_manager.py -- Subprocess lifecycle: record() runs rr record, ReplayServer starts/stops rr replay gdbserver, helper functions for rr ps, rr traceinfo, rr rm, listing traces. Port allocation via ephemeral socket binding.
• gdb_session.py -- Wraps pygdbmi GdbController. _write_until() is the core I/O loop: sends a GDB/MI command and keeps reading responses until a predicate is satisfied or deadline expires. All timeouts are configurable via RR_MCP_TIMEOUT_* environment variables.
• mi_commands.py -- Pure functions returning GDB/MI command strings. Reverse execution uses CLI commands (rc, reverse-step, etc.) because MI has no reverse equivalents. rr_when, rr_seek, checkpoint_save/restore use -interpreter-exec console.
• response_parser.py -- Pure functions parsing pygdbmi response dicts into domain dataclasses. Key helpers: find_result_response(), find_stop_event(), get_console_output(), is_error().
• types.py -- Dataclasses for all MCP tool return types (Breakpoint, Frame, StopEvent, Variable, ThreadInfo, etc.).