MCP Server for rr Reverse Debugging (karellen-rr-mcp)

Overview

karellen-rr-mcp is an MCP (Model Context Protocol) server that enables any MCP-compliant LLM client to use rr for reverse debugging. Instead of iteratively adding debug output and rebuilding, the LLM can record a failing test with rr, then replay it with full forward and reverse debugging via GDB/MI, inspecting program state without modifying source code.

Requirements

• Linux on x86-64 (rr only supports Linux; aarch64 is experimental)

• rr installed and on PATH

• GDB installed and on PATH (used by rr for debugging)

• Python >= 3.10

• perf_event_paranoid set to 1 to allow recording:

  sudo sysctl kernel.perf_event_paranoid=1

Installing rr and GDB

Via pip (using karellen-rr):

pip install karellen-rr-mcp[rr]

This installs rr as a pip package alongside karellen-rr-mcp. GDB must still be installed separately via your system package manager.

Fedora / RHEL / CentOS:

sudo dnf install rr gdb

Ubuntu / Debian:

sudo apt install rr gdb

Arch Linux:

sudo pacman -S rr gdb

Configuring perf_event_paranoid

rr requires access to hardware performance counters. Set perf_event_paranoid to 1:

sudo sysctl kernel.perf_event_paranoid=1

To make this persistent across reboots:

echo 'kernel.perf_event_paranoid=1' | sudo tee /etc/sysctl.d/50-rr.conf

Verify the setup

rr record /bin/true && echo "rr is working"

If this fails with a permissions error, check perf_event_paranoid. If it fails inside a container or VM, note that rr requires access to CPU performance counters — it does not work in most containers (Docker, Podman) or VMs unless hardware PMU passthrough is configured.

Installation

pip install karellen-rr-mcp

Or with pipx for an isolated environment:

pipx install karellen-rr-mcp

Claude Code Integration

Claude Code plugin (recommended)

The plugin automatically configures the MCP server and includes:

• Crash detection hook that suggests rr when a Bash command exits with a signal (SIGSEGV, SIGABRT, SIGBUS, etc.) or output contains crash/sanitizer signatures

• /karellen-rr-mcp:rr-debug skill that walks through the full record-replay-analyze workflow step by step

• rr-investigator agent that Claude can spawn to autonomously investigate crashes using rr reverse execution

From the Karellen plugins marketplace:

claude plugin marketplace add karellen/claude-plugins
claude plugin install karellen-rr-mcp@karellen-plugins

Or from the official Anthropic marketplace (if accepted):

claude plugin install karellen-rr-mcp@claude-plugins-official

Or load directly from a local checkout for testing:

claude --plugin-dir /path/to/karellen-rr-mcp

Manual MCP server configuration

If you prefer not to use the plugin, you can configure the MCP server directly. This gives you the MCP tools but not the skill, agent, or crash detection hook.

Using the CLI:

claude mcp add --transport stdio karellen-rr-mcp -- karellen-rr-mcp

Or manually add to ~/.claude.json (user scope) or .mcp.json in your project root (project scope, shared via version control):

{
  "mcpServers": {
    "karellen-rr-mcp": {
      "type": "stdio",
      "command": "karellen-rr-mcp"
    }
  }
}

If installed with pipx:

claude mcp add --transport stdio karellen-rr-mcp -- pipx run karellen-rr-mcp

or manually:

{
  "mcpServers": {
    "karellen-rr-mcp": {
      "type": "stdio",
      "command": "pipx",
      "args": ["run", "karellen-rr-mcp"]
    }
  }
}

Auto-approve rr tools

By default Claude Code will prompt for confirmation before each rr_* tool call. You can approve individually by selecting "Yes, and don't ask again" when prompted.

To auto-approve all tools upfront, add a permission rule to your user settings (~/.claude/settings.json):

{
  "permissions": {
    "allow": [
      "mcp__plugin_karellen-rr-mcp_karellen-rr-mcp__*",
      "mcp__karellen-rr-mcp__*"
    ]
  }
}

The first rule covers plugin-loaded tools, the second covers manual MCP configuration.

Or for a project-scoped setting, add the same rule to .claude/settings.json in your project root (this file can be committed to version control so all team members get it).

Available Tools

Session Lifecycle

Breakpoints

Execution Control

Thread and Frame Navigation

State Inspection

Checkpoints

Configuration

Timeouts

All timeouts are configurable via environment variables (in seconds). Set them in your MCP server configuration:

{
  "mcpServers": {
    "karellen-rr-mcp": {
      "type": "stdio",
      "command": "karellen-rr-mcp",
      "env": {
        "RR_MCP_TIMEOUT_FORWARD": "300",
        "RR_MCP_TIMEOUT_REVERSE": "600"
      }
    }
  }
}

For large binaries (e.g. MariaDB, Firefox), you may need to increase RR_MCP_TIMEOUT_CONNECT (symbol loading can take 20+ seconds) and RR_MCP_TIMEOUT_FORWARD (replaying to a breakpoint deep in execution can take minutes).

Troubleshooting

AMD Zen CPUs

rr does not work reliably on AMD Zen CPUs unless the hardware SpecLockMap optimization is disabled. When running rr on Zen you may see:

On Zen CPUs, rr will not work reliably unless you disable the hardware SpecLockMap optimization.

Workaround: run the zen_workaround.py script from the rr source tree as root:

sudo python3 scripts/zen_workaround.py

This fix must be reapplied after each reboot or suspend. To make it persist, you must also stabilize the Speculative Store Bypass (SSB) mitigation by adding one of the following kernel command-line parameters:

• spec_store_bypass_disable=on — fully enables SSB mitigation (has performance implications)

• nospec_store_bypass_disable — fully disables SSB mitigation (has security implications)

Alternatively, build and load the zen_workaround.ko kernel module from the rr source tree, which prevents SSB mitigation from resetting the workaround without requiring kernel parameters.

See the rr Zen wiki page for full details.

MSR kernel module not loaded

The zen_workaround.py script accesses CPU model-specific registers via /dev/cpu/0/msr, which requires the msr kernel module. On many distributions this module is not loaded by default. If the script fails, load it manually:

sudo modprobe msr

To make this persistent across reboots:

echo 'msr' | sudo tee /etc/modules-load.d/msr.conf

Note: on systems with Secure Boot enabled, the msr module may fail to load because it is not signed. You may need to either disable Secure Boot in your UEFI/BIOS settings, or sign the module with your own Machine Owner Key (MOK).

MADV_GUARD_INSTALL crash on kernel 6.13+ with glibc 2.42+

Linux 6.13 introduced MADV_GUARD_INSTALL (madvise advice 102) for lightweight stack guard pages. glibc 2.42+ (e.g. Fedora 43) uses this in pthread_create. rr 5.9.0 (the latest release, from February 2025) does not recognize this madvise advice value and crashes with:

Assertion `t->regs().syscall_result_signed() == -syscall_state.expect_errno' failed to hold.
Expected EINVAL for 'madvise' but got result 0 (errno SUCCESS); unknown madvise(102)

This was fixed in rr git master (commit 34ff3a7, August 2025) but has not been included in a release yet. You must build rr from source to get the fix:

git clone https://github.com/rr-debugger/rr.git
cd rr
mkdir build && cd build
cmake ..
make -j$(nproc)
sudo make install

See rr-debugger/rr#4044 and rr-debugger/rr#3995 for details.

License

Apache-2.0