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Debugging with undo

Skill undoio/addons/skills/debugging-with-undo

Debug programs using Undo time travel debugging. Record flaky tests, intermittent failures, race conditions, or bugs that don't reproduce reliably. Analyze recordings by traveling backwards through execution history. Use when the user mentions debugging, flaky tests, intermittent bugs, race conditions, `.undo` files (Undo recordings), or asks why something happened in an actual run. This can provide similar results as adding logging statements, but without modifying the code or needing to re-run the program multiple times.From its SKILL.md

Install
npx -y skills add undoio/addons --skill debugging-with-undo

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SKILL.md

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Debugging with Undo time travel debugger

Undo provides time travel debugging for Linux programs. This Skill guides you to:

  1. Record program execution to capture failures (especially intermittent ones)
  2. Debug recordings by traveling backwards through execution history

When to record (use record MCP tool)

Record program execution when the user:

  • Has a flaky test or intermittent failure that doesn't reproduce reliably
  • Mentions race conditions, threading issues, or concurrency bugs
  • Wants to capture a failure for later analysis
  • Needs deterministic replay of a bug

Undo captures the exact execution including thread interleavings, making even race conditions reproducible.

When to debug (use debugging MCP tools)

Debug a recording when the user:

  • Has a .undo recording file to investigate
  • Asks "why did this happen" or "how did this value get set"
  • Wants root cause analysis on a captured failure
  • Needs to understand execution backwards from a crash or assertion

Workflow

  1. If no recording exists: Use record MCP tool to capture the failure
  2. If recording exists: Use debugging MCP tools to analyze it
  3. Work backwards: Start at the end (where the bug manifested), trace backwards

Important

  • Never access .undo files directly - they are opaque binary recordings
  • Never invoke udb, live-record, or undo via Bash - use the Undo MCP tools

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