Debug Session

debug-session

Skill metrics: Emit a start event to the activity log:

echo "{\"type\":\"skill\",\"event\":\"start\",\"skill\":\"debug-session\",\"ts\":\"$(date -u +%Y-%m-%dT%H:%M:%SZ)\"}" >> docs/sessions/.activity-log.jsonl

Debug session for: $ARGUMENTS

Failure State Persistence

At each phase transition, write docs/sessions/.failure-state.md with YAML frontmatter so the Stop hook and /continue can programmatically detect incomplete workflows.

At workflow start (Phase 1 entry):

---
status: active
skill: debug-session
phase: "1"
phase_name: "Root Cause Investigation"
started_at: "[ISO-8601 timestamp from date -u +%Y-%m-%dT%H:%M:%SZ]"
story: "[from $ARGUMENTS — the error description]"
branch: "[from git branch --show-current]"
next_action: "Reproduce error and trace root cause"
files_modified: []
---

## Context
Error: [error description]
Root cause: not yet identified
Hypothesis: none
Fix attempts: 0
Files investigated: [list as investigation proceeds]

At each phase transition: Update the frontmatter fields: phase, phase_name, next_action, and append to files_modified. Update the Context section with investigation progress (root cause, hypothesis, fix attempts).

On successful fix (Phase 5 complete): Delete .failure-state.md — clean state means no failure to recover from.

Phase 1: Root Cause Investigation (MANDATORY)

Do NOT skip this phase. Do NOT jump to a fix.

1a. Understand the Error

• Read the FULL error message — every line, including stack trace
• What is the exact error? (copy it verbatim)
• Where does it occur? (file:line from stack trace)
• When does it occur? (always, intermittently, under specific conditions)

1b. Reproduce Consistently

Run the relevant test or command to reproduce:

# Run the failing test or trigger the error

If it cannot be reproduced, gather more context before proceeding.

1c. Check Recent Changes and ADR Constraints

git log --oneline -10
git diff HEAD~3

Did a recent change introduce this? Use git bisect for non-obvious regressions.

1d. Trace Backward from Symptom

Apply the failure_diagnosis reasoning tool from .claude/skills/story-cycle/references/reasoning-tools.md:

1. Read the FULL error output — every line, including stack trace and surrounding context
2. Identify: what was expected vs. what actually happened
3. Trace backward: which function produced the wrong result? What were its inputs?
4. If inputs are correct: the bug is in this function — inspect its logic
5. If inputs are wrong: trace one level further back — where do those inputs come from?
6. Repeat until you find the point where correct data becomes incorrect

For multi-component systems, trace across component boundaries — check API calls, database queries, event handlers.

See references/root-cause-tracing.md — search for ## Steps for the detailed backward trace procedure.

DO / DON'T:

• DO trace backward from the symptom to the source before forming a hypothesis.
• DON'T jump to a fix based on the error message alone — the message describes the symptom, not the cause.
• DO add temporary logging to confirm data flow before changing logic.
• DON'T change multiple things at once hoping one will fix it.

Phase 2: Pattern Analysis

1. Find a WORKING example of similar functionality in the codebase
2. Compare the working example against the broken code line by line
3. Identify specific differences — which difference explains the bug?
4. Understand the dependency chain — are all dependencies correct and available?

2b. Search for Known Issues (Optional)

1. Identify the dependency name and version from package.json, requirements.txt, or equivalent
2. Use WebSearch to search for: "[dependency] [error message or key phrase]"
3. If a known issue or fix is found, verify it applies to your version and context
4. If no results: proceed to Phase 3 — the bug is likely project-specific

Phase 3: Hypothesis and Testing

Form a Single Hypothesis

Based on evidence from Phases 1-2, state:

• Hypothesis: "The bug is caused by [specific cause] because [evidence]"
• Prediction: "If I change [specific thing], the error should [specific expected change]"

Test Minimally

• Change ONE variable at a time
• Run the reproduction command after each change
• If the hypothesis is wrong, REVERT the change and form a new hypothesis

Stopping Points — STOP If:

See references/condition-based-waiting.md — search for ## Patterns by Language and load only your language's section.

Phase 4: Fix Implementation (TDD)

1. Write a test that reproduces the bug (MUST fail before fix)
2. Verify the test fails for the RIGHT reason (matches reported behavior)
3. Implement the MINIMAL fix — change as little as possible
4. Verify the reproduction test passes
5. Run the FULL test suite for regressions

Phase 4.5: Error Learning

If the root cause was initially misdiagnosed or the fix required changing approach, invoke the record-failure micro-component from .claude/prompts/record-failure.md to record the pattern in docs/context/error-patterns.md. This helps future sessions avoid the same diagnostic mistakes.

Phase 5: Verify and Document

• All tests pass (show output)
• The original error is no longer reproducible
• The fix addresses the root cause, not just the symptom
• Commit message explains what caused the bug and why

git add <files>
git commit -m "fix(<scope>): <what was fixed>

Root cause: <explanation>

Co-Authored-By: Claude <noreply@anthropic.com>"

Skill metrics: Emit a completion event:

echo "{\"type\":\"skill\",\"event\":\"end\",\"skill\":\"debug-session\",\"outcome\":\"success\",\"ts\":\"$(date -u +%Y-%m-%dT%H:%M:%SZ)\"}" >> docs/sessions/.activity-log.jsonl

Example

Input:  /debug-session "TypeError: Cannot read properties of undefined (reading 'map')"
Output: Phase 1 → traced to `fetchUsers()` returning `undefined` when API returns 404
        Phase 3 → hypothesis: missing error handling for non-200 responses
        Phase 4 → added guard clause + regression test
        Phase 5 → commit: "fix(api): handle non-200 response in fetchUsers"

Next Steps:
→ /story-cycle "[next story]" — continue with the next story
→ /sprint-end — if the fix was the last item in the sprint
→ /handoff — if ending the session

Recovery

For phase-specific error recovery, consult references/error-recovery.md — search for the relevant ## Phase N section.

General recovery:

• Cannot reproduce: Gather more context. Check environment differences. Ask user for exact reproduction steps.
• Root cause unclear after investigation: Document findings so far. Ask user for additional context. Do NOT guess.
• Fix introduces new failures: Revert the fix. The original bug is better than two bugs.
• Timing/race condition: See references/condition-based-waiting.md. Replace arbitrary sleeps with condition polling.

Rules

• NEVER apply a fix without understanding the root cause
• NEVER make multiple changes at once — one hypothesis, one change, one verification
• NEVER weaken or delete tests to make them pass
• NEVER say "it should work now" without showing test output
• If 3+ fixes fail, STOP and discuss the approach with the user