Systematic Debugging

Skill: Systematic Debugging

This skill enforces root-cause investigation before any fix attempt. Random fixes waste time and create new bugs.

When to Use

Use for any technical issue:

• Test failures
• Bugs in production or development
• Unexpected behavior
• Performance problems
• Build failures
• Integration issues

Do not skip this skill when:

• The issue seems simple (simple bugs have root causes too)
• You are under time pressure (systematic debugging is faster than thrashing)
• A fix seems obvious (obvious fixes often mask the real problem)

Execution Model

• Primary agent runs this skill directly.
• Rationale: Debugging requires iterative investigation in the same context. Delegating to a subagent loses the accumulated understanding.
• Exception: For multi-component issues spanning independent subsystems, delegate per-subsystem investigation to separate agents via the dispatch-parallel-agents skill.

Workflow

Phase 1: Root Cause Investigation

No fixes are allowed until this phase is complete.

1. Read error messages carefully -- stack traces, line numbers, error codes. Do not skip past them.
2. Reproduce consistently -- determine the exact steps. If not reproducible, gather more data instead of guessing.
3. Check recent changes -- git diff, recent commits, new dependencies, config changes, environmental differences.
4. Gather evidence at component boundaries -- for multi-layer systems (CI -> build -> deploy, API -> service -> database), add diagnostic logging at each boundary to identify where the failure occurs.
5. Trace data flow -- follow the bad value backward through the call stack to its origin. Fix at the source, not at the symptom.

Phase 2: Pattern Analysis

1. Find working examples -- locate similar working code in the same codebase.
2. Compare against references -- if implementing a pattern, read the reference implementation completely. Do not skim.
3. Identify differences -- list every difference between working and broken code, however small.
4. Understand dependencies -- what components, settings, config, and environment does this code need?

Phase 3: Hypothesis and Testing

1. Form a single hypothesis -- state clearly: "I think X is the root cause because Y."
2. Test minimally -- make the smallest possible change to test the hypothesis. One variable at a time.
3. Verify before continuing -- if the hypothesis is wrong, form a new one. Do not stack fixes.
4. Acknowledge unknowns -- if you do not understand something, say so. Do not pretend.

Phase 4: Implementation

1. Fix the root cause, not the symptom.
2. One change at a time -- no "while I'm here" improvements, no bundled refactoring.
3. Verify the fix -- run the relevant tests, confirm the issue is resolved, confirm no regressions.
4. If 3+ fixes have failed -- stop. The issue is likely architectural, not a bug. Discuss with the user before attempting more fixes.

Rules

1. No fixes without investigation: Phase 1 must be complete before proposing any fix.
2. One hypothesis at a time: Do not apply multiple changes simultaneously.
3. Evidence over intuition: Every fix must be justified by evidence from the investigation, not by "it might work."
4. Escalate after 3 failed attempts: Three failed fixes indicate an architectural problem. Stop fixing and discuss with the user.
5. Do not increase timeouts as a fix: Find the real timing issue instead.
6. Read error messages completely: Stack traces contain the answer more often than not.