name: sh:debug
description: "Structured root-cause diagnosis before proposing fixes"

Systematic Debugging

Diagnose bugs, test failures, and unexpected behavior using structured root-cause analysis.
No fixes without understanding. Symptom fixes are failure.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

When to Use

Any technical issue: test failures, runtime bugs, unexpected behavior, performance problems.
Use ESPECIALLY when under time pressure or when "just one quick fix" seems obvious.

The Four Phases

Complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

1. Read error messages carefully -- full stack traces, line numbers, error codes
2. Reproduce consistently -- exact steps, reliable trigger, or gather more data
3. Check recent changes -- git diff, new deps, config changes, env differences
4. Gather evidence at component boundaries -- log inputs/outputs at each layer
5. Trace data flow -- where does the bad value originate? Trace backward to source

Phase 2: Pattern Analysis

1. Find working examples -- similar working code in the same codebase
2. Compare against references -- read reference implementations completely
3. Identify differences -- list every difference, however small
4. Understand dependencies -- components, config, environment assumptions

Phase 3: Hypothesis and Testing

1. Form single hypothesis -- "X is root cause because Y"
2. Test minimally -- smallest possible change, one variable at a time
3. Verify -- worked? Phase 4. Failed? New hypothesis. Never stack fixes

Phase 4: Implementation

1. Create failing test -- simplest reproduction, automated if possible
2. Implement single fix -- address root cause, ONE change, no "while I'm here"
3. Verify fix -- test passes, no regressions, issue resolved
4. If 3+ fixes failed -- STOP. Question the architecture. Discuss with user

Red Flags -- STOP and Return to Phase 1

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• Proposing solutions before tracing data flow
• Each fix reveals new problem in different place
• "One more fix attempt" after 2+ failures

Quick Reference

Boundaries

Will: Systematic diagnosis, evidence gathering, root-cause identification, targeted fix.
Will not: Apply fixes without investigation, stack multiple fixes, skip reproduction.

Next Step

After diagnosis, apply the fix. Use /sh:test to verify. Use /sh:analyze if issue reveals broader quality concerns.