start-learning

Systematic Debugging

Find the cause from evidence before changing behavior. Match the depth of the investigation to the problem; a clear local failure does not need a formal incident process. Use the following phases to avoid speculative fixes.

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read Error Messages Carefully

   • Don't skip past errors or warnings
   • They often contain the exact solution
   • Read stack traces completely
   • Note line numbers, file paths, error codes

2. Reproduce Consistently

   • Can you trigger it reliably?
   • What are the exact steps?
   • Does it happen every time?
   • If not reproducible -> gather more data, don't guess

3. Check Recent Changes

   • What changed that could cause this?
   • Git diff, recent commits
   • New dependencies, config changes
   • Environmental differences

4. Gather Evidence in Multi-Component Systems

   WHEN system has multiple components (CI -> build -> signing, API -> service -> database):

   When existing evidence does not locate the failure, add targeted diagnostics:

   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer
   
   Run once to gather evidence showing WHERE it breaks
   THEN analyze evidence to identify failing component
   THEN investigate that specific component
   

   For example, compare a request's identifier, input shape, and result at the API, worker, and database boundaries to locate where behavior diverges. Record redacted state or presence, never secret values or broad environment dumps.

5. Trace Data Flow

   WHEN error is deep in call stack:

   See root-cause-tracing.md in this directory for the complete backward tracing technique.

   Quick version:

   • Where does bad value originate?
   • What called this with bad value?
   • Keep tracing up until you find the source
   • Fix at source, not at symptom

Phase 2: Pattern Analysis

Find the pattern before fixing:

1. Find Working Examples

   • Locate similar working code in same codebase
   • What works that's similar to what's broken?

2. Compare Against References

   • Read the relevant reference implementation and version-specific contracts
   • Understand the dependencies and assumptions before applying the pattern

3. Identify Differences

   • What's different between working and broken?
   • List every difference, however small
   • Don't assume "that can't matter"

4. Understand Dependencies

   • What other components does this need?
   • What settings, config, environment?
   • What assumptions does it make?

Phase 3: Hypothesis and Testing

Scientific method:

1. Form Single Hypothesis

   • State clearly: "I think X is the root cause because Y"
   • Write it down
   • Be specific, not vague

2. Test Minimally

   • Make the SMALLEST possible change to test hypothesis
   • One variable at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes -> Phase 4
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

4. When You Don't Know

   • Say "I don't understand X"
   • Don't pretend to know
   • Ask for help
   • Research more

Phase 4: Implementation

1. Create a focused reproduction or regression check for meaningful behavior or security changes. Confirm that it demonstrates the original failure. For low-impact changes, use appropriate existing checks or a manual reproduction.
2. Fix the identified cause with a focused change. Avoid unrelated refactoring or stacking several speculative fixes together.
3. Verify the original failure and related behavior with verification-before-completion, then remove temporary diagnostics.
4. If the fix fails, return to the evidence and reassess the hypothesis. Repeated failures may reveal shared state, coupling, or a flawed assumption. Discuss material design changes with the user; do not infer architectural failure solely from an arbitrary attempt count.

When Process Reveals "No Root Cause"

If systematic investigation reveals issue is truly environmental, timing-dependent, or external:

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

Supporting Techniques

These techniques are part of systematic debugging and available in this directory:

• root-cause-tracing.md - Trace bugs backward through call stack to find original trigger
• defense-in-depth.md - Add validation at multiple layers after finding root cause
• condition-based-waiting.md - Replace arbitrary timeouts with condition polling

Related skills:

• verification-before-completion - Verify fix worked before claiming success