Systematic debugging

Systematic Debugging

<!-- Adapted from obra/superpowers — Tessera graph_impact added to Phase 1 -->

Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

Core principle: ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

Violating the letter of this process is violating the spirit of debugging.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed Phase 1, you cannot propose fixes.

Tessera: Map Blast Radius First

Before starting Phase 1 — if tessera MCP is configured in this session:

1. graph_continue (mandatory first call)
2. graph_impact(changed_files=["<file where bug was reported>"])

graph_impact shows which other files depend on the broken code. This tells you:

• How far a fix might ripple
• Which tests are likely affected
• Whether this is a leaf-level bug or a core abstraction issue

Surface this impact map before investigating root cause.

The Four Phases

You MUST complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read error messages carefully — stack traces, line numbers, error codes
2. Reproduce consistently — can you trigger it reliably every time?
3. Check recent changes — git diff, recent commits, new dependencies
4. Gather evidence in multi-component systems
   • Add diagnostic logging at each component boundary
   • Run once to gather evidence showing WHERE it breaks
   • Then analyze to identify the failing component
5. Trace data flow — where does the bad value originate? Trace backward through the call stack

Phase 2: Pattern Analysis

1. Find working examples — use graph_retrieve(query="<working component similar to broken one>") if tessera MCP is configured to find similar code in the codebase
2. Read the reference — use graph_read to read the working implementation completely, not just skim
3. Identify differences — list every difference, however small
4. Understand dependencies — what settings, config, environment does this need?

Phase 3: Hypothesis and Testing

1. Form single hypothesis — "I think X is the root cause because Y"
2. Test minimally — smallest possible change to test hypothesis
3. Verify before continuing — did it work? If not, form NEW hypothesis. Don't stack fixes.
4. When you don't know — say so; don't pretend

Phase 4: Implementation

1. Create failing test case — use test-driven-development skill
2. Implement single fix — address root cause, ONE change at a time
3. Verify fix — test passes, no regressions, issue actually resolved
4. If tessera MCP is configured: register the fix:

   graph_register_edit(
     files   = ["path/to/fixed/file::function_name"],
     summary = "Fixed: <root cause description>",
     checklist_item_id = <id if this maps to an active plan item>
   )
   

5. If fix doesn't work — return to Phase 1 with new information
6. If 3+ fixes failed — STOP. Question the architecture. Discuss with user.

Red Flags — STOP and Follow Process

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

All of these mean: Return to Phase 1.

Quick Reference

Phase	Key Activities	Success Criteria
1. Root Cause	Read errors, reproduce, check changes, gather evidence	Understand WHAT and WHY
2. Pattern	Find working examples, compare	Identify differences
3. Hypothesis	Form theory, test minimally	Confirmed or new hypothesis
4. Implementation	Create test, fix, verify	Bug resolved, tests pass

Related Skills

• test-driven-development — for creating failing test case (Phase 4, Step 1)
• verification-before-completion — verify fix worked before claiming success