Debug

Debug

A discipline for hard bugs. Skip phases only when explicitly justified. Use the project's domain glossary (module names, struct names) to build a clear mental model before diving in.

Phase 1 — Build a Feedback Loop

This is the skill. If you have a fast, deterministic, agent-runnable pass/fail signal for the bug, you will find the cause. If you don't, no amount of staring at code will save you.

Spend disproportionate effort here. Be aggressive. Be creative. Refuse to give up.

Ways to Build One (try in order)

1. Reproduce via server log — find the exact error in the game server log, capture the call stack or error code
2. CLI invocation / GM command — trigger the bug via a GM command on the dev server
3. Reproduce via client — replicate the exact client sequence that triggers the bug
4. Throwaway harness — add a temporary GM command that exercises the broken code path in isolation
5. Instrument the code path — add tagged LOG calls: LOG_DEBUG("...[DEBUG-a4f2] value=%d", iValue), recompile, redeploy
6. Property loop — if the bug is "sometimes wrong output", trigger it 100× and look for the failure mode
7. Differential loop — compare behavior between two SVN revisions by checking out each and redeploying
8. Bisection — if the bug appeared between two known SVN revisions, use svn log -r N:M --limit 50 to find suspect commits, then svn update -r <REV> + redeploy to binary-search for the culprit

Iterate on the Loop

Once you have a loop:

• Can I make it faster? (Narrow the trigger to fewer steps)
• Can I make the signal sharper? (Assert on the specific symptom, not just "server crashed")
• Can I make it more deterministic? (Eliminate timing or state dependencies)

When You Cannot Build a Loop

Stop and say so explicitly. List what you tried. Ask the user for: (a) a captured log file or crash dump, (b) exact reproduction steps, or (c) permission to add temporary production instrumentation.

Do not proceed to Phase 2 without a loop.

Phase 2 — Reproduce

Run the loop. Confirm:

• The failure mode matches what the user described — not a different error nearby
• Reproducible across multiple attempts (or at a high enough rate)
• Exact symptom captured (error code, wrong value, crash location)

Phase 3 — Hypothesise

Generate 3–5 ranked hypotheses before testing any of them.

Each hypothesis must be falsifiable:

"If X is the cause, then changing Y will make the bug disappear / changing Z will make it worse."

Show the ranked list to the user before testing. They often have domain knowledge that re-ranks instantly. Proceed with your ranking if the user is AFK.

Phase 4 — Instrument

Each probe maps to a specific prediction from Phase 3. Change one variable at a time.

Tool preference:

1. Targeted LOG_DEBUG calls at the boundaries that distinguish hypotheses
2. Breakpoint via gdb if the environment supports it
3. Never "log everything and grep"

Tag every debug log with a unique prefix, e.g. [DEBUG-a4f2]. Cleanup becomes a single grep. Remove all tagged logs before committing the fix.

For performance regressions: Establish a baseline timing measurement first; then bisect to find which revision introduced the slowdown. Measure first, fix second.

Phase 5 — Fix

Apply the minimal fix for the confirmed root cause. Don't expand scope.

After fixing:

1. Re-run the Phase 1 feedback loop — confirm the bug no longer reproduces
2. Invoke /review — get a code review on the fix before committing
3. Remove all [DEBUG-...] instrumentation (grep -r "DEBUG-a4f2" .)
4. svn commit -m "fix: <root cause description>"

Phase 6 — Post-Mortem

Required before declaring done:

• Original reproduction no longer works
• All [DEBUG-...] instrumentation removed
• Throwaway GM commands deleted
• The root cause is stated in the SVN commit message
• Ask: what would have prevented this bug? If architectural change is needed, invoke /architect with the specifics