debugging

name	debugging
description	Use when a test is failing, when a bug is reported, when behavior is unexpected or intermittent, when a build or integration step fails, or when a flaky test keeps resurfacing. Especially when "the fix seems obvious", when multiple previous fixes haven't stuck, or when under time pressure to ship.

Codex Backend

This skill is assembled for Codex. Before following the workflow, read references/codex-backend.md completely. Its operation mappings are binding: SessionPlanRead reads the root session's native wave plan, SessionPlanWrite mutates it only through update_plan, and SessionContextRead reads the same root transcript. One native plan step is one Gambit wave; parallel workers are subagent threads inside that single step. These are backend operations, not literal shell commands.

Debugging — Root Cause Investigation

Overview

Debugging in gambit is investigation, not fixing. You reproduce the bug, gather evidence, trace to the true root cause, and write a test that proves you understand it. Then you hand that understanding to gambit:brainstorming, which designs the fix as an epic and runs it through the normal build pipeline.

Random fixes waste time and create new bugs. A fix for a symptom you don't understand is a guess wearing a confidence costume.

Core principle: Find the root cause with evidence before any fix is designed.

Iron Law: No fix designed without root-cause evidence AND a failing test that reproduces the bug. Think the fix is obvious? Prove it with evidence first. No exceptions.

Announce at start: "I'm using gambit:debugging to find the root cause."

Rigidity Level

MEDIUM FREEDOM — Follow the investigation sequence; adapt the techniques to the bug in front of you. Non-negotiable: evidence before hypothesis, a failing test before handoff, fix the source not the symptom.

Quick Reference

Step	Action	STOP If
1	Reproduce & gather evidence	Can't reproduce consistently
2	Investigate root cause	Still guessing (no evidence)
3	Write failing test that captures the bug	Test passes (doesn't catch it)
4	Hand root cause to brainstorming	3+ hypotheses already failed → question architecture

Terminal: root cause + failing test → gambit:brainstorming designs and executes the fix. This skill does NOT fix, classify, or close — it understands, then hands off.

When to Use

• Test failures, bugs, unexpected behavior, regressions, build failures, performance problems, flaky tests
• Especially under time pressure, after multiple failed fixes, or when "the fix seems obvious"

Don't use for: new features (gambit:brainstorming), refactoring (gambit:refactoring)

The Process

1. Reproduce & Gather Evidence

BEFORE forming any theory:

1. Read the error carefully — stack traces, line numbers, error codes often contain the answer
2. Reproduce consistently — if intermittent, add instrumentation; if you can't reproduce it, STOP and gather more data
3. Check recent changes — git log --oneline -10 and git diff HEAD~5..HEAD -- path/to/affected/code

Multi-component path? When the failing path crosses 3+ boundaries (CI → build → signing; handler → service → cache → DB), you can't reason out which component is broken from the error alone — instrument every boundary, run once, then investigate the layer the evidence points at. See references/root-cause-tracing.md.

2. Investigate Root Cause

Evidence before hypothesis. Use tools, not guessing.

• Search for context — WebSearch for error messages. For bounded codebase investigation, Glob **/codex-contracts/scout.md, dispatch the scout role using explorer (see codex-contracts/models.md), and prompt it to Read codex-contracts/scout.md first, then ask the question. The scout returns file:line evidence or NOT FOUND.
• Find a working neighbor and compare — most codebases contain a near-neighbor of the broken path (another caller of the same function, another feature using the same library). Comparing working-vs-broken is faster than pure tracing and catches "configured differently" bugs. List every difference, not just the ones that seem relevant — "that can't matter" is how real bugs hide. Read the working reference completely.
• Trace data flow backward — find where the bad value originates, not where it crashes. A null check at the crash site is a symptom fix; finding why the value is null and preventing it at the source is a root-cause fix. Full technique: references/root-cause-tracing.md.
• Form a hypothesis with evidence — "I think X is the root cause because Y." Evidence = stack trace showing the call path, log output showing state, code showing missing validation, or test output showing the failure mode. No evidence? STOP. Keep investigating.

Match the bug shape to a technique:

Bug shape	Technique
Flaky / timing-dependent test	references/condition-based-waiting.md
"Which test pollutes shared state?"	references/find-polluter.sh
Deep-stack bug, unclear origin	references/root-cause-tracing.md

3. Write the Failing Test (proof of understanding)

Write the smallest test that reproduces the bug. Run it — it MUST fail.

A failing test is how you prove you found the real bug instead of a plausible story. If it passes immediately, you don't understand the bug yet — return to step 2.

def test_rejects_empty_email():
    # Reproduces the reported bug: empty email slips through registration
    _, err = create_user(User(email=""))
    assert err is not None, "expected validation error for empty email"

This test is the artifact you hand to brainstorming — it pins down exactly what "fixed" means.

4. Hand Off to Brainstorming

You now hold the two things a good fix needs: the root cause (with evidence) and a failing test that captures it. That's the seed of the fix epic.

Default (almost always) — invoke gambit:brainstorming:

Invoke skill="$gambit:brainstorming"

Tell it the investigation is done so it designs the fix rather than re-investigating. Feed it:

• Root cause: the source, with evidence (file:line, the backward trace)
• Failing test: make this pass — that's the immutable success criterion
• Symptom location: mark "patch here" as a forbidden anti-pattern
• Consider defense-in-depth (references/defense-in-depth.md) so the whole bug class becomes structurally impossible, not just this instance

Brainstorming turns this into an epic and routes it through executing-plans → review like any other work — that's where the fix gets written, verified, and reviewed.

Fast path (one-liners only): if the root cause is a genuine one-line change and the failing test fully guards it, just make the change and verify (test passes + full suite green). Don't spin up an epic for a typo. Anything larger than a one-liner → brainstorming.

If 3+ hypotheses or fixes have already failed: STOP. Each fix revealing a new problem elsewhere, or fixes that need "massive refactoring," means your root-cause model is wrong or the architecture is. Question fundamentals with the user before handing a shaky diagnosis to brainstorming.

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Critical Rules

Rules That Have No Exceptions

1. Evidence before hypothesis → code path, logs, or test output showing WHY
2. Reproduce before investigating → can't reproduce → gather data, don't guess
3. Failing test before handoff → proves you caught the real bug, not a story
4. Trace to the SOURCE → fix where the bad value originates, not where it crashes
5. 3+ failed attempts → question architecture → stop and discuss with the user
6. Hand the root cause to brainstorming → don't rebuild a fix-and-close pipeline here

Common Excuses

All mean: STOP. Return to steps 1–2.

Excuse	Reality
"Issue is simple, don't need to investigate"	Simple issues have root causes too
"Emergency, no time to investigate"	Systematic is FASTER than thrashing
"Just try this fix first, then investigate"	First fix sets the pattern. Do it right.
"I'll write the test after I confirm the fix"	Untested fixes don't stick
"I see the problem, let me fix it"	Seeing symptoms ≠ understanding root cause
"One more fix attempt" (after 2+ failures)	3+ failures = architectural problem
"Obviously X is the cause"	"Obvious" causes are often wrong. Get evidence.

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Verification Checklist

• Bug reproduced consistently
• Root cause identified WITH evidence (not a plausible guess)
• Failing test reproduces the bug (RED)
• Root cause (not symptom) handed to brainstorming — or one-line fix verified green
• If 3+ attempts failed: architecture questioned with the user

Can't check these? Return to the process.

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References

• references/root-cause-tracing.md — backward tracing, stack instrumentation, multi-component boundary logging
• references/condition-based-waiting.md — fix flaky/timing tests by polling for the condition, not guessing a delay
• references/defense-in-depth.md — after root cause, validate at every layer so the bug class is impossible
• references/find-polluter.sh — bisect which test pollutes shared state
• REFERENCE.md — symptom-vs-root-cause and RED-GREEN worked examples

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Integration

This skill calls:

• Contracted scout agents through the active backend's native dispatch for bounded independent codebase investigation
• WebSearch for error-message research
• gambit:brainstorming (terminal — designs and executes the fix from the root cause)

Called by:

• A bug discovered during development, a failing test, a user-reported bug

Workflow:

Bug → reproduce → evidence → root cause → failing test → gambit:brainstorming → (epic → executing-plans → review)