dvy1987 Fault Localize
You find the earliest decisive divergence in a trace, hypothesize cause from evidence, apply a targeted repair, replay from that point, and verify an outcome flip. No vague retries.
Hard Rules
Require a trace file (.agent-loom/traces/<run-id>.jsonl) or reconstruct from session tool history.
Follow references/LOCALIZATION.md — earliest error/divergence wins.
Hypothesis must cite output_ref or error from the trace record.
Replay from suspected step — not full blind restart unless plan-layer fault.
Record attribution with outcome_flip boolean.
Route code defects to debug-and-fix; plan faults to dynamic-routing.
Workflow
Step 1 — Load trace
Run trace_query.py timeline and errors. If no trace, build minimal timeline from session history.
Step 2 — Identify suspected step
Earliest record with error or output contradicting expected. State suspected_step_id.
Step 3 — Write hypothesis
Three lines max — cite evidence. Classify layer.
Step 4 — Propose targeted repair
One intervention matched to layer. Not a kitchen-sink fix.
Step 5 — Replay and verify
Re-run from suspected step. Compare outcome.
Step 6 — Attribute
Write attribution record. If no flip, consider next candidate step.
Gotchas
- Latest error ≠ earliest cause — always scan chronologically.
- Fixing symptoms downstream masks upstream plan errors.
- Flaky tests need two replays before attributing.
Output Format
## Fault localization — [run_id]
Suspected step: **Sx** @ [ts]
Hypothesis: [evidence-cited]
Layer: [tooling|code|plan|environment]
Proposed repair: [one intervention]
Replay result: [pass|fail]
Outcome flip: [true|false]
Attribution: `.agent-loom/traces/[run_id]-attribution.json`
Next: [resume plan | debug-and-fix | escalate]
Examples
Teaser: Trace shows S1 pass, S2 npm test fail → hypothesize missing env → repair .env.example → replay S2 → flip true.
Full pairs: references/examples.md
Common Rationalizations
| Excuse | Reality |
|---|---|
| "Last error is the cause" | Downstream errors are often symptoms. |
| "Retry whole run" | Expensive; masks localized fix opportunity. |
| "Guess and patch" | Hypothesis without evidence → wrong attribution. |
| "Trace is optional" | Localization without trace is reconstruction guesswork. |
| "One flip proves root cause" | Flaky tests need confirmatory replay. |
Verification
- Suspected step is earliest divergence
- Hypothesis cites trace evidence
- Attribution record written
- Layer-appropriate handoff if not fixed
Red Flags
- Hypothesis with no trace citation
- Full rerun without identifying suspected step
- outcome_flip true on single flaky test
Prune Log
Last pruned: 2026-07-05
- Initial release from high-leverage skill spec (Skill 4 family)
Impact Report
Localize: [run_id] | Suspected: [step] | Flip: [bool] | Layer: [layer]