Logic-Lens — Fault Locate

When to Use

Use this skill when you need locate the root cause of a CONFIRMED failure via backward-then-forward semi-formal tracing. Trigger when the user provides a stack trace, failing assertion, error message, or specific wrong-value observation — "find the bug", "this test is failing", "track down this crash", "why is...

Setup

Use lazy loading per ../_shared/common.md §13:

1. Read ../_shared/common.md only for language, Iron Law, Fault Confidence, scope routing, Remedy discipline, config fields, and loading budget.
2. Read only the relevant step in logic-locate-guide.md as you reach it.
3. Load ../_shared/logic-risks.md, ../_shared/semiformal-guide.md, ../_shared/semiformal-checklist.md, and ../_shared/report-template.md on demand when the current step needs them.

Process

Step 0. Language + scope routing. Detect language per common.md §1. Confirm a concrete failure exists (stack trace, failing assertion, specific wrong value). If only a suspicion, switch to logic-review.

Step 1. Understand the failure (guide Step 1) — observed behavior, expected behavior, reproduction path.

Step 2. Identify the entry point (guide Step 2) — failing test, outermost application frame, or request handler — whichever is closest to the failure. Stay inside the failure cone first: stack frames, failing test fixture, directly called local functions, and config/env values read on that path. Do not scan unrelated modules unless the trace crosses into them.

Step 3. Trace backward from the failure point (guide Step 3) — walk each value and state back to its origin, building premises at every hop.

Step 4. Trace forward to confirm (guide Step 4) — from the suspected root, verify the trace reaches the observed symptom.

Step 5. Interprocedural tracing if a callee is implicated (guide Step 5) — trace into the callee; check return values under observed conditions, unhandled exceptions, shared-state mutation. Apply the depth limit and Call-Chain Context Label format defined in semiformal-guide.md §Call-Chain Context Labels; at the limit, state the remaining callee path as a premise assumption and downgrade to Medium confidence (per common.md §7).

Step 6. Identify the root divergence and classify (guide Step 6) — state the exact line/expression, the violated premise, the actual behavior, the propagation chain to the symptom; pick the L-code.

Step 7. Output the focused report (guide Step 7) — Fault Confidence (High/Medium/Low, per common.md §7); Primary Fault (single five-field finding); optionally Contributing Factors; a minimal Remedy per common.md §10. Format is mandatory even for simple one-function bugs: always emit the labeled Premises / Trace / Divergence / Trigger / Remedy fields and the Fault Confidence line. Never answer with a plain fix suggestion.

Mode line in report: Fault Locate (Chinese: 故障定位).

Output format: the Findings section has ONE Primary Fault, not a full Critical/Warning/Suggestion split. The Logic Score line is replaced by Fault Confidence: High / Medium / Low.

Limitations

• Use this skill only when the task clearly matches its upstream source and local project context.
• Verify commands, generated code, dependencies, credentials, and external service behavior before applying changes.
• Do not treat examples as a substitute for environment-specific tests, security review, or user approval for destructive or costly actions.