Diagnose Bug

Use this skill to diagnose backend and algorithmic failures from evidence inward: reproduce with concrete inputs, establish the intended invariant or contract, correlate runtime signals, then inspect code. Keep it globally usable; discover the project's actual runtimes, test harnesses, and observability instead of assuming a stack.

This skill is for non-UI surfaces: APIs, services, CLIs, libraries, workers/queues, ETL/pipelines, compilers/analyzers, and algorithms (correctness, complexity, numeric stability). This is quality regime A (deterministic compute).

• Live browser/product UI failures → troubleshoot-app (regime B)
• Agent loops / stochastic LLM quality → agents analyze (+ Langfuse traces/evals), not this skill as the primary cut (regime C)

Use existing specs, types, contracts, property tests, unit/integration tests, DocSlime/TESTING, and docs as the definition of correct behavior (handbook/concepts/13-quality-trace.md). Prefer refining an existing definition over inventing a parallel one. If none fits, propose a concise contract or Given/When/Then scenario and say where it should live. Wrong answers, broken invariants, and silent data lies are bugs — including data/test/observability debt labels when those name the interest (handbook/concepts/12-bugs-and-debt.md).

Operating contract

Shared by every DecisionNerd/dev-skills skill. Canonical copy: handbook/concepts/14-operating-contract.md.