Debug complex issues using competing hypotheses with parallel investigation, evidence collection, and root cause arbitration. Use this skill when debugging bugs with multiple potential causes, performing root cause analysis, or organizing parallel investigation workflows.

Parallel Debugging Framework for debugging complex issues using the Analysis of Competing Hypotheses (ACH) methodology with parallel agent investigation. When to Use This Skill Bug has multiple plausible root causes Initial debugging attempts haven't identified the issue Issue spans multiple modules or components Need systematic root cause analysis with evidence Want to avoid confirmation bias in debugging Hypothesis Generation Framework Generate hypotheses across 6 failure mode categories: 1. Logic Error Incorrect conditional logic (wrong operator, missing case) Off by one errors in loops or array access Missing edge case handling Incorrect algorithm implementation 2. Data Issue Invalid or unexpected input data Type mismatch or coercion error Null/undefined/None where value expected Encoding or serialization problem Data truncation or overflow 3. State Problem Race condition between concurrent operations Stale cache returning outdated data Incorrect initialization or default values Unintended mutation of shared state State machine transition error 4. Integration Failure API contract violation (request/response mismatch) Version incompatibility between components Configuration mismatch between environments Missing or incorrect environment variables Network timeout or connection failure 5. Resource Issue Memory leak causing gradual degradation Connection pool exhaustion File descriptor or handle leak Disk space or quota exceeded CPU saturation from inefficient processing 6. Environment Missing runtime dependency Wrong library or framework version Platform specific behavior difference Permission or access control issue Timezone or locale related behavior Evidence Collection Standards What Constitutes Evidence Evidence Type Strength Example Direct Strong Code at file.ts:42 shows if (x 0) should be if (x = 0) Correlational Medium Error rate increased after commit abc123 Testimonial Weak "It works on my machine" Absence Variable No null check found in the code path Citation Format Always cite evidence with file:line references: Confidence Levels Level Criteria High ( 80%) Multiple direct evidence pieces, clear causal chain, no contradicting evidence Medium (50 80%) Some direct evidence, plausible causal chain, minor ambiguities Low (<50%) Mostly correlational evidence, incomplete causal chain, some contradicting evidence Result Arbitration Protocol After all investigators report: Step 1: Categorize Results Confirmed : High confidence, strong evidence, clear causal chain Plausible : Medium confidence, some evidence, reasonable causal chain Falsified : Evidence contradicts the hypothesis Inconclusive : Insufficient evidence to confirm or falsify Step 2: Compare Confirmed Hypotheses If multiple hypotheses are confirmed, rank by: 1. Confidence level 2. Number of supporting evidence pieces 3. Strength of causal chain 4. Absence of contradicting evidence Step 3: Determine Root Cause If one hypothesis clearly dominates: declare as root cause If multiple hypotheses are equally likely: may be compound issue (multiple contributing causes) If no hypotheses confirmed: generate new hypotheses based on evidence gathered Step 4: Validate Fix Before declaring the bug fixed: [ ] Fix addresses the identified root cause [ ] Fix doesn't introduce new issues [ ] Original reproduction case no longer fails [ ] Related edge cases are covered [ ] Relevant tests are added or updated

parallel-debugging