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Stale cache returning outdated data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:

**Evidence**: The validation function at `src/validators/user.ts:87`
does not check for empty strings, only null/undefined. This allows
empty email addresses to pass validation.

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:

Confidence level
Number of supporting evidence pieces
Strength of causal chain
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