Edge Case Analysis

Overview

This skill defines how to systematically identify and test edge cases - unusual conditions, extreme values, or unexpected combinations that can cause failures. Finding edge cases early prevents production bugs and improves robustness.

When to Use

• When designing functions, APIs, or user inputs
• Before delivering any solution that handles variable input
• When reviewing code or specifications for completeness
• When creating test plans
• Don't use when: Solution only handles fixed, known, single value

Core Procedures

Step 1: Identify Dimensions

For each input, parameter, or condition, identify its dimensions:

• Range: Minimum to maximum values
• Type: String, number, list, object, null, undefined
• Length: Empty, single, typical, maximum
• Encoding: Unicode, special characters, binary, emojis
• State: New, existing, deleted, locked, expired
• Access: Owner, admin, guest, anonymous, unauthorized

Step 2: Apply Analysis Techniques

For each dimension:

BOUNDARY VALUES: Test exact boundaries and just outside

• Min-1, Min, Min+1, Typical, Max-1, Max, Max+1
• Zero, one, many, too many

COMBINATIONS: Test pairs of conditions

• Valid input + invalid state
• Normal operation at boundary conditions
• Multiple edge cases occurring simultaneously

NEGATIVE TESTING: Test what should NOT work

• Invalid types for each parameter
• Missing required fields
• Unauthorized access attempts
• Malformed input (truncated, corrupted)

TEMPORAL: Test time-based edge cases

• Midnight, end of month, leap day, DST changes
• Timezone conversions, expired data
• Race conditions, concurrent operations
• Rapid succession of same operation

Step 3: Document and Prioritize

EDGE CASE
=========
Description: [what condition could cause a problem]
Category: [boundary/combination/negative/temporal/state]
Likelihood: [RARE/UNLIKELY/POSSIBLE/LIKELY]
Impact: [LOW/MEDIUM/HIGH/CRITICAL]
Test: [how to validate this edge case]
Status: [OPEN / TESTED PASS / TESTED FAIL / FIXED]

Step 4: Validate Fixes

For each edge case that fails:

• Understand WHY it fails (root cause)
• Fix the root cause, not just the symptom
• Re-test the edge case
• Add to regression test suite

Quality Checklist

• All input dimensions analyzed
• Boundary values identified for each dimension
• Negative testing scenarios defined
• Temporal edge cases considered (if applicable)
• High-impact edge cases added to test suite
• Root cause identified for any failing edge cases

Error Handling

• Error: Too many edge cases to test all Response: Prioritize by Impact × Likelihood, test HIGH and CRITICAL first
• Error: Edge case requires conditions hard to reproduce Response: Simulate or mock the condition, document in automated test

Cross-Team Integration

Related Skills: testing-strategy, test-driven-development, testing-verification, output-validation-checklist Used By: Testers, developers, QA agents, validators, especially Performance, Load, Security testing agents