Edge Case Discovery

Edge Case Discovery

Systematic 5-category analysis for exhaustive edge case enumeration.

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MANDATORY FIRST STEP

CREATE TodoWrite with these 5 categories (15+ items total):

Do not proceed to implementation or testing until TodoWrite is verified.

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Verification Checkpoint

After creating TodoWrite, verify 3 random items pass this test:

Each item must have ALL THREE:

• ✓ Specific input/​condition ("empty string", "null value", "overflow at 2^31")
• ✓ Expected behavior ("returns error", "throws exception", "defaults to X")
• ✓ Test scenario ("when user submits form with...", "if API returns...")

DO NOT PROCEED until 15+ items AND quality check passes.

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Framework

1. Boundary Value Analysis

Test min/​max/​zero/​null/​empty/​overflow for ALL inputs:

• Numeric: min, max, zero, negative, overflow
• Strings: empty, null, very long, special chars, encoding
• Collections: empty, single item, max size, duplicates
• Time: past, future, now, leap years, timezones

2. Equivalence Partitioning

Group inputs into classes, test one representative from each:

• Valid inputs (happy path)
• Invalid inputs (expected errors)
• Boundary inputs (edge of valid/​invalid)

3. State Transition Analysis

Map ALL valid + invalid state transitions:

• Valid transitions
• Invalid transitions (should reject)
• Edge states (init, cleanup)
• Concurrent access scenarios

4. Error Condition Enumeration

List ALL failure modes for each operation:

• Network failures, timeouts
• Resource exhaustion (memory, disk, connections)
• Concurrent access conflicts
• Partial failures, external dependency failures

5. Assumption Challenging

Question every "always", "never", "impossible":

• "This will always be valid" → What if not?
• "Users won't do that" → What if they do?
• "This is impossible" → Under what conditions could it happen?

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Red Flags - STOP When You Think:

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Response Templates

"Just test the happy path"

❌ BLOCKED: Happy path testing leaves production vulnerable to edge case failures.

Required to override:

1. Documented risk acceptance from technical lead
2. Explicit scope limitation with rationale ("internal tool with controlled inputs")
3. Post-release budget allocated for edge case fixes (2-4 engineer-weeks)
4. Monitoring/​alerting plan for unexpected edge cases in production

Reality check:

• 60% of production incidents stem from unhandled edge cases
• Average incident costs: $5-50K in lost productivity, reputation damage
• Systematic edge case analysis: 30-60 minutes upfront

"We don't have time for this"

❌ BLOCKED: Edge case discovery is NOT optional for production code.

Time investment:

• TodoWrite creation: 10-15 minutes
• Analysis per category: 5-10 minutes (25-50 minutes total)
• Total: 35-65 minutes

Compared to:

• One production incident investigation: 4-8 hours
• Hotfix deployment: 2-4 hours
• Customer impact/​reputation damage: immeasurable

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Verification Before Complete

After completing TodoWrite and analysis, verify:

If any category incomplete, do not proceed to implementation.

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Output

Edge case catalog with test scenarios for each category.