Edge Case Discovery

Systematic 5-category analysis for exhaustive edge case enumeration.

MANDATORY FIRST STEP

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

Category	Minimum Items
Boundary Value Analysis	3+
Equivalence Partitioning	3+
State Transition Analysis	3+
Error Condition Enumeration	3+
Assumption Challenging	3+

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

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...")

❌ FAILS	✅ PASSES
"Test boundaries"	"Test numeric input: min (-2^31), max (2^31-1), zero, negative (-1), overflow (2^31)"
"Check errors"	"Network timeout after 30s: retry 3x with exponential backoff, return error 503 to client"
"Test state changes"	"User logout during file upload: cancel upload, clean temp files, redirect to login"

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

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?

Red Flags - STOP When You Think:

Thought	Reality
"Main path is sufficient"	Edge cases cause 60% of production bugs - exhaustive analysis prevents incidents
"This is overkill for a simple feature"	15 items takes 10 minutes, prevents hours of debugging and hotfixes
"We'll catch issues in testing"	Manual testing misses 70% of edge cases - systematic enumeration is essential
"Users won't do that"	Users do unexpected things constantly - assume nothing about behavior
"This analysis takes too long"	Post-release bug fixes cost 10-100x more than upfront analysis

Response Templates

"Just test the happy path"

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

Required to override:

Documented risk acceptance from technical lead
Explicit scope limitation with rationale ("internal tool with controlled inputs")
Post-release budget allocated for edge case fixes (2-4 engineer-weeks)
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

Verification Before Complete

After completing TodoWrite and analysis, verify:

Category	Requirements
Boundary Value	✓ Min/max/zero/null tested ✓ Overflow scenarios identified ✓ Empty collections handled
Equivalence	✓ Valid inputs ✓ Invalid inputs ✓ Boundary cases
State Transition	✓ Valid transitions ✓ Invalid transitions ✓ Concurrent access
Error Conditions	✓ Network failures ✓ Resource exhaustion ✓ Partial failures
Assumptions	✓ All "always"/"never" challenged ✓ Impossible scenarios explored

If any category incomplete, do not proceed to implementation.

Output

Edge case catalog with test scenarios for each category.

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