Deliver Edge Cases

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Edge Cases

An edge cases document systematically catalogs the unusual, boundary, and error scenarios for a feature. While happy-path flows are typically well-specified, edge cases often get discovered in production - causing bugs, poor user experience, and support burden. Documenting edge cases upfront ensures engineering handles them intentionally and QA knows what to test.

When to Use

• When you need to enumerate failure modes, race conditions, timeouts, and boundary or limit scenarios - everything that can go wrong - and define a recovery path for each
• During feature specification before engineering begins
• When preparing QA test plans
• After discovering production bugs to prevent similar issues
• When reviewing PRDs or user stories for completeness
• Before launch to ensure error states have been designed

When NOT to Use

• You need story-scoped Given/When/Then checks for handoff -> use deliver-acceptance-criteria; this skill catalogs the whole feature's failure surface
• The feature is not specified enough to enumerate inputs, states, and limits -> use deliver-prd first
• A production incident already happened and you want the learning banked -> use iterate-lessons-log, then update this catalog with the new case
• You need readiness coordination for a launch, not failure analysis -> use deliver-launch-checklist

Instructions

When asked to document edge cases, follow these steps:

• Define the Feature Scope

Clearly describe what feature or flow you're analyzing. Edge cases are specific to context - the same input might be valid in one feature and invalid in another.

• Walk Through Input Validation

Consider every user input: What if it's empty? Too long? Wrong format? Contains special characters? What are the minimum and maximum valid values?

• Explore Boundary Conditions

Find the edges of acceptable ranges. If a field accepts 1-100, test 0, 1, 100, and 101. Consider pagination boundaries, timeout thresholds, and rate limits.

• Map Error States

Identify what can go wrong: network failures, permission denied, resource not found, concurrent modifications, expired sessions. Document both the scenario and expected behavior.

• Consider Concurrency Issues

What if two users act simultaneously? What if the user double-clicks? What if data changes between load and save? Race conditions often cause subtle bugs.

• Define Recovery Paths

For each error, specify how users recover. What message do they see? Can they retry? Is data preserved? Good error handling turns frustration into confidence.

• Prioritize by Likelihood and Impact

Not all edge cases need the same attention. High-likelihood + high-impact cases need robust handling; rare + low-impact cases might just need graceful failure.

Output Format

Use the template in references/TEMPLATE.md to structure the output. A complete edge-case catalog fills every template section: Feature Overview; Edge Case Categories; Error Messages; Recovery Paths; and Test Scenarios.

Quality Checklist

Before finalizing, verify:

• [ ] All user inputs have validation edge cases documented
• [ ] Boundary conditions are explicitly listed
• [ ] Network/system failure scenarios are covered
• [ ] Each error state has a defined user-facing message
• [ ] Recovery paths are specified (not just error detection)
• [ ] Edge cases are prioritized by likelihood and impact

Examples

See references/EXAMPLE.md for a completed example.