<!-- PM-Skills | https://github.com/product-on-purpose/pm-skills | Apache 2.0 -->
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:
Examples
See references/EXAMPLE.md for a completed example.
