---

name: regression-and-parity-check description: Validates that changes maintain parity with previous system behavior and don't introduce regressions. Use after implementation or migration. triggers: [pre-release, post-task-check, migration-parity-test] outputs: [parity-report, regression-log]

Regression and Parity Check

Purpose

Ensures that the new solution is at least as good as the old one. In migrations, it guarantees functional parity; in new features, it ensures existing paths remain unaffected.

When to use this skill

• During the final stage of feature implementation
• Before cutover in a migration project
• When refactoring critical path logic

Verification Steps

1. Compare Behavior, Not Code: Focus on the observable output and side effects defined in the behavior-catalog.
2. Identify Mismatches: Flag any difference in status codes, data formats, or side-effect ordering.
3. Classify Deviations:
   • Bug: Unintended change. (Fix needed)
   • Spec Gap: Implementation is better/different but spec wasn't updated. (Update spec)
   • Accepted Deviation: Intentional change due to new constraints. (Document)
4. Enforce Parity: Rejection is the default for any unclassified mismatch.

Decision Tree

Copyflowchart TD
    A[Start Comparison] --> B{Parity Observed?}
    B -->|Yes| C[Check Performance Parity]
    B -->|No| D{Intentional Change?}
    D -->|No| E[Reject: Regression Detected]
    D -->|Yes| F{Approved in Spec?}
    F -->|No| G[Hold: Update Spec First]
    F -->|Yes| H[Approve: Documented Deviation]
    C --> I[Final Approval]

Review Checklist

1. Input Parity: Does the system handle all legacy input formats?
2. Error Parity: Are error messages and codes consistent (unless intentionally improved)?
3. Side-Effect Parity: Are emails/logs/DB updates still happening as expected?
4. Performance Parity: Is the new system within 10% of legacy latency?

How to provide feedback

• Be specific: "The parity check failed on 'Order Cancel'; old system returned 204, new returns 200."
• Explain why: "Client libraries may depend on the specific 204 No Content status."
• Suggest alternatives: "Update the controller to return a 204 for cancel operations to maintain parity."

Parity is success.