Module Regression

# Module Regression Ledger

## When to Activate

- The repository has at least three interdependent modules, or a previous change to A has broken B.
- A change modifies a module's externally observable behavior and must prove both that module and downstream consumers still work.
- The project already has executable tests or reconciliation commands that should be organized by dependency.

## Anti-Patterns

- Creating a regression ledger for a small repository with no cross-module dependencies.
- Guessing dependencies manually or treating one scan as permanent truth.
- Replacing executable acceptance-command exit codes with model review.

## Related Skills

- `test-collaboration`: TEST-IDs, test assets, and defect-regression evidence.
- `ai-regression-testing`: executable regression tests derived from important defects.
- `contract-first`: cross-service or cross-client contract verification.
- `change-impact`: pre- and post-implementation blast-radius analysis.

## Problem

In large projects, modules depend on one another. People and agents often verify only the changed module while downstream consumers fail silently. The defect appears days later when an aggregate, export, or external behavior is wrong.

The remedy is a **regression ledger** (`REGRESSION.md`) plus a deterministic audit: after any relevant change, run the changed module and all affected downstream consumers. Delivery requires every command to pass.

## Three Required Fields per Module