# Policy Engine 30-001 Prep — Overlay Projection

Status: **Ready for implementation** (2025-11-22)
Owners: Policy Guild · Cartographer Guild
Scope: Freeze overlay projection contract needed for Policy Engine phase III.

## Inputs
- Path/scope schema: `docs/modules/policy/prep/2025-11-20-policy-engine-29-002-prep.md`.
- Metrics/log schema: `docs/modules/policy/prep/2025-11-20-policy-engine-29-004-prep.md`.

## Overlay Projection Contract
- Input: evaluation records `{tenantId, policyId, pathScope[], evidenceDigest, decision, reasons[]}`.
- Output overlay entry:
  - `overlayId` (deterministic hash of `tenantId+policyId+pathScope+evidenceDigest`)
  - `subject` (`purl` or `pkg` tuple), `pathScope`, `decision`, `rationale`, `inputs` (evidence digests), `policyVersion`, `createdAt`.
- Determinism: pathScope sorted; reasons sorted by `code`; timestamps set to evaluation clock.
- Export shape for downstream components via `/overlays/{overlayId}` and `/overlays/search` with paging.

## Acceptance Criteria
- Overlay schema above referenced in Sprint 125 and 0125 trackers; downstream simulation/bridge tasks can consume it without further schema.
- Any future field additions require version bump `overlaySchemaVersion`.

## Notes
- Completes PREP-POLICY-ENGINE-30-001-WAITING-ON-29-004-M.