# API Contract Template

A contract is the **frozen, reviewed interface** between two or more components. When task A produces a shared model, DTO, schema, event payload, or public API, and task B consumes it, they must not reverse-engineer each other's implementation — they must read the contract.

Save the filled contract at `_docs/02_document/contracts/<component>/<name>.md`. Reference it from the producing task's `## Contract` section and from every consuming task's `## Dependencies` section.

---

```markdown
# Contract: [contract-name]

**Component**: [component-name]
**Producer task**: [TRACKER-ID] — [task filename]
**Consumer tasks**: [list of TRACKER-IDs or "TBD at decompose time"]
**Version**: 1.0.0
**Status**: [draft | frozen | deprecated]
**Last Updated**: [YYYY-MM-DD]

## Purpose

Short statement of what this contract represents and why it is shared (1–3 sentences).

## Shape

Choose ONE of the following shape forms per the contract type:

### For data models (DTO / schema / event)

```[language]
// language-native type definitions — e.g., Python dataclass, C# record, TypeScript interface, Rust struct, JSON Schema
```

For each field:

| Field | Type | Required | Description | Constraints |
|-------|------|----------|-------------|-------------|
| `id` | `string` (UUID) | yes | Unique identifier | RFC 4122 v4 |
| `created_at` | `datetime` (ISO 8601 UTC) | yes | Creation timestamp | |
| `...` | ... | ... | ... | ... |

### For function / method APIs

| Name | Signature | Throws / Errors | Blocking? |
|------|-----------|-----------------|-----------|
| `do_x` | `(input: InputDto) -> Result<OutputDto, XError>` | `XError::NotFound`, `XError::Invalid` | sync |
| ... | ... | ... | ... |

### For HTTP / RPC endpoints

| Method | Path | Request body | Response | Status codes |
|--------|------|--------------|----------|--------------|
| `POST` | `/api/v1/resource` | `CreateResource` | `Resource` | 201, 400, 409 |
| ... | ... | ... | ... | ... |

## Invariants

Properties that MUST hold for every valid instance or every allowed interaction. These survive refactors.

- Invariant 1: [statement]
- Invariant 2: [statement]

## Non-Goals

Things this contract intentionally does NOT cover. Helps prevent scope creep.

- Not covered: [statement]

## Versioning Rules

- **Breaking changes** (field renamed/removed, type changed, required→optional flipped) require a new major version and a deprecation path for consumers.
- **Non-breaking additions** (new optional field, new error variant consumers already tolerate) require a minor version bump.

## Test Cases

Representative cases that both producer and consumer tests must cover. Keep short — this is the contract test surface, not an exhaustive suite.

| Case | Input | Expected | Notes |
|------|-------|----------|-------|
| valid-minimal | minimal valid instance | accepted | |
| invalid-missing-required | missing `id` | rejected with specific error | |
| edge-case-x | ... | ... | |

## Change Log

| Version | Date | Change | Author |
|---------|------|--------|--------|
| 1.0.0 | YYYY-MM-DD | Initial contract | [agent/user] |
```

---

## Decompose-skill rules for emitting contracts

A task is a **shared-models / shared-API task** when ANY of the following is true:

- The component spec lists it as a shared component (under `shared/*` in `module-layout.md`).
- The task's **Scope.Included** mentions any of: "public interface", "DTO", "schema", "event", "contract", "API endpoint", "shared model".
- The task is parented to a cross-cutting epic (`epic_type: cross-cutting`).
- The task is depended on by ≥2 other tasks across different components.

For every shared-models / shared-API task:

1. Create a contract file at `_docs/02_document/contracts/<component>/<name>.md` using this template.
2. Fill in Shape, Invariants, Non-Goals, Versioning Rules, and at least 3 Test Cases.
3. Add a mandatory `## Contract` section to the task spec that links to the contract file:

   ```markdown
   ## Contract

   This task produces/implements the contract at `_docs/02_document/contracts/<component>/<name>.md`.
   Consumers MUST read that file — not this task spec — to discover the interface.
   ```

4. For every consuming task, add the contract path to its `## Dependencies` section as a document dependency (not a task dependency):

   ```markdown
   ### Document Dependencies
   - `_docs/02_document/contracts/<component>/<name>.md` — API contract produced by [TRACKER-ID].
   ```

5. If the contract changes after it was frozen, the producer task must bump the `Version` and note the change in `Change Log`. Consumers referenced in the contract header must be notified (surface to user via Choose format).

## Code-review-skill rules for verifying contracts

Phase 2 (Spec Compliance) adds a check:

- For every task with a `## Contract` section:
  - Verify the referenced contract file exists at the stated path.
  - Verify the implementation's public signatures (types, method shapes, endpoint paths) match the contract's Shape section.
  - If they diverge, emit a `Spec-Gap` finding with High severity.
- For every consuming task's Document Dependencies that reference a contract:
  - Verify the consumer's imports / calls match the contract's Shape.
  - If they diverge, emit a `Spec-Gap` finding with High severity and a hint that either the contract or the consumer is drifting.
