---
id: wiki-2026-0508-api-contract-definition
title: API Contract Definition
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [API Contract, OpenAPI Contract, Schema-First API, Contract-First Design]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [api, openapi, contract, schema, design]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: yaml
  framework: openapi-3.1
---

# API Contract Definition

## 매 한 줄
> **"매 API 의 shape (path, method, schema, error, auth) 을 매 machine-readable spec 으로 매 먼저 정의하고 매 그 후 server/client 가 매 그것을 따른다."** OpenAPI 3.1 (REST), gRPC `.proto`, GraphQL SDL, AsyncAPI (event), JSON Schema 가 매 dominant. 2026 현재 매 LLM 이 매 spec 을 읽어 매 client / mock / test 를 매 generate — 매 contract-first 가 매 AI 시대의 매 default.

## 매 핵심

### 매 contract-first vs code-first
- **Contract-first**: 매 spec → 매 server stub + 매 client SDK. 매 multi-team / multi-language 의 default.
- **Code-first**: 매 code → 매 spec generated (FastAPI, NestJS Swagger). 매 single team 에서 매 빠름.
- 2026 추세: 매 contract-first 가 매 LLM-friendly + 매 mock-driven dev 에 유리.

### 매 standard 별 사용처
- **OpenAPI 3.1**: 매 REST/HTTP. 매 JSON Schema 2020-12 align.
- **gRPC + protobuf**: 매 internal high-perf, 매 strong typing.
- **GraphQL SDL**: 매 client-driven query, 매 BFF.
- **AsyncAPI 3**: 매 Kafka / NATS / WebSocket event.
- **JSON Schema**: 매 payload validation, 매 LLM structured output.

### 매 contract testing
- **Provider tests**: 매 server 가 매 spec 을 만족하는지 검증 (매 schemathesis, dredd, prism).
- **Consumer-driven contracts**: 매 Pact — 매 client 가 매 expectation 을 매 publish, 매 server 가 매 verify.

### 매 응용
1. SDK auto-generation (Stainless, Speakeasy, OpenAPI Generator).
2. Mock server (Prism, Mockoon).
3. Fuzz / property test (Schemathesis).

## 💻 패턴

### 1) OpenAPI 3.1 minimal
```yaml
openapi: 3.1.0
info: { title: Orders API, version: 1.2.0 }
servers: [{ url: https://api.example.com/v1 }]
paths:
  /orders/{id}:
    get:
      operationId: getOrder
      parameters:
        - { name: id, in: path, required: true, schema: { type: string, format: uuid } }
      responses:
        '200':
          content: { application/json: { schema: { $ref: '#/components/schemas/Order' } } }
        '404': { description: Not found }
components:
  schemas:
    Order:
      type: object
      required: [id, total]
      properties:
        id:    { type: string, format: uuid }
        total: { type: number, minimum: 0 }
        status: { type: string, enum: [pending, paid, shipped, canceled] }
```

### 2) gRPC proto
```proto
syntax = "proto3";
package orders.v1;

service Orders {
  rpc Get (GetReq) returns (Order);
  rpc Stream (StreamReq) returns (stream Event);
}
message GetReq  { string id = 1; }
message Order   { string id = 1; double total = 2; Status status = 3; }
enum Status { PENDING = 0; PAID = 1; SHIPPED = 2; CANCELED = 3; }
```

### 3) GraphQL SDL
```graphql
type Order {
  id: ID!
  total: Float!
  status: Status!
}
enum Status { PENDING PAID SHIPPED CANCELED }
type Query { order(id: ID!): Order }
```

### 4) AsyncAPI 3 (Kafka event)
```yaml
asyncapi: 3.0.0
info: { title: Orders Events, version: 1.0.0 }
channels:
  orders.created:
    address: orders.created
    messages:
      OrderCreated:
        payload: { $ref: '#/components/schemas/Order' }
operations:
  publishOrderCreated:
    action: send
    channel: { $ref: '#/channels/orders.created' }
```

### 5) Server stub generation
```bash
# 매 OpenAPI → TypeScript Express
npx openapi-generator-cli generate -i openapi.yaml -g typescript-node -o ./gen
# 매 OpenAPI → Python (FastAPI server)
npx openapi-typescript openapi.yaml -o gen/types.ts
```

### 6) Schemathesis property-based test
```bash
schemathesis run openapi.yaml --base-url=http://localhost:8000 \
  --checks=all --hypothesis-deadline=2000
```

### 7) Pact consumer test (TS)
```ts
import { Pact } from '@pact-foundation/pact';
const provider = new Pact({ consumer: 'web', provider: 'orders' });
await provider.addInteraction({
  state: 'order 1 exists',
  uponReceiving: 'a get for order 1',
  withRequest: { method: 'GET', path: '/v1/orders/1' },
  willRespondWith: { status: 200, body: { id: '1', total: 99 } }
});
```

### 8) JSON Schema for LLM structured output
```python
order_schema = {
  "type": "object",
  "required": ["id", "total"],
  "properties": {"id":{"type":"string"},"total":{"type":"number"}}
}
client.messages.create(
  model="claude-opus-4-7",
  tools=[{"name":"return_order","input_schema":order_schema}],
  tool_choice={"type":"tool","name":"return_order"},
  messages=[...]
)
```

## 매 결정 기준
| 상황 | Standard |
|---|---|
| 매 public REST | OpenAPI 3.1 |
| 매 internal microservices | gRPC + protobuf |
| 매 client-shaped query | GraphQL |
| 매 event-driven | AsyncAPI 3 |
| 매 LLM structured output | JSON Schema |

**기본값**: 매 contract-first + 매 OpenAPI 3.1 + 매 SDK auto-gen + 매 schemathesis CI. 매 internal 은 gRPC.

## 🔗 Graph
- 부모: [[API Design]]
- 변형: [[OpenAPI]] · [[gRPC]]
- 응용: [[Contract Testing]]
- Adjacent: [[JSON Schema]] · [[Pact]]

## 🤖 LLM 활용
**언제**: 매 spec → SDK / mock / test scaffold 자동 생성, 매 spec lint, 매 changelog diff.
**언제 X**: 매 LLM 이 매 spec 을 매 invent — 매 source-of-truth 는 매 git-tracked spec file.

## ❌ 안티패턴
- **Spec drift**: 매 server 가 매 spec 과 매 다름. 매 schemathesis CI 로 차단.
- **No versioning**: 매 breaking change 를 매 same path 에 push. 매 `/v1` → `/v2` 또는 매 deprecated header.
- **Anyof everywhere**: 매 spec 이 매 너무 permissive → 매 client 의 매 type narrowing 불가.
- **Code-first without lint**: 매 generated spec 이 매 inconsistent (매 nullable, 매 enum).
- **Mock-only test**: 매 contract test 없이 매 mock 만 사용 → 매 prod fail.

## 🧪 검증 / 중복
- Verified (OpenAPI 3.1 spec, AsyncAPI 3.0 spec, Pact docs, Schemathesis docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — OpenAPI/gRPC/GraphQL/AsyncAPI 4-standard + contract testing |