API Versioning Patterns

Evolve your API confidently. Version correctly, deprecate gracefully, migrate safely — without breaking existing consumers.

Versioning Strategies

Pick one strategy and apply it consistently across your entire API surface.

Strategy	Format	Visibility	Cacheability	Best For
URL Path	/api/v1/users	High	Excellent	Public APIs, third-party integrations
Query Param	/api/users?v=1	Medium	Moderate	Simple APIs, prototyping
Header	Accept-Version: v1	Low	Good	Internal APIs, coordinated consumers
Content Negotiation	Accept: application/vnd.api.v1+json	Low	Good	Enterprise, strict REST compliance

Installation

OpenClaw / Moltbot / Clawbot

npx clawhub@latest install api-versioning

---

URL Path Versioning

The most common strategy. Version lives in the URL, making it immediately visible.

from fastapi import FastAPI, APIRouter

v1 = APIRouter(prefix="/api/v1")
v2 = APIRouter(prefix="/api/v2")

@v1.get("/users")
async def list_users_v1():
    return {"users": [...]}

@v2.get("/users")
async def list_users_v2():
    return {"data": {"users": [...]}, "meta": {...}}

app = FastAPI()
app.include_router(v1)
app.include_router(v2)

Rules:

• Always prefix: /api/v1/... not /v1/api/...
• Major version only: /api/v1/, never /api/v1.2/ or /api/v1.2.3/
• Every endpoint must be versioned — no mixing versioned and unversioned paths

Header Versioning

Version specified via request headers, keeping URLs clean.

function versionRouter(req, res, next) {
  const version = req.headers['accept-version'] || 'v2'; // default to latest
  req.apiVersion = version;
  next();
}

app.get('/api/users', versionRouter, (req, res) => {
  if (req.apiVersion === 'v1') return res.json({ users: [...] });
  if (req.apiVersion === 'v2') return res.json({ data: { users: [...] }, meta: {} });
  return res.status(400).json({ error: `Unsupported version: ${req.apiVersion}` });
});

Always define fallback behavior when no version header is sent — default to latest stable or return 400 Bad Request.

Semantic Versioning for APIs

SemVer Component	API Meaning	Action Required
MAJOR (v1 → v2)	Breaking changes — remove field, rename endpoint, change auth	Clients must migrate
MINOR (v1.1 → v1.2)	Additive, backward-compatible — new optional field, new endpoint	No client changes
PATCH (v1.1.0 → v1.1.1)	Bug fixes, no behavior change	No client changes

Only MAJOR versions appear in URL paths. Communicate MINOR and PATCH through changelogs.

---

Breaking vs Non-Breaking Changes

Breaking — Require New Version

Change	Why It Breaks
Remove a response field	Clients reading that field get undefined
Rename a field	Same as removal from the client's perspective
Change a field's type	"id": 123 → "id": "123" breaks typed clients
Remove an endpoint	Clients calling it get 404
Make optional param required	Existing requests missing it start failing
Change URL structure	Bookmarked/hardcoded URLs break
Change error response format	Client error-handling logic breaks
Change authentication mechanism	Existing credentials stop working

Non-Breaking — Safe Under Same Version

Change	Why It's Safe
Add new optional response field	Clients ignore unknown fields
Add new endpoint	Doesn't affect existing endpoints
Add new optional query/body param	Existing requests work without it
Add new enum value	Safe if clients handle unknown values gracefully
Relax a validation constraint	Previously valid requests remain valid
Improve performance	Same interface, faster response

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Deprecation Strategy

Never remove a version without warning. Follow this timeline:

Phase 1: ANNOUNCE
  • Sunset header on responses  • Changelog entry
  • Email/webhook to consumers  • Docs marked "deprecated"

Phase 2: SUNSET PERIOD
  • v1 still works but warns     • Monitor v1 traffic
  • Contact remaining consumers  • Provide migration support

Phase 3: REMOVAL
  • v1 returns 410 Gone
  • Response body includes migration guide URL
  • Redirect docs to v2

Minimum deprecation periods: Public API: 12 months · Partner API: 6 months · Internal API: 1–3 months

Sunset HTTP Header (RFC 8594)

Include on every response from a deprecated version:

HTTP/1.1 200 OK
Sunset: Sat, 01 Mar 2025 00:00:00 GMT
Deprecation: true
Link: <https://api.example.com/docs/migrate-v1-v2>; rel="sunset"
X-API-Warn: "v1 is deprecated. Migrate to v2 by 2025-03-01."

Retired Version Response

When past sunset, return 410 Gone:

{
  "error": "VersionRetired",
  "message": "API v1 was retired on 2025-03-01.",
  "migration_guide": "https://api.example.com/docs/migrate-v1-v2",
  "current_version": "v2"
}

---

Migration Patterns

Adapter Pattern

Shared business logic, version-specific serialization:

class UserService:
    async def get_user(self, user_id: str) -> User:
        return await self.repo.find(user_id)

def to_v1(user: User) -> dict:
    return {"id": user.id, "name": user.full_name, "email": user.email}

def to_v2(user: User) -> dict:
    return {
        "id": user.id,
        "name": {"first": user.first_name, "last": user.last_name},
        "emails": [{"address": e, "primary": i == 0} for i, e in enumerate(user.emails)],
        "created_at": user.created_at.isoformat(),
    }

Facade Pattern

Single entry point delegates to the correct versioned handler:

async def get_user(user_id: str, version: int):
    user = await user_service.get_user(user_id)
    serializers = {1: to_v1, 2: to_v2}
    serialize = serializers.get(version)
    if not serialize:
        raise UnsupportedVersionError(version)
    return serialize(user)

Versioned Controllers

Separate controller files per version, shared service layer:

api/
  v1/
    users.py      # v1 request/response shapes
    orders.py
  v2/
    users.py      # v2 request/response shapes
    orders.py
  services/
    user_service.py   # version-agnostic business logic
    order_service.py

API Gateway Routing

Route versions at infrastructure layer:

routes:
  - match: /api/v1/*
    upstream: api-v1-service:8080
  - match: /api/v2/*
    upstream: api-v2-service:8080

---

Multi-Version Support

Architecture:

Request → API Gateway → Version Router → v1 Handler → Shared Service Layer → DB
                                        → v2 Handler ↗

Principles:

1. Business logic is version-agnostic. Services, repositories, and domain models are shared.
2. Serialization is version-specific. Each version has its own request validators and response serializers.
3. Transformations are explicit. A v1_to_v2 transformer documents every field mapping.
4. Tests cover all active versions. Every supported version has its own integration test suite.

Maximum concurrent versions: 2–3 active (current + 1–2 deprecated). More than 3 creates unsustainable maintenance burden.

---

Client Communication

Changelog

Publish a changelog for every release, tagged by version and change type:

## v2.3.0 — 2025-02-01
### Added
- `avatar_url` field on User response
- `GET /api/v2/users/{id}/activity` endpoint
### Deprecated
- `name` field on User — use `first_name` and `last_name` (removal in v3)

Migration Guides

For every major version bump, provide:

• Field-by-field mapping table (v1 → v2)
• Before/after request and response examples
• Code snippets for common languages/SDKs
• Timeline with key dates (announcement, sunset, removal)

SDK Versioning

Align SDK major versions with API major versions:

api-client@1.x  →  /api/v1
api-client@2.x  →  /api/v2

Ship the new SDK before announcing API deprecation.

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Anti-Patterns

Anti-Pattern	Fix
Versioning too frequently	Batch breaking changes into infrequent major releases
Breaking without notice	Always follow the deprecation timeline
Eternal version support	Set and enforce sunset dates
Inconsistent versioning	One version scheme, applied uniformly
Version per endpoint	Version the entire API surface together
Using versions to gate features	Use feature flags separately; versions are for contracts
No default version	Always define a default or return explicit 400

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NEVER Do

1. NEVER remove a field, endpoint, or change a type without bumping the major version
2. NEVER sunset a public API version with less than 6 months notice
3. NEVER mix versioning strategies in the same API (URL path for some, headers for others)
4. NEVER use minor or patch versions in URL paths (/api/v1.2/ is wrong — use /api/v1/)
5. NEVER version individual endpoints independently — version the entire API surface as a unit
6. NEVER deploy a breaking change under an existing version number, even if "nobody uses that field"
7. NEVER skip documenting differences between versions — every breaking change needs a migration guide entry