---

name: api-versioning-strategy description: "Write an API versioning strategy document for a service or API platform. Use when asked to define versioning policy, plan API deprecation, classify breaking changes, or document version lifecycle. Produces a complete versioning strategy with breaking-change classification table, deprecation timeline, migration guide template, and client communication template."

API Versioning Strategy

Produce a complete API versioning strategy document that gives a service team durable, consistent rules for evolving their API without breaking consumers. This document covers the versioning scheme selection (with rationale), lifecycle policy from introduction through sunset, a precise breaking-change classification, and all the communication artifacts a team needs when deprecating a version. Engineers should be able to hand this document to a new team member or external consumer and have them understand exactly what to expect.

Required Inputs

Ask for these if not already provided:

• API type — REST, GraphQL, or gRPC (each has different versioning mechanics)
• Current versioning approach — URL path (/v1/), request header, query parameter, or none; if none, document starts fresh
• Number of existing versions and active consumer count — needed to size the lifecycle policy and migration scope
• Deprecation timeline constraints — any hard deadlines (contract SLAs, compliance windows, annual release cycles)
• Consumer type — internal teams only, external partners, public API, or mix (affects communication channel choices)

If any input is missing, ask before producing the document. For GraphQL, note that the versioning approach differs substantially (schema evolution over versioning) and tailor the scheme section accordingly.

Output Format

---

API Versioning Strategy: [Service Name]

Owner: [Team Name] API Type: [REST / GraphQL / gRPC] Document Version: 1.0 Last Reviewed: [Date] Next Review: [Date + 6 months]

---

1. Versioning Scheme

Selected Approach: [URL Path / Request Header / Query Parameter]

| Scheme | Example | Pros | Cons | Verdict | |--------|---------|------|------|---------| | URL Path | /v2/orders | Visible in logs and bookmarks; trivial to route | Violates strict REST resource identity; clutters URL space | Recommended for public-facing REST APIs | | Accept Header | Accept: application/vnd.[service].v2+json | Keeps URLs clean; proper content negotiation | Harder to test in browser; less visible in logs | Recommended for internal APIs with controlled clients | | Query Parameter | /orders?version=2 | Easy to retrofit without URL restructuring | Often missed in client code; cache-key complications | Acceptable only for read-heavy APIs already in production | | GraphQL Schema Evolution | Field deprecation + @deprecated directive | No versioning needed for additive changes | Requires disciplined schema design | Recommended for GraphQL APIs |

Rationale for [chosen scheme]: [One paragraph explaining why this scheme fits the API type, consumer type, and operational context provided. Reference the specific inputs — e.g., "Because this API has external partners who integrate via generated clients, URL path versioning provides the most predictable routing behavior and eliminates header negotiation complexity."]

Version Format

[Base URL]/v{MAJOR}/{resource}

Examples:
  https://api.[company].com/v1/orders
  https://api.[company].com/v2/orders/{id}/items

Version identifier: integer only (v1, v2, v3)
No minor versions in the URL — minor/patch changes are non-breaking and deployed continuously.

---

2. Version Lifecycle Policy

Lifecycle Stages

  STABLE ──────────────────────────────────────────────────►
      │
      ├─ STABLE        Active development, full SLA, new consumers allowed
      │
      ├─ DEPRECATED    Announced, timeline posted, migration docs live.
      │                New consumers blocked. Existing consumers receive warnings.
      │
      ├─ SUNSET        Requests return HTTP 410 Gone + migration pointer.
      │                30-day window before routing is removed.
      │
      └─ RETIRED       Routing removed, docs archived, no traffic accepted.

| Stage | Duration | SLA Applies | New Consumers Allowed | Required Action | |-------|----------|-------------|----------------------|-----------------| | Stable | Until superseded | Yes — full | Yes | None | | Deprecated | [12 months / adjust per constraint] | Yes — degraded acceptable | No | Migrate before sunset date | | Sunset | 30-day window | Best-effort only | No | Migrate immediately | | Retired | Permanent | None | No | — |

Minimum Stable Period: A version must remain Stable for at least [6 / 12] months before deprecation can be announced.

Maximum Simultaneous Versions: No more than [2] versions in Stable or Deprecated status at any time. Releasing v3 requires committing to a sunset date for v1 in the same announcement.

---

3. Breaking vs. Non-Breaking Change Classification

Apply this table before every API change. If a change is marked Breaking, it requires a new major version. When uncertain, default to Breaking.

| Change Type | Specific Example | Classification | Rationale | |-------------|-----------------|----------------|-----------| | Remove a response field | Delete order.legacy_id from response | Breaking | Clients reading this field will null-pointer or fail | | Rename a field | user_name → username | Breaking | Clients referencing old name receive null | | Change field type | "amount": "10.00" → "amount": 10.00 | Breaking | Type mismatch at deserialization | | Make optional field required | email required in POST body | Breaking | Existing callers omitting it receive 400 | | Remove an endpoint | DELETE /v1/widgets/{id} removed | Breaking | Existing callers receive 404 | | Change HTTP method | GET /search → POST /search | Breaking | Bookmarked or cached GET calls fail | | Change authentication scheme | API key → OAuth2 | Breaking | All clients must re-authenticate | | Restructure error response shape | Error JSON schema changed | Breaking | Error-handling code misparses responses | | Expand enum values (response) | New status: "on_hold" value returned | Breaking | Switch statements with no default fall through | | Change pagination defaults | page_size default 20 → 50 | Breaking | Response length changes unexpectedly | | Tighten input validation | Max length 100 → 50 | Breaking | Previously valid inputs now rejected | | Add new optional field to response | Add order.tax_breakdown | Non-Breaking | Clients ignore unknown fields per spec | | Add new optional request parameter | Add ?include_archived=true | Non-Breaking | Ignored by existing clients | | Add a new endpoint | GET /v1/orders/{id}/audit | Non-Breaking | No existing client references it | | Relax input validation | Min length 10 → 5 | Non-Breaking | Existing valid inputs remain valid | | Performance or latency improvement | Response time reduced | Non-Breaking | — | | Add new enum value (request-only) | Accept new type: "express" | Non-Breaking | Existing values still accepted |

---

4. Deprecation Process

Step-by-Step Deprecation Checklist

• [ ] T-0 (Decision day): Engineering lead approves deprecation. New version confirmed Stable. Sunset date set.
• [ ] T-0: Update API docs — add deprecation banner to all v[N] endpoint pages.
• [ ] T-0: Add Deprecation and Sunset response headers to all v[N] responses (see format below).
• [ ] T-0: Block new consumer onboarding for v[N] in API gateway and developer portal.
• [ ] T-0: Send initial deprecation notice to all registered consumers (see Section 5 template).
• [ ] T-0: Open tracking issue in engineering backlog linking all known consumers to their migration status.
• [ ] T minus 30 days: Send 30-day warning to all consumers still sending v[N] traffic.
• [ ] T minus 7 days: Send final warning. If consumer traffic > 100 req/day, escalate directly to their engineering lead.
• [ ] Sunset date: Switch v[N] routing to return HTTP 410 Gone with body pointing to migration guide.
• [ ] T plus 30 days: Remove routing rules. Archive documentation. Close tracking issue.

Deprecation Response Headers

HTTP/1.1 200 OK
Deprecation: true
Sunset: Sat, 01 Jan 2027 00:00:00 GMT
Link: <https://docs.[company].com/api/migration/v1-to-v2>; rel="successor-version"

Sunset Response Body

HTTP/1.1 410 Gone
Content-Type: application/json

{
  "error": "api_version_sunset",
  "message": "API v1 was sunset on 2027-01-01. Please migrate to v2.",
  "migration_guide": "https://docs.[company].com/api/migration/v1-to-v2",
  "support": "api-support@[company].com"
}

---

5. Client Communication Templates

Initial Deprecation Notice

Subject: [Action Required] [Service Name] API v[N] Deprecation — Sunset [Date]

Hi [Team / Partner Name],

We are deprecating [Service Name] API v[N], effective [Sunset Date].

What this means for you:
- v[N] continues to work normally until [Sunset Date]
- After [Sunset Date], all v[N] requests return HTTP 410 Gone
- v[N+1] is available today and fully stable

Your current usage: approximately [X] requests/day as of [Date].
Estimated migration effort: [Small: < 1 day | Medium: 1–3 days | Large: 3–10 days]

Migration resources:
  Migration guide:  [URL]
  Changelog:        [URL]
  Office hours:     [Date/Time/Link]
  Support:          [Slack channel or email]

Key dates:
  [Date]          Deprecation announced (today)
  [Date]          New consumer onboarding blocked for v[N]
  [Date]          30-day warning sent to remaining consumers
  [Sunset Date]   v[N] returns 410 Gone

Reply to this message or contact us at [channel] with questions.

[Your Name], [Team Name]

30-Day Warning

Subject: [30 Days Remaining] [Service Name] API v[N] sunsets [Date]

Hi [Team / Partner Name],

[Service Name] API v[N] sunsets in 30 days on [Date].

Your current v[N] traffic: [X] requests/day — migration is not yet complete.

If you have a technical blocker requiring an extension, contact us before
[Date minus 14 days]. Extensions require a documented blocker and a committed
migration completion date.

Migration guide: [URL] | Support: [channel]

---

6. Migration Guide Template

Publish one migration guide per version transition at docs.[company].com/api/migration/v[N]-to-v[N+1].

# Migration Guide: v[N] → v[N+1]

**Estimated effort:** [Small: < 1 day | Medium: 1–3 days | Large: 3–10 days]
**Breaking changes in this guide:** [count]

## Quick Start

Update your base URL:
  Before: https://api.[company].com/v[N]/
  After:  https://api.[company].com/v[N+1]/

## Breaking Changes

### 1. [Field Rename: user_name → username]

**Affected endpoints:** `GET /users/{id}`, `POST /users`

Before (v[N]):
{ "user_name": "alice" }

After (v[N+1]):
{ "username": "alice" }

Migration: Replace all references to `user_name` with `username` in request
builders and response parsers.

### 2. [Next breaking change — repeat structure]

## New Capabilities in v[N+1]

| Feature | Description | Docs |
|---------|-------------|------|
| [Feature name] | [Brief description] | [Link] |

## SDK Upgrade Reference

| Language | Package | v[N+1] Version | Install Command |
|----------|---------|----------------|-----------------|
| Python | `[company]-sdk` | `2.0.0` | `pip install [company]-sdk==2.0.0` |
| Node.js | `@[company]/sdk` | `2.0.0` | `npm install @[company]/sdk@2.0.0` |
| Go | `github.com/[company]/sdk-go` | `v2.0.0` | `go get github.com/[company]/sdk-go/v2` |
| Java | `com.[company]:sdk` | `2.0.0` | Update pom.xml / build.gradle |

## Migration Validation Checklist

- [ ] Base URL updated to v[N+1]
- [ ] All renamed fields updated in request serializers
- [ ]