Versioning APIs

Overview

Implement API versioning strategies -- URL path (/v1/, /v2/), header-based (Accept: application/vnd.api.v2+json), or query parameter (?version=2) -- with backward compatibility layers, deprecation notices, and automated migration paths. Manage concurrent version support, sunset timelines, and breaking change detection across the API surface.

Prerequisites

Existing API codebase with route definitions and controller implementations
Version control history for tracking breaking changes across releases
OpenAPI specs for each supported API version (or ability to generate them)
API gateway or reverse proxy capable of version-based routing (optional: Kong, AWS API Gateway)
Consumer notification channel for deprecation announcements (changelog, email, response headers)

Instructions

Audit existing endpoints using Grep and Read to identify current versioning approach (if any) and catalog all public-facing endpoints with their request/response contracts.
Select a versioning strategy based on API consumer patterns: URL path versioning for public APIs, header versioning for APIs needing clean URLs, or content negotiation for advanced use cases.
Create a version router that directs requests to the appropriate version handler set based on the extracted version identifier from URL, header, or query parameter.
Implement version-specific controller directories (/v1/controllers/, /v2/controllers/) with shared business logic extracted into version-independent service layers.
Build a compatibility layer that transforms v1 requests into v2 format and v2 responses back to v1 format, enabling older versions to run on the latest business logic.
Add deprecation headers to sunset versions: Deprecation: true, Sunset: <date>, and Link: <migration-guide-url>; rel="sunset" per the Sunset HTTP header RFC.
Create a breaking change detector that compares OpenAPI specs between versions and flags removed fields, changed types, new required parameters, and altered response structures.
Write version compatibility tests that send v1-formatted requests and verify correct responses, ensuring the compatibility layer preserves backward compatibility.

See ${CLAUDE_SKILL_DIR}/references/implementation.md for the full implementation guide.

Output

${CLAUDE_SKILL_DIR}/src/routes/v1/ - Version 1 route definitions
${CLAUDE_SKILL_DIR}/src/routes/v2/ - Version 2 route definitions
${CLAUDE_SKILL_DIR}/src/middleware/version-router.js - Version extraction and routing middleware
${CLAUDE_SKILL_DIR}/src/compatibility/ - Request/response transformation layers between versions
${CLAUDE_SKILL_DIR}/src/utils/breaking-change-detector.js - OpenAPI diff tool for breaking changes
${CLAUDE_SKILL_DIR}/docs/migration-guide-v1-to-v2.md - Consumer migration documentation

Error Handling

Error	Cause	Solution
400 Unsupported Version	Client requests a version that does not exist	Return available versions list in error body; suggest closest valid version
410 Gone	Client requests a sunset version past its end-of-life date	Return migration guide URL and recommended current version in error body
Compatibility layer failure	v1-to-v2 transform encounters a field with no mapping	Log unmapped fields; return partial response with warning header; alert API team
Version header ignored	Client sets version header but reverse proxy strips custom headers	Document required proxy configuration; add URL path fallback for header-based versioning
Breaking change undetected	Semantic change (same field name, different meaning) not caught by schema diff	Add contract tests with business-logic assertions beyond schema structure

Refer to ${CLAUDE_SKILL_DIR}/references/errors.md for comprehensive error patterns.

Examples

URL path versioning: Migrate from /api/users to /api/v1/users and /api/v2/users where v2 changes name (string) to firstName/lastName (object), with v1 compatibility layer that joins the fields.

Header-based versioning: Route requests using Accept: application/vnd.myapi.v2+json with a default version fallback for clients omitting the header, and version discovery via OPTIONS responses.

Sunset lifecycle management: Announce v1 deprecation with 6-month sunset timeline, add Sunset headers immediately, log v1 usage metrics to track migration progress, and auto-disable after sunset date.

See ${CLAUDE_SKILL_DIR}/references/examples.md for additional examples.

Resources

RFC 8594 The Sunset HTTP Header Field
API Versioning strategies comparison (Stripe, GitHub, Twilio approaches)
OpenAPI diff tools: oasdiff, openapi-diff
Semantic Versioning 2.0.0: https://semver.org/

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