API Deprecation and Sunset: Build a Migration Plan

An API can keep working after it has been deprecated. That is useful transition time, not a reason to ignore the signal until the endpoint disappears. Convert the notice into a consumer inventory and a tested migration plan while you still have both versions available.

The practical distinction is between a discouraged interface and an interface expected to become unavailable. Track both the replacement behavior and the timeline. A date in a response header does not migrate queued jobs, generated clients or integrations owned by another team.

Understand the two signals

RFC 9745 defines the Deprecation response header as a way to communicate a resource's deprecation status. RFC 8594 defines Sunset as a signal about when a resource is expected to become unresponsive. They serve related but different purposes and use different date representations.

Don't parse both with one improvised date rule. Follow the relevant specification or a supported parser. Review linked documentation for the notice's scope: it may apply to a version, a resource or a documented group of endpoints.

Keep the notice itself and the interpretation in your migration record. If the provider changes the date, you should be able to identify what changed.

Inventory actual consumers

Search application code, scheduled jobs, dashboards, webhooks and generated clients for the affected interface. Include scripts used only during releases or recovery. A rarely used integration can remain undiscovered until the old endpoint is gone.

Identify an owner for each consumer and record its release process. A browser frontend can update quickly, while an installed mobile client may remain in use for much longer. The migration window must account for how consumers really reach users.

Use a synthetic planning record:

{
  "consumer":"daily-report-job",
  "old_contract":"v1-report",
  "replacement_contract":"v2-report",
  "owner":"platform-team",
  "migration_state":"testing"
}

Inspect the inventory in JSON Viewer. Don't add private API tokens or customer payloads to a migration spreadsheet merely because it is convenient.

Compare behavior rather than names

The replacement endpoint may differ in field types, pagination, error handling or permissions. A new URL with a similar name is not proof of a compatible contract.

Create fixtures for the real operations your consumer performs: empty results, maximum supported input, ordinary successful responses and documented errors. Compare sanitized outputs with JSON Diff.

Classify differences as intentional or unexpected. An omitted field might be a documented change, while a changed timestamp unit might be a client bug. Write the required client action beside each meaningful difference.

Inspect signals through the actual client path

Public endpoints can be checked with the HTTP Header Checker. Private endpoints require your authorized client or integration test environment. The public checker does not carry your authenticated request context.

Verify whether the SDK or gateway exposes the relevant response headers. If it hides them, add an appropriate internal observation path. Avoid logging complete responses merely to capture one notice.

Check success and error responses where the provider documents signals. Don't assume a notice will appear in every method or representation. Use the provider's migration documentation as the source of scope.

Plan a staged cutover

Implement the replacement client behind a controlled configuration switch. Run it against synthetic data first. Then use a limited real workload where authorized and appropriate, monitoring correctness and completion rather than only response status.

For read-only operations, a comparison run can be useful. For writes, don't send the same business action to both endpoints without an explicit safe design. A dual-write experiment can create duplicates or inconsistent state.

Keep a rollback path while the old interface remains supported. Document what happens if the old interface is already unavailable; rollback cannot depend on an endpoint that has passed its operational retirement point.

Update delayed work and stored contracts

Queued jobs can contain old request shapes. Saved configurations may select the old version. Review these persisted consumers before declaring the application migrated.

Choose whether to translate old queued work, finish it within the supported window or reject it with a clear recovery action. Test translation using fixtures and preserve the intended resource identifiers.

For AI tools that call the API, update the tool schema and examples as part of the migration. See AI tool schema compatibility for handling older call shapes and saved conversations.

Define completion with evidence

A migration is complete when supported consumers use the new interface, critical fixtures pass and remaining old traffic is understood. “The new code was deployed” is an intermediate milestone.

Monitor old-version usage through the transition window where your access permits it. Distinguish active users from internal probes and forgotten jobs. Contact owners through your normal team process rather than silently changing unrelated integrations.

Update documentation, examples and recovery scripts. A support guide that still recommends the retired endpoint can reintroduce the dependency after the migration.

Conclusion

Treat deprecation as time to investigate and Sunset as an operational deadline signal. Inventory consumers, test replacement semantics and include delayed work in the cutover. Completion requires verified behavior and understood old traffic, not just a changed URL.