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Anthropic’s current model-status documentation identifies claude-sonnet-5 as an active model; the available official material does not confirm a public “Claude Sonnet 5.5” API model. Verify the exact identifier you intend to deploy before building a migration test around it. For a move from Sonnet 4.5 or earlier to Sonnet 5, Anthropic’s migration guidance calls for reviewing the effort default and removing manual extended-thinking configuration with budget_tokens.
Verify the target model and find every old-model caller
Do not assume that “Sonnet 5.5” is an API model identifier. The Anthropic model-status material available here names Sonnet 5 as claude-sonnet-5, and its migration guidance describes moving to Sonnet 5 from Sonnet 4.5 or earlier. Check Anthropic’s current model-status and migration documentation for the exact model identifier and instructions that apply to your account, API, and SDK version.
Before changing code, inventory where the old model is used. Anthropic says developers can review model usage on the Claude Console Usage page by exporting usage and examining the CSV breakdown by API key and model.
- In the Claude Console, open Usage and export usage data.
- Review the CSV by API key and model to identify keys still associated with the old model.
- Map those keys to the services, scheduled jobs, environments, and owners that use them. Confirm the mapping with the teams responsible; a key-level usage record does not by itself identify every application caller.
- Record the request settings and SDK/API versions used by each caller so the migration test covers the configurations that will actually change.
Review thinking configuration as a migration change
For migration from Sonnet 4.5 or earlier to Sonnet 5, Anthropic’s prompting guidance says to account for an effort-default change and remove manual extended thinking configured with budget_tokens. Treat these as explicit items in the migration diff rather than carrying old settings forward by habit.
- Check each caller for a manually configured
budget_tokensvalue and remove the old extended-thinking configuration as the migration guidance directs. - Review the effort setting and its default for the target model. Make the intended behavior explicit where your application depends on it, using the syntax supported by the API and SDK version in use.
- Validate the final request configuration against current documentation for the exact model and API/SDK version. Do not infer request fields or compatibility rules from a different model generation.
- Keep the old and new configurations identifiable in test records so a failure can be tied to a specific request change.
The official material cited here does not establish a particular thinking-block or signature failure mode. If your application has experienced one, reproduce it with the affected model, request, API/SDK version, and response before describing it as a migration defect.
Design a regression harness around your application’s behavior
Anthropic recommends testing applications thoroughly with newer models well before the current model’s retirement date. It does not prescribe a harness, fixtures, metrics, thresholds, or specific assertions for thinking blocks. The checks below are engineering recommendations: adapt them to the response contract and business risks of your own application.
Build representative test cases
- Select requests from the services and workflows identified in the usage inventory, including ordinary cases and cases where output handling is business-critical.
- For each case, preserve the relevant input, request parameters, model identifier, and expected application behavior. Avoid treating one prompt or one successful response as sufficient coverage.
- Where the output can be judged against a stable expected result, define that expectation. For subjective or open-ended work, route samples to human review rather than pretending there is a universal pass score.
Exercise parsing and persistence paths
If your application reads, transforms, or stores structured content blocks, test those code paths using responses from the intended target model and the exact API/SDK versions you plan to deploy. Verify the behavior your application requires: parsing succeeds when expected, validation errors are handled, and unexpected content does not silently become a valid business result. Treat any assertion about a specific block shape, signature, or ordering as application- and API-specific; the official migration material here does not define those failure cases.
Compare outcomes without inventing a universal threshold
Run the same representative cases against the old and intended new configurations where feasible, then assess outcomes using criteria relevant to the workflow. Useful measures include task success against fixtures, output parsing and validation results, latency, and cost in your own environment. The available official material publishes no comparative results or pass thresholds for these measures, so establish acceptable limits from your application’s requirements rather than presenting a made-up score as Anthropic guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Gate rollout on evidence and a recovery plan
Use the test results to decide whether each caller is ready, not just whether requests complete successfully. A practical release gate should account for both business output quality and the application’s ability to safely handle returned content.
- Run pre-production tests: exercise representative cases with the target model identifier and reviewed thinking configuration.
- Inspect failures: separate model-output differences from request-configuration, SDK/API, parsing, validation, and persistence problems.
- Set application-specific acceptance criteria: document which failures block release and how latency or cost changes affect the decision. There is no universal threshold established by the cited migration guidance.
- Deploy in controlled stages: move callers in a sequence that lets the team observe each service’s behavior before migrating the next one.
- Define rollback conditions in advance: identify the owner, the conditions that trigger a rollback, and the verified way to restore the prior working configuration. Keep the old model available only while it remains supported and appropriate under current Anthropic model-status guidance.
Anthropic’s recommendation is to test well before the current model retires; the available material does not give a retirement date here. Check the current status information when scheduling the migration rather than assuming a date.
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