Define contract tests between two services

Two services that integrate fine in staging and break on deploy day.

Define the contract between the consumer and provider described below.

1. State the contract explicitly: endpoints, request shapes, response shapes, status codes, and the error responses the consumer actually handles.
2. Separate what the consumer genuinely depends on from what the provider happens to return. The contract is only the former, and getting this wrong is how contracts become brittle.
3. Write the consumer-side expectations as test code in my framework.
4. Write the provider-side verification, including the states the provider must be able to set up.
5. List the changes the provider could make that would break this contract, and those that would be safe.
6. Describe how the contract is verified in CI, and what should block a deploy.

Consumer and provider details:

Beyond unit tests. Manual QA, exploratory charters, accessibility, load, and contracts between services.