Before approving an API contract, check that intended consumers can understand it, every request and outcome is defined, compatibility and lifecycle rules are explicit, security boundaries can be reviewed, and tests will verify that the running API matches the contract. Ask for evidence for each check—not just a valid specification file.
1. Can intended consumers understand and use the API?
Start with the developers or systems that will call the API and the tasks they need to complete. An API is easier to use when its design reflects those needs, rather than assumptions known only to its authors. GOV.UK guidance recommends understanding user needs before building an API and says ease of understanding affects whether it is used: GDS API technical and data standards.
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Review operation names, resource boundaries, terminology, and examples as a consumer would. Ask whether someone can tell what an operation does, which resource it affects, and what information they need to supply without relying on undocumented context. A written specification gives consumers something concrete to review while design changes are still practical; the UK Home Office API guidance also recommends developing the API specification during design: Designing and maintaining an API.
- Can consumers map each operation to a clear task?
- Are names and terms used consistently?
- Do examples illustrate realistic use without being the only place important behavior is defined?
- Have representative consumers reviewed the design or confirmed that it meets their needs?
2. Are requests, responses, and failures explicit?
For every operation, inspect its parameters, request body, constraints, responses, status codes, and error behavior. The OpenAPI Specification is a standard, language-agnostic way to describe HTTP APIs for both people and tools; it can support discovery and understanding without requiring access to source code: OpenAPI Specification 3.2.1.
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Do not make consumers guess whether a field is required, which values are accepted, what a response contains, or what happens when input is invalid. Check that success and failure outcomes are meaningful and that status codes reflect the situation. The Home Office guidance calls for appropriate status codes and input validation; for example, a 403 response can indicate that the caller lacks access.
- Are required and optional fields distinguished?
- Are formats, ranges, allowed values, and validation behavior described?
- Are expected responses documented, including relevant error responses?
- Can consumers distinguish invalid input, missing resources, and lack of permission where those cases apply?
An example can clarify a contract, but it should not be the only source for a rule. If behavior matters to a client, it belongs in the contract or accompanying authoritative documentation.
3. Are compatibility and lifecycle expectations clear?
Approval should establish how the API changes over time, not just how it works today. Look for a versioning policy, a definition of breaking change, deprecation and support expectations, and a migration path when existing consumers could be affected.
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GOV.UK advises avoiding changes that stop older versions working where possible; when older versions cannot be maintained, a new URI version is one possible approach. The Home Office guidance recommends choosing a versioning strategy and communicating deprecation. Neither source establishes one scheme as right for every API.
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The Home Office identifies URI path, query parameter, and header approaches. Compare them against the API’s context rather than selecting one by habit:
| Review axis | Question to resolve |
|---|---|
| Consumer compatibility and migration | How much client work will a change require, and can existing clients continue to operate? |
| Version scope | Does a version apply to the whole API or vary by endpoint? |
| Discoverability | Can clients readily identify which version they are using? |
| Deprecation and support | How will consumers be notified, and how long will an older version remain supported? |
| Operational cost | What is required to maintain older versions while consumers migrate? |
URI versioning is described by GOV.UK as simple and commonly used, but that is not a mandate. The approval decision should record the chosen policy and how breaking changes, notices, and migrations will be handled.
4. Can reviewers assess permissions and security boundaries?
Check what callers must prove, what they are allowed to do, and which data or operations require tighter controls. GOV.UK frames API security across data, application, and network access, as well as auditing, and recommends considering security from the beginning of design: GDS API technical and data standards.
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Western Australia’s API design guidance adds risk-based authentication and authorization, input validation, rate or resource controls, logging, and extra safeguards for administrative operations: Architecture decision record: API design.
- Are authentication and authorization requirements declared?
- Does each operation expose only the access it needs, including access to individual records?
- Are sensitive and administrative operations identified for additional safeguards?
- Are input validation, resource limits, and relevant logging requirements addressed?
A declaration in a contract is evidence of intended behavior, not proof that a deployed service enforces it. Ask how authorization, validation, and other material controls are tested.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.5. Is there evidence the implementation will match the contract?
A clear contract can still drift from the API that ships. Ask how the specification is version-controlled, validated, and checked against implementation. Western Australia’s guidance calls for automated contract-conformance, behavior, and security testing in CI/CD, with coverage of material operations and risks; it also recommends reviewing generated or maintained contracts for drift.
Before approval, request an evidence package that identifies the contract version, shows relevant test results, and explains how changes—especially breaking changes—will be communicated. Scale the depth of review and testing to the consumers affected, the sensitivity of the data, and the operational risk.
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- Validation and conformance evidence tied to that version
- Behavior and security test evidence for material operations and risks
- A defined owner and process for detecting drift and communicating breaking changes
What these checks apply to
OpenAPI describes HTTP APIs. Other interface types may need a protocol-native schema or contract instead. Western Australia’s OpenAPI-specific requirement excludes non-HTTP protocols, event streams, GraphQL schemas, and unchangeable third-party APIs. The government guidance cited here offers review recommendations; it is not a universal regulatory mandate. Whatever format you use, documentation can state intended behavior, but test evidence and operational controls are needed to establish whether a running service follows it.
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