An API contract describes how one application can request an action or exchange data with another. It establishes the shape of requests and responses, authentication and important error behaviour. A successful connection still needs agreed business rules about ownership, meaning and completion.
This guide explains the decisions a business should review before relying on an integration. It applies to internal applications and connected services without assuming that any particular supplier's field names or version policy apply everywhere.
Define the operation and business outcome
Start with the task, such as submitting an enquiry or updating a delivery address. Describe what the receiving service will do and what the caller should expect in return. Distinguish a request being accepted for later processing from the requested action being completed.
Record the responsible system and owner. If a CRM receives a customer record, but staff must qualify it before assigning work, that review is part of the workflow. An interface specification does not automatically describe every operational step around an API call.
Declare request fields and their meaning
Define field names, types, required values and limits. Separate a property's internal identifier from the label shown to staff. A field labelled Contact in a user interface may not use that word in the API. Record which values are stable identifiers and which are editable descriptions.
Specify how blank, missing and null values behave. A missing value might leave existing information unchanged, while an explicit blank might erase it. These meanings depend on the operation and implementation. Test the actual rule instead of treating all empty values as equivalent.
Validate structure and business rules separately
A schema can check that a payload contains the expected types and structure. A JSON number and a string containing digits are different types. A declared format may also require configuration to be enforced by the validator, so verify the behaviour of the chosen implementation.
Structural validity does not prove business correctness. A well-formed appointment time can still fall outside opening hours. Validate permissions, state transitions and relevant business limits separately. Return an actionable explanation without exposing unnecessary internal or customer information.
Describe relationships as well as records
A contact record and its relationship to an organisation are separate concerns in many systems. List the associations needed by the workflow and how they are created, changed or removed. A successful contact update does not prove that the contact is linked to the intended company or booking.
Choose a reliable matching strategy using appropriate identifiers. Do not silently merge two people because their names resemble each other. Establish what happens when a referenced record is absent or ambiguous, and assign the exception to someone who can resolve it.
Document responses and known failures
- Show the normal success response and what it confirms.
- Define validation failures and the relevant field information.
- Describe authentication and permission failures.
- Record duplicate or conflicting request behaviour.
- Explain rate limits and temporary unavailability.
- Identify failures that require staff review rather than automatic retry.
Keep error identifiers stable where applications depend on them. Human-readable wording can change without being a suitable machine decision rule. Log a safe correlation reference, so support teams can connect a failed request with its processing history.
Use an interface description proportionately
OpenAPI can describe HTTP API operations, parameters, payloads and responses in a structured form. It helps teams understand the declared interface and supports appropriate tooling. It does not establish that the running service follows the description or that the surrounding customer journey is complete.
Maintain useful examples of ordinary requests and important exceptions. Keep credentials and personal records out of shared documentation. Assign ownership to the contract and update it alongside implementation changes so it remains a practical reference.
Plan versions and deprecation
API versions, client-library versions and webhook destination versions can be separate settings. Record each dependency where relevant and identify which changes are breaking. A client package update does not necessarily change the version used by an existing event destination.
Agree how consumers are notified, how long an old contract remains available and what evidence is needed before retiring it. Inspect actual usage rather than assuming every caller has upgraded. Test both intended compatibility and the failure behaviour of unsupported requests.
Verify both sides of the contract
Testing a consumer against a local mock demonstrates its handling of the simulated response. It does not prove that the real provider returns that response. Contract verification should also check the provider against the interactions consumers rely on, using an appropriate controlled environment.
Combine those checks with integration and business acceptance tests. Our requirements and acceptance criteria guide helps define the intended outcome. Use Giraffe Digital's digital strategy service to align integration scope, responsibilities and delivery priorities before implementation.
