Making an API contract the single source of truth
Making an API contract the single source of truth, and deriving clients, types and validation from it automatically rather than copying them by hand, is a practice I apply as soon as a project has a well-defined API.
The typical need
On a project with an API consumed by several clients — a frontend, a mobile app, an external partner — the description of the exchange contract often ends up copied in several places: in the backend code, in the frontend code, in a separate document. These copies drift apart almost inevitably over time: a field renamed on the backend without the frontend being updated at the same time, documentation that becomes wrong without anyone noticing.
How I approach this kind of work
The OpenAPI specification precisely describes an API's routes, parameters, and request and response formats, in a structured format that tools can read. Rather than writing it after the fact purely for documentation purposes, I treat it as the project's source of truth: backend code can be generated or validated from it, and, more importantly, client code (types, call functions, validation) is generated automatically for each consumer, with no manual copying.
This approach shifts a category of errors from runtime in production to compile time or generation time: a field renamed in the specification immediately breaks client code generation, with an explicit, localised error, rather than a bug discovered by a user weeks later. Getting set up initially requires the discipline of updating the specification with every API change, which is more a change of habit than a technical difficulty.
Factors that affect the estimate
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Existing API or one still to design
Documenting an API already in place requires a different reconstruction effort than designing a specification as the API is being created.
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Number of consumers of the API
The benefit of automatic generation grows with the number of different clients that need to stay in sync with the contract.
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Complexity of the data types
Nested data structures, or ones with many validation cases, require a more detailed specification to write and maintain.
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Integration into the development pipeline
Automating generation on every specification change, rather than occasional manual generation, requires extra initial setup work in the project's tooling.
Frequently asked questions
Does this approach apply to an API already in production?
Do I need to change language or framework to adopt this practice?
What happens if the specification isn't kept up to date?
Describe your need in one minute
A few targeted questions so I can reply with an estimate rather than another questionnaire.