The contract written
before the frontend has to guess
A backend built API-first means the contract, every endpoint, every request and response shape, is written down in OpenAPI before the frontend team starts guessing at what comes back, and the documentation generates from that same contract instead of living in a wiki page that quietly stops matching reality. We build this way whenever more than one client, a web app, a mobile app, a partner integration, talks to the same backend.
What it is
API-first means the API’s contract, every endpoint, its request parameters, its response shape, its error cases, is designed and written down in the OpenAPI specification before the backend implementation is built out, rather than being discovered by reading the code after the fact. The documentation a frontend or partner team sees is generated directly from that same specification, so it cannot drift out of sync with reality the way a hand-maintained wiki page reliably does.
When you need it (and when you do not)
It earns its cost the moment more than one client talks to the same backend: a web app and a mobile app, an internal tool and a partner integration, or a team building the frontend in parallel with the backend rather than waiting for it to finish first. A multichain crypto wallet backend we built needed a clear, versioned contract precisely because several clients, the wallet app and supporting services, all depended on the same API staying predictable as it evolved.
It is the wrong tool, or at least overhead without payoff, for a backend with exactly one client built by the same team at the same time; writing a formal spec for an API nobody outside the immediate team will ever consume independently is process for its own sake. We build those backends with good internal structure but skip the formal OpenAPI-first ceremony when there is no real audience for the contract.
How we build it
We write the OpenAPI specification first, in a working session with whoever is building the frontend or integrating as a partner, deciding real questions before any code exists: what does a validation error look like across every endpoint, is a field optional or required, what does pagination look like for a list that can get large. This is where the actual design work happens, and skipping it just moves these decisions into ad hoc Slack messages during implementation, which is slower and less consistent.
The backend is built in FastAPI for Python projects, which generates its OpenAPI spec directly from the code’s own type annotations, keeping the written contract and the actual behavior from diverging, or in Node with an equivalent setup when the project’s ecosystem calls for TypeScript. Every endpoint gets consistent request validation and error response shapes, since an API where one endpoint returns a different error format than another is an API every client has to special-case around.
Versioning is planned from the start, not improvised when the first breaking change comes up: a clear strategy (URL versioning, header versioning, or additive-only changes) decided before it is needed means existing clients do not break the day you need to change something. Where several independent clients consume the API, we generate typed client SDKs directly from the spec, so a mobile or frontend team gets working, type-safe code instead of hand-written requests guessing at the response shape.
What to watch
A spec that is written once and never revisited drifts from the real implementation just as badly as no spec at all; we wire automated tests that validate the running API against its own OpenAPI contract, so a change that breaks the contract fails a test before it reaches anyone depending on it. Over-versioning is its own trap, maintaining five live API versions because nobody ever deprecated the old ones costs real maintenance time; we set a deprecation policy as part of the versioning strategy, not as an afterthought once the backend is already carrying four unused versions.
Price and timeline
| Option | Price | What it covers | Timeline |
|---|---|---|---|
| Core API with contract | from $3,500 | OpenAPI spec, backend implementation, generated docs | 3 to 5 weeks |
| Multi-client API with SDKs | from $8,000 | Versioning, typed client SDKs, several consuming applications | 5 to 7 weeks |
Running cost is backend hosting, typically $30 to $200 a month depending on traffic.
Related
This pairs with GraphQL API as an alternative contract style for clients needing flexible queries, and with serverless backend architecture when the API’s endpoints map naturally to individual functions. See the development service page for our full build process. For real examples, see digital goods marketplace automation and the multichain crypto wallet.
Have more than one client guessing at the same backend’s behavior? Get in touch and we will write the contract before any more code does.
FAQ
How much does an API-first backend cost?
From $3,500 for a backend with a clear contract covering your core resources, 3 to 7 weeks. A larger API serving several clients with versioning and SDK generation runs $7,000 to $15,000 depending on the number of endpoints and integrations.
What is the actual benefit over just building the backend and documenting it after?
Writing the contract first forces decisions, what does an error actually look like, what fields are required, before any code commits to an answer, which avoids frontend and backend teams building against two different mental models of the same endpoint. Documenting after the fact tends to describe what the code happens to do, including accidents, rather than what it should do.
FastAPI or another framework?
FastAPI for Python backends, since it generates an OpenAPI spec directly from the code's type annotations, keeping the contract and the implementation from drifting apart. We use Node with an equivalent OpenAPI-first setup when the project's team or ecosystem calls for TypeScript instead.
Can this generate client code for our mobile app or frontend?
Yes. An OpenAPI spec can generate typed client libraries for TypeScript, Swift, Kotlin and others, so a frontend team gets a typed client instead of hand-writing fetch calls and guessing at response shapes from a wiki page.
Who owns the API and its documentation?
You. The OpenAPI spec and the backend code live in your repository, and the documentation is generated from that spec, so it is never a separate document someone forgot to update.