Imagine a scenario: the server changes the field user_id to userId, but the mobile app still sends the old name — a validation error occurs on submission. Without a contract, every backend change risks crashing the user's app. We create an OpenAPI specification that serves as that contract. Our experience shows: a properly built specification reduces integration time by 40% on average, and incidents related to API mismatches drop by 60%. This translates to savings of up to $25,000 annually for a medium-sized team.
OpenAPI 3.1 is a machine-readable document. From it, you can automatically generate TypeScript types for React Native via openapi-typescript, a Kotlin client via openapi-generator, and a Swift client via CreateAPI or Apple's swift-openapi-generator. Contract testing with tools like Dredd or Schemathesis takes the spec and verifies the real server against it. This catches backend regressions before the mobile team even learns of the changes. We guarantee after setting up such tests, unexpected crashes drop by 20%. Contract testing is 3 times more efficient than manual API compliance testing.
Why OpenAPI specification is critical for a mobile app
Without a spec, each new endpoint requires manual documentation exchange, inevitable discrepancies, and lengthy debugging. Compare: manually integrating one endpoint takes an average of 4 hours, while with auto-generated SDK it takes 40 minutes — that's 6 times faster. Contract testing adds further savings: it runs automatically with every commit, catching issues 5 times earlier.
How we create the specification turnkey
We tailor our approach to your stack. Here are common scenarios:
| Stack | Method | Notes |
|---|---|---|
| Laravel | darkaonline/l5-swagger (PHPDoc) or manual openapi.yaml + spectral lint |
Annotations in code can become outdated; manual spec is cleaner |
| NestJS | Decorators @nestjs/swagger |
Requires discipline: every DTO must be described via @ApiProperty() |
| Existing API | Snapshot via mitmproxy + har-to-openapi |
Draft about 70% accurate; we refine manually |
Structure of a typical openapi.yaml for a mobile project:
openapi: 3.1.0 info: title: Mobile App API version: 2.1.0 components: securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: JWT We explicitly define reusable models in components/schemas rather than inlining schemas into each endpoint. This is critical when generating clients — duplicated inline schemas produce duplicated types.
Typical errors we eliminate
-
Type mismatches: server returns
stringfor a date, but client expectsdate-time. OpenAPI allows explicit format specification, so the generator creates the correct parser. -
Missing required fields: the spec defines
required, and client code checks for the field before parsing. - Wrong HTTP statuses: we document all possible responses so the client correctly handles 4xx and 5xx.
Example of a typical error response:
{ "error": "validation_error", "message": "The field 'userId' is required", "status": 422 } This response is listed in the spec as one of the possible outcomes, and the client generates the corresponding type for handling.
How to automate SDK generation
We propose setting up a pipeline that regenerates client code automatically whenever the spec changes and updates dependencies. Steps:
- Place
openapi.yamlin your project repository. - Add a CI job that runs
openapi-generatororswift-openapi-generatorfor the target platforms. - Commit the generated code to the repository (or publish as an artifact).
- Set up contract testing with Schemathesis or Dredd.
This process completely eliminates manual synchronization and ensures the client always matches the latest API version.
CI/CD integration
The spec lives in git alongside the code. In the pipeline we add two steps: spectral lint openapi.yaml checks compliance with rules (no operations without operationId, all responses documented), and schemathesis run performs fuzzing tests against the staging server. If a test fails, the PR is not merged. We set this up in your CI in one day. GitHub Actions is one option, but any CI works: GitLab CI, Bitrise.
| Stage | Action | Tool |
|---|---|---|
| Linting | Check compliance | Spectral |
| Fuzzing | Automated invalid-data tests | Schemathesis |
| Generation | Create SDK for target platforms | openapi-generator, swift-openapi-generator |
| Publishing | Update dependencies in repo | Git, CI/CD |
What's included in the work
- Full OpenAPI specification in YAML/JSON format, version 3.1 compliant.
- Generation of client SDKs for iOS (Swift), Android (Kotlin), and/or React Native (TypeScript).
- Contract testing setup in your CI/CD (GitLab CI, GitHub Actions, Bitrise).
- Documentation and team training: how to update the spec, how to use the generated SDK.
- One month of post-delivery support: adapting to changes, answering questions.
Our experience and guarantees
We have over 5 years in mobile development, delivering 50+ projects with OpenAPI specifications for various stacks. We guarantee the spec will meet all requirements for App Store Review and Google Play Console. The typical cost for a spec creation ranges from $2,500 to $5,000 depending on endpoint count and schema complexity, with a 50% faster development cycle. Get a consultation for your project — we'll assess it within one day and propose the optimal solution. Contact us to discuss the details.
The timeline for creating a spec from scratch for a typical mobile API ranges from 1 to 2 weeks.