A coding agent can help build a small REST API, but “make an API” is too broad to produce a reliable result. Start with one resource and a written contract. Decide what requests and responses should look like, how invalid input is handled, and which tests prove the endpoint behaves as expected.
Write the API contract before asking for code
Describe one resource, its fields, and the operations the first version needs. Specify required and optional values, response status codes, error format, authentication expectations, and whether the data is stored or temporary. If the project already has a framework or API style, tell the agent to follow it rather than adding a second stack.
Ask the agent to inspect the repository and propose the route, handler, validation, and test locations before it edits. Confirm that the proposed approach matches the existing architecture. A short plan is cheaper to correct than a large implementation built on a wrong assumption.
| Contract detail | Example question |
|---|---|
| Resource | What does one record represent? |
| Input | Which fields are required, optional, or rejected? |
| Response | What does success and each expected error look like? |
| Access | Who may read, create, update, or delete the record? |
| Persistence | Is data stored in the existing database or kept temporarily? |
Build one complete path before adding endpoints
Implement one request from route to response, including validation and a focused test. Check that malformed data receives a predictable error and that unexpected failures do not reveal stack traces or secrets to callers. If the endpoint changes data, test that the stored state matches the contract.
Only after the first path is understandable should you add the next operation. This makes it easier to see where routing, validation, authentication, persistence, and error handling belong. It also gives a reviewer a smaller diff.
- Write the request and response contract.
- Ask the agent to locate existing framework patterns.
- Implement one endpoint with validation and a regression test.
- Run the project checks and inspect the response behavior.
- Add authentication, persistence, and more routes as separate changes.
Review security and failure behavior
Check authorization for every operation, not only authentication at the route. Confirm that a caller cannot read or change another user’s record by changing an identifier. Validate inputs at the boundary and keep secrets in environment configuration rather than source files or logs.
Test malformed JSON, missing fields, unexpected identifiers, duplicate requests, and database errors. Do not treat the happy path as the whole API. If the endpoint handles sensitive data, payment, or account changes, ask a responsible human to review the design before deployment.
Use the agent as an implementation partner
The best agent prompt names the contract, the project files to inspect, what not to change, and the checks to run. Ask it to report any assumption it could not confirm. Review the diff, run the tests yourself, and keep each change small enough to understand.
For a larger app, use the web app build guide. If this is a new prototype, the vibe coding guides explain how to move from a quick idea toward something maintainable.