Validate Inputs at the API Boundary

A practical approach

Step 1

Define a request schema with limits on length, size and allowed values. Decide how unknown fields and missing optional fields are handled.

Step 2

Normalize carefully before applying domain rules. Preserve distinctions that matter, such as unknown versus explicitly empty values.

Step 3

Reject invalid data with a consistent error contract and test the endpoint directly, bypassing the user interface.

Worked example

A contact form sends a topic and email address. A modified client can submit an unsupported topic or a very large body. Server checks protect the workflow even when the front-end controls normally prevent those inputs.

A mistake to avoid

TypeScript types do not validate arbitrary network data at runtime by themselves. Another mistake is accepting a client-supplied price or role as authoritative simply because its type is correct.

Try it yourself

Send one valid request and four invalid variants to a local test endpoint: missing field, wrong type, excessive length and unsupported value. Confirm that each is rejected before business side effects run.

Your implementation checklist

• Runtime validation runs on the server.
• Size and length limits are explicit.
• Domain checks follow structural checks.
• Authorization is not confused with validation.