Build an API backend with an AI coding agent

What an API backend does

A backend is the server side of an app. It holds the data, enforces the rules, and exposes endpoints that a front end, a mobile app, or another service can call. When someone loads a page and sees their orders, a backend answered the request that fetched them.

Most backends handle a few core jobs:

• Routes: named addresses like /users or /orders that accept requests.
• Data storage: a database or file that keeps information between requests.
• Validation: checks that reject bad or incomplete input.
• Responses: structured data, usually JSON, sent back to whatever made the request.

Describe the endpoints you need

The clearest way to start is to list the operations your app performs. For a simple task app you might need to create a task, list all tasks, update one, and delete one. Those four operations map directly onto endpoints.

Describe them to the agent in plain language, including what data each one accepts and returns. The agent writes the routes, sets up storage, and runs the server in your browser so you can call the endpoints and see the responses. Because mobileCoder needs no local setup, you skip installing runtimes, package managers, or databases before you can test anything.

Test the endpoints as they are built

An API is easy to get subtly wrong, so test each endpoint while the details are fresh. Send a request, look at the response, and confirm it matches what you described.

• Create a record, then list records and check that the new one appears.
• Send incomplete or malformed input and confirm the endpoint rejects it cleanly instead of crashing.
• Update and delete a record, then verify the change actually took effect.

Running the backend as it is built means you catch these issues early, when they are cheap to fix, rather than after a front end is depending on them.

Handle keys and secrets carefully

Backends often talk to other services, such as a payment provider or an email sender, and those services need credentials. mobileCoder supports bringing your own API keys, so calls to external services run under your own account.

Keep secrets out of the client. A key that belongs on the server should never be sent to the browser, because anyone can read what the browser receives. Ask the agent where each key is used and confirm that sensitive values stay on the backend.

Review before anything depends on it

AI-generated backend code can be wrong in ways that matter, including weak input checks or a query that returns more than it should. Read the code before you build on top of it.

• Check that every endpoint validates its input rather than trusting it.
• Confirm errors return a sensible response instead of leaking internal details.
• Look at how data is read and written, and test what happens when two requests touch the same record.

A backend is the foundation other parts of your app stand on. The time you spend reviewing it now saves you from chasing strange bugs later.

Grow the backend one endpoint at a time

Once the core endpoints work and survive a few bad inputs, you can extend the backend the same way you started it. Describe the next operation, let the agent build and run it, then test it before moving on.

Adding one endpoint at a time keeps each change small enough to verify. If you ask for several new routes at once and something breaks, it is harder to tell which one caused it. A steady rhythm of describe, run, test, and review is slower than it feels but far easier to trust, and it keeps the whole backend in a state you understand rather than a pile of code you are hoping works.

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