Most backend bugs don’t happen because developers are careless.
They happen because APIs are easy to misuse.
If an API allows the wrong thing, someone will eventually do it — intentionally or not. Senior engineers know this, and they design APIs that guide correct usage and make incorrect usage difficult.
This post is about how experienced engineers think when designing APIs that survive real-world use.
The Core Principle: Assume Misuse, Not Malice
When designing an API, don’t assume:
- consumers will read the docs carefully,
- inputs will always be valid,
- calls will be made in the correct order.
Assume instead:
- someone will misunderstand it,
- someone will skip validation,
- someone will copy-paste an example without thinking.
A good API doesn’t rely on “being used correctly.”
It enforces correctness by design.
Clear Inputs, Clear Outputs
Ambiguous APIs invite misuse.
Bad example:
POST /updateUser
What does this update?
- name?
- email?
- password?
- all of them?
Better:
PATCH /users/{id}/email
Now:
- the intent is clear,
- the scope is limited,
- misuse becomes harder.
If an API endpoint can do “too many things,” it will eventually do the wrong one.
Make Invalid States Impossible
Senior engineers try to design APIs where invalid states cannot exist.
For example, instead of:
{
"status": "active",
"deleted": true
}
Design it so:
- a user cannot be both active and deleted,
- the API does not accept conflicting inputs.
If your API allows contradictory data, the bug isn’t in the client — it’s in the design.
Use Explicit Errors, Not Silent Defaults
Silent behavior is dangerous.
Bad design:
- missing field → default value applied
- invalid input → ignored
- partial failure → success response
This makes bugs invisible.
Better design:
- reject invalid input loudly,
- return clear error messages,
- fail fast and early.
APIs should teach consumers how to use them correctly through errors.
Avoid “Magic” Behavior
Magic APIs feel convenient at first — and painful later.
Examples of magic behavior:
- auto-creating resources without saying so,
- silently retrying without limits,
- changing behavior based on hidden flags.
If something important happens, make it explicit.
Good APIs are boring.
Boring APIs are predictable.
Idempotency Matters More Than You Think
In real systems:
- requests get retried,
- clients time out,
- networks fail.
If calling an API twice causes unintended side effects, it will break in production.
Design APIs so:
- repeated requests are safe,
- duplicate calls don’t corrupt state,
- clients don’t need complex retry logic.
This alone prevents many production incidents.
Design for the Future Consumer
The person using your API in six months:
- might not be you,
- might not know the context,
- might be under pressure.
Ask yourself:
- Can this API be misunderstood?
- Can it be called in the wrong order?
- Can misuse cause data corruption?
If the answer is “yes,” redesign.
A Real-World Example
Many production bugs come from APIs like:
POST /processPayment
Called twice → double charge.
A better design:
- separate intent from execution,
- require idempotency keys,
- make side effects explicit.
The extra effort up front saves incidents later.
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Final Takeaway
APIs are contracts, not just endpoints.
A well-designed API:
- limits what can go wrong,
- makes correct usage obvious,
- makes incorrect usage difficult,
- protects the system from human error.
Senior engineers don’t just design APIs that work —
they design APIs that are hard to misuse.
Top comments (2)
Thanks for sharing this article! I found it very informative and enjoyable to read.
Thank you!