# Guarantees

URL: /platform/guarantees · Id: `page:/platform/guarantees`

> The promises every API on the platform keeps, each enforced by code and checked by a test.

Every API on the platform keeps the promises below, because the code that serves it enforces each one.

| Promise | Concept | How it holds |
| --- | --- | --- |
| <a id="interfaces-agree"></a>**Interfaces cannot disagree** | [Definition](/reference/concepts#definition) | Every [Interface](/interfaces) reads the one [Definition](/reference/concepts#definition), and a contract test runs each example through each Interface. |
| <a id="posted-is-stated"></a>**The posted rate is the rate every call states** | [Price](/reference/concepts#price) | The cost header and the usage Event of a call carry the rate the pricing document, OpenAPI and the docs post. |
| <a id="unpriced-never-free"></a>**An unpriced operation is never served free** | [Price](/reference/concepts#price) | When the Offer posts no rate for an operation, the whole API answers 503 until it does. |
| <a id="miss-is-free"></a>**A miss costs nothing** | [Price](/reference/concepts#price) | A miss, an error and every generated Interface answer with a price of 0. |
| <a id="retry-counted-once"></a>**A retry is counted once** | [Meter](/reference/concepts#meter) | The usage token is the Idempotency-[Key](/callers/keys), or else the Key, operation, input and hour of the call. |
| <a id="limit-never-skipped"></a>**A Limit is never skipped** | [Limit](/callers/keys#checks) | When the [Limits](/callers/keys#checks) cannot be checked, the call answers 503 and nothing runs. |
| <a id="every-call-kept"></a>**Every call is kept** | [Call record](/evidence/call-record) | Each call is a record that never expires, holding a digest of the Key and never the Key. |
| <a id="test-never-touches-live"></a>**A test call never touches live books** | [Test twin](/callers/keys#kinds) | A test or dev Key keeps its calls and usage in a separate namespace whose books never post. |
| <a id="own-host-only"></a>**Every link is on the API's host** | [Host](/publish/hosting) | Every generated link is built from the origin the request reached. |
| <a id="validated-on-write-and-read"></a>**A Definition is checked when it is written and when it is read** | [Definition](/reference/concepts#definition) | A Definition with any problem is refused on write and on read back, so a bad one never serves. |