OpenAPI gave API definitions a formal model. This repository does the same for API changes: a machine-readable classification of every possible change to an OpenAPI contract, and whether each one breaks existing API consumers.

The model is one file, openapi-changes-model.yaml. It contains:

• The vocabulary: every term the model uses, defined in the file itself: the location grammar and the claim syntax built on it, the seven actions a document edit can perform (add, remove, set, unset, increase, decrease, change), the wire directions, the areas and kinds that situate each change in the OpenAPI object model, the six effects a change can have on the set of valid payloads, the guards (document states that qualify a verdict), the severity levels, and the statuses and categories of the coverage listing.
• The severity law: one rule that derives every verdict. Guards apply first; then narrowing breaks request consumers, widening breaks response consumers, an incomparable change breaks both, and an unknown one is a warning. When a change cannot be proven safe, it is breaking.
• 681 named changes, each with its direction, effect, guards, derived severity, human-readable message, and its claims: the exact document locations and actions it covers, as location:action patterns over the OpenAPI object model.
• The full edit space: 15,255 possible edits, enumerated mechanically from the OpenAPI specification's object model. Every edit is covered by named changes, waived with a written reason, or classified as non-contract (unable to affect which payloads are valid). None are undecided.

See SPEC.md for an explanation of the concepts.

Draft (0.1.0). The model is exported from oasdiff, its reference implementation, at every oasdiff release; the generator in generator/ pins the exact version. The direction of authority is deliberate for now: verdicts are debated here, decided changes land in the reference implementation, and the next export carries them. As the model stabilizes, the intent is to reverse this: the model becomes the source of truth and implementations, oasdiff included, derive from it.

Disagreement with a verdict is the most valuable contribution. If you believe a change is classified wrongly, wrongly waived, or missing:

• Open an issue naming the change id (or the edit's location and action) and the payload that demonstrates your case: a request or response that was valid before and breaks after, or the reason none can exist.
• Pull requests are welcome against SPEC.md and the documentation. The model file itself is generated, so classification changes are agreed in an issue first and then land via the reference implementation.

The severity law is the part most worth challenging: it is short, explicit, and every one of the 681 verdicts follows from it. An argument that changes the law changes everything downstream, on purpose.

Each named change carries claims, the edits it covers. A claim is a path into the OpenAPI definition plus the action performed there: paths.*.*.requestBody.content.*.schema.maxLength:decrease reads as "the maxLength keyword of any request body schema, decreased", with * standing in for a name the API author chooses, such as a specific path, method, or media type, as opposed to the fixed field names of the specification.

- id: request-property-max-length-decreased
  level: error
  direction: request
  effect: narrows
  claims:
    - paths.*.*.requestBody.content.*.schema.maxLength:decrease
  message: the %s request property's maxLength was decreased to %s

The coverage section is the exhaustiveness proof: every edit's disposition, including the written reason for every exclusion. The reference implementation's build fails if an edit is left undecided or a reason goes stale.

Apache-2.0, the same license as the OpenAPI Specification.