Models

De facto standards for the model layer, published by the API Commons.

OpenAI published an API. The industry turned it into a wire format. Today a chat completion request written for one provider runs against hundreds of others, and "OpenAI-compatible" appears in the documentation of 211 providers in the API Evangelist catalog — without anyone ever having written down what the phrase commits you to.

This repository writes it down.

What "compatible" turns out to mean

Two operations.

Operation	Adopters	Share
POST /chat/completions	105	87.5%
GET /models	86	71.7%
POST /embeddings	39	32.5%

Nothing else clears a third of the cohort. The gap between 71.7% and 32.5% is the tier boundary, and it was measured rather than chosen: every one of OpenAI's 338 operations is graded by the share of adopters who declare it in their own published OpenAPI.

Of the 211 providers claiming compatibility, 67 demonstrably reach the core tier. 101 publish a spec we can read; 34 of those declare part of the profile but not all of it. The rest say it in prose. None of that is a failing grade — a partial implementation is a normal thing to be. It is recorded rather than rounded up.

The second dialect

A growing share of model hosts speak Anthropic's Messages API alongside OpenAI's. Profiling it turned out to need a different denominator, and the reason is worth stating plainly.

29 of Anthropic's operations share a path shape with the OpenAI interface — /models, /files, /skills, /vaults, /agents. Measured against everyone who publishes a spec, GET /models came out at 86.3% for the Anthropic profile. That number was measuring OpenAI. The cohort is therefore restricted to the 29 providers declaring at least one operation unique to Anthropic, and every shared operation carries ambiguous: true in the profile and a warning in the matrix.

Operation	Adopters	Share
GET /v1/models	21	72.4% (shared shape)
POST /v1/messages	20	69.0%
POST /v1/messages/count_tokens	4	13.8%

The dialects differ in a way that breaks shims: Anthropic requires max_tokens where OpenAI treats it as optional, and returns content blocks where OpenAI returns choices. Both are rules in the Anthropic ruleset, and both fire on real providers — 0g-labs and friendliai declare a messages request without max_tokens.

What's here

standard/openai-v1/              338 operations graded, 2 core
standard/anthropic-messages-v1/  244 operations graded, 2 core
  profile.yml        every operation with its tier and the evidence for it
  openapi.yml        the core + extended surface, self-contained, x-tier on every operation
  overlay.yml        an OpenAPI Overlay that stamps the tiers onto the upstream spec
  schema/            JSON Schema 2020-12 for the core request and response bodies
  mcp/tools.json     MCP tool definitions for the core tier
  arazzo/            the flow that proves a core claim: list models, then use one
  spectral/          lint any spec against the profile, with fixtures and a harness
adopters/
  openai-v1.yml                    175 entries
  anthropic-messages-v1.yml         20 entries
  *-matrix.md                      generated: operation x adopter
scripts/             every artifact above is generated; none is hand-maintained

Check a provider

npx @stoplight/spectral-cli lint their-openapi.yaml \
  -r https://raw.githubusercontent.com/api-commons/models/main/standard/openai-v1/spectral/openai-profile.yaml

npx @stoplight/spectral-cli lint their-openapi.yaml \
  -r https://raw.githubusercontent.com/api-commons/models/main/standard/anthropic-messages-v1/spectral/anthropic-profile.yaml

It reports whether they reach the core tier and whether the operations they declare have the shape a client written against OpenAI expects. Run against real providers on 2026-09-13: Groq and OpenRouter lint clean, Cerebras declares no model listing, and DeepInfra's response shapes differ from the profile.

What it does not check

Runtime behaviour — a spec is a claim about a server, not the server. Streaming semantics, because SSE framing is not expressible in OpenAPI. Whether gpt-4o means the same thing at two providers, which no ruleset can answer. Extended and vendor tiers beyond one informational notice, because declaring them is not a defect.

A known upstream limitation

spectral:oas cannot lint this profile. Spectral 6.16.3 crashes on it with Cannot read properties of null (reading 'enum') — and crashes identically on OpenAI's own untouched upstream spec, which is how we know it is not caused by anything done here. It is not the null default or null example values either; both were ruled out by elimination. The profile's own ruleset runs clean over the same document, and the generated Arazzo and JSON Schema artifacts validate normally.

Regenerate

python3 scripts/build-profile.py   --spec <upstream> --sha <commit> --census <census> \
                                   --cohort <n> --claimants <n> [--standard <id>] ...
python3 scripts/build-artifacts.py --standard <id>
python3 scripts/harvest-adopters.py --standard <id> --candidates-key "<claim>" --census <census>
node scripts/test-ruleset.mjs      # every profile, every rule, both clean fixtures

The upstream spec moves weekly, so re-pinning is a deliberate versioned act rather than a silent drift.

Provenance

The OpenAI interface is derived from openai/openai-openapi at commit 38170fdddbb6a1813eae6c6587ee17cf2987185b — OpenAPI 3.1.0, info.version 2.3.0, 215 paths, 338 operations. That repository is MIT; this profile carries its attribution and does not republish it.

The Anthropic Messages dialect is derived from scripts/mock-spec.json.gz in anthropics/anthropic-sdk-python — OpenAPI 3.1.0, 165 paths, 244 operations, no declared info.version. That repository is MIT. Anthropic bundled this spec into their own repo on 2026-09-03, in the same commit that removed the Stainless CDN URL from .stats.yml; the in-repo file is a better source than the CDN copy, because it is first-party and pinnable by commit.

Not published by, affiliated with, or endorsed by OpenAI or Anthropic. Both names appear here as factual references to the interfaces being described.

Part of API Commons

Related

• adopters — the schema behind the registry
• storage · open-data · blockchain — the other standards from practice
• api-validator — lint in the browser

License

Artifacts — the profile, schemas, and API descriptions — are licensed CC BY-NC-SA 4.0. Code — everything in scripts/ — is licensed Apache-2.0.