Performance & quality
21 distinct extensions were invented to do this job. Each one is a provider deciding OpenAPI would not carry something and building their own way through. Where a job has many names and few uses each, nobody won — the problem was solved privately, over and over.
21 extensions
10732 occurrences
5 vendor-named
The extensions
| Extension | What it appears to do | Uses | Providers |
|---|---|---|---|
| x-validate | `x-validate` appears on schema objects and properties. Its value is a string. Used by a single provider across 12 OpenAPI documents. Observed values include `null`. Consistent with performance & quali | 3505 | 1 |
| x-logErrorCode | `x-logErrorCode` appears on response objects. Its value is an integer. Used by 2 providers across 21 OpenAPI documents. Observed values include `401`, `500`, `404`, `429`. Consistent with performance | 2018 | 2 |
| x-errorCodes | `x-errorCodes` appears on response objects. Its value is a list. Used by a single provider across 3 OpenAPI documents. Consistent with performance & quality — this is inferred from where the key appea | 1860 | 1 |
|
x-ms-error-code
vendor |
`x-ms-error-code` appears on header objects. Its value is a structured object. Used by a single provider across 60 OpenAPI documents. It sits in the `x-ms-` namespace, so its meaning is defined by ms | 1136 | 1 |
| x-latency-category | `x-latency-category` appears on operations. Its value is a string. Used by a single provider across 1 OpenAPI document. Observed values include `responsive`, `interactive`, `background`. Consistent wi | 1038 | 1 |
| x-cfPlanAvailability | `x-cfPlanAvailability` appears on operations. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where | 480 | 1 |
| x-availability | `x-availability` appears on operations. Its value is a string. Used by a single provider across 1 OpenAPI document. Observed values include `High`, `Limited`. Consistent with performance & quality — t | 310 | 1 |
|
x-fern-availability
vendor |
`x-fern-availability` appears on operations. Its value is a string. Used by a single provider across 7 OpenAPI documents. It sits in the `x-fern-` namespace, so its meaning is defined by fern tooling | 186 | 1 |
| x-unstable | `x-unstable` appears on operations. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where the key a | 92 | 1 |
| x-monitor | `x-monitor` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where the key appears | 49 | 1 |
|
x-portone-unstable
vendor |
`x-portone-unstable` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. It sits in the `x-portone-` namespace, so its meaning is defined by porton | 27 | 1 |
| x-idempotency-key | `x-idempotency-key` appears on parameter objects. Its value is a structured object. Used by 3 providers across 10 OpenAPI documents. Consistent with performance & quality — this is inferred from where | 10 | 3 |
| x-400 | `x-400` appears on response objects. Its value is a string. Used by 3 providers across 3 OpenAPI documents. Observed values include `Reference No Not found:`, `Transaction already canceled:`, `From Da | 9 | 3 |
| x-korbit-error-codes | `x-korbit-error-codes` appears on operations. Its value is a list. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where the key appe | 4 | 1 |
| x-ably-errorcode | `x-ably-errorcode` appears on header objects. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where | 2 | 1 |
| x-403 | `x-403` appears on operations. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where the key appear | 1 | 1 |
| x-api-availability | `x-api-availability` appears in the info block. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from whe | 1 | 1 |
| x-error-codes | `x-error-codes` appears at the root of the document. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred fro | 1 | 1 |
| x-idempotent | `x-idempotent` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with performance & quality — this is inferred from where the key appe | 1 | 1 |
|
x-ms-response-latency-in-milliseconds
vendor |
`x-ms-response-latency-in-milliseconds` appears on header objects. Its value is a structured object. Used by a single provider across 1 OpenAPI document. It sits in the `x-ms-` namespace, so its meani | 1 | 1 |
|
x-square-generic-error-codes
vendor |
`x-square-generic-error-codes` appears in the info block. Its value is a list. Used by a single provider across 1 OpenAPI document. It sits in the `x-square-` namespace, so its meaning is defined by s | 1 | 1 |
Work with this as data
Every extension here is available over the APIs.io API and to AI agents over MCP. OpenAPI Extensions is not yet its own endpoint on the v1 API. Reach it through catalog search and the tag graph, or the MCP server.
MCP server
One button, every client — Claude, Cursor, VS Code and the rest.
https://apis.io/mcp
Tools for openapi extensions
3 MCP tools reach this
apis_io_searchSTART HERE — APIs, providers and tags for one query, each with its total.resolveTurn a domain, URL or GitHub org into the provider it belongs to.find_cohortsEvery scored population of providers in the catalog.
Call it yourself
curl for this page
Search the catalog
curl "https://apis.io/api/v1/search?q=performance-quality&limit=10"
Everything under a tag
curl "https://apis.io/api/v1/tags/performance-quality"Discovery needs no key. Ratings and market analysis are Pro.