Capability declaration
23 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.
23 extensions
4846 occurrences
3 vendor-named
The extensions
| Extension | What it appears to do | Uses | Providers |
|---|---|---|---|
| x-undo | `x-undo` appears on operations. Its value is a structured object. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where the key appe | 523 | 1 |
| x-capability | `x-capability` appears on operations. Its value is a list. Used by a single provider across 1 OpenAPI document. Observed values include `transformationsAcl:READ`, `transformationsAcl:WRITE`, `subscrib | 426 | 1 |
| x-requiresInternalClient | `x-requiresInternalClient` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferred from whe | 424 | 1 |
| x-requiresUser | `x-requiresUser` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferred from where the key | 424 | 1 |
| x-supportsCursorBasedPaging | `x-supportsCursorBasedPaging` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferred from | 424 | 1 |
| x-supportsCursorBasedPagingWithoutTotal | `x-supportsCursorBasedPagingWithoutTotal` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is in | 424 | 1 |
| x-supportsOffsetBasedPaging | `x-supportsOffsetBasedPaging` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferred from | 424 | 1 |
| x-supportsOffsetBasedPagingWithoutTotal | `x-supportsOffsetBasedPagingWithoutTotal` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is in | 424 | 1 |
| x-supportsPermissionDeniedResponse | `x-supportsPermissionDeniedResponse` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferre | 424 | 1 |
| x-supportsPush | `x-supportsPush` appears on operations. Its value is a boolean flag. Used by a single provider across 17 OpenAPI documents. Consistent with capability declaration — this is inferred from where the key | 424 | 1 |
| x-pd-requires-scope | `x-pd-requires-scope` appears on operations. Its value is a string. Used by a single provider across 1 OpenAPI document. Observed values include `services.write`, `services.read`, `incidents.read`, `s | 310 | 1 |
| x-requires-authentication | `x-requires-authentication` appears on operations. Its value is a boolean flag. Used by a single provider across 12 OpenAPI documents. Consistent with capability declaration — this is inferred from wh | 60 | 1 |
|
x-konfig-operation-can-have-single-parameter
vendor |
`x-konfig-operation-can-have-single-parameter` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. It sits in the `x-konfig-` namespace, so its mea | 53 | 1 |
| x-supports-streaming | `x-supports-streaming` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where the | 25 | 1 |
| x-requires-administrator | `x-requires-administrator` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where | 20 | 1 |
| x-feature-flag | `x-feature-flag` appears on operations. Its value is a string. Used by 2 providers across 2 OpenAPI documents. Observed values include `user-groups-enabled`, `p1ax-1984-configuration-promotion`, `user | 15 | 2 |
| x-cdm-capability | `x-cdm-capability` appears on operations. Its value is a list. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where the key appears | 7 | 1 |
| x-gremlin-feature-flag | `x-gremlin-feature-flag` appears on operations. Its value is a string. Used by a single provider across 1 OpenAPI document. Observed values include `gcp-ihc`. Consistent with capability declaration — | 4 | 1 |
| x-requires-api-key | `x-requires-api-key` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where the k | 4 | 1 |
| x-requires-owner | `x-requires-owner` appears on operations. Its value is a boolean flag. Used by a single provider across 1 OpenAPI document. Consistent with capability declaration — this is inferred from where the key | 3 | 1 |
|
x-box-requires-admin
vendor |
`x-box-requires-admin` appears on operations. Its value is a boolean flag. Used by a single provider across 2 OpenAPI documents. It sits in the `x-box-` namespace, so its meaning is defined by box too | 2 | 1 |
|
x-ms-has-immutability-policy
vendor |
`x-ms-has-immutability-policy` 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 meaning is def | 1 | 1 |
| x-requiresEnv | `x-requiresEnv` appears on parameter objects. Its value is a string. Used by a single provider across 1 OpenAPI document. Observed values include `API_ALLOW_SKIP_PAGINATION`. Consistent with capabilit | 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=capability-declaration&limit=10"
Everything under a tag
curl "https://apis.io/api/v1/tags/capability-declaration"Discovery needs no key. Ratings and market analysis are Pro.