matrix-spec-proposals/api/openapi_extensions.md

# OpenAPI Extensions

For some functionality that is not directly provided by the OpenAPI v2
specification, some extensions have been added that are to be consistent
across the specification. The defined extensions are listed below. Extensions
should not break parsers, however if extra functionality is required, aware
parsers should be able to take advantage of the added syntax.

## Extensible Query Parameters

<!-- TODO: Remove and change instances to 'explode' after OpenAPI/Swagger v3 update -->

If a unknown amount of query parameters can be added to a request, the `name`
must be `fields...`, with the trailing ellipses representing the possibility
of more fields.

Example:

```
  - in: query
    name: fields...
    type: string
```