How to specify a constant HTTP header value in a Smithy model?

I’d like to model an API operation that requires a specific header value to be provided; I’d prefer not to pass the complexity on to the API consumer.

In my mind this should go into the @http trait applied to the operation shape, but from the docs that’s not supported.

Is there an existing trait that would be an appropriate way to do this?

@http(method:"POST", uri: "/", headers: { "api-version": "v1" })
operation DescribeThing {
  ...
}

[glb]

@mtdowling got it, thanks. I like the idea of supporting this at both levels. I’d suggest semantics of “if you specify the same header name on a service and an operation then the operation will take precedence”. I don’t like the idea of it being impossible to override the default service value for an operation, and I expect that folks won’t accidentally add the trait to an operation. Thoughts?

[eduardomourar]

@glb, when I first started using Smithy I expected to do something like this:

/// Some description to be added to the constant
@const("v1")
string ApiVersionV1

@http(method:"POST", uri: "/")
operation DescribeThing {
  @httpHeader("api-version")
  apiVersion: ApiVersionV1,
  ...
}

As you can see, if the const trait were available in Smithy IDL, we would be able to have constant values anywhere we want not only in headers.