model.APIFormat is deliberately open. A custom label is valid when the composition root supplies a matching codec and router; the model package does not pretend to know provider policy for an unknown string.

Open label

model := model.CustomModel(
	"acme", model.APIFormat("acme-json"),
	"https://api.acme.example/v1", "acme-small",
)
if err := model.Validate(); err != nil {
	return err
}

Model.Validate still enforces a non-empty name, safe HTTPS or loopback HTTP base URL, context-limit relationships, and capability relationships. It does not add a provider/format allowlist.

Implementation

Implement codec.RequestEncoder and codec.ResponseDecoder; add codec.StreamDecoder only if the target streams. Supply a route.Router that owns method, URL, and route headers. If a custom stream body is not SSE, use a custom framer behind StreamDecoder; do not make the generic transport parse the dialect. Add tests for request bytes, response normalization, body close on decoder failure, and typed unsupported-feature errors.

var _ codec.Codec = acmeCodec{}
var _ route.Router = acmeRouter{}

Source and proof

Run the custom package’s tests together with go test ./model ./codec ./route ./transport.