diff --git a/internal/domain/response_type.go b/internal/domain/response_type.go
new file mode 100644
index 0000000..41a688e
--- /dev/null
+++ b/internal/domain/response_type.go
@@ -0,0 +1,63 @@
+package domain
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// NOTE(toby3d): Encapsulate enums in structs for extra compile-time safety:
+// https://threedots.tech/post/safer-enums-in-go/#struct-based-enums
+type ResponseType struct {
+	slug string
+}
+
+//nolint: gochecknoglobals // NOTE(toby3d): structs cannot be constants
+var (
+	ResponseTypeUndefined ResponseType = ResponseType{
+		slug: "",
+	}
+
+	// Deprecated(toby3d): Only accept response_type=code requests, and for
+	// backwards-compatible support, treat response_type=id requests as
+	// response_type=code requests:
+	// https://aaronparecki.com/2020/12/03/1/indieauth-2020#response-type
+	ResponseTypeID ResponseType = ResponseType{
+		slug: "id",
+	}
+
+	// Indicates to the authorization server that an authorization code
+	// should be returned as the response:
+	// https://indieauth.net/source/#authorization-request-li-1
+	ResponseTypeCode ResponseType = ResponseType{
+		slug: "code",
+	}
+)
+
+var ErrResponseTypeUnknown = errors.New("unknown grant type")
+
+func ParseResponseType(slug string) (ResponseType, error) {
+	switch strings.ToLower(slug) {
+	case ResponseTypeCode.slug:
+		return ResponseTypeCode, nil
+	case ResponseTypeID.slug:
+		return ResponseTypeID, nil
+	}
+
+	return ResponseTypeUndefined, fmt.Errorf("%w: %s", ErrResponseTypeUnknown, slug)
+}
+
+func (rt *ResponseType) UnmarshalForm(src []byte) error {
+	responseType, err := ParseResponseType(string(src))
+	if err != nil {
+		return fmt.Errorf("response_type: %w", err)
+	}
+
+	*rt = responseType
+
+	return nil
+}
+
+func (rt ResponseType) String() string {
+	return rt.slug
+}