diff --git a/internal/domain/grant_type.go b/internal/domain/grant_type.go
new file mode 100644
index 0000000..2dea568
--- /dev/null
+++ b/internal/domain/grant_type.go
@@ -0,0 +1,44 @@
+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 GrantType struct {
+	slug string
+}
+
+//nolint: gochecknoglobals // NOTE(toby3d): structs cannot be constants
+var (
+	GrantTypeUndefined         = GrantType{slug: ""}
+	GrantTypeAuthorizationCode = GrantType{slug: "authorization_code"}
+)
+
+var ErrGrantTypeUnknown = errors.New("unknown grant type")
+
+func ParseGrantType(slug string) (GrantType, error) {
+	if strings.ToLower(slug) == GrantTypeAuthorizationCode.slug {
+		return GrantTypeAuthorizationCode, nil
+	}
+
+	return GrantTypeUndefined, fmt.Errorf("%w: %s", ErrGrantTypeUnknown, slug)
+}
+
+func (gt *GrantType) UnmarshalForm(src []byte) error {
+	responseType, err := ParseGrantType(string(src))
+	if err != nil {
+		return fmt.Errorf("grant_type: %w", err)
+	}
+
+	*gt = responseType
+
+	return nil
+}
+
+func (gt GrantType) String() string {
+	return gt.slug
+}