diff --git a/packages/extension-form/src/internal/FormField.ts b/packages/extension-form/src/internal/FormField.ts
index 759b563..f33818f 100644
--- a/packages/extension-form/src/internal/FormField.ts
+++ b/packages/extension-form/src/internal/FormField.ts
@@ -35,12 +35,19 @@ export interface ArrayFormField<
     readonly elements: readonly Formify.Formify<Value>[]
 }
 
-export interface StructFormField<
+export type StructFormField<
     S extends Schema.Struct<Fields>,
     Fields extends Schema.Struct.Fields,
-> extends FormField<S> {
-    readonly fields: { readonly [K in keyof Fields]: Formify.Formify<Fields[K]> }
-}
+> = (
+    & FormField<S>
+    & { readonly fields: { readonly [K in keyof Fields]: Formify.Formify<Fields[K]> } }
+    & {
+        [K in keyof Fields as Fields[K] extends
+            Schema.tag<infer _> ? K : never
+        ]: Fields[K] extends
+            Schema.tag<infer Tag> ? Tag : never
+    }
+)
 
 export interface GenericFormField<S extends Schema.Schema.Any> extends FormField<S> {
     readonly value: S["Type"]
diff --git a/packages/extension-form/src/internal/Formify.ts b/packages/extension-form/src/internal/Formify.ts
index 79a9254..77a4bd1 100644
--- a/packages/extension-form/src/internal/Formify.ts
+++ b/packages/extension-form/src/internal/Formify.ts
@@ -22,7 +22,7 @@ export type Formify<S> = (
 )
 
 
-const LoginForm = Schema.Union(
+const Login = Schema.Union(
     Schema.Struct({
         _tag: Schema.tag("ByEmail"),
         email: Schema.String,
@@ -34,12 +34,18 @@ const LoginForm = Schema.Union(
         phone: Schema.String,
         password: Schema.RedactedFromSelf(Schema.String),
     }),
-)
-type LoginFormTree = Formify<typeof LoginForm>
-declare const loginFormTree: LoginFormTree
 
-switch (loginFormTree.member.fields._tag.value) {
+    Schema.TaggedStruct("ByKey", {
+        id: Schema.String,
+        password: Schema.RedactedFromSelf(Schema.String),
+    }),
+)
+type LoginForm = Formify<typeof Login>
+declare const loginForm: LoginForm
+
+switch (loginForm.member._tag) {
     case "ByEmail":
+        loginForm.member
     break
     case "ByPhone":
     break