diff --git a/lib/config/builder/value.ts b/lib/config/builder/value.ts
index 7f442be..cca028b 100644
--- a/lib/config/builder/value.ts
+++ b/lib/config/builder/value.ts
@@ -145,6 +145,7 @@ export class Value<A extends ValueSpec> extends IBuilder<A> {
       name: string;
       description: string | null;
       warning: string | null;
+      selectKey: string;
       variants: Variants<{ [key: string]: { name: string; spec: InputSpec } }>;
       nullable: boolean;
       default: string | null;
diff --git a/lib/config/config-types.ts b/lib/config/config-types.ts
index 5dfbb62..ce4df9c 100644
--- a/lib/config/config-types.ts
+++ b/lib/config/config-types.ts
@@ -66,6 +66,7 @@ export interface ValueSpecBoolean extends WithStandalone {
 
 export interface ValueSpecUnion extends WithStandalone {
   type: "union";
+  selectKey: string;
   nullable: boolean;
   variants: Record<string, { name: string; spec: InputSpec }>;
 }
diff --git a/scripts/oldSpecToBuilder.ts b/scripts/oldSpecToBuilder.ts
index 093e772..b6f1398 100644
--- a/scripts/oldSpecToBuilder.ts
+++ b/scripts/oldSpecToBuilder.ts
@@ -136,13 +136,14 @@ export default async function makeFileContent(
       case "union": {
         const variants = newConst(
           value.name + "_variants",
-          convertVariants(value.variants, value["variant-names"] || {})
+          convertVariants(value.variants, value.tag["variant-names"] || {})
         );
 
         return `Value.union({
         name: ${JSON.stringify(value.name || null)},
-        description: ${JSON.stringify(value.description || null)},
-        warning: ${JSON.stringify(value.warning || null)},
+        description: ${JSON.stringify(value.tag.description || null)},
+        warning: ${JSON.stringify(value.tag.warning || null)},
+        selectKey: ${JSON.stringify(value.tag.id || null)},
         nullable: false,
         default: ${JSON.stringify(value.default || null)},
         variants: ${variants},