Fix double-suffix and oneOf variant mutation bugs

- Add withInputSuffix helper to prevent "PetInput" → "PetInputInput"
- Fix oneOf input unions to mutate variant types with input context
  (model variants now correctly get their Input suffix, e.g. Cat → CatInput)
- Refactor model.ts to use shared withInputSuffix helper

Author: FionaBronwen

packages/graphql/src/lib/type-utils.ts

@@ -113,6 +113,10 @@ export function sanitizeNameForGraphQL(name: string, prefix: string = ""): strin
   return name;
 }
 
+/** Add "Input" suffix to a name if it doesn't already have one. */
+export function withInputSuffix(name: string): string {
+  return name.endsWith("Input") ? name : name + "Input";
+}
 
 /** Convert a name to CONSTANT_CASE for GraphQL enum members. */
 export function toEnumMemberName(enumName: string, name: string) {

packages/graphql/src/mutation-engine/mutations/model.ts

@@ -6,7 +6,7 @@ import {
   type SimpleMutationOptions,
   type SimpleMutations,
 } from "@typespec/mutator-framework";
-import { sanitizeNameForGraphQL } from "../../lib/type-utils.js";
+import { sanitizeNameForGraphQL, withInputSuffix } from "../../lib/type-utils.js";
 import { GraphQLMutationOptions, GraphQLTypeContext } from "../options.js";
 
 /**
@@ -38,8 +38,8 @@ export class GraphQLModelMutation extends SimpleModelMutation<SimpleMutationOpti
       // Apply GraphQL name sanitization
       model.name = sanitizeNameForGraphQL(model.name);
       // Input models get "Input" suffix (unless they already have it)
-      if (this.typeContext === GraphQLTypeContext.Input && !model.name.endsWith("Input")) {
-        model.name = model.name + "Input";
+      if (this.typeContext === GraphQLTypeContext.Input) {
+        model.name = withInputSuffix(model.name);
       }
     });
     super.mutate();

packages/graphql/src/mutation-engine/mutations/union.ts

@@ -18,6 +18,7 @@ import {
   sanitizeNameForGraphQL,
   stripNullVariants,
   toTypeName,
+  withInputSuffix,
 } from "../../lib/type-utils.js";
 import { GraphQLMutationOptions, GraphQLTypeContext } from "../options.js";
 
@@ -196,15 +197,18 @@ export class GraphQLUnionMutation extends UnionMutation<MutationOptions, any, Mu
     const properties: Record<string, ReturnType<typeof tk.modelProperty.create>> = {};
     for (const variant of flattenedVariants) {
       const fieldName = sanitizeNameForGraphQL(variantNameToString(variant.name));
+      // Mutate the variant type with the same options (preserving input context)
+      // so that model variants get their Input suffix.
+      const variantMutation = this.engine.mutate(variant.type, this.options);
       properties[fieldName] = tk.modelProperty.create({
         name: fieldName,
-        type: variant.type,
+        type: variantMutation.mutationNode.mutatedType,
         optional: true, // oneOf: exactly one must be provided
       });
     }
 
     const unionName = getUnionName(this.sourceType, program);
-    const modelName = sanitizeNameForGraphQL(unionName) + "Input";
+    const modelName = withInputSuffix(sanitizeNameForGraphQL(unionName));
 
     const oneOfModel = tk.model.create({
       name: modelName,