Update follow-up-items: mark nullability bug as resolved

Author: FionaBronwen

packages/graphql/.claude/follow-up-items.md

@@ -0,0 +1,72 @@
+# GraphQL Emitter Follow-up Items
+
+## ~~Input Type Nullability Bug~~ (RESOLVED)
+
+**Status**: Fixed in PR #77 (commit ea85714d2)
+
+**Original issue**: Optional fields in input types were being made non-null.
+
+**Resolution**: Per the GraphQL spec, "nullability directly determines whether a field is required." Optional fields (`?`) should be nullable in both input and output contexts. This also enables circular references in input types (e.g., `Author.friend?: Author` → `friend: AuthorInput` is valid because it's nullable).
+
+**Correct behavior** (now implemented):
+
+| TypeSpec           | Output   | Input    |
+|--------------------|----------|----------|
+| a: string          | String!  | String!  |
+| b?: string         | String   | String   |
+| c: string \| null  | String   | String   |
+
+---
+
+## @oneOf Input Union Bug (Priority: Medium)
+
+**Issue**: Unions used in input context (e.g., mutation parameters) should be converted to `@oneOf` input objects per the GraphQL spec, but the emitter crashes with "Unknown GraphQL type" when attempting this.
+
+**Test case that exposes the bug**:
+```typespec
+@schema
+namespace TestNamespace {
+  model TextContent {
+    text: string;
+  }
+
+  model ImageContent {
+    url: string;
+  }
+
+  union Content {
+    text: TextContent,
+    image: ImageContent,
+  }
+
+  model Post {
+    id: string;
+    content: Content;
+  }
+
+  @query
+  op getPost(id: string): Post;
+
+  @mutation
+  op createPost(content: Content): Post;
+}
+```
+
+**Error**:
+```
+Error: Unknown GraphQL type "ContentInput".
+    at resolveNamedType (src/schema/build/types.ts:100:11)
+```
+
+**Root cause**: The `GraphQLUnionMutation.mutateAsOneOfInput()` correctly creates the `@oneOf` input model (e.g., `ContentInput`), but this synthetic model is not being registered in the type registry that `buildArgsMap` uses to resolve argument types.
+
+**Expected behavior**:
+- In output context: `union Content = TextContent | ImageContent`
+- In input context: `input ContentInput @oneOf { text: TextContentInput, image: ImageContentInput }`
+
+**Files to investigate**:
+- `src/mutation-engine/mutations/union.ts` - `mutateAsOneOfInput()` creates the @oneOf model
+- `src/schema/build/types.ts` - where the type registry is built and `resolveNamedType` fails
+- `src/mutation-engine/schema-mutator.ts` - may need to register synthetic types
+
+**Related**: Similar bug exists for complex input types used in `@operationFields` arguments (the `FilterOptions` model isn't discovered when used only as an operation field argument type).