fix(openapi3): resolve "Duplicate type name" error for named union with bytes in multipart body

.chronus/changes/fix-openapi3-multipart-union-bytes-2026-3-16.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Fix OpenAPI emitter failing with "Duplicate type name" error when using a named union with a `bytes` variant in a multipart body (e.g. `HttpPart<MyUnion>` where `MyUnion` includes `bytes`).

packages/openapi3/src/visibility-usage.ts

@@ -152,6 +152,16 @@ function addUsagesInOperation(
     navigateReferencedTypes(httpOperation.parameters.body.type, visibility, (type, vis) =>
       trackUsage(metadataInfo, usages, type, vis),
     );
+    // For multipart bodies, also navigate part types directly. HttpPart<T> wrappers are
+    // empty models with no properties, so navigateReferencedTypes won't reach T through
+    // normal property traversal, causing T to be incorrectly treated as unreachable.
+    if (httpOperation.parameters.body.bodyKind === "multipart") {
+      for (const part of httpOperation.parameters.body.parts) {
+        navigateReferencedTypes(part.body.type, visibility, (type, vis) =>
+          trackUsage(metadataInfo, usages, type, vis),
+        );
+      }
+    }
   }
   for (const param of httpOperation.parameters.parameters) {
     navigateReferencedTypes(param.param, visibility, (type, vis) =>

packages/openapi3/test/multipart.test.ts

@@ -91,6 +91,33 @@ worksFor(supportedVersions, ({ openApiFor }) => {
       description: "My doc",
     });
   });
+
+  it("named union with bytes variant does not cause 'Duplicate type name' error", async () => {
+    const res = await openApiFor(
+      `
+    union BinaryOrJson {
+      bytes,
+      { file_id: string },
+    }
+    op upload(@header contentType: "multipart/form-data", @multipartBody body: { attachment: HttpPart<BinaryOrJson> }): void;
+    `,
+    );
+    const op = res.paths["/"].post;
+    // The union should be referenced as a $ref (not inlined), and should be available in components
+    expect(op.requestBody.content["multipart/form-data"].schema.properties.attachment.$ref).toEqual(
+      "#/components/schemas/BinaryOrJson",
+    );
+    const schema = res.components.schemas.BinaryOrJson;
+    expect(schema).toBeDefined();
+    // The schema should use anyOf with 2 variants
+    expect(schema.anyOf).toHaveLength(2);
+    // The object variant ({file_id: string}) should appear in the schema regardless of version
+    expect(schema.anyOf).toContainEqual({
+      type: "object",
+      properties: { file_id: { type: "string" } },
+      required: ["file_id"],
+    });
+  });
 });
 
 worksFor(["3.0.0"], ({ openApiFor }) => {