From 768364411f2416dad3a0d403a28fd4879e13ed16 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 16 Mar 2026 19:07:55 +0000
Subject: [PATCH 1/3] Initial plan


From 8cc1d72bfd82d1eec749f5a581330db83ac51757 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 16 Mar 2026 19:41:08 +0000
Subject: [PATCH 2/3] fix(openapi3): fix 'Duplicate type name' error for union
 with bytes in multipart body

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>
---
 ...penapi3-multipart-union-bytes-2026-3-16.md |  7 +++++
 packages/openapi3/src/visibility-usage.ts     | 10 +++++++
 packages/openapi3/test/multipart.test.ts      | 27 +++++++++++++++++++
 3 files changed, 44 insertions(+)
 create mode 100644 .chronus/changes/fix-openapi3-multipart-union-bytes-2026-3-16.md

diff --git a/.chronus/changes/fix-openapi3-multipart-union-bytes-2026-3-16.md b/.chronus/changes/fix-openapi3-multipart-union-bytes-2026-3-16.md
new file mode 100644
index 00000000000..264194d9ed4
--- /dev/null
+++ b/.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`).
diff --git a/packages/openapi3/src/visibility-usage.ts b/packages/openapi3/src/visibility-usage.ts
index b01ed33dedb..2e30d48499b 100644
--- a/packages/openapi3/src/visibility-usage.ts
+++ b/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) =>
diff --git a/packages/openapi3/test/multipart.test.ts b/packages/openapi3/test/multipart.test.ts
index 1858f48e0f7..bdb81e55af3 100644
--- a/packages/openapi3/test/multipart.test.ts
+++ b/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 }) => {

From 7019adddb7584252f918e3d1be8bf46810bb78fa Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 16 Mar 2026 20:57:49 +0000
Subject: [PATCH 3/3] style: apply prettier formatting

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>
---
 packages/openapi3/test/multipart.test.ts | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/openapi3/test/multipart.test.ts b/packages/openapi3/test/multipart.test.ts
index bdb81e55af3..b5a53caa25d 100644
--- a/packages/openapi3/test/multipart.test.ts
+++ b/packages/openapi3/test/multipart.test.ts
@@ -104,9 +104,9 @@ worksFor(supportedVersions, ({ openApiFor }) => {
     );
     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");
+    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