From 2d0b9fb01e7a5f9d5792c7449cdb91c8ff7b5fa9 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 27 Mar 2026 02:28:50 +0000
Subject: [PATCH 1/2] Initial plan


From d993f90c6eb04c875690459bedc46fd5b021c5df Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 27 Mar 2026 02:41:25 +0000
Subject: [PATCH 2/2] fix: emit C# class for models extending another model
 with no additional properties

Agent-Logs-Url: https://github.com/microsoft/typespec/sessions/606be68b-2055-4873-8f57-07dc41156fd0

Co-authored-by: markcowl <1054056+markcowl@users.noreply.github.com>
---
 ...-empty-derived-model-emission-2026-3-27.md |  7 +++++
 packages/http-server-csharp/src/lib/utils.ts  |  2 +-
 .../test/generation.test.ts                   | 27 +++++++++++++++++++
 3 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 .chronus/changes/fix-empty-derived-model-emission-2026-3-27.md

diff --git a/.chronus/changes/fix-empty-derived-model-emission-2026-3-27.md b/.chronus/changes/fix-empty-derived-model-emission-2026-3-27.md
new file mode 100644
index 00000000000..5853e64894b
--- /dev/null
+++ b/.chronus/changes/fix-empty-derived-model-emission-2026-3-27.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Emit C# class for models that extend another model with no additional properties
diff --git a/packages/http-server-csharp/src/lib/utils.ts b/packages/http-server-csharp/src/lib/utils.ts
index 2b64cb84fea..10955435bb7 100644
--- a/packages/http-server-csharp/src/lib/utils.ts
+++ b/packages/http-server-csharp/src/lib/utils.ts
@@ -1059,7 +1059,7 @@ export function getControllerReturnStatement(
 
 export function isEmptyResponseModel(program: Program, model: Type): boolean {
   if (model.kind !== "Model") return false;
-  if (model.properties.size === 0) return true;
+  if (model.properties.size === 0 && !model.baseModel) return true;
 
   return (
     model.properties.size === 1 &&
diff --git a/packages/http-server-csharp/test/generation.test.ts b/packages/http-server-csharp/test/generation.test.ts
index 311289ba764..5cb6dfacfca 100644
--- a/packages/http-server-csharp/test/generation.test.ts
+++ b/packages/http-server-csharp/test/generation.test.ts
@@ -3395,3 +3395,30 @@ describe("collection type: defined as emitter option", () => {
     );
   });
 });
+
+it("emits class for model extending another model with no additional properties", async () => {
+  await compileAndValidateMultiple(
+    tester,
+    `
+      model Foo {
+        id: int32;
+        name: string;
+      }
+
+      model Baz extends Foo {}
+
+      @route("/foo/{id}") @get op getFoo(id: int32): Foo;
+      `,
+    [
+      [
+        "Foo.cs",
+        [
+          "public partial class Foo",
+          "public int Id { get; set; }",
+          "public string Name { get; set; }",
+        ],
+      ],
+      ["Baz.cs", ["public partial class Baz : Foo"]],
+    ],
+  );
+});