[tcgc] fix @markAsLro for nullable response types

.chronus/changes/tcgc-fixMarkAsLro-2025-9-23-16-23-7.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Fix usage of `@markAsLro` for lro operations with nullable overall response types

.chronus/changes/tcgc-fixMarkAsLro-2025-9-24-13-16-5.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@azure-tools/azure-http-specs"
+---
+
+merge into release branch

packages/typespec-client-generator-core/src/internal-utils.ts

@@ -917,11 +917,24 @@ export function getTcgcLroMetadata<TServiceOperation extends SdkServiceOperation
   if (getMarkAsLro(context, operation)) {
     // we guard against this in the setting of `@markAsLro`
     const sdkMethod = ignoreDiagnostics(getSdkBasicServiceMethod(context, operation, client));
-    const returnType = sdkMethod.response.type!.__raw! as Model;
+    let returnType: Model;
+    const sdkMethodResponseType = sdkMethod.response.type!;
+    switch (sdkMethodResponseType.kind) {
+      case "nullable":
+        returnType = sdkMethodResponseType.type.__raw! as Model;
+        break;
+      case "model":
+        returnType = sdkMethodResponseType.__raw! as Model;
+        break;
+      default:
+        throw new Error(
+          `LRO method ${operation.name} with @markAsLro must have a model return type.`,
+        );
+    }
     return {
       operation,
       logicalResult: returnType,
-      finalStateVia: FinalStateValue.originalUri, // doesn't really matter, but for get LRO
+      finalStateVia: FinalStateValue.location,
       pollingInfo: {
         kind: "pollingOperationStep",
         responseModel: returnType,

packages/typespec-client-generator-core/src/methods.ts

@@ -389,9 +389,11 @@ function getServiceMethodLroMetadata<TServiceOperation extends SdkServiceOperati
     finalResponse: getFinalResponse(),
     finalStep: getSdkLroServiceFinalStep(context, rawMetadata.finalStep),
     pollingStep: {
-      responseBody: diagnostics.pipe(
-        getClientTypeWithDiagnostics(context, rawMetadata.pollingInfo.responseModel),
-      ) as SdkModelType,
+      responseBody: rawMetadata.pollingInfo.responseModel
+        ? (diagnostics.pipe(
+            getClientTypeWithDiagnostics(context, rawMetadata.pollingInfo.responseModel),
+          ) as SdkModelType)
+        : undefined,
     },
     operation: ignoreDiagnostics(getSdkBasicServiceMethod(context, rawMetadata.operation, client))
       .operation,

packages/typespec-client-generator-core/test/decorators/mark-as-lro.test.ts

@@ -181,8 +181,8 @@ it("should work with ArmResourceRead", async () => {
       }
       @Azure.ClientGenerator.Core.Legacy.markAsLro
       op getProductionSiteDeploymentStatus is ArmResourceRead<
-      Employee
-    >;
+        Employee
+      >;
     `);
 
   const methods = armRunner.context.sdkPackage.clients[0].methods;
@@ -201,3 +201,40 @@ it("should work with ArmResourceRead", async () => {
   strictEqual(metadata.finalResponse?.result?.name, "Employee");
   ok(!metadata.finalResponse?.resultSegments);
 });
+
+it("Extension.Read", async () => {
+  const armRunner = await createSdkTestRunner({
+    librariesToAdd: [AzureResourceManagerTestLibrary, AzureCoreTestLibrary, OpenAPITestLibrary],
+    autoUsings: ["Azure.ResourceManager", "Azure.Core"],
+    emitterName: "@azure-tools/typespec-python",
+  });
+  await armRunner.compileWithBuiltInAzureResourceManagerService(`
+    /** A ContosoProviderHub resource */
+    model Employee is TrackedResource<{}> {
+      ...ResourceNameParameter<Employee>;
+    }
+    @Azure.ClientGenerator.Core.Legacy.markAsLro
+    op get is Extension.Read<
+      Extension.ScopeParameter,
+      Employee,
+      Response = ArmResponse<Employee> | ArmAcceptedResponse,
+      Error = ErrorResponse
+    >;
+    `);
+
+  const methods = armRunner.context.sdkPackage.clients[0].methods;
+  strictEqual(methods.length, 1);
+  const method = methods[0];
+  strictEqual(method.kind, "lro");
+  strictEqual(method.name, "get");
+
+  const metadata = method.lroMetadata;
+  ok(metadata);
+  strictEqual(metadata.finalStateVia, FinalStateValue.originalUri);
+  strictEqual(method.response.type?.kind, "model");
+  strictEqual(method.response.type?.name, "Employee");
+  strictEqual(metadata.envelopeResult?.name, "Employee");
+  strictEqual(metadata.finalResponse?.envelopeResult?.name, "Employee");
+  strictEqual(metadata.finalResponse?.result?.name, "Employee");
+  ok(!metadata.finalResponse?.resultSegments);
+});