diff --git a/.chronus/changes/fix-flatten-property-tests-2026-03-04-10-56-42.md b/.chronus/changes/fix-flatten-property-tests-2026-03-04-10-56-42.md
new file mode 100644
index 0000000000..2fbe56ce69
--- /dev/null
+++ b/.chronus/changes/fix-flatten-property-tests-2026-03-04-10-56-42.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/azure-http-specs"
+---
+
+Fix test cases for flatten property with read-only properties.
diff --git a/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/main.tsp b/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/main.tsp
index 7caf563c6b..5a92e439b4 100644
--- a/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/main.tsp
+++ b/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/main.tsp
@@ -172,11 +172,7 @@ model Solution {
 
   #suppress "@azure-tools/typespec-azure-core/no-legacy-usage" "Testing backcompat"
   @global.Azure.ClientGenerator.Core.Legacy.flattenProperty
-  properties: SolutionProperties;
-
-  #suppress "@azure-tools/typespec-azure-core/no-legacy-usage" "Testing backcompat"
-  @global.Azure.ClientGenerator.Core.Legacy.flattenProperty
-  propertiesOptional?: SolutionProperties;
+  properties?: SolutionProperties;
 }
 
 @scenario
@@ -186,7 +182,6 @@ model Solution {
   
   Expected behavior: When flattening a property that contains only read-only properties:
   - For input (write): Read-only properties should not appear in the input model, regardless of flattening
-  - For response (read): Read-only properties should be flattened to the parent model level
   
   The `properties` and `propertiesOptional` fields contain only read-only properties (`solutionId`, 
   `title`, `content`). These should be flattened into the parent `Solution` model in the response.
@@ -198,13 +193,15 @@ model Solution {
   }
   ```
   
-  Expected response body (properties flattened to parent level):
+  Expected response body:
   ```json
   {
     "name": "foo",
-    "solutionId": "solution1",
-    "title": "Solution Title",
-    "content": "Solution Content"
+    "properties": {
+      "solutionId": "solution1",
+      "title": "Solution Title",
+      "content": "Solution Content"
+    }
   }
   ```
   """)
diff --git a/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/mockapi.ts b/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/mockapi.ts
index 67547ad2b9..38d70be79a 100644
--- a/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/mockapi.ts
+++ b/packages/azure-http-specs/specs/azure/client-generator-core/flatten-property/mockapi.ts
@@ -86,9 +86,11 @@ Scenarios.Azure_ClientGenerator_Core_FlattenProperty_putFlattenReadOnlyModel = p
     },
     {
       name: "foo",
-      solutionId: "solution1",
-      title: "Solution Title",
-      content: "Solution Content",
+      properties: {
+        solutionId: "solution1",
+        title: "Solution Title",
+        content: "Solution Content",
+      },
     },
   ),
 );