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 242d49ece4..8bccd8dc31 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
@@ -110,3 +110,91 @@ op putFlattenModel(@body input: FlattenModel): FlattenModel;
   """)
 @put
 op putNestedFlattenModel(@body input: NestedFlattenModel): NestedFlattenModel;
+
+@doc("This is the model with unknown type properties to be flattened.")
+model FlattenUnknownModel {
+  name: string;
+
+  #suppress "@azure-tools/typespec-azure-core/no-unknown" "For testing purposes"
+  #suppress "@azure-tools/typespec-azure-core/no-legacy-usage" "Testing backcompat"
+  @global.Azure.ClientGenerator.Core.Legacy.flattenProperty
+  properties?: unknown;
+}
+
+@scenario
+@route("/flattenUnknownModel")
+@scenarioDoc("""
+  Update and receive model with flatten property of unknown type.
+  Expected input body:
+  ```json
+  {
+    "name": "foo"
+  }
+  ```
+  
+  Expected response body:
+  ```json
+  {
+    "name": "test",
+    "properties": {
+      "key1": "value1",
+      "key2": "value2"
+    }
+  }
+  ```
+  """)
+@put
+op putFlattenUnknownModel(@body input: FlattenUnknownModel): FlattenUnknownModel;
+
+@doc("This is the model with all read-only properties to be flattened.")
+model SolutionProperties {
+  @visibility(Lifecycle.Read)
+  solutionId?: string;
+
+  @visibility(Lifecycle.Read)
+  title?: string;
+
+  @visibility(Lifecycle.Read)
+  content?: string;
+}
+
+@doc("This is the model with flattened properties that are all read-only.")
+model Solution {
+  name: string;
+
+  #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;
+}
+
+@scenario
+@route("/flattenReadOnlyModel")
+@scenarioDoc("""
+  Test model with flatten property containing all read-only properties.
+  Expected input body:
+  ```json
+  {
+    "name": "foo",
+    "properties": {},
+    "propertiesOptional": {}
+  }
+  ```
+  
+  Expected response body:
+  ```json
+  {
+    "name": "test",
+    "properties": {
+      "solutionId": "solution1",
+      "title": "Solution Title",
+      "content": "Solution Content"
+    }
+  }
+  ```
+  """)
+@put
+op putFlattenReadOnlyModel(@body body: Solution): Solution;
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 8b5205d8f9..422f19579e 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
@@ -61,3 +61,38 @@ Scenarios.Azure_ClientGenerator_Core_FlattenProperty_putNestedFlattenModel = pas
     },
   ),
 );
+
+Scenarios.Azure_ClientGenerator_Core_FlattenProperty_putFlattenUnknownModel = passOnSuccess(
+  createMockApiDefinitions(
+    "flattenUnknownModel",
+    {
+      name: "foo",
+    },
+    {
+      name: "test",
+      properties: {
+        key1: "value1",
+        key2: "value2",
+      },
+    },
+  ),
+);
+
+Scenarios.Azure_ClientGenerator_Core_FlattenProperty_putFlattenReadOnlyModel = passOnSuccess(
+  createMockApiDefinitions(
+    "flattenReadOnlyModel",
+    {
+      name: "foo",
+      properties: {},
+      propertiesOptional: {},
+    },
+    {
+      name: "test",
+      properties: {
+        solutionId: "solution1",
+        title: "Solution Title",
+        content: "Solution Content",
+      },
+    },
+  ),
+);