diff --git a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs
index d3ec7ab8f0b..bf5d2c44b4a 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs
@@ -36,6 +36,7 @@ public FunctionApprovalRequestContent(string id, FunctionCallContent functionCal
     /// Creates a <see cref="FunctionApprovalResponseContent"/> to indicate whether the function call is approved or rejected based on the value of <paramref name="approved"/>.
     /// </summary>
     /// <param name="approved"><see langword="true"/> if the function call is approved; otherwise, <see langword="false"/>.</param>
+    /// <param name="reason">An optional reason for the approval or rejection.</param>
     /// <returns>The <see cref="FunctionApprovalResponseContent"/> representing the approval response.</returns>
-    public FunctionApprovalResponseContent CreateResponse(bool approved) => new(Id, approved, FunctionCall);
+    public FunctionApprovalResponseContent CreateResponse(bool approved, string? reason = null) => new(Id, approved, FunctionCall) { Reason = reason };
 }
diff --git a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalResponseContent.cs b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalResponseContent.cs
index 948dc6a1347..3af623e9549 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalResponseContent.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalResponseContent.cs
@@ -38,4 +38,9 @@ public FunctionApprovalResponseContent(string id, bool approved, FunctionCallCon
     /// Gets the function call for which approval was requested.
     /// </summary>
     public FunctionCallContent FunctionCall { get; }
+
+    /// <summary>
+    /// Gets or sets the optional reason for the approval or rejection.
+    /// </summary>
+    public string? Reason { get; set; }
 }
diff --git a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
index b3885542de4..cc43c192f89 100644
--- a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
@@ -1418,7 +1418,16 @@ private static (List<ApprovalResultWithRequestMessage>? approvals, List<Approval
     /// <returns>The <see cref="AIContent"/> for the rejected function calls.</returns>
     private static List<AIContent>? GenerateRejectedFunctionResults(List<ApprovalResultWithRequestMessage>? rejections) =>
         rejections is { Count: > 0 } ?
-            rejections.ConvertAll(static m => (AIContent)new FunctionResultContent(m.Response.FunctionCall.CallId, "Error: Tool call invocation was rejected by user.")) :
+            rejections.ConvertAll(m =>
+            {
+                string result = "Tool call invocation rejected.";
+                if (!string.IsNullOrWhiteSpace(m.Response.Reason))
+                {
+                    result = $"{result} {m.Response.Reason}";
+                }
+
+                return (AIContent)new FunctionResultContent(m.Response.FunctionCall.CallId, result);
+            }) :
             null;
 
     /// <summary>
diff --git a/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalRequestContentTests.cs b/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalRequestContentTests.cs
index 924243a7d1c..cc5cc1dd8d9 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalRequestContentTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalRequestContentTests.cs
@@ -52,6 +52,28 @@ public void CreateResponse_ReturnsExpectedResponse(bool approved)
         Assert.Same(id, response.Id);
         Assert.Equal(approved, response.Approved);
         Assert.Same(functionCall, response.FunctionCall);
+        Assert.Null(response.Reason);
+    }
+
+    [Theory]
+    [InlineData(true, "Approved for testing")]
+    [InlineData(false, "Rejected due to security concerns")]
+    [InlineData(true, null)]
+    [InlineData(false, null)]
+    public void CreateResponse_WithReason_ReturnsExpectedResponse(bool approved, string? reason)
+    {
+        string id = "req-1";
+        FunctionCallContent functionCall = new("FCC1", "TestFunction");
+
+        FunctionApprovalRequestContent content = new(id, functionCall);
+
+        var response = content.CreateResponse(approved, reason);
+
+        Assert.NotNull(response);
+        Assert.Same(id, response.Id);
+        Assert.Equal(approved, response.Approved);
+        Assert.Same(functionCall, response.FunctionCall);
+        Assert.Equal(reason, response.Reason);
     }
 
     [Fact]
diff --git a/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalResponseContentTests.cs b/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalResponseContentTests.cs
index 67d2f13cf49..405955463a1 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalResponseContentTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalResponseContentTests.cs
@@ -35,10 +35,15 @@ public void Constructor_Roundtrips(string id, bool approved)
         Assert.Same(functionCall, content.FunctionCall);
     }
 
-    [Fact]
-    public void Serialization_Roundtrips()
+    [Theory]
+    [InlineData(null)]
+    [InlineData("Custom rejection reason")]
+    public void Serialization_Roundtrips(string? reason)
     {
-        var content = new FunctionApprovalResponseContent("request123", true, new FunctionCallContent("call123", "functionName"));
+        var content = new FunctionApprovalResponseContent("request123", true, new FunctionCallContent("call123", "functionName"))
+        {
+            Reason = reason
+        };
 
         var json = JsonSerializer.Serialize(content, AIJsonUtilities.DefaultOptions);
         var deserializedContent = JsonSerializer.Deserialize<FunctionApprovalResponseContent>(json, AIJsonUtilities.DefaultOptions);
@@ -46,6 +51,7 @@ public void Serialization_Roundtrips()
         Assert.NotNull(deserializedContent);
         Assert.Equal(content.Id, deserializedContent.Id);
         Assert.Equal(content.Approved, deserializedContent.Approved);
+        Assert.Equal(content.Reason, deserializedContent.Reason);
         Assert.NotNull(deserializedContent.FunctionCall);
         Assert.Equal(content.FunctionCall.CallId, deserializedContent.FunctionCall.CallId);
         Assert.Equal(content.FunctionCall.Name, deserializedContent.FunctionCall.Name);
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs
index 93298ba2ac1..01f2e111447 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs
@@ -331,8 +331,8 @@ public async Task RejectedApprovalResponsesAreFailedAsync()
             new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
             new ChatMessage(ChatRole.Tool,
             [
-                new FunctionResultContent("callId1", result: "Error: Tool call invocation was rejected by user."),
-                new FunctionResultContent("callId2", result: "Error: Tool call invocation was rejected by user.")
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected."),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected.")
             ]),
         ];
 
@@ -346,8 +346,8 @@ public async Task RejectedApprovalResponsesAreFailedAsync()
             new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
             new ChatMessage(ChatRole.Tool,
             [
-                new FunctionResultContent("callId1", result: "Error: Tool call invocation was rejected by user."),
-                new FunctionResultContent("callId2", result: "Error: Tool call invocation was rejected by user.")
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected."),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected.")
             ]),
             new ChatMessage(ChatRole.Assistant, "world"),
         ];
@@ -388,7 +388,7 @@ public async Task MixedApprovedAndRejectedApprovalResponsesAreExecutedAndFailedA
         [
             new ChatMessage(ChatRole.User, "hello"),
             new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
-            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Error: Tool call invocation was rejected by user.")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Tool call invocation rejected.")]),
             new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId2", result: "Result 2: 42")]),
         ];
 
@@ -400,7 +400,7 @@ public async Task MixedApprovedAndRejectedApprovalResponsesAreExecutedAndFailedA
         List<ChatMessage> nonStreamingOutput =
         [
             new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
-            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Error: Tool call invocation was rejected by user.")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Tool call invocation rejected.")]),
             new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId2", result: "Result 2: 42")]),
             new ChatMessage(ChatRole.Assistant, "world"),
         ];
@@ -410,7 +410,7 @@ public async Task MixedApprovedAndRejectedApprovalResponsesAreExecutedAndFailedA
             new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
             new ChatMessage(ChatRole.Tool,
             [
-                new FunctionResultContent("callId1", result: "Error: Tool call invocation was rejected by user."),
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected."),
                 new FunctionResultContent("callId2", result: "Result 2: 42")
             ]),
             new ChatMessage(ChatRole.Assistant, "world"),
@@ -421,6 +421,222 @@ public async Task MixedApprovedAndRejectedApprovalResponsesAreExecutedAndFailedA
         await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, streamingOutput, expectedDownstreamClientInput);
     }
 
+    [Fact]
+    public async Task RejectedApprovalResponsesWithCustomReasonAsync()
+    {
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                new ApprovalRequiredAIFunction(AIFunctionFactory.Create(() => "Result 1", "Func1")),
+                AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
+            ]
+        };
+
+        List<ChatMessage> input =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionApprovalRequestContent("callId1", new FunctionCallContent("callId1", "Func1")),
+                new FunctionApprovalRequestContent("callId2", new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } }))
+            ]) { MessageId = "resp1" },
+            new ChatMessage(ChatRole.User,
+            [
+                new FunctionApprovalResponseContent("callId1", false, new FunctionCallContent("callId1", "Func1"))
+                {
+                    Reason = "User denied permission for this operation"
+                },
+                new FunctionApprovalResponseContent("callId2", false, new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } }))
+                {
+                    Reason = "Function Func2 is not allowed at this time"
+                }
+            ]),
+        ];
+
+        List<ChatMessage> expectedDownstreamClientInput =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected. User denied permission for this operation"),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected. Function Func2 is not allowed at this time")
+            ]),
+        ];
+
+        List<ChatMessage> downstreamClientOutput =
+        [
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        List<ChatMessage> output =
+        [
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1"), new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected. User denied permission for this operation"),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected. Function Func2 is not allowed at this time")
+            ]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+
+        await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+    }
+
+    [Fact]
+    public async Task MixedApprovalResponsesWithCustomAndDefaultReasonsAsync()
+    {
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                new ApprovalRequiredAIFunction(AIFunctionFactory.Create(() => "Result 1", "Func1")),
+                AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
+                AIFunctionFactory.Create((string s) => $"Result 3: {s}", "Func3"),
+            ]
+        };
+
+        List<ChatMessage> input =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionApprovalRequestContent("callId1", new FunctionCallContent("callId1", "Func1")),
+                new FunctionApprovalRequestContent("callId2", new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })),
+                new FunctionApprovalRequestContent("callId3", new FunctionCallContent("callId3", "Func3", arguments: new Dictionary<string, object?> { { "s", "test" } }))
+            ]) { MessageId = "resp1" },
+            new ChatMessage(ChatRole.User,
+            [
+                new FunctionApprovalResponseContent("callId1", false, new FunctionCallContent("callId1", "Func1")) { Reason = "Custom rejection for Func1" },
+                new FunctionApprovalResponseContent("callId2", false, new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })),
+                new FunctionApprovalResponseContent("callId3", true, new FunctionCallContent("callId3", "Func3", arguments: new Dictionary<string, object?> { { "s", "test" } }))
+            ]),
+        ];
+
+        List<ChatMessage> expectedDownstreamClientInput =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("callId1", "Func1"),
+                new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } }),
+                new FunctionCallContent("callId3", "Func3", arguments: new Dictionary<string, object?> { { "s", "test" } })
+            ]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected. Custom rejection for Func1"),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected.")
+            ]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId3", result: "Result 3: test")]),
+        ];
+
+        List<ChatMessage> downstreamClientOutput =
+        [
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        List<ChatMessage> nonStreamingOutput =
+        [
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("callId1", "Func1"),
+                new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } }),
+                new FunctionCallContent("callId3", "Func3", arguments: new Dictionary<string, object?> { { "s", "test" } })
+            ]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected. Custom rejection for Func1"),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected.")
+            ]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId3", result: "Result 3: test")]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        List<ChatMessage> streamingOutput =
+        [
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("callId1", "Func1"),
+                new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } }),
+                new FunctionCallContent("callId3", "Func3", arguments: new Dictionary<string, object?> { { "s", "test" } })
+            ]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected. Custom rejection for Func1"),
+                new FunctionResultContent("callId2", result: "Tool call invocation rejected."),
+                new FunctionResultContent("callId3", result: "Result 3: test")
+            ]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, input, downstreamClientOutput, nonStreamingOutput, expectedDownstreamClientInput);
+
+        await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, streamingOutput, expectedDownstreamClientInput);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public async Task RejectedApprovalResponsesWithEmptyOrWhitespaceReasonUsesDefaultMessageAsync(string? reason)
+    {
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                new ApprovalRequiredAIFunction(AIFunctionFactory.Create(() => "Result 1", "Func1")),
+            ]
+        };
+
+        List<ChatMessage> input =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionApprovalRequestContent("callId1", new FunctionCallContent("callId1", "Func1")),
+            ]) { MessageId = "resp1" },
+            new ChatMessage(ChatRole.User,
+            [
+                new FunctionApprovalResponseContent("callId1", false, new FunctionCallContent("callId1", "Func1"))
+                {
+                    Reason = reason
+                },
+            ]),
+        ];
+
+        List<ChatMessage> expectedDownstreamClientInput =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected.")
+            ]),
+        ];
+
+        List<ChatMessage> downstreamClientOutput =
+        [
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        List<ChatMessage> output =
+        [
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected.")
+            ]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+
+        await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+    }
+
     [Fact]
     public async Task ApprovedInputsAreExecutedAndFunctionResultsAreConvertedAsync()
     {