diff --git a/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalAgent.cs b/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalAgent.cs
index 0c9e36f392..16a4b04098 100644
--- a/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalAgent.cs
+++ b/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalAgent.cs
@@ -741,11 +741,19 @@ private static bool ArgumentsMatch(IDictionary<string, string> ruleArguments, ID
     /// <summary>
     /// Serializes function call arguments to a string dictionary for storage and comparison.
     /// </summary>
-    private static Dictionary<string, string>? SerializeArguments(IDictionary<string, object?>? arguments, JsonSerializerOptions jsonSerializerOptions)
+    /// <remarks>
+    /// Always returns a non-null dictionary so that an argument-scoped standing approval
+    /// (the <see cref="AlwaysApproveToolApprovalResponseContent.AlwaysApproveToolWithArguments"/>
+    /// path) records an exact-arguments rule. A <see langword="null"/> or empty source dictionary
+    /// yields an empty dictionary, which matches only future no-argument calls. A <see langword="null"/>
+    /// value is reserved on <see cref="ToolApprovalRule.Arguments"/> for tool-level rules and is never
+    /// produced here, preventing an exact-arguments approval from widening into a tool-level approval.
+    /// </remarks>
+    private static Dictionary<string, string> SerializeArguments(IDictionary<string, object?>? arguments, JsonSerializerOptions jsonSerializerOptions)
     {
         if (arguments is null || arguments.Count == 0)
         {
-            return null;
+            return new Dictionary<string, string>(StringComparer.Ordinal);
         }
 
         var serialized = new Dictionary<string, string>(arguments.Count, StringComparer.Ordinal);
diff --git a/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalRule.cs b/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalRule.cs
index e633d1e003..2a8ab690bb 100644
--- a/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalRule.cs
+++ b/dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalRule.cs
@@ -18,9 +18,16 @@ namespace Microsoft.Agents.AI;
 /// <item><b>Tool-level</b>: When <see cref="Arguments"/> is <see langword="null"/>,
 /// all calls to the tool identified by <see cref="ToolName"/> are auto-approved.</item>
 /// <item><b>Tool+arguments</b>: When <see cref="Arguments"/> is non-null,
-/// only calls to the specified tool with exactly matching argument values are auto-approved.</item>
+/// only calls to the specified tool with exactly matching argument values are auto-approved.
+/// A non-null but <b>empty</b> dictionary matches only calls that supply no arguments; it does
+/// not widen into a tool-level rule.</item>
 /// </list>
 /// </para>
+/// <para>
+/// <see langword="null"/> is therefore reserved exclusively for tool-level approval. An
+/// argument-scoped approval (including one created from a no-argument call) is always stored
+/// as a non-null dictionary so it cannot be silently broadened to all invocations of the tool.
+/// </para>
 /// </remarks>
 [Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
 internal sealed class ToolApprovalRule
@@ -34,7 +41,8 @@ internal sealed class ToolApprovalRule
     /// <summary>
     /// Gets or sets the specific argument values that must match for this rule to apply.
     /// When <see langword="null"/>, the rule applies to all invocations of the tool
-    /// regardless of arguments.
+    /// regardless of arguments. A non-null but empty dictionary applies only to
+    /// invocations that supply no arguments.
     /// </summary>
     /// <remarks>
     /// Argument values are stored as their JSON-serialized string representations
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalAgentTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalAgentTests.cs
index 6e497d8bea..3a5c00ff7b 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalAgentTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalAgentTests.cs
@@ -262,6 +262,95 @@ public async Task RunAsync_ToolWithArgsRule_DoesNotAutoApproveDifferentArgsAsync
         Assert.Equal("ReadFile", ((FunctionCallContent)requests[0].ToolCall).Name);
     }
 
+    /// <summary>
+    /// Verify that approving a no-argument call with the "always approve with exact arguments"
+    /// option does NOT auto-approve a later same-tool call that supplies arguments. The later
+    /// call must still surface for explicit approval (security regression for #6486).
+    /// </summary>
+    [Fact]
+    public async Task RunAsync_EmptyArgsToolWithArgsRule_DoesNotAutoApproveCallWithArgumentsAsync()
+    {
+        // Arrange
+        var session = new ChatClientAgentSession();
+
+        // Approve a no-argument SendPayment call with "always approve with exact arguments".
+        var ruleRequest = new ToolApprovalRequestContent("req0", new FunctionCallContent("call0", "SendPayment"));
+        var alwaysApproveResponse = ruleRequest.CreateAlwaysApproveToolWithArgumentsResponse();
+
+        // The inner agent then requests SendPayment WITH sensitive arguments.
+        var sensitiveArgs = new Dictionary<string, object?>
+        {
+            ["recipient"] = "attacker@example.test",
+            ["amount"] = 5000,
+        };
+        var newApprovalRequest = new ToolApprovalRequestContent("req1", new FunctionCallContent("call1", "SendPayment", sensitiveArgs));
+        var approvalResponseMsg = new AgentResponse([new ChatMessage(ChatRole.Assistant, [newApprovalRequest])]);
+
+        var innerAgent = CreateMockAgent(approvalResponseMsg);
+        var agent = new ToolApprovalAgent(innerAgent.Object);
+        var inputMessages = new List<ChatMessage>
+        {
+            new(ChatRole.User, [alwaysApproveResponse]),
+        };
+
+        // Act
+        var response = await agent.RunAsync(inputMessages, session);
+
+        // Assert — the argument-bearing request must surface, not be auto-approved.
+        var requests = response.Messages.SelectMany(m => m.Contents).OfType<ToolApprovalRequestContent>().ToList();
+        Assert.Single(requests);
+        Assert.Equal("SendPayment", ((FunctionCallContent)requests[0].ToolCall).Name);
+    }
+
+    /// <summary>
+    /// Verify that approving a no-argument call with the "always approve with exact arguments"
+    /// option still auto-approves a later same-tool call that also supplies no arguments,
+    /// preserving the intended narrow behavior.
+    /// </summary>
+    [Fact]
+    public async Task RunAsync_EmptyArgsToolWithArgsRule_AutoApprovesLaterEmptyArgsCallAsync()
+    {
+        // Arrange
+        var session = new ChatClientAgentSession();
+
+        var ruleRequest = new ToolApprovalRequestContent("req0", new FunctionCallContent("call0", "SendPayment"));
+        var alwaysApproveResponse = ruleRequest.CreateAlwaysApproveToolWithArgumentsResponse();
+
+        // Inner agent first re-requests SendPayment with no arguments, then returns a final response.
+        var emptyArgsRequest = new ToolApprovalRequestContent("req1", new FunctionCallContent("call1", "SendPayment"));
+        var approvalResponse = new AgentResponse([new ChatMessage(ChatRole.Assistant, [emptyArgsRequest])]);
+        var finalResponse = new AgentResponse([new ChatMessage(ChatRole.Assistant, "Payment sent")]);
+
+        var callCount = 0;
+        var innerAgent = new Mock<AIAgent>();
+        innerAgent
+            .Protected()
+            .Setup<Task<AgentResponse>>("RunCoreAsync",
+                ItExpr.IsAny<IEnumerable<ChatMessage>>(),
+                ItExpr.IsAny<AgentSession?>(),
+                ItExpr.IsAny<AgentRunOptions?>(),
+                ItExpr.IsAny<CancellationToken>())
+            .ReturnsAsync(() =>
+            {
+                callCount++;
+                return callCount == 1 ? approvalResponse : finalResponse;
+            });
+
+        var agent = new ToolApprovalAgent(innerAgent.Object);
+        var inputMessages = new List<ChatMessage>
+        {
+            new(ChatRole.User, [alwaysApproveResponse]),
+        };
+
+        // Act
+        var response = await agent.RunAsync(inputMessages, session);
+
+        // Assert — the no-argument call is auto-approved and the inner agent reaches its final response.
+        Assert.Equal("Payment sent", response.Text);
+        var requests = response.Messages.SelectMany(m => m.Contents).OfType<ToolApprovalRequestContent>().ToList();
+        Assert.Empty(requests);
+    }
+
     #endregion
 
     #region Mixed Auto-Approve
@@ -728,6 +817,56 @@ public void MatchesRule_JsonElementArgs_MatchesCorrectly()
         Assert.True(ToolApprovalAgent.MatchesRule(request, rules, AgentJsonUtilities.DefaultOptions));
     }
 
+    /// <summary>
+    /// Verify that an empty-arguments rule (non-null but empty) matches only a call that
+    /// supplies no arguments, and never widens into a tool-level match.
+    /// </summary>
+    [Fact]
+    public void MatchesRule_EmptyArgumentsRule_MatchesEmptyArgumentCall_ReturnsTrue()
+    {
+        // Arrange — an exact-arguments approval of a no-argument call is stored as an empty dictionary.
+        var rules = new List<ToolApprovalRule>
+        {
+            new()
+            {
+                ToolName = "SendPayment",
+                Arguments = new Dictionary<string, string>(),
+            },
+        };
+        var request = new ToolApprovalRequestContent("req1", new FunctionCallContent("call1", "SendPayment"));
+
+        // Act & Assert
+        Assert.True(ToolApprovalAgent.MatchesRule(request, rules, AgentJsonUtilities.DefaultOptions));
+    }
+
+    /// <summary>
+    /// Verify that an empty-arguments rule does NOT match a later same-tool call that supplies
+    /// arguments. This guards against an exact-arguments approval of a no-argument call being
+    /// widened into a tool-level approval.
+    /// </summary>
+    [Fact]
+    public void MatchesRule_EmptyArgumentsRule_DoesNotMatchCallWithArguments_ReturnsFalse()
+    {
+        // Arrange
+        var rules = new List<ToolApprovalRule>
+        {
+            new()
+            {
+                ToolName = "SendPayment",
+                Arguments = new Dictionary<string, string>(),
+            },
+        };
+        var request = new ToolApprovalRequestContent("req1",
+            new FunctionCallContent("call1", "SendPayment", new Dictionary<string, object?>
+            {
+                ["recipient"] = "attacker@example.test",
+                ["amount"] = 5000,
+            }));
+
+        // Act & Assert
+        Assert.False(ToolApprovalAgent.MatchesRule(request, rules, AgentJsonUtilities.DefaultOptions));
+    }
+
     #endregion
 
     #region Extension Methods
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalRuleTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalRuleTests.cs
index 9cb9e617fe..02cbca665d 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalRuleTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalRuleTests.cs
@@ -103,6 +103,32 @@ public void Serialize_ToolWithArgsRule_RoundTrips()
         Assert.Equal("utf-8", deserialized.Arguments["encoding"]);
     }
 
+    /// <summary>
+    /// Verify that an empty-arguments rule round-trips as a non-null empty dictionary, keeping it
+    /// distinct from a tool-level (null arguments) rule so persisted session state preserves the
+    /// narrower scope.
+    /// </summary>
+    [Fact]
+    public void Serialize_EmptyArgsRule_RoundTrips()
+    {
+        // Arrange
+        var rule = new ToolApprovalRule
+        {
+            ToolName = "SendPayment",
+            Arguments = new Dictionary<string, string>(),
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(rule, AgentJsonUtilities.DefaultOptions);
+        var deserialized = JsonSerializer.Deserialize<ToolApprovalRule>(json, AgentJsonUtilities.DefaultOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal("SendPayment", deserialized!.ToolName);
+        Assert.NotNull(deserialized.Arguments);
+        Assert.Empty(deserialized.Arguments!);
+    }
+
     /// <summary>
     /// Verify that JSON property names are correctly applied.
     /// </summary>