diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/DefaultMcpToolHandler.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/DefaultMcpToolHandler.cs
index 61aedec347f..9bd67e55eb4 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/DefaultMcpToolHandler.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/DefaultMcpToolHandler.cs
@@ -104,6 +104,21 @@ public async Task<McpServerToolResultContent> InvokeToolAsync(
             ThrowIfListToolsArgumentsSpecified(arguments);
         }
 
+        if (headers is not null)
+        {
+            foreach (KeyValuePair<string, string> header in headers)
+            {
+                HttpHeaderValidation.ValidateNoProhibitedCharacters(
+                    header.Key,
+                    nameof(headers),
+                    "HTTP header name contains invalid characters.");
+                HttpHeaderValidation.ValidateNoProhibitedCharacters(
+                    header.Value,
+                    nameof(headers),
+                    "HTTP header value contains invalid characters.");
+            }
+        }
+
         if (this._httpClientProvider is not null)
         {
             TaskCompletionSource<bool> invocationCompleted = new(TaskCreationOptions.RunContinuationsAsynchronously);
diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/Microsoft.Agents.AI.Workflows.Declarative.Mcp.csproj b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/Microsoft.Agents.AI.Workflows.Declarative.Mcp.csproj
index 08738266c09..0bb04cc608e 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/Microsoft.Agents.AI.Workflows.Declarative.Mcp.csproj
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Mcp/Microsoft.Agents.AI.Workflows.Declarative.Mcp.csproj
@@ -7,6 +7,7 @@
 
   <PropertyGroup>
     <InjectSharedThrow>true</InjectSharedThrow>
+    <InjectSharedHttpHeaderValidation>true</InjectSharedHttpHeaderValidation>
     <InjectSharedOriginPinning>true</InjectSharedOriginPinning>
     <InjectIsExternalInitOnLegacy>true</InjectIsExternalInitOnLegacy>
     <InjectTrimAttributesOnLegacy>true</InjectTrimAttributesOnLegacy>
diff --git a/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.Mcp.UnitTests/DefaultMcpToolHandlerTests.cs b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.Mcp.UnitTests/DefaultMcpToolHandlerTests.cs
index 610f7393eda..aa1fb0440aa 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.Mcp.UnitTests/DefaultMcpToolHandlerTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.Mcp.UnitTests/DefaultMcpToolHandlerTests.cs
@@ -90,6 +90,59 @@ public async Task DisposeAsync_WhenCalledMultipleTimes_ShouldHandleGracefullyAsy
 
     #region HttpClientProvider Tests
 
+    [Theory]
+    [InlineData("\r", false, false)]
+    [InlineData("\n", false, false)]
+    [InlineData("\0", false, false)]
+    [InlineData("\r", true, false)]
+    [InlineData("\n", true, false)]
+    [InlineData("\0", true, false)]
+    [InlineData("\r", false, true)]
+    [InlineData("\n", false, true)]
+    [InlineData("\0", false, true)]
+    [InlineData("\r", true, true)]
+    [InlineData("\n", true, true)]
+    [InlineData("\0", true, true)]
+    public async Task InvokeToolAsync_WithInvalidHeader_ShouldRejectBeforeClientCreationAsync(
+        string invalidCharacter,
+        bool invalidName,
+        bool useProvider)
+    {
+        // Arrange
+        bool clientCreationAttempted = false;
+        Task<HttpClient?> ProviderAsync(string url, CancellationToken ct)
+        {
+            clientCreationAttempted = true;
+            throw new InvalidOperationException("Client creation should not be reached.");
+        }
+
+        HttpMessageHandler CreateHandler()
+        {
+            clientCreationAttempted = true;
+            throw new InvalidOperationException("Client creation should not be reached.");
+        }
+
+        await using DefaultMcpToolHandler handler = new(useProvider ? ProviderAsync : null, CreateHandler);
+        Dictionary<string, string> headers = new()
+        {
+            [invalidName ? $"X-Test{invalidCharacter}" : "X-Test"] = invalidName ? "value" : $"value{invalidCharacter}"
+        };
+
+        // Act
+        async Task actAsync() => await handler.InvokeToolAsync(
+            serverUrl: "https://example.com/mcp",
+            serverLabel: null,
+            toolName: "testTool",
+            arguments: null,
+            headers: headers,
+            connectionName: null);
+
+        // Assert
+        ArgumentException exception = await Assert.ThrowsAsync<ArgumentException>(actAsync);
+        Assert.Equal("headers", exception.ParamName);
+        Assert.False(clientCreationAttempted);
+    }
+
     [Fact]
     public async Task InvokeToolAsync_WithHttpClientProvider_ShouldCallProviderAsync()
     {