diff --git a/dotnet/samples/Concepts/ChatCompletion/Connectors_CustomHttpClient.cs b/dotnet/samples/Concepts/ChatCompletion/Connectors_CustomHttpClient.cs
index 54de56688cdd..30df3f43870a 100644
--- a/dotnet/samples/Concepts/ChatCompletion/Connectors_CustomHttpClient.cs
+++ b/dotnet/samples/Concepts/ChatCompletion/Connectors_CustomHttpClient.cs
@@ -1,6 +1,14 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
+using System;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.SemanticKernel;
+using Xunit;
+using Xunit.Abstractions;
 
 namespace ChatCompletion;
 
@@ -36,4 +44,126 @@ public void UseCustomHttpClient()
                 httpClient: httpClient)
             .Build();
     }
+
+    /// <summary>
+    /// Demonstrates how to use a custom DelegatingHandler to send the 'api-key' header
+    /// and 'api-version' parameter required by Azure AI Foundry / Model Catalog deployments (resolves #10455).
+    /// </summary>
+    [Fact]
+    public void UseCustomHttpClientWithApiKeyHeaderForAzureAIFoundry()
+    {
+        // Azure AI Foundry model inference endpoints (e.g. Phi-3, Mistral, Llama) require an 'api-key' header
+        // and an 'api-version' query parameter rather than the standard OpenAI 'Authorization: Bearer' token.
+        using var handler = new AzureAIFoundryApiKeyHandler("test-api-key");
+        using var httpClient = new HttpClient(handler);
+
+        var kernel = Kernel.CreateBuilder()
+            .AddOpenAIChatCompletion(
+                modelId: "Phi-3-small-8k-instruct",
+                endpoint: new Uri("https://my-resource.services.ai.azure.com/models"),
+                apiKey: "test-api-key",
+                httpClient: httpClient)
+            .Build();
+
+        Assert.NotNull(kernel);
+    }
+
+    /// <summary>
+    /// Verifies that AzureAIFoundryApiKeyHandler injects the 'api-key' header and 'api-version'
+    /// query parameter, and preserves existing values without contacting external services.
+    /// </summary>
+    [Fact]
+    public async Task AzureAIFoundryApiKeyHandler_InjectsHeaderAndApiVersionAsync()
+    {
+        HttpRequestMessage? capturedRequest = null;
+        var innerHandler = new RecordingHandler(req => capturedRequest = req);
+        using var handler = new AzureAIFoundryApiKeyHandler("my-secret-key", "2024-05-01-preview", innerHandler);
+        using var invoker = new HttpMessageInvoker(handler);
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "https://my-resource.services.ai.azure.com/models/chat/completions");
+        using var response = await invoker.SendAsync(request, CancellationToken.None);
+
+        Assert.NotNull(capturedRequest);
+        Assert.True(capturedRequest.Headers.Contains("api-key"));
+        Assert.Equal("my-secret-key", capturedRequest.Headers.GetValues("api-key").First());
+        Assert.NotNull(capturedRequest.RequestUri);
+        Assert.Contains("api-version=2024-05-01-preview", capturedRequest.RequestUri.Query);
+    }
+
+    /// <summary>
+    /// Verifies that an existing 'api-key' header is preserved if already set on the request.
+    /// </summary>
+    [Fact]
+    public async Task AzureAIFoundryApiKeyHandler_PreservesExistingHeaderAsync()
+    {
+        HttpRequestMessage? capturedRequest = null;
+        var innerHandler = new RecordingHandler(req => capturedRequest = req);
+        using var handler = new AzureAIFoundryApiKeyHandler("default-key", "2024-05-01-preview", innerHandler);
+        using var invoker = new HttpMessageInvoker(handler);
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "https://my-resource.services.ai.azure.com/models/chat/completions?existing=1");
+        request.Headers.Add("api-key", "explicit-key");
+        using var response = await invoker.SendAsync(request, CancellationToken.None);
+
+        Assert.NotNull(capturedRequest);
+        Assert.Equal("explicit-key", capturedRequest.Headers.GetValues("api-key").First());
+        Assert.NotNull(capturedRequest.RequestUri);
+        Assert.Contains("existing=1", capturedRequest.RequestUri.Query);
+        Assert.Contains("api-version=2024-05-01-preview", capturedRequest.RequestUri.Query);
+    }
+
+    /// <summary>
+    /// DelegatingHandler that injects the 'api-key' header and 'api-version' query parameter
+    /// for Azure AI Foundry and OpenAI-compatible endpoints.
+    /// </summary>
+    private sealed class AzureAIFoundryApiKeyHandler : DelegatingHandler
+    {
+        private readonly string _apiKey;
+        private readonly string _apiVersion;
+
+        public AzureAIFoundryApiKeyHandler(string apiKey, string apiVersion = "2024-05-01-preview", HttpMessageHandler? innerHandler = null)
+            : base(innerHandler ?? new HttpClientHandler())
+        {
+            this._apiKey = apiKey;
+            this._apiVersion = apiVersion;
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            // 1. Ensure required 'api-key' header is present
+            if (!request.Headers.Contains("api-key"))
+            {
+                request.Headers.Add("api-key", this._apiKey);
+            }
+
+            // 2. Ensure required 'api-version' query parameter is present on inference endpoint
+            if (request.RequestUri is not null)
+            {
+                var uriBuilder = new UriBuilder(request.RequestUri);
+                if (!uriBuilder.Query.Contains("api-version="))
+                {
+                    var query = string.IsNullOrEmpty(uriBuilder.Query) ? string.Empty : uriBuilder.Query.TrimStart('?') + "&";
+                    query += $"api-version={Uri.EscapeDataString(this._apiVersion)}";
+                    uriBuilder.Query = query;
+                    request.RequestUri = uriBuilder.Uri;
+                }
+            }
+
+            return base.SendAsync(request, cancellationToken);
+        }
+    }
+
+    /// <summary>
+    /// In-memory handler for testing HTTP request transformations without live network calls.
+    /// </summary>
+    private sealed class RecordingHandler(Action<HttpRequestMessage> onRequest) : HttpMessageHandler
+    {
+        private readonly Action<HttpRequestMessage> _onRequest = onRequest;
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            this._onRequest(request);
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK));
+        }
+    }
 }