diff --git a/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIClientContainerFileExtensions.cs b/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIClientContainerFileExtensions.cs
new file mode 100644
index 0000000000..192bdcb379
--- /dev/null
+++ b/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIClientContainerFileExtensions.cs
@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.ClientModel;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace OpenAI;
+
+/// <summary>
+/// Provides extension methods for <see cref="OpenAIClient"/> to download files from OpenAI containers.
+/// </summary>
+/// <remarks>
+/// Container files are generated by tools such as Code Interpreter and have IDs prefixed with
+/// <c>cfile_</c>. These files reside within a specific container (identified by a <c>cntr_</c>
+/// prefixed ID) and cannot be downloaded using the standard Files API
+/// (<see cref="OpenAIClient.GetOpenAIFileClient"/>). Use these methods to download container-scoped
+/// files through the Containers API instead.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AIOpenAIResponses)]
+public static class OpenAIClientContainerFileExtensions
+{
+    /// <summary>
+    /// Downloads a file from an OpenAI container.
+    /// </summary>
+    /// <param name="client">The <see cref="OpenAIClient"/> to use for the download.</param>
+    /// <param name="containerId">The ID of the container (e.g., <c>cntr_...</c>) that holds the file.</param>
+    /// <param name="fileId">The ID of the file to download (e.g., <c>cfile_...</c>).</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <returns>A <see cref="Task{ClientResult{BinaryData}}"/> containing the downloaded file data as <see cref="BinaryData"/>.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when <paramref name="client"/>, <paramref name="containerId"/>, or <paramref name="fileId"/> is <see langword="null"/>.</exception>
+    /// <exception cref="ArgumentException">Thrown when <paramref name="containerId"/> or <paramref name="fileId"/> is empty or consists only of whitespace characters.</exception>
+    public static async Task<ClientResult<BinaryData>> DownloadContainerFileAsync(
+        this OpenAIClient client,
+        string containerId,
+        string fileId,
+        CancellationToken cancellationToken = default)
+    {
+        Throw.IfNull(client);
+        Throw.IfNullOrWhitespace(containerId);
+        Throw.IfNullOrWhitespace(fileId);
+
+        var containerClient = client.GetContainerClient();
+        return await containerClient.DownloadContainerFileAsync(containerId, fileId, cancellationToken).ConfigureAwait(false);
+    }
+}
diff --git a/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIClientContainerFileExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIClientContainerFileExtensionsTests.cs
new file mode 100644
index 0000000000..0a6e6500a5
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIClientContainerFileExtensionsTests.cs
@@ -0,0 +1,132 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading.Tasks;
+using OpenAI;
+
+namespace Microsoft.Agents.AI.OpenAI.UnitTests.Extensions;
+
+/// <summary>
+/// Unit tests for the <see cref="OpenAIClientContainerFileExtensions"/> class.
+/// </summary>
+public sealed class OpenAIClientContainerFileExtensionsTests
+{
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentNullException when client is null.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithNullClient_ThrowsArgumentNullExceptionAsync()
+    {
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentNullException>(
+            () => ((OpenAIClient)null!).DownloadContainerFileAsync("cntr_123", "cfile_456"));
+
+        Assert.Equal("client", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentNullException when containerId is null.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithNullContainerId_ThrowsArgumentNullExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentNullException>(
+            () => client.DownloadContainerFileAsync(null!, "cfile_456"));
+
+        Assert.Equal("containerId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentException when containerId is empty.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithEmptyContainerId_ThrowsArgumentExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentException>(
+            () => client.DownloadContainerFileAsync("", "cfile_456"));
+
+        Assert.Equal("containerId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentException when containerId is whitespace.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithWhitespaceContainerId_ThrowsArgumentExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentException>(
+            () => client.DownloadContainerFileAsync("   ", "cfile_456"));
+
+        Assert.Equal("containerId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentNullException when fileId is null.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithNullFileId_ThrowsArgumentNullExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentNullException>(
+            () => client.DownloadContainerFileAsync("cntr_123", null!));
+
+        Assert.Equal("fileId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentException when fileId is empty.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithEmptyFileId_ThrowsArgumentExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentException>(
+            () => client.DownloadContainerFileAsync("cntr_123", ""));
+
+        Assert.Equal("fileId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// Verify that DownloadContainerFileAsync throws ArgumentException when fileId is whitespace.
+    /// </summary>
+    [Fact]
+    public async Task DownloadContainerFileAsync_WithWhitespaceFileId_ThrowsArgumentExceptionAsync()
+    {
+        // Arrange
+        var client = new TestOpenAIClient();
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentException>(
+            () => client.DownloadContainerFileAsync("cntr_123", "   "));
+
+        Assert.Equal("fileId", exception.ParamName);
+    }
+
+    /// <summary>
+    /// A test <see cref="OpenAIClient"/> subclass for unit testing without requiring an API key.
+    /// </summary>
+    private sealed class TestOpenAIClient : OpenAIClient
+    {
+        public TestOpenAIClient()
+        {
+        }
+    }
+}