diff --git a/dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs b/dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
index e20d1ab448..aabd012be4 100644
--- a/dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
@@ -63,9 +63,13 @@ public static IEndpointConventionBuilder MapAGUI(
                 }
             };
 
+            // Create a session so middleware can access it (fixes #3823)
+            var session = await aiAgent.CreateSessionAsync(cancellationToken).ConfigureAwait(false);
+
             // Run the agent and convert to AG-UI events
             var events = aiAgent.RunStreamingAsync(
                 messages,
+                session,
                 options: runOptions,
                 cancellationToken: cancellationToken)
                 .AsChatResponseUpdatesAsync()
diff --git a/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SessionMiddlewareTests.cs b/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SessionMiddlewareTests.cs
new file mode 100644
index 0000000000..df400a37cd
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SessionMiddlewareTests.cs
@@ -0,0 +1,314 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Net.Http;
+using System.Runtime.CompilerServices;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Microsoft.Agents.AI.AGUI;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests;
+
+/// <summary>
+/// Integration tests that verify session propagation through the AGUI middleware pipeline.
+/// Regression tests for <see href="https://github.com/microsoft/agent-framework/issues/3823">#3823</see>:
+/// "Session is always null in the middleware".
+/// </summary>
+public sealed class SessionMiddlewareTests : IAsyncDisposable
+{
+    private WebApplication? _app;
+    private HttpClient? _client;
+
+    /// <summary>
+    /// Verifies that when a workflow is configured with middleware via <c>AIAgentBuilder.Use</c>,
+    /// the session parameter passed to the middleware during an AGUI-initiated run is not null.
+    /// </summary>
+    /// <remarks>
+    /// Regression test for #3823. The bug was that <c>MapAGUI</c> called
+    /// <c>RunStreamingAsync(messages, options: runOptions, ...)</c>, skipping the <c>session</c> parameter
+    /// entirely, which caused it to be <c>null</c> for all middleware decorators.
+    /// </remarks>
+    [Fact]
+    public async Task AGUIMiddleware_WithWorkflow_SessionIsNotNullAsync()
+    {
+        // Arrange
+        AgentSession? capturedSession = null;
+        var innerAgent = new FakeSessionCapturingAgent();
+
+        // Wrap the agent with middleware that captures the session parameter
+        AIAgent agentWithMiddleware = new AIAgentBuilder(innerAgent)
+            .Use(
+                runFunc: null,
+                runStreamingFunc: (messages, session, options, innerAgent, ct) =>
+                {
+                    // Capture session in middleware — this is what #3823 tests
+                    capturedSession = session;
+                    return innerAgent.RunStreamingAsync(messages, session, options, ct);
+                })
+            .Build();
+
+        await this.SetupTestServerAsync(agentWithMiddleware);
+        var chatClient = new AGUIChatClient(this._client!, "", null);
+        AIAgent clientAgent = chatClient.AsAIAgent(instructions: null, name: "assistant", description: "Session test assistant", tools: []);
+        ChatClientAgentSession clientSession = (ChatClientAgentSession)await clientAgent.CreateSessionAsync();
+        ChatMessage userMessage = new(ChatRole.User, "hello");
+
+        List<AgentResponseUpdate> updates = [];
+
+        // Act — run through the AGUI endpoint (full round-trip: AGUIChatClient → HTTP → MapAGUI → middleware → agent)
+        await foreach (AgentResponseUpdate update in clientAgent.RunStreamingAsync([userMessage], clientSession, new AgentRunOptions(), CancellationToken.None))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        updates.Should().NotBeEmpty("the agent should have produced streaming updates");
+        capturedSession.Should().NotBeNull("session should be propagated to middleware when invoked via AGUI endpoint (regression #3823)");
+    }
+
+    /// <summary>
+    /// Verifies that the session is also available via <see cref="AIAgent.CurrentRunContext"/>
+    /// when the agent is invoked through the AGUI endpoint.
+    /// </summary>
+    [Fact]
+    public async Task AGUIMiddleware_CurrentRunContext_SessionIsNotNullAsync()
+    {
+        // Arrange
+        AgentRunContext? capturedRunContext = null;
+        var innerAgent = new FakeSessionCapturingAgent();
+
+        // Wrap the agent with middleware that captures CurrentRunContext
+        AIAgent agentWithMiddleware = new AIAgentBuilder(innerAgent)
+            .Use(
+                runFunc: null,
+                runStreamingFunc: (messages, session, options, innerAgent, ct) =>
+                {
+                    // Capture the run context — its Session should not be null
+                    capturedRunContext = AIAgent.CurrentRunContext;
+                    return innerAgent.RunStreamingAsync(messages, session, options, ct);
+                })
+            .Build();
+
+        await this.SetupTestServerAsync(agentWithMiddleware);
+        var chatClient = new AGUIChatClient(this._client!, "", null);
+        AIAgent clientAgent = chatClient.AsAIAgent(instructions: null, name: "assistant", description: "Context test assistant", tools: []);
+        ChatClientAgentSession clientSession = (ChatClientAgentSession)await clientAgent.CreateSessionAsync();
+        ChatMessage userMessage = new(ChatRole.User, "test context");
+
+        List<AgentResponseUpdate> updates = [];
+
+        // Act
+        await foreach (AgentResponseUpdate update in clientAgent.RunStreamingAsync([userMessage], clientSession, new AgentRunOptions(), CancellationToken.None))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        updates.Should().NotBeEmpty();
+        capturedRunContext.Should().NotBeNull("CurrentRunContext should be set during AGUI-initiated runs");
+        capturedRunContext!.Session.Should().NotBeNull("CurrentRunContext.Session should not be null when invoked via AGUI endpoint (regression #3823)");
+    }
+
+    /// <summary>
+    /// Verifies that the shared pre/post-processing middleware (via the single-delegate <c>AIAgentBuilder.Use</c> overload)
+    /// also receives a non-null session when invoked through the AGUI endpoint.
+    /// </summary>
+    [Fact]
+    public async Task AGUIMiddleware_SharedUseOverload_SessionIsNotNullAsync()
+    {
+        // Arrange
+        AgentSession? capturedSession = null;
+        var innerAgent = new FakeSessionCapturingAgent();
+
+        // Use the shared (pre/post-processing) middleware overload
+        AIAgent agentWithMiddleware = new AIAgentBuilder(innerAgent)
+            .Use(async (messages, session, options, next, ct) =>
+            {
+                capturedSession = session;
+                await next(messages, session, options, ct);
+            })
+            .Build();
+
+        await this.SetupTestServerAsync(agentWithMiddleware);
+        var chatClient = new AGUIChatClient(this._client!, "", null);
+        AIAgent clientAgent = chatClient.AsAIAgent(instructions: null, name: "assistant", description: "Shared middleware test", tools: []);
+        ChatClientAgentSession clientSession = (ChatClientAgentSession)await clientAgent.CreateSessionAsync();
+        ChatMessage userMessage = new(ChatRole.User, "hello shared");
+
+        List<AgentResponseUpdate> updates = [];
+
+        // Act
+        await foreach (AgentResponseUpdate update in clientAgent.RunStreamingAsync([userMessage], clientSession, new AgentRunOptions(), CancellationToken.None))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        updates.Should().NotBeEmpty();
+        capturedSession.Should().NotBeNull("session should be propagated to shared middleware when invoked via AGUI endpoint (regression #3823)");
+    }
+
+    /// <summary>
+    /// Verifies that the session received by middleware can be used to store and retrieve
+    /// state via <see cref="AgentSessionStateBag"/>, proving it is a functional session instance.
+    /// </summary>
+    [Fact]
+    public async Task AGUIMiddleware_SessionStateBag_IsAccessibleAsync()
+    {
+        // Arrange
+        bool stateBagAccessible = false;
+        var innerAgent = new FakeSessionCapturingAgent();
+
+        AIAgent agentWithMiddleware = new AIAgentBuilder(innerAgent)
+            .Use(
+                runFunc: null,
+                runStreamingFunc: (messages, session, options, innerAgent, ct) =>
+                {
+                    if (session is not null)
+                    {
+                        // Verify StateBag is usable
+                        session.StateBag.SetValue("test_key", "test_value");
+                        stateBagAccessible = session.StateBag.TryGetValue<string>("test_key", out var retrieved)
+                            && retrieved == "test_value";
+                    }
+
+                    return innerAgent.RunStreamingAsync(messages, session, options, ct);
+                })
+            .Build();
+
+        await this.SetupTestServerAsync(agentWithMiddleware);
+        var chatClient = new AGUIChatClient(this._client!, "", null);
+        AIAgent clientAgent = chatClient.AsAIAgent(instructions: null, name: "assistant", description: "StateBag test", tools: []);
+        ChatClientAgentSession clientSession = (ChatClientAgentSession)await clientAgent.CreateSessionAsync();
+        ChatMessage userMessage = new(ChatRole.User, "test statebag");
+
+        List<AgentResponseUpdate> updates = [];
+
+        // Act
+        await foreach (AgentResponseUpdate update in clientAgent.RunStreamingAsync([userMessage], clientSession, new AgentRunOptions(), CancellationToken.None))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        updates.Should().NotBeEmpty();
+        stateBagAccessible.Should().BeTrue("middleware should be able to use Session.StateBag when invoked via AGUI endpoint");
+    }
+
+    private async Task SetupTestServerAsync(AIAgent agent)
+    {
+        WebApplicationBuilder builder = WebApplication.CreateBuilder();
+        builder.WebHost.UseTestServer();
+        builder.Services.AddAGUI();
+
+        this._app = builder.Build();
+        this._app.MapAGUI("/agent", agent);
+
+        await this._app.StartAsync();
+
+        TestServer testServer = this._app.Services.GetRequiredService<IServer>() as TestServer
+            ?? throw new InvalidOperationException("TestServer not found");
+
+        this._client = testServer.CreateClient();
+        this._client.BaseAddress = new Uri("http://localhost/agent");
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        this._client?.Dispose();
+        if (this._app != null)
+        {
+            await this._app.DisposeAsync();
+        }
+    }
+}
+
+/// <summary>
+/// A fake agent for testing session propagation through middleware.
+/// This agent implements <see cref="CreateSessionCoreAsync"/> to return a valid session,
+/// and its <see cref="RunCoreStreamingAsync"/> produces a deterministic response.
+/// </summary>
+[SuppressMessage("Performance", "CA1812:Avoid uninstantiated internal classes", Justification = "Instantiated directly in tests")]
+internal sealed class FakeSessionCapturingAgent : AIAgent
+{
+    protected override string? IdCore => "fake-session-agent";
+
+    public override string? Description => "A fake agent for testing session propagation through middleware";
+
+    protected override ValueTask<AgentSession> CreateSessionCoreAsync(CancellationToken cancellationToken = default) =>
+        new(new FakeAgentSession());
+
+    protected override ValueTask<AgentSession> DeserializeSessionCoreAsync(JsonElement serializedState, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default) =>
+        new(serializedState.Deserialize<FakeAgentSession>(jsonSerializerOptions)!);
+
+    protected override ValueTask<JsonElement> SerializeSessionCoreAsync(AgentSession session, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default)
+    {
+        if (session is not FakeAgentSession fakeSession)
+        {
+            throw new InvalidOperationException(
+                $"The provided session type '{session.GetType().Name}' is not compatible with this agent. " +
+                $"Only sessions of type '{nameof(FakeAgentSession)}' can be serialized by this agent.");
+        }
+
+        return new(JsonSerializer.SerializeToElement(fakeSession, jsonSerializerOptions));
+    }
+
+    protected override async Task<AgentResponse> RunCoreAsync(
+        IEnumerable<ChatMessage> messages,
+        AgentSession? session = null,
+        AgentRunOptions? options = null,
+        CancellationToken cancellationToken = default)
+    {
+        List<AgentResponseUpdate> updates = [];
+        await foreach (AgentResponseUpdate update in this.RunStreamingAsync(messages, session, options, cancellationToken).ConfigureAwait(false))
+        {
+            updates.Add(update);
+        }
+
+        return updates.ToAgentResponse();
+    }
+
+    protected override async IAsyncEnumerable<AgentResponseUpdate> RunCoreStreamingAsync(
+        IEnumerable<ChatMessage> messages,
+        AgentSession? session = null,
+        AgentRunOptions? options = null,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        string messageId = Guid.NewGuid().ToString("N");
+
+        foreach (string chunk in new[] { "Session", " ", "test", " ", "response" })
+        {
+            yield return new AgentResponseUpdate
+            {
+                MessageId = messageId,
+                Role = ChatRole.Assistant,
+                Contents = [new TextContent(chunk)]
+            };
+
+            await Task.Yield();
+        }
+    }
+
+    private sealed class FakeAgentSession : AgentSession
+    {
+        public FakeAgentSession()
+        {
+        }
+
+        [JsonConstructor]
+        public FakeAgentSession(AgentSessionStateBag stateBag) : base(stateBag)
+        {
+        }
+    }
+}