diff --git a/dotnet/src/Agents/UnitTests/Core/AgentThreadTests.cs b/dotnet/src/Agents/UnitTests/Core/AgentThreadTests.cs
new file mode 100644
index 000000000000..0f22756037bb
--- /dev/null
+++ b/dotnet/src/Agents/UnitTests/Core/AgentThreadTests.cs
@@ -0,0 +1,168 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.SemanticKernel;
+using Microsoft.SemanticKernel.Agents;
+using Xunit;
+
+namespace SemanticKernel.Agents.UnitTests.Core;
+
+/// <summary>
+/// Contains tests for the <see cref="AgentThread"/> class.
+/// </summary>
+public class AgentThreadTests
+{
+    /// <summary>
+    /// Tests that the CreateAsync method sets the Id and invokes CreateInternalAsync once.
+    /// </summary>
+    [Fact]
+    public async Task CreateShouldSetIdAndInvokeCreateInternalOnceAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+
+        // Act
+        await thread.CreateAsync();
+        await thread.CreateAsync();
+
+        // Assert
+        Assert.Equal("test-thread-id", thread.Id);
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the CreateAsync method throws an InvalidOperationException if the thread is deleted.
+    /// </summary>
+    [Fact]
+    public async Task CreateShouldThrowIfThreadDeletedAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+        await thread.CreateAsync();
+        await thread.DeleteAsync();
+
+        // Act & Assert
+        await Assert.ThrowsAsync<InvalidOperationException>(() => thread.CreateAsync());
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+        Assert.Equal(1, thread.DeleteInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the DeleteAsync method sets IsDeleted and invokes DeleteInternalAsync.
+    /// </summary>
+    [Fact]
+    public async Task DeleteShouldSetIsDeletedAndInvokeDeleteInternalAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+        await thread.CreateAsync();
+
+        // Act
+        await thread.DeleteAsync();
+
+        // Assert
+        Assert.True(thread.IsDeleted);
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+        Assert.Equal(1, thread.DeleteInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the DeleteAsync method does not invoke DeleteInternalAsync if the thread is already deleted.
+    /// </summary>
+    [Fact]
+    public async Task DeleteShouldNotInvokeDeleteInternalIfAlreadyDeletedAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+        await thread.CreateAsync();
+        await thread.DeleteAsync();
+
+        // Act
+        await thread.DeleteAsync();
+
+        // Assert
+        Assert.True(thread.IsDeleted);
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+        Assert.Equal(1, thread.DeleteInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the DeleteAsync method throws an InvalidOperationException if the thread was never created.
+    /// </summary>
+    [Fact]
+    public async Task DeleteShouldThrowIfNeverCreatedAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+
+        // Act & Assert
+        await Assert.ThrowsAsync<InvalidOperationException>(() => thread.DeleteAsync());
+        Assert.Equal(0, thread.CreateInternalAsyncCount);
+        Assert.Equal(0, thread.DeleteInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the OnNewMessageAsync method creates the thread if it is not already created.
+    /// </summary>
+    [Fact]
+    public async Task OnNewMessageShouldCreateThreadIfNotCreatedAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+        var message = new ChatMessageContent();
+
+        // Act
+        await thread.OnNewMessageAsync(message);
+
+        // Assert
+        Assert.Equal("test-thread-id", thread.Id);
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+        Assert.Equal(1, thread.OnNewMessageInternalAsyncCount);
+    }
+
+    /// <summary>
+    /// Tests that the OnNewMessageAsync method throws an InvalidOperationException if the thread is deleted.
+    /// </summary>
+    [Fact]
+    public async Task OnNewMessageShouldThrowIfThreadDeletedAsync()
+    {
+        // Arrange
+        var thread = new TestAgentThread();
+        await thread.CreateAsync();
+        await thread.DeleteAsync();
+        var message = new ChatMessageContent();
+
+        // Act & Assert
+        await Assert.ThrowsAsync<InvalidOperationException>(() => thread.OnNewMessageAsync(message));
+        Assert.Equal(1, thread.CreateInternalAsyncCount);
+        Assert.Equal(1, thread.DeleteInternalAsyncCount);
+        Assert.Equal(0, thread.OnNewMessageInternalAsyncCount);
+    }
+
+    private sealed class TestAgentThread : AgentThread
+    {
+        public int CreateInternalAsyncCount { get; private set; }
+        public int DeleteInternalAsyncCount { get; private set; }
+        public int OnNewMessageInternalAsyncCount { get; private set; }
+
+        protected override Task<string?> CreateInternalAsync(CancellationToken cancellationToken)
+        {
+            this.CreateInternalAsyncCount++;
+            return Task.FromResult<string?>("test-thread-id");
+        }
+
+        protected override Task DeleteInternalAsync(CancellationToken cancellationToken)
+        {
+            this.DeleteInternalAsyncCount++;
+            return Task.CompletedTask;
+        }
+
+        protected override Task OnNewMessageInternalAsync(ChatMessageContent newMessage, CancellationToken cancellationToken = default)
+        {
+            this.OnNewMessageInternalAsyncCount++;
+            return Task.CompletedTask;
+        }
+    }
+}
diff --git a/dotnet/src/Agents/UnitTests/Core/ChatHistoryAgentThreadTests.cs b/dotnet/src/Agents/UnitTests/Core/ChatHistoryAgentThreadTests.cs
new file mode 100644
index 000000000000..c5fa720adc50
--- /dev/null
+++ b/dotnet/src/Agents/UnitTests/Core/ChatHistoryAgentThreadTests.cs
@@ -0,0 +1,126 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.SemanticKernel;
+using Microsoft.SemanticKernel.Agents;
+using Microsoft.SemanticKernel.ChatCompletion;
+using Xunit;
+
+namespace SemanticKernel.Agents.UnitTests.Core;
+
+/// <summary>
+/// Contains tests for the <see cref="ChatHistoryAgentThread"/> class.
+/// </summary>
+public class ChatHistoryAgentThreadTests
+{
+    /// <summary>
+    /// Tests that creating a thread generates a unique Id and doesn't change IsDeleted.
+    /// </summary>
+    [Fact]
+    public async Task CreateShouldGenerateIdAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+
+        // Act
+        await thread.CreateAsync();
+
+        // Assert
+        Assert.NotNull(thread.Id);
+        Assert.False(thread.IsDeleted);
+    }
+
+    /// <summary>
+    /// Tests that deleting a thread marks it as deleted.
+    /// </summary>
+    [Fact]
+    public async Task DeleteShouldMarkThreadAsDeletedAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+        await thread.CreateAsync();
+
+        // Act
+        await thread.DeleteAsync();
+
+        // Assert
+        Assert.True(thread.IsDeleted);
+    }
+
+    /// <summary>
+    /// Tests that adding a new message to the thread adds it to the message history.
+    /// </summary>
+    [Fact]
+    public async Task OnNewMessageShouldAddMessageToHistoryAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+        var message = new ChatMessageContent(AuthorRole.User, "Hello");
+
+        // Act
+        await thread.OnNewMessageAsync(message);
+
+        // Assert
+        var messages = await thread.GetMessagesAsync().ToListAsync();
+        Assert.Single(messages);
+        Assert.Equal("Hello", messages[0].Content);
+    }
+
+    /// <summary>
+    /// Tests that GetMessagesAsync returns all messages in the thread.
+    /// </summary>
+    [Fact]
+    public async Task GetMessagesShouldReturnAllMessagesAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+        var message1 = new ChatMessageContent(AuthorRole.User, "Hello");
+        var message2 = new ChatMessageContent(AuthorRole.Assistant, "Hi there");
+
+        await thread.OnNewMessageAsync(message1);
+        await thread.OnNewMessageAsync(message2);
+
+        // Act
+        var messages = await thread.GetMessagesAsync().ToListAsync();
+
+        // Assert
+        Assert.Equal(2, messages.Count);
+        Assert.Equal("Hello", messages[0].Content);
+        Assert.Equal("Hi there", messages[1].Content);
+    }
+
+    /// <summary>
+    /// Tests that GetMessagesAsync throws an InvalidOperationException if the thread is deleted.
+    /// </summary>
+    [Fact]
+    public async Task GetMessagesShouldThrowIfThreadIsDeletedAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+        await thread.CreateAsync();
+        await thread.DeleteAsync();
+
+        // Act & Assert
+        await Assert.ThrowsAsync<InvalidOperationException>(async () => await thread.GetMessagesAsync().ToListAsync());
+    }
+
+    /// <summary>
+    /// Tests that GetMessagesAsync creates the thread if it has not been created yet.
+    /// </summary>
+    [Fact]
+    public async Task GetMessagesShouldCreateThreadIfNotCreatedAsync()
+    {
+        // Arrange
+        var thread = new ChatHistoryAgentThread();
+
+        // Act
+        var messages = await thread.GetMessagesAsync().ToListAsync();
+
+        // Assert
+        Assert.NotNull(thread.Id);
+        Assert.False(thread.IsDeleted);
+        Assert.Empty(messages);
+    }
+}
diff --git a/dotnet/src/Agents/UnitTests/OpenAI/OpenAIAssistantAgentThreadTests.cs b/dotnet/src/Agents/UnitTests/OpenAI/OpenAIAssistantAgentThreadTests.cs
new file mode 100644
index 000000000000..b6443fc05845
--- /dev/null
+++ b/dotnet/src/Agents/UnitTests/OpenAI/OpenAIAssistantAgentThreadTests.cs
@@ -0,0 +1,123 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.ClientModel;
+using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Microsoft.SemanticKernel.Agents.OpenAI;
+using Moq;
+using OpenAI.Assistants;
+using Xunit;
+
+namespace SemanticKernel.Agents.UnitTests.OpenAI;
+
+/// <summary>
+/// Tests for the <see cref="OpenAIAssistantAgentThread"/> class.
+/// </summary>
+public class OpenAIAssistantAgentThreadTests : IDisposable
+{
+    private readonly HttpMessageHandlerStub _messageHandlerStub;
+    private readonly HttpClient _httpClient;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="OpenAIAssistantAgentThreadTests"/> class.
+    /// </summary>
+    public OpenAIAssistantAgentThreadTests()
+    {
+        this._messageHandlerStub = new HttpMessageHandlerStub();
+        this._httpClient = new HttpClient(this._messageHandlerStub, disposeHandler: false);
+    }
+
+    /// <summary>
+    /// Tests that the constructor verifies parameters and throws <see cref="ArgumentNullException"/> when necessary.
+    /// </summary>
+    [Fact]
+    public void ConstructorShouldVerifyParams()
+    {
+        // Arrange
+        var mockClient = new Mock<AssistantClient>();
+
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() => new OpenAIAssistantAgentThread(null!));
+        Assert.Throws<ArgumentNullException>(() => new OpenAIAssistantAgentThread(null!, "threadId"));
+        Assert.Throws<ArgumentNullException>(() => new OpenAIAssistantAgentThread(mockClient.Object, id: null!));
+
+        var thread = new OpenAIAssistantAgentThread(mockClient.Object);
+        Assert.NotNull(thread);
+    }
+
+    /// <summary>
+    /// Tests that the constructor for resuming a thread uses the provided parameters.
+    /// </summary>
+    [Fact]
+    public void ConstructorForResumingThreadShouldUseParams()
+    {
+        // Arrange
+        var mockClient = new Mock<AssistantClient>();
+
+        // Act
+        var threadWithId = new OpenAIAssistantAgentThread(mockClient.Object, "threadId");
+
+        // Assert
+        Assert.NotNull(threadWithId);
+        Assert.Equal("threadId", threadWithId.Id);
+    }
+
+    /// <summary>
+    /// Tests that the CreateAsync method invokes the client and sets the thread ID.
+    /// </summary>
+    [Fact]
+    public async Task CreateShouldInvokeClientAsync()
+    {
+        // Arrange
+        this._messageHandlerStub.SetupResponses(HttpStatusCode.OK, OpenAIAssistantResponseContent.CreateThread);
+
+        var provider = this.CreateTestProvider();
+        var assistantClient = provider.AssistantClient;
+
+        var thread = new OpenAIAssistantAgentThread(assistantClient);
+
+        // Act
+        await thread.CreateAsync();
+
+        // Assert
+        Assert.Equal("thread_abc123", thread.Id);
+        Assert.Empty(this._messageHandlerStub.ResponseQueue);
+    }
+
+    /// <summary>
+    /// Tests that the DeleteAsync method invokes the client.
+    /// </summary>
+    [Fact]
+    public async Task DeleteShouldInvokeClientAsync()
+    {
+        // Arrange
+        this._messageHandlerStub.SetupResponses(HttpStatusCode.OK, OpenAIAssistantResponseContent.CreateThread);
+        this._messageHandlerStub.SetupResponses(HttpStatusCode.OK, OpenAIAssistantResponseContent.DeleteThread);
+
+        var provider = this.CreateTestProvider();
+        var assistantClient = provider.AssistantClient;
+
+        var thread = new OpenAIAssistantAgentThread(assistantClient);
+        await thread.CreateAsync();
+
+        // Act
+        await thread.DeleteAsync();
+
+        // Assert
+        Assert.Empty(this._messageHandlerStub.ResponseQueue);
+    }
+
+    /// <inheritdoc/>
+    public void Dispose()
+    {
+        this._messageHandlerStub.Dispose();
+        this._httpClient.Dispose();
+
+        GC.SuppressFinalize(this);
+    }
+
+    private OpenAIClientProvider CreateTestProvider()
+        => OpenAIClientProvider.ForOpenAI(apiKey: new ApiKeyCredential("fakekey"), endpoint: null, this._httpClient);
+}