[BREAKING] Remove NotifyThreadOfNewMessagesAsync AIAgent helper

[westey-m]

Motivation and Context

The NotifyThreadOfNewMessagesAsync helper is not required anymore, since all agents know their thread types and can call the internals of the thread to set messages directly. Previously this was not the case, but we have moved away from this, so removing the unnecessary code.

This is potentially breaking for AIAgent implementers.

Description

• Remove NotifyThreadOfNewMessagesAsync helper method on AIAgent, and changing AIAgent implementations to update their threads directly if needed.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[rogerbarreto]

Should this considered [Breaking] ?

Wonder which scenarios will stop working after this change.

[westey-m]

Should this considered [Breaking] ?

Wonder which scenarios will stop working after this change.

It's only potentially breaking for AIAgent implementers. It really depends on their scenario though. You'll see that only two of our currently agent types actually use this, so impact should be low. I've updated it with [breaking] anyway. Thanks!

[Copilot]

Pull request overview

This PR removes the NotifyThreadOfNewMessagesAsync helper method from the AIAgent base class and refactors agent implementations to directly update their thread message stores. This simplifies the architecture by eliminating an abstraction layer that is no longer needed now that all agents have typed knowledge of their threads.

Key Changes

• Removed MessagesReceivedAsync virtual method from AgentThread base class and its overrides in ChatClientAgentThread, InMemoryAgentThread, and WorkflowThread
• Removed NotifyThreadOfNewMessagesAsync protected helper method from AIAgent base class
• Added new private NotifyMessageStoreOfNewMessagesAsync method in ChatClientAgent that directly calls AddMessagesAsync on the message store
• Updated sample code to directly call MessageStore.AddMessagesAsync instead of using the removed helper

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentThread.cs	Removed the MessagesReceivedAsync virtual method that threads used to override
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Removed the NotifyThreadOfNewMessagesAsync protected helper method
dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryAgentThread.cs	Removed the MessagesReceivedAsync override that added messages to the store
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Added new private NotifyMessageStoreOfNewMessagesAsync method and updated message store initialization to include default fallback
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentThread.cs	Removed the MessagesReceivedAsync override that handled the three-case logic for message storage
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowThread.cs	Removed the MessagesReceivedAsync override
dotnet/samples/GettingStarted/AgentProviders/Agent_With_CustomImplementation/Program.cs	Updated to directly call MessageStore.AddMessagesAsync with added type validation
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentThreadTests.cs	Removed tests for the removed OnNewMessagesAsync functionality
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentThreadTests.cs	Removed test for the MessagesReceivedAsync method
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AIAgentTests.cs	Removed test for the NotifyThreadOfNewMessagesAsync helper method