.NET: Fix function call and tool result messages not stored in session when using ChatReducer with AfterMessageAdded trigger

[Copilot]

Motivation and Context

When InMemoryChatHistoryProvider is configured with a ChatReducer and ReducerTriggerEvent = AfterMessageAdded, function call and tool result messages from the current run are immediately stripped from the session — even when the reducer's target count far exceeds the actual message count (e.g., MessageCountingChatReducer(20) with only 4 messages).

Fixes #issue.

Description

Root cause: MessageCountingChatReducer (from MEAI) always normalizes the conversation by stripping function call/result pairs regardless of count — it treats them as ephemeral intermediate messages. With the previous AfterMessageAdded implementation, the reducer ran on the combined state (old + newly added messages), so current-turn tool calls were immediately removed.

Before (order of operations in StoreChatHistoryAsync):

1. Add all new messages (user + function calls + tool results + final assistant) to state.Messages
2. Run reducer → strips current turn's tool calls

After:

1. Run reducer on existing (pre-turn) messages only → strips old history as expected
2. Add new current-turn messages → always preserved in full

This makes AfterMessageAdded behaviorally consistent with BeforeMessagesRetrieval: both reduce old messages while preserving the complete current-turn message set. The practical distinction remains: BeforeMessagesRetrieval limits the context window sent to the LLM; AfterMessageAdded limits what's persisted in storage without affecting the current invocation's context.

Changes:

• InMemoryChatHistoryProvider.StoreChatHistoryAsync — reorder: reduce existing messages, then append new ones
• AddMessagesAsync_WithReducer_AfterMessageAdded_InvokesReducerAsync test — updated to pre-populate state with existing messages and verify reducer operates only on those, not newly added ones
• Added AddMessagesAsync_WithReducer_AfterMessageAdded_PreservesCurrentTurnFunctionCallsAsync — regression test verifying function call and tool result messages are preserved when an aggressive reducer is configured

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.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET: [Bug]: Function call and tool result messages are not stored in the session when using ChatReducer</issue_title>
<issue_description>### Description

Consider the following code:

#!/usr/bin/env dotnet

#:sdk Microsoft.NET.Sdk

#:property OutputType=Exe
#:property TargetFramework=net10.0
#:property ImplicitUsings=enable
#:property Nullable=enable
#:property NoWarn=$(NoWarn);MEAI001
#:property PublishAot=false

#:package Azure.AI.OpenAI@2.1.0
#:package Azure.Identity@1.18.0
#:package Microsoft.Agents.AI.OpenAI@1.0.0-rc3

using System.ComponentModel;
using Azure.AI.OpenAI;
using Azure.Identity;
using Microsoft.Agents.AI;
using Microsoft.Extensions.AI;
using OpenAI.Chat;

var endpoint = Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT") ?? throw new InvalidOperationException("AZURE_OPENAI_ENDPOINT is not set.");
var deploymentName = Environment.GetEnvironmentVariable("AZURE_OPENAI_DEPLOYMENT_NAME") ?? "gpt-4.1";

[Description("Get the weather for a given location.")]
static string GetWeather([Description("The location to get the weather for.")] string location)
    => $"The weather in {location} is cloudy with a high of 15°C.";

AIAgent agent = new AzureOpenAIClient(
    new Uri(endpoint),
    new DefaultAzureCredential())
    .GetChatClient(deploymentName).AsIChatClient()
    .AsAIAgent(new ChatClientAgentOptions
    {
        Name = "Assistant",
        //ChatHistoryProvider = new InMemoryChatHistoryProvider(new()
        //{
        //    ChatReducer = new MessageCountingChatReducer(20),
        //    ReducerTriggerEvent = InMemoryChatHistoryProviderOptions.ChatReducerTriggerEvent.AfterMessageAdded,
        //}),
        ChatOptions = new ChatOptions
        {
            Tools = [AIFunctionFactory.Create(GetWeather)]
        }
    });

var session = await agent.CreateSessionAsync();
var response = await agent.RunAsync("What is the weather like in Taggia?", session);
Console.WriteLine(response);

session.TryGetInMemoryChatHistory(out var messages);
Console.WriteLine($"Messages in session: {messages.Count}");

In particular, the setting of ChatHistoryProvider has been commented out. With this configuration, after calling RunAsync, I have 4 messages in the session:

{
   "messages":[
      {
         "role":"user",
         "contents":[
            {
               "$type":"text",
               "text":"What is the weather like in Taggia?"
            }
         ]
      },
      {
         "authorName":"Assistant",
         "createdAt":"2026-03-05T09:14:51+00:00",
         "role":"assistant",
         "contents":[
            {
               "$type":"functionCall",
               "callId":"call_19muVFBQeEth4xsPu3OAckUP",
               "name":"_Main_g_GetWeather_0_0",
               "arguments":{
                  "location":"Taggia"
               },
               "informationalOnly":true
            }
         ],
         "messageId":"chatcmpl-DFz59TbFUrm2bR3ZeRogTiO4W0vpb"
      },
      {
         "authorName":"Assistant",
         "role":"tool",
         "contents":[
            {
               "$type":"functionResult",
               "callId":"call_19muVFBQeEth4xsPu3OAckUP",
               "result":"The weather in Taggia is cloudy with a high of 15°C."
            }
         ]
      },
      {
         "authorName":"Assistant",
         "createdAt":"2026-03-05T09:14:52+00:00",
         "role":"assistant",
         "contents":[
            {
               "$type":"text",
               "text":"The weather in Taggia is currently cloudy with a high temperature of 15°C."
            }
         ],
         "messageId":"chatcmpl-DFz5A3EVZ0egQlwhVFv11oHYgqnXP"
      }
   ]
}

However, if I uncomment the setting of ChatHistoryProvider to use a ChatReducer, when I call RunAsync, I get only 2 messages:

{
   "messages":[
      {
         "role":"user",
         "contents":[
            {
               "$type":"text",
               "text":"What is the weather like in Taggia?"
            }
         ]
      },
      {
         "authorName":"Assistant",
         "createdAt":"2026-03-05T09:16:15+00:00",
         "role":"assistant",
         "contents":[
            {
               "$type":"text",
               "text":"The weather in Taggia is currently cloudy, with a high temperature of 15°C."
            }
         ],
         "messageId":"chatcmpl-DFz6VO7mqr1FNCQLTgJpYROGGolJo"
      }
   ]
}

Function call and Function result messages aren't present in the session. I have also tried to explicitly set StorageInputRequestMessageFilter and StorageInputResponseMessageFilter properties:

ChatHistoryProvider = new InMemoryChatHistoryProvider(new()
{
    ChatReducer = new MessageCountingChatReducer(20),
    ReducerTriggerEvent = InMemoryChatHistoryProviderOptions.ChatReducerTriggerEvent.AfterMessageAdded,
    StorageInputRequestMessageFilter = messages...

</details>



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- Fixes microsoft/agent-framework#4494

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