.NET: Duplicate System Instruction when creating a ChatClientAgentOptions instance with the constructor

[htilssu]

Motivation and Context

1. Why is this change required?

Creating a ChatClientAgentOptions instance with the constructor that accepts only the instruction argument caused the instruction text to be duplicated internally when the agent options were later merged into chat options.

2. What problem does it solve?

This change removes redundant instruction concatenation logic to ensure that Instructions are applied only once.
It prevents recursive or repeated values being passed into ChatOptions during agent initialization.

3. What scenario does it contribute to?

Improves consistency and correctness when initializing ChatClientAgent instances with predefined instructions, especially in scenarios where low-level options are already configured through CreateConfiguredChatOptions().

Description

Motivation and Context

When ChatClientAgent.RunAsync() is called with requestOptions == null,
the logic at

agent-framework/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs

Lines 609 to 613 in 7e1fd67

	if (!string.IsNullOrWhiteSpace(this.Instructions))
	{
	chatOptions ??= new();
	chatOptions.Instructions = string.IsNullOrWhiteSpace(chatOptions.Instructions) ? this.Instructions : $"{this.Instructions}\n{chatOptions.Instructions}";
	}

manually concatenates this.Instructions with chatOptions.Instructions.

However, this.Instructions is just a proxy for
_agentOptions.Instructions

agent-framework/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs

Lines 143 to 145 in 7e1fd67

	/// </remarks>
	public string? Instructions => this._agentOptions?.Instructions;

and during ChatClientAgentOptions construction, the same instruction value is already
propagated into its ChatOptions field:

agent-framework/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentOptions.cs

Lines 48 to 51 in 7e1fd67

	if (instructions is not null)
	{
	(this.ChatOptions ??= new()).Instructions = instructions;
	}

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.

[Copilot]

Pull Request Overview

Fixes a bug where system instructions were duplicated when creating a ChatClientAgentOptions instance using the constructor that accepts only the instruction parameter. The issue occurred because instructions were being set both in the constructor and again during chat options merging.

• Removed redundant instruction assignment in ChatClientAgentOptions constructor
• Added test case to verify single instruction assignment behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentOptions.cs	Removes duplicate instruction assignment logic from constructor
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Adds test case to verify instructions are not duplicated when using constructor

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[htilssu]

I noticed that six tests are currently failing because they depend on the code in constructor I just removed.
Would you prefer that I update those tests to match the new implementation, or remove them if they’re no longer applicable?

[westey-m]

I noticed that six tests are currently failing because they depend on the code in constructor I just removed. Would you prefer that I update those tests to match the new implementation, or remove them if they’re no longer applicable?

@htilssu, thanks for the contribution!
Yes, please update those tests to match the new implementation. From what I can tell they should still be applicable, just the part that checks for instructions in both the ChatClientAgentOptions and ChatOptions is wrong.

[htilssu]

@htilssu, thanks for the contribution! Yes, please update those tests to match the new implementation. From what I can tell they should still be applicable, just the part that checks for instructions in both the ChatClientAgentOptions and ChatOptions is wrong.

@westey-m Thanks for the feedback! I've updated the unit tests accordingly to align with the new implementation.

[htilssu]

@TaoChenOSU Could you please review the changes in this PR when you have a moment?