Add Reason property to FunctionApprovalResponseContent for custom rejection messages

[Copilot]

Implementation: Add Reason property to FunctionApprovalResponseContent

Changes Completed

• Add nullable Reason property to FunctionApprovalResponseContent class

  • Added "optional" to property documentation as suggested
  • Removed verbose <remarks> section for cleaner documentation

• Update FunctionInvokingChatClient.GenerateRejectedFunctionResults method

  • Changed rejection message format to always include "Tool call invocation rejected."
  • Appends custom reason when provided: "Tool call invocation rejected. {Reason}"
  • Uses IsNullOrWhiteSpace check as suggested

• Update all tests for new message format

  • Updated default rejection message expectations
  • Updated custom reason expectations to include prefix
  • All FunctionInvokingChatClientApprovalsTests pass across .NET 8, 9, and 10

• Add serialization tests for Reason property

  • Updated FunctionApprovalResponseContentTests.Serialization_Roundtrips to test with both null and custom reason
  • Ensures Reason property correctly serializes and deserializes through JSON

• Add edge case tests for Reason property

  • Added RejectedApprovalResponsesWithEmptyOrWhitespaceReasonUsesDefaultMessageAsync test
  • Tests null, empty string, and whitespace-only values
  • Validates that IsNullOrWhiteSpace logic treats all as default message

• Build and test validation

  • All builds passed successfully (Debug configuration)
  • All 22 FunctionInvokingChatClientApprovalsTests pass
  • All 6 FunctionApprovalResponseContentTests pass

• Code review feedback addressed

  • Simplified property documentation
  • Improved rejection message format to avoid ambiguity
  • Added comprehensive test coverage per reviewer suggestions

Summary

All PR review feedback has been addressed. The rejection message now clearly indicates it's a rejection while allowing custom context to be appended. The implementation includes proper serialization support and comprehensive edge case testing to document behavior with empty/whitespace/null reasons.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Provide ability to specify reject message for FunctionInvokingChatClient</issue_title>
<issue_description>The reject message for FunctionApprovalResponseContent with Approved = false is currently a hard-coded string:

"Error: Tool call invocation was rejected by user."

However, this situation is not necessarily an error. It should be possible to provide a custom rejection message, allowing callers to specify the actual reason for rejection.

Relevant code:

extensions/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs

Line 1421 in cc328a7

rejections.ConvertAll(static m => (AIContent)new FunctionResultContent(m.Response.FunctionCall.CallId, "Error: Tool call invocation was rejected by user.")) :

</issue_description>

<agent_instructions>Please add a nullable string Reason get/set property to FunctionApprovalResponseContent. If such a reason is set, FunctionInvokingChatClient should factor that into the FunctionResultContent message it creates for rejected tool calls. The property can currently be ignored for Approval=true, though it's conceivable the message might be used for something in the future, and if nothing else, it'll show up in tracing/logging/etc. when the content is serialized.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Provide ability to specify reject message for FunctionInvokingChatClient #7139

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Microsoft Reviewers: Open in CodeFlow

[stephentoub]

@copilot

[stephentoub]

@NoofSaeidh, does this address your scenarios / concerns from #7139?

[stephentoub]

cc: @rogerbarreto, @westey-m, @jozkee

[Copilot]

Pull request overview

This PR adds a nullable Reason property to FunctionApprovalResponseContent to enable custom rejection messages for function call approvals. Previously, rejected function calls used a hard-coded error message. Now, users can provide specific reasons for rejections that will be included in the function result content sent to the downstream client.

Key Changes:

• Added Reason property (nullable string, get/set) to FunctionApprovalResponseContent with documentation indicating it's optional and can be used for both approval and rejection
• Updated rejection message format in FunctionInvokingChatClient.GenerateRejectedFunctionResults from "Error: Tool call invocation was rejected by user." to "Tool call invocation rejected." with optional custom reason appended when provided
• Updated all existing test expectations to match the new default rejection message format and added comprehensive tests for custom rejection reasons

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalResponseContent.cs	Adds nullable Reason property to allow custom messages for approval/rejection responses
src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs	Updates GenerateRejectedFunctionResults to use new rejection message format and append custom reason when provided using IsNullOrWhiteSpace check
test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs	Updates existing test expectations for new default message format and adds two comprehensive tests (RejectedApprovalResponsesWithCustomReasonAsync and MixedApprovalResponsesWithCustomAndDefaultReasonsAsync) covering custom rejection reasons

[stephentoub]

@copilot, please address the feedback

[NoofSaeidh]

@NoofSaeidh, does this address your scenarios / concerns from #7139?

Yes, thank you!

[NoofSaeidh]

@stephentoub actually looks like it is worth to add Reason into the CreateResponse method in FunctionApprovalRequestContent

extensions/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs

Line 40 in eedc644

public FunctionApprovalResponseContent CreateResponse(bool approved) => new(Id, approved, FunctionCall);

[stephentoub]

@stephentoub actually looks like it is worth to add Reason into the CreateResponse method in FunctionApprovalRequestContent

extensions/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionApprovalRequestContent.cs

Line 40 in eedc644

public FunctionApprovalResponseContent CreateResponse(bool approved) => new(Id, approved, FunctionCall);

Why not just set the property on the resulting object?