Handle F# optional parameters in AIFunctionFactory schema generation

[eiriktsarpalis]

Summary

F# optional parameters declared with the ?param syntax are compiled with Microsoft.FSharp.Core.OptionalArgumentAttribute but do not set the ParameterAttributes.Optional flag, so ParameterInfo.IsOptional returns false. This caused AIFunctionFactory to treat them as required parameters, producing incorrect JSON schemas and throwing ArgumentException when the parameter was omitted during invocation.

Changes

Fix (AIJsonUtilities.Schema.Create.cs)

Updated TryGetEffectiveDefaultValue to:

1. Check parameterInfo.IsOptional (covers COM interop [Optional] parameters)
2. Check for OptionalArgumentAttribute by name via IsFSharpOptionalParameter() -- avoids a hard dependency on FSharp.Core
3. When either is detected, return true with a null default value (equivalent to FSharpOption.None)

This single change fixes both schema generation (parameter no longer in the required array, gets "default": null) and invocation (marshaller returns null instead of throwing).

New F# test project

Added test/Libraries/Microsoft.Extensions.AI.Abstractions.FSharp.Tests/ with 6 tests covering:

• Schema correctly marks F# optional parameters as not required
• Schema includes "default": null for optional parameters
• Invocation works when optional parameters are both omitted and provided
• Multiple optional parameters work correctly

Test Results

• 6/6 new F# tests pass
• 664/664 existing AI tests pass
• 1611/1611 existing AI Abstractions tests pass

Context

Addresses modelcontextprotocol/csharp-sdk#1476 (comment) -- F# MCP tool methods with optional parameters threw errors when clients omitted those parameters.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Updates AIFunctionFactory schema generation and invocation so that F# optional parameters (compiled with Microsoft.FSharp.Core.OptionalArgumentAttribute, but not ParameterInfo.IsOptional) are treated as optional with a null default, and adds an F# test project to validate the behavior.

Changes:

• Extend default-value detection to treat F# ?param optional parameters (and IsOptional parameters without a declared default) as having an effective default of null.
• Add a new F# test project validating schema required/default behavior and invocation with omitted/provided optional args.

Reviewed changes

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

File	Description
test/Libraries/Microsoft.Extensions.AI.Abstractions.FSharp.Tests/Microsoft.Extensions.AI.Abstractions.FSharp.Tests.fsproj	Introduces a new F# test project to cover F# optional parameter behavior.
test/Libraries/Microsoft.Extensions.AI.Abstractions.FSharp.Tests/FSharpOptionalParameterTests.fs	Adds F# tests validating schema generation and invocation with F# optional parameters.
src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs	Updates default-value inference to recognize F# optional parameters via OptionalArgumentAttribute.