Generated tool input schema for reflected C# parameter objects does not satisfy OpenAI strict function schema requirements in VS Copilot.

[KKMicrosoft]

Select Topic Area

Bug

Copilot Feature Area

Visual Studio

Body

Platform: Visual Studio
Model: GPT-5.3-Codex (But any GPT model breaks with this)

Create a model with nested custom model property

Example
public class InputModel
{
public CustomClass Property

public string? Property2

public IReadOnlyList? Property3
}

I use this as an input parameter in the MCP tool

[McpServerTool(
    Name = "ToolName",
    Title = "Tool function",
    UseStructuredContent = true,
    ReadOnly = true,
    Destructive = false,
    Idempotent = true,
    OpenWorld = false)]
[Description("Perform some function")]
public async Task<CustomResponse> Tool(InputModel model)

{

return CustomResponse;
}

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