[Microsoft.Extensions.AI.OpenAI] Tool schema with object-valued additionalProperties throws JsonException (ToolJson.AdditionalProperties typed as bool)

[scottt732]

Package: Microsoft.Extensions.AI.OpenAI 10.6.0

Summary

OpenAIClientExtensions.ToOpenAIFunctionParameters deserializes every tool's schema via element.Deserialize(OpenAIJsonContext.Default.ToolJson), and the internal ToolJson type declares:

[JsonPropertyName("additionalProperties")] public bool AdditionalProperties { get; set; }

JSON Schema permits additionalProperties to be either a boolean or a sub-schema object. Any tool whose schema uses the object form throws:

System.Text.Json.JsonException: The JSON value could not be converted to System.Boolean. Path: $.additionalProperties | LineNumber: 0 | BytePositionInLine: 261.

…aborting the request. This happens for every function tool regardless of strict mode — ToOpenAIFunctionParameters deserializes into ToolJson on both the strict-transformed and the raw-schema branch — and on both the Chat Completions (ToOpenAIChatTool) and Responses/Assistants tool-conversion paths.

Repro

Create an AIFunction whose JsonSchema is:

{ "type": "object", "properties": { "labels": { "type": "object" } }, "additionalProperties": { "type": "string" } }

…and pass it as a tool to an OpenAI IChatClient. The request build throws the JsonException above. This schema shape is common in OpenAPI/MCP-derived tools (e.g. map/dictionary parameters render additionalProperties as a sub-schema).

Expected

The object form of additionalProperties is accepted (handled or normalized), not a hard JsonException.

Suggested fix

Type ToolJson.AdditionalProperties so it tolerates both forms (e.g. JsonElement, or bool? with a converter), or normalize the schema before deserialization.

Workaround

Wrap the IChatClient in a DelegatingChatClient that recursively coerces object-valued additionalProperties to false in each tool's schema before it reaches the bridge.