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Validate and filter inputSchema for McpServerTool so it is spec compliant #343

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Validate and filter inputSchema for McpServerTool so it is spec compliant #343

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41 changes: 40 additions & 1 deletion src/ModelContextProtocol/Server/AIFunctionMcpServerTool.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -162,7 +162,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
{
Name = options?.Name ?? function.Name,
Description = options?.Description ?? function.Description,
InputSchema = function.JsonSchema,
InputSchema = FilterJsonSchema(function.JsonSchema),
};

if (options is not null)
Expand Down Expand Up @@ -220,6 +220,45 @@ options.OpenWorld is not null ||
return newOptions;
}

/// <summary>
/// Filters a JsonElement containing a schema to only include allowed properties: "type", "properties", and "required".
/// </summary>
private static JsonElement FilterJsonSchema(JsonElement schema)
{
using var memoryStream = new MemoryStream();
using var writer = new Utf8JsonWriter(memoryStream);
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@halter73 halter73 Apr 22, 2025 •
edited
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I think it would probably be better to use JsonObject to build up the JSON DOM and then convert that to a JsonElement using jsonNode.Deserialize<JsonElement>(typeInfo).

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/use-dom#create-a-jsonnode-dom-with-object-initializers-and-make-changes

@eiriktsarpalis is probably the best person to ask about the right approach here though.

Edit: Could we also use AIFunctionFactoryOptions.JsonSchemaCreateOptions.TransformSchemaNode for the AIFunctions we create ourselves from Delegates and the like and avoid generating the intermediate schema in that case?

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Edit: Could we also use AIFunctionFactoryOptions.JsonSchemaCreateOptions.TransformSchemaNode for the AIFunctions we create ourselves from Delegates and the like and avoid generating the intermediate schema in that case?

Yep, that's the best way to achieve that. Although per my remarks in #343 (comment) this should most likely be left to individual implementers as I think the metaschema is not meant to be interpreted literally.


writer.WriteStartObject();

// Include "type" property if it exists
if (schema.TryGetProperty("type", out var typeElement))
{
writer.WritePropertyName("type");
typeElement.WriteTo(writer);
}

// Include "properties" property if it exists
if (schema.TryGetProperty("properties", out var propertiesElement))
{
writer.WritePropertyName("properties");
propertiesElement.WriteTo(writer);
}

// Include "required" property if it exists
if (schema.TryGetProperty("required", out var requiredElement))
{
writer.WritePropertyName("required");
requiredElement.WriteTo(writer);
}

writer.WriteEndObject();
writer.Flush();

memoryStream.Position = 0;
using var document = JsonDocument.Parse(memoryStream.ToArray());
return document.RootElement.Clone();
}

/// <summary>Gets the <see cref="AIFunction"/> wrapped by this tool.</summary>
internal AIFunction AIFunction { get; }

Expand Down
8 changes: 6 additions & 2 deletions src/ModelContextProtocol/Utils/Json/McpJsonUtilities.cs
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Expand Up @@ -68,8 +68,12 @@ internal static bool IsValidMcpToolSchema(JsonElement element)
{
return false;
}

return true; // No need to check other properties
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Now we need to update the return false; at the end of the loop to return true if the "type" property was found and know unknown properties were found.

}
else if (!(property.NameEquals("properties") ||
property.NameEquals("required")))
{
// Only "type", "properties", and "required" are allowed.
return false;
}
}

Expand Down
3 changes: 2 additions & 1 deletion tests/ModelContextProtocol.Tests/Protocol/ProtocolTypeTests.cs
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Expand Up @@ -30,6 +30,7 @@ public static void ToolInputSchema_HasValidDefaultSchema()
[InlineData("""{"type":"number"}""")]
[InlineData("""{"type":"array"}""")]
[InlineData("""{"type":["object"]}""")]
[InlineData("""{"type":"object", "title": "MyAwesomeTool", "description": "It's awesome!", "properties": {}, "required" : ["NotAParam"] }""")]
public static void ToolInputSchema_RejectsInvalidSchemaDocuments(string invalidSchema)
{
using var document = JsonDocument.Parse(invalidSchema);
Expand All @@ -41,7 +42,7 @@ public static void ToolInputSchema_RejectsInvalidSchemaDocuments(string invalidS
[Theory]
[InlineData("""{"type":"object"}""")]
[InlineData("""{"type":"object", "properties": {}, "required" : [] }""")]
[InlineData("""{"type":"object", "title": "MyAwesomeTool", "description": "It's awesome!", "properties": {}, "required" : ["NotAParam"] }""")]
[InlineData("""{"type":"object", "properties": {}, "required" : ["NotAParam"] }""")]
public static void ToolInputSchema_AcceptsValidSchemaDocuments(string validSchema)
{
using var document = JsonDocument.Parse(validSchema);
Expand Down
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