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Preserve nulls required by structured output schemas #1929

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13 changes: 11 additions & 2 deletions src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
using System.Reflection;
using System.Text.Json;
using System.Text.Json.Nodes;
using System.Text.Json.Serialization;
using System.Text.RegularExpressions;

namespace ModelContextProtocol.Server;
Expand All @@ -14,6 +15,7 @@ namespace ModelContextProtocol.Server;
internal sealed partial class AIFunctionMcpServerTool : McpServerTool
{
private readonly IReadOnlyList<object> _metadata;
private readonly JsonSerializerOptions _structuredContentSerializerOptions;

/// <summary>
/// Creates an <see cref="McpServerTool"/> instance for a method, specified via a <see cref="Delegate"/> instance.
Expand Down Expand Up @@ -225,6 +227,13 @@ private AIFunctionMcpServerTool(AIFunction function, Tool tool, IServiceProvider
ProtocolTool = tool;

_metadata = metadata;
_structuredContentSerializerOptions = tool.OutputSchema is not null &&
function.JsonSerializerOptions.DefaultIgnoreCondition is JsonIgnoreCondition.WhenWritingNull
? new JsonSerializerOptions(function.JsonSerializerOptions)
{
DefaultIgnoreCondition = JsonIgnoreCondition.Never,
}
: function.JsonSerializerOptions;
}

/// <inheritdoc />
Expand Down Expand Up @@ -677,7 +686,7 @@ private static void RewriteRefPointers(JsonNode? node)
JsonElement jsonElement => jsonElement,
JsonNode node => JsonSerializer.SerializeToElement(node, McpJsonUtilities.JsonContext.Default.JsonNode),
null => null,
_ => JsonSerializer.SerializeToElement(aiFunctionResult, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
_ => JsonSerializer.SerializeToElement(aiFunctionResult, _structuredContentSerializerOptions.GetTypeInfo(typeof(object))),
};

// Pre-SEP-2106 clients expect the {"result": <value>} envelope for non-object
Expand Down Expand Up @@ -805,4 +814,4 @@ private static bool IsPrimitiveHeaderType(Type type)
type == typeof(uint) ||
type == typeof(long);
}
}
}
61 changes: 61 additions & 0 deletions tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -481,6 +481,58 @@ public async Task StructuredOutput_Enabled_ReturnsExpectedSchema<T>(T value)
AssertMatchesJsonSchema(tool.ProtocolTool.OutputSchema.Value, result.StructuredContent);
}

[Theory]
[InlineData(null, Sep2106ProtocolVersion)]
[InlineData("note", Sep2106ProtocolVersion)]
[InlineData(null, "2025-11-25")]
[InlineData("note", "2025-11-25")]
public async Task StructuredOutput_Enabled_RequiredNullableProperties_MatchesOutputSchema(string? note, string protocolVersion)
{
JsonSerializerOptions serializerOptions = new(McpJsonUtilities.DefaultOptions)
{
TypeInfoResolver = JsonTypeInfoResolver.Combine(
McpJsonUtilities.DefaultOptions.TypeInfoResolver,
new DefaultJsonTypeInfoResolver()),
};
McpServerTool tool = McpServerTool.Create(
() => new RequiredNullableOutput("42", note),
new() { Name = "tool", UseStructuredContent = true, SerializerOptions = serializerOptions });
var request = CreateRequestContextWithProtocolVersion(protocolVersion);

var result = await tool.InvokeAsync(request, TestContext.Current.CancellationToken);

Assert.NotNull(tool.ProtocolTool.OutputSchema);
Assert.NotNull(result.StructuredContent);
Assert.Equal(note is null ? JsonValueKind.Null : JsonValueKind.String, result.StructuredContent.Value.GetProperty("note").ValueKind);
AssertMatchesJsonSchema(tool.ProtocolTool.OutputSchema.Value, result.StructuredContent);
}

[Theory]
[InlineData(null, Sep2106ProtocolVersion)]
[InlineData("note", Sep2106ProtocolVersion)]
[InlineData(null, "2025-11-25")]
[InlineData("note", "2025-11-25")]
public async Task StructuredOutput_Enabled_RequiredNullablePropertiesInClass_MatchesOutputSchema(string? note, string protocolVersion)
{
JsonSerializerOptions serializerOptions = new(McpJsonUtilities.DefaultOptions)
{
TypeInfoResolver = JsonTypeInfoResolver.Combine(
McpJsonUtilities.DefaultOptions.TypeInfoResolver,
new DefaultJsonTypeInfoResolver()),
};
McpServerTool tool = McpServerTool.Create(
() => new RequiredNullablePropertiesOutput { Id = "42", Note = note },
new() { Name = "tool", UseStructuredContent = true, SerializerOptions = serializerOptions });
var request = CreateRequestContextWithProtocolVersion(protocolVersion);

var result = await tool.InvokeAsync(request, TestContext.Current.CancellationToken);

Assert.NotNull(tool.ProtocolTool.OutputSchema);
Assert.NotNull(result.StructuredContent);
Assert.Equal(note is null ? JsonValueKind.Null : JsonValueKind.String, result.StructuredContent.Value.GetProperty("note").ValueKind);
AssertMatchesJsonSchema(tool.ProtocolTool.OutputSchema.Value, result.StructuredContent);
}

[Fact]
public async Task StructuredOutput_Enabled_VoidReturningTools_ReturnsExpectedSchema()
{
Expand Down Expand Up @@ -1091,6 +1143,15 @@ Instance JSON document does not match the specified schema.

record Person(string Name, int Age);

private sealed record RequiredNullableOutput(string Id, string? Note);

private sealed class RequiredNullablePropertiesOutput
{
public required string Id { get; init; }

public required string? Note { get; init; }
}

private static JsonSerializerOptions CreateSerializerOptionsWithPerson()
{
JsonSerializerOptions options = new(McpJsonUtilities.DefaultOptions);
Expand Down