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Fix structured output backwards compatibility for string results #748

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Fix structured output backwards compatibility for string results #748

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fc571e2
Fix structured output backwards compatibility for string literals
PederHP Sep 1, 2025
b9468e2
Serialize structured content not result
PederHP Sep 1, 2025
b015e38
Update src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs
PederHP Sep 4, 2025
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7 changes: 5 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 @@ -257,10 +257,13 @@ public override async ValueTask<CallToolResult> InvokeAsync(
Content = [],
StructuredContent = structuredContent,
},

string text => new()
{
Content = [new TextContentBlock { Text = text }],
// If there is structuredOutput we must return that in stringified form,
// as required for backward compatibility by the MCP specification.
Content = [new TextContentBlock { Text = structuredContent == null ? text :
JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
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Why not just resolve the type of structuredContent itself and skip one degree of indirection?

Suggested change
JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo<JsonNode>()))}],

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Why not just resolve the type of structuredContent itself and skip one degree of indirection?

Suggested change
JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo<JsonNode>()))}],

Good point, but this goes for the default switch case too then?

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Yeah, I suppose we can leave it for consistency.

StructuredContent = structuredContent,
},

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