NotSupportedException when returning an IEnumerable<ContentBlock>

[asklar]

Repro:

1. Create an McpServerTool that returns IEnumerable
2. Observe the following exception:

JsonTypeInfo metadata for type 'System.Collections.Generic.IEnumerable`1[ModelContextProtocol.Protocol.ContentBlock]' was not provided by TypeInfoResolver of type '[ModelContextProtocol.McpJsonUtilities+JsonContext, Microsoft.Extensions.AI.AIJsonUtilities+JsonContext]'. If using source generation, ensure that all root types passed to the serializer have been annotated with 'JsonSerializableAttribute', along with any types that might be serialized polymorphically

Looks like IEnumerable and similar for ContentBlock and derived types is not part of the json serializer context:

csharp-sdk/src/ModelContextProtocol.Core/McpJsonUtilities.cs

Line 220 in 5e5b1af

internal sealed partial class JsonContext : JsonSerializerContext;

[eiriktsarpalis]

This is a known issue when working with the source generator and AOT: the exact types used by the tool in question must be provided explicitly in a JsonSerializerContext. This means that even if McpJsonUtilities includes ContentBlock, List<ContentBlock>, or ContentBlock[] it would still not work for IEnumerable<ContentBlock> or ContentBlock[][].

Since it is not feasible to include every possible permutation of generic type combinations in McpJsonUtilities, the best way forward is to author your own JsonSerializerContext that includes the necessary types. You can then feed the resultant JsonSerializerOptions into the relevant WithTool method call.

[stephentoub]

@eiriktsarpalis, given IEnumerable is the common type for all collectons and we document it here:

csharp-sdk/src/ModelContextProtocol.Core/Server/McpServerTool.cs

Line 121 in 5e5b1af

/// <term><see cref="IEnumerable{ContentBlock}"/> of <see cref="ContentBlock"/></term>

I think it'd make sense to include it

[eiriktsarpalis]

Sure, although fundamentally it wouldn't improve the underlying experience.