Fix ToJsonObject serialization failure with anonymous types in dictionary values

[Copilot]

Fix ToJsonObject serialization failure with anonymous types

• Address all PR feedback from @stephentoub
• Use GetTypeInfo instead of passing options directly
• Revert McpJsonUtilities changes (no DefaultJsonTypeInfoResolver)
• Revert FunctionResultContent and default case to original code
• Move tests to existing test files (colocated with content block tests)
• All tests passing on .NET 8, 9, and 10

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ToJsonObject fails when dictionary values contain anonymous types (sampling pipeline regression in 0.5.0-preview.1)</issue_title>
<issue_description>Describe the bug
When using Semantic Kernel sampling via MCP, the internal payload passed from Semantic Kernel to MCP contains a dictionary whose values may include anonymous types (objects generated by Semantic Kernel and not controllable by the application).

In ModelContextProtocol 0.5.0-preview.1, calling
ModelContextProtocol.AIContentExtensions.ToJsonObject(IReadOnlyDictionary<string, object?> properties)
throws a System.NotSupportedException when any dictionary value is an anonymous type.

This previously worked in 0.4.0-preview.3 (stdio transport only) but now fails in 0.5.0-preview.1, including stdio, indicating that stdio and web transports now share the same serialization pipeline.

To Reproduce
Use Semantic Kernel with MCP sampling (AddSamplingChatClient)

Trigger a prompt invocation where Semantic Kernel generates arguments/metadata internally

Ensure the dictionary passed into
ModelContextProtocol.AIContentExtensions.ToJsonObject(...)
contains any anonymous type as a value (structure does not matter)

Run with ModelContextProtocol 0.5.0-preview.1

Observe serialization failure

Minimal conceptual example:

var dict = new Dictionary<string, object?>
{
["data"] = new { X = 1.0, Y = 2.0 }
};

AIContentExtensions.ToJsonObject(dict);

Expected behavior
ToJsonObject should be able to serialize dictionaries whose values are anonymous types, producing a valid JsonObject / JsonNode.

At minimum, anonymous types should be handled as plain object graphs rather than requiring source-generated JsonTypeInfo, which cannot be provided for anonymous types.

This is especially important because:

These anonymous types are generated internally by Semantic Kernel

Application developers cannot intercept or replace them

Logs

System.NotSupportedException: JsonTypeInfo metadata for type
'<>f__AnonymousType0`5[...]' was not provided by TypeInfoResolver
of type 'ModelContextProtocol.McpJsonUtilities+JsonContext'
   at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetadataForType
   at System.Text.Json.JsonSerializer.SerializeToNode
   at ModelContextProtocol.AIContentExtensions.ToJsonObject
   at ModelContextProtocol.Server.McpServer.SampleAsync(...)

Additional context

In 0.4.0-preview.3:

stdio transport worked

web transport failed (likely due to different serialization pipelines)

In 0.5.0-preview.1:

Both stdio and web transports fail

Indicates a unified pipeline that now enforces the same resolver behavior

Request / suggestion:
Enhance ModelContextProtocol.AIContentExtensions.ToJsonObject to support
dictionary values containing anonymous types, either by:

Falling back to a non–source-generated resolver for such values, or

Detecting anonymous types and serializing them via a compatible JsonSerializerOptions

This would restore compatibility with Semantic Kernel sampling scenarios
where developers cannot control the generated object shapes.</issue_description>

Comments on the Issue (you are @copilot in this section)

@eiriktsarpalis Have you turned on `PublishAOT` in your application? Unfortunately, anonymous types cannot be (and probably never will be) supported by source generators. You could either turn off AOT or replace your anonymous types with equivalent named types (it should be straightforward nowadays with record types). @stephentoub @eiriktsarpalis, this fails without PublishAOT. Here's a minimal repro: ```csharp using Microsoft.Extensions.AI; using ModelContextProtocol;

AIContent c = new()
{
AdditionalProperties = new()
{
["data"] = new { X = 1.0, Y = 2.0 }
}
};

c.ToContentBlock();

That fails with:
```csharp
Unhandled exception. System.NotSupportedException: JsonTypeInfo metadata for type '<>f__AnonymousType0`2[System.Double,System.Double]' was not provided by TypeInfoResolver of type 'ModelContextProtocol.McpJsonUtilities+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. The unsupported member type is located on type 'System.Object'. Path: $.
 ---> System.NotSupportedException: JsonTypeInfo metadata for type '<>f__AnonymousType0`2[System.Double,System.Double]' was not provided by TypeInfoResolver of type 'ModelContextProtocol.McpJsonUtilities+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.
   at System.Text.Json.ThrowHelper.ThrowNotSupportedException_NoMetad...

</details>



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- Fixes modelcontextprotocol/csharp-sdk#1112

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[stephentoub]

@copilot, these attributes must not be used and shouldn't be necessary.

[Copilot]

Removed the UnconditionalSuppressMessage attributes in commit 7041a89.

[stephentoub]

@copilot, this should pass in .GetTypeInfo(typeof(object)) (or typeof(relevantType)), not the options. Same goes for any other place you were passing in the options directly.

[Copilot]

Changed to use GetTypeInfo(typeof(IReadOnlyDictionary<string, object?>)) instead of passing options directly in commit 7041a89.

[stephentoub]

@copilot, the previous code was correct. Did a test fail with it? Note that anonymous types will fail in the core test suite because reflection serialization is dialed.

[Copilot]

Reverted to the original code that serializes content in commit 7041a89. The test failure was due to missing reflection support in .NET 9 by default, which is now handled by skipping tests when reflection is disabled.

[stephentoub]

@copilot, no, revert this

[Copilot]

Reverted the default case to use JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object))) in commit 7041a89.

[stephentoub]

@copilot, revert this

[Copilot]

Reverted McpJsonUtilities.cs to remove the DefaultJsonTypeInfoResolver in commit 7041a89.

[stephentoub]

@copilot, all of the new tests should be added to existing test files, eg colocated with the content block tests, not adding new test files

[Copilot]

Moved all anonymous type tests into AIContentExtensionsTests.cs and deleted the separate test files in commit 867d044.