mcpcompat drops entire tool/prompt result when it contains an embedded resource

[jhrozek]

Summary

mcpcompat.EmbeddedResource has a Resource ResourceContents field where ResourceContents is a non-empty interface, and EmbeddedResource has no custom UnmarshalJSON. So json.Unmarshal of any "type":"resource" content item errors:

json: cannot unmarshal object into Go struct field EmbeddedResource.resource of type mcp.ResourceContents

Because CallToolResult.UnmarshalJSON (and the client's convertResult) propagate that error, any tool result — or prompt message — containing an embedded resource is lost in its entirety, including any sibling text/image parts in the same content array.

Impact

Discovered from a ToolHive vMCP MCP-conformance run (the mcp-go → mcpcompat swap). Two conformance scenarios regressed:

• tools-call-embedded-resource → "No resource content found"
• tools-call-mixed-content → "Expected multiple content items; Missing image content; Missing resource content"

The prompts/get path is affected too, since convertGetPromptResult also decodes content via mcp.UnmarshalContent.

Call chain

1. client.CallTool converts the go-sdk result via the generic convertResult → jsonConvert.
2. That runs CallToolResult.UnmarshalJSON, which calls UnmarshalContent per item.
3. UnmarshalContent, case ContentTypeResource, does json.Unmarshal(data, &EmbeddedResource{}) — stdlib cannot populate the ResourceContents interface field → error.
4. The error aborts the whole decode; convertResult discards the partially-decoded result.

ReadResource and GetPrompt were hand-written to avoid this exact trap (convertReadResourceResult maps concrete text/blob types explicitly); CallTool was left on the generic path.

Fix

Add a custom UnmarshalJSON to EmbeddedResource that resolves the polymorphic Resource field to a concrete TextResourceContents/BlobResourceContents (blob-precedence, mirroring convertReadResourceResult). UnmarshalContent is the single chokepoint backing both tools/call and prompts/get.

Repro

wire := []byte(`{"type":"resource","resource":{"uri":"file:///x.txt","mimeType":"text/plain","text":"body"}}`)
_, err := mcp.UnmarshalContent(wire)
// err: json: cannot unmarshal object into Go struct field EmbeddedResource.resource of type mcp.ResourceContents