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### Dify version
Source checkout of main at `43af5082d3c4ab8a96067370d2f86f214e3c85ae`.
### Cloud or Self Hosted
Self Hosted (Source)
### Steps to reproduce
This was reproduced through `CallToolResult.model_validate`, `MCPTool.invoke`, and `ToolEngine.tool_response_to_str`, with the remote call mocked to return this valid protocol result:
```json
{"content":[{"type":"resource_link","uri":"https://example.com/report.txt","name":"report","description":"Generated report","mimeType":"text/plain"}],"structuredContent":null}
```
1. Return a resource-link-only result from an MCP tool without an output schema or `structuredContent`.
2. Inspect the messages yielded by `MCPTool.invoke` and the resulting tool observation.
3. Repeat with mixed text and resource-link content.
The protocol parser recognizes `ResourceLink`, but the dispatch in `MCPTool._invoke` has no case for it. The accompanying regressions use actual protocol model validation and the production observation conversion.
### ✔️ Expected Behavior
The resource link and its metadata are retained in the tool observation, including when mixed with text. Retaining a link does not require fetching its URI.
### ❌ Actual Behavior
The resource link is logged as unsupported and dropped. A link-only result yields no messages; a mixed result keeps only the supported content and loses the resource link.
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<!-- Answer -->
The code at the commit you cite matches your report. `MCPTool._invoke` handles `TextContent`, `ImageContent`/`AudioContent` and `EmbeddedResource`, and sends every other content type to a `case _` branch that only logs "Unsupported content type" and yields nothing ([tool.py](https://github.com/langgenius/dify/blob/43af5082d3c4ab8a96067370d2f86f214e3c85ae/api/core/tools/mcp_tool/tool.py#L84-L105)). `ResourceLink` is a valid member of the `ContentBlock` union, so the protocol model parses it ([types.py](https://github.com/langgenius/dify/blob/43af5082d3c4ab8a96067370d2f86f214e3c85ae/api/core/mcp/types.py#L737-L747)), but it then reaches that fallback and is dropped. The docs describe the same dispatch table and don't list `ResourceLink` either [[1]](https://app.dosu.dev/6088a4f4-073a-43f3-b3d4-4f06fb0c05a0/documents/59781843-a5b1-40c0-8244-ab755355ee52).
A fix could add a `case ResourceLink():` branch before the fallback that yields a text or JSON message with the `uri`, `name`, `description` and `mimeType`. That would keep the link in the observation, including in mixed results, without fetching the URI. I haven't tested this. Choosing between text and JSON is a maintainer call, since the two message types end up in the observation differently.
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