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Is validating of data in the MCP writer always the right thing to do?

  • June 3, 2026
  • 4 replies
  • 165 views
martinkoch
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Hello all,

Just as many of us, I am working on a PoC with MCP.

Now, the following occours.

My mcp-tool writes a file, and I try to respond, using the MCP-Writer, with a ‘Resource Link’ to that file.

Only, the file is written to sharepoint. Unlike the SharepointConnector, the MCP-Writer has no access to the file-location, and therefore refuses to write out the link giving the error message:

MCP Writer: Unable to write resource file, please verify the file path https:/[ourName].sharepoint.com/teams/…….

The Resource Link send, contains the other attributes and the file-name, but not the filepath I try to return.

I can imagine there are more situations where one wants to return something which can not be validated by FME Flow. Perhaps this validating should be optional, default ‘on’?

 

Kind regards,

Martin

 

      4 replies

      NickAtSafe
      Safer
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      • NickAtSafeSafer
      • Safer
      • June 3, 2026

      Hi ​@martinkoch - thanks for sharing this.

      The MCP Writer currently tries to access and download the file at the provided filepath in order to host it as a Resource on the MCP Server. That works fine for files within FME Flow's scope (local or Shared Resources), but it can't reach external URLs like SharePoint — hence the error.

      The MCP spec actually distinguishes between these two scenarios. For https:// URIs, the spec notes that servers should only use that scheme when the client can fetch the resource directly from the web on its own — meaning it doesn't need to go via the MCP server. Your case is exactly that: Claude (or whichever MCP client) could fetch a SharePoint link directly, so the MCP Server doesn't need to hold the file at all. The current behavior may be a bit more restrictive than is required.

      The most reliable path today is to download the SharePoint file within the workspace first (for example using the SharePoint Connector), then write the local copy as the Resource Link. Not ideal, but it keeps the resource within Flow's reach.

      On the topic of optional validation, this is something we're actively looking at. The direction we're considering is that when an https:// URI is provided, the MCP Writer simply passes it through as a resource link without trying to access the underlying file — leaving it to the client to resolve.

      No timeline to share yet but I'll make sure it's tracked.

      Always appreciate the detailed questions/thoughts!

      danielleatsafe
      Safer
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      Highlighting ​@martinkoch’s related Idea in case others want to upvote it: 

       

      NickAtSafe
      Safer
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      • NickAtSafeSafer
      • Safer
      • June 8, 2026

      Hi ​@martinkoch - just a quick update. We’re hoping to add the functionality I described in my last post into the 2026.3 release. We’re doing some work on Resources in MCP overall for this release, and it’ll fit nicely into it.

        martinkoch
        Enthusiast
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        • martinkochEnthusiast
        • Author
        • Enthusiast
        • June 8, 2026

        Hi ​@martinkoch - just a quick update. We’re hoping to add the functionality I described in my last post into the 2026.3 release. We’re doing some work on Resources in MCP overall for this release, and it’ll fit nicely into it.

        I’ll closely follow the MCP developments in FME Flow 🤓

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