How do I return file in current implementation?

[vshapenko]

Hello! Just checked the docs and tests and did not find any sample on how I can return file as tool answer, for example. Could anyone shed some light on it?

[stephentoub]

There are a variety of ways, but probably the easiest is to just return a Microsoft.Extensions.AI.DataContent, e.g.

[McpServerTool]
public static DataContent GetMyImage()
{
    byte[] bytes = File.ReadAllBytes("path/to/my/image.png");
    return new DataContent(bytes, "image/png");
}

[danijerez]

Can "DataContent" also be used as input? I was trying a base64 string because I wasn't familiar with it.

example:

[McpServerTool]
public static bool UploadDocument(DataContent fileContent)
{
    using var stream = new MemoryStream(fileContent.Data.ToArray());
    var filePart = new StreamPart(stream, fileName, fileContent.MediaType);
    ....
}

[stephentoub]

You should be able to, but DataContent isn't just bytes, it also contains a media type, so its natural representation is as a data URI. And a schema generated for it will look something like:

{"type":"object","properties":{"uri":{"type":"string"}}

so while I think it should work, you would need to actually send your base64 string instead as a data URI on a json object, e.g.:

{
    "uri": "data:text/plain;base64,SGVsbG8sIFdvcmxkIQ=="
}

[vshapenko]

Thanks a lot, will try

[McTristan]

Depending on the client not alle mimetypes and data sizes are supported though, i.e. Claude AI doesn't like Excel files (even if they are small) and produces weird errors when trying to return zip files. Finding out what exactly works and what not is a challenge right now as it is constantly changing ...