[API Proposal]: Allow MarkItDownMcpReader to send `file:` / `http:` URIs instead of inlining data

[mldisibio]

Background and motivation

MarkItDownMcpReader always sends the document as an inline base64 data: URI, so large files exceed the MCP message-size limit (413/-32600) with no alternative — even though the convert_to_markdown tool accepts file:/http: URIs. Callers whose file is reachable by the server (mounted volume, shared path, URL) should be able to have the server read it directly.

API Proposal

namespace Microsoft.Extensions.DataIngestion;

public sealed class MarkItDownMcpReaderOptions
{
    public Func<FileInfo, Uri>? SourceUriResolver { get; set; }
}

public class MarkItDownMcpReader : IngestionDocumentReader
{
    public MarkItDownMcpReader(
        Uri mcpServerUri,
        ModelContextProtocol.Client.McpClientOptions? options = null,
        MarkItDownMcpReaderOptions? readerOptions = null);
}

API Usage

// Input dir mounted into the MarkItDown container at /data
var reader = new MarkItDownMcpReader(
    new Uri("http://localhost:3001/mcp"),
    readerOptions: new MarkItDownMcpReaderOptions
    {
        SourceUriResolver = fi => new Uri($"file:///data/{fi.Name}")
    });

var doc = await reader.ReadAsync(new FileInfo("data/input/very_large.docx"), ct); // no inline bytes, no 413

Alternative Designs

overload ReadAsync(Uri sourceUri, string identifier, …) so callers pass a server-reachable URI directly; or add a bool InlineContent toggle plus a base-path map.

Risks

additive/opt-in — default (inline data:) is unchanged, so no breaking change. Resolver misconfiguration yields a server-side "file not found," which pairs naturally with fixing the IsError handling so it surfaces as a real error.