microsoft/autogen · error · ArgumentException

Invalid DataUri format, expected data:[<mediatype>][;base64]

Error message

Invalid DataUri format, expected data:[<mediatype>][;base64],<data>

What it means

ImageMessage's string-url constructor detects data URIs by the 'data:' prefix and validates them against s_DataUriRegex for the shape data:[<mediatype>][;base64],<data>. A prefix match that fails the regex throws ArgumentException('Invalid DataUri format...', paramName: url). The parser then base64-decodes the data group, so malformed base64 will subsequently throw FormatException from Convert.FromBase64String.

Source

Thrown at dotnet/src/AutoGen.Core/Message/ImageMessage.cs:31

    /// <summary>
    /// Create an ImageMessage from a url.
    /// The url can be a regular url or a data uri.
    /// If the url is a data uri, the scheme must be "data" and the format must be data:[<mediatype>][;base64],<data>
    /// </summary>
    public ImageMessage(Role role, string url, string? from = null, string? mimeType = null)
    {
        this.Role = role;
        this.From = from;

        // url might be a data uri or a regular url
        if (url.StartsWith("data:", StringComparison.OrdinalIgnoreCase))
        {
            // the url must be in the format of data:[<mediatype>][;base64],<data>
            var match = s_DataUriRegex.Match(url);

            if (!match.Success)
            {
                throw new ArgumentException("Invalid DataUri format, expected data:[<mediatype>][;base64],<data>", nameof(url));
            }

            this.Data = new BinaryData(Convert.FromBase64String(match.Groups["data"].Value), match.Groups["mediatype"].Value);

            this.MimeType = match.Groups["mediatype"].Value;
        }
        else
        {
            this.Url = url;
            // try infer mimeType from uri extension if not provided
            if (mimeType is null)
            {
                mimeType = url switch
                {
                    _ when url.EndsWith(".png", StringComparison.OrdinalIgnoreCase) => "image/png",
                    _ when url.EndsWith(".jpg", StringComparison.OrdinalIgnoreCase) => "image/jpeg",
                    _ when url.EndsWith(".jpeg", StringComparison.OrdinalIgnoreCase) => "image/jpeg",
                    _ when url.EndsWith(".gif", StringComparison.OrdinalIgnoreCase) => "image/gif",

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Format data URIs as data:<mediatype>;base64,<payload>, e.g. $"data:image/png;base64,{Convert.ToBase64String(bytes)}".
2. Strip whitespace/newlines from the base64 payload before constructing the ImageMessage.
3. Pre-validate with your own regex match on the data URI before passing it in.
4. If you already have raw bytes plus a media type, use the BinaryData constructor instead, which sidesteps URI formatting entirely.

Example fix

// before
var msg = new ImageMessage(Role.User, $"data:image/png {base64}"); // missing ';base64,'

// after
var msg = new ImageMessage(Role.User, $"data:image/png;base64,{base64}");
// or better: bypass URI formatting
var msg = new ImageMessage(Role.User, new BinaryData(bytes, "image/png"));

using System.Text.RegularExpressions;
static readonly Regex DataUri = new(@"^data:(?<mediatype>[^;,]+)?(?:;base64)?,(?<data>[A-Za-z0-9+/=]+)$", RegexOptions.Compiled);

bool IsValidDataUri(string url) => !url.StartsWith("data:", StringComparison.OrdinalIgnoreCase) || DataUri.IsMatch(url);

static bool IsWellFormedDataUri(string url) => url.StartsWith("data:", StringComparison.OrdinalIgnoreCase)
    && url.Contains(',')
    && url.Contains(";base64");

try { var msg = new ImageMessage(Role.User, url); }
catch (ArgumentException ex) when (ex.Message.Contains("DataUri"))
{
    // fall back to fetching/downloading the image and using the BinaryData constructor
}

When it happens

Trigger: Passing strings like 'data:image/png' (missing comma and payload), 'data:,hello' with base64 expected, 'data:image/png;base64,' (empty data), or a data URI with whitespace/line breaks in the payload; URL-escaped data URIs that were never decoded.

Common situations: Hand-assembling data URIs from byte arrays or base64 strings and forgetting the ';base64' marker or comma separator; copying data URIs from HTML/CSS that contain newline formatting; truncation of long base64 strings by config files or logs.

Related errors

• Data cannot be empty
• The from property of the message {message} is different from
• MimeType is required for ImageMessage
• MediaType is needed for DataUri Images
• Invalid aggregate message {reason}

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/5697da3be6c0a0b0. Report an issue: GitHub.