iOfficeAI/OfficeCLI · error · ArgumentException

Only base64-encoded data URIs are supported

Error message

Only base64-encoded data URIs are supported

What it means

Thrown by ImageSource.ResolveDataUri when a data URI contains a comma but its header portion (the part before the comma) does not contain 'base64'. OfficeCLI only supports base64-encoded data URIs — URL-encoded (percent-encoded) data URIs are rejected because the decoder expects raw base64. The check is case-insensitive and substring-based against the header.

Source

Thrown at src/officecli/Core/ImageSource.cs:128

        if (type == ImagePartType.Jpeg) return "JPEG";
        if (type == ImagePartType.Gif) return "GIF";
        if (type == ImagePartType.Bmp) return "BMP";
        if (type == ImagePartType.Tiff) return "TIFF";
        return type.ContentType ?? "unknown";
    }

    private static (Stream, PartTypeInfo) ResolveDataUri(string dataUri)
    {
        // Format: data:[<mediatype>][;base64],<data>
        var commaIdx = dataUri.IndexOf(',');
        if (commaIdx < 0)
            throw new ArgumentException("Invalid data URI: missing comma separator");

        var header = dataUri[..commaIdx]; // e.g. "data:image/png;base64"
        var data = dataUri[(commaIdx + 1)..];

        if (!header.Contains("base64", StringComparison.OrdinalIgnoreCase))
            throw new ArgumentException("Only base64-encoded data URIs are supported");

        // Extract MIME type
        var mimeStart = header.IndexOf(':') + 1;
        var mimeEnd = header.IndexOf(';');
        var mime = mimeEnd > mimeStart ? header[mimeStart..mimeEnd] : header[mimeStart..];

        var contentType = MimeToContentType(mime);
        var bytes = Convert.FromBase64String(data);
        // An empty payload decodes "successfully" to zero bytes and would
        // flow all the way into a 0-byte media part — schema-valid, but the
        // picture renders broken in Word/PowerPoint. Reject up front, like
        // the extpart carrier does for empty data.
        if (bytes.Length == 0)
            throw new ArgumentException("Invalid data URI: empty base64 payload (would produce a 0-byte image).");
        return (new MemoryStream(bytes), contentType);
    }

    private static (Stream, PartTypeInfo) ResolveUrl(string url)

View on GitHub (pinned to 1ced45e900)

Solutions

  1. Base64-encode the image payload and add ';base64' to the data URI header: 'data:image/png;base64,<base64data>'.
  2. Use a tool or library function to produce base64 data URIs (e.g. base64 command, or Convert.ToBase64String in C#).
  3. If you have a URL-encoded data URI, decode it first, then re-encode as base64.

Example fix

// before — URL-encoded data URI rejected
add image src='data:image/png,%89PNG%0D%0A...' path='/body'

// after — base64-encoded data URI accepted
add image src='data:image/png;base64,iVBORw0KGgoAAAANSUhEUg...' path='/body'
Defensive patterns

Strategy: validation

Validate before calling

// Pre-check that data URI is base64-encoded
if (source.StartsWith("data:", StringComparison.OrdinalIgnoreCase))
{
    int commaIdx = source.IndexOf(',');
    if (commaIdx > 0)
    {
        string header = source[..commaIdx];
        if (!header.Contains("base64", StringComparison.OrdinalIgnoreCase))
        {
            Console.Error.WriteLine("Only base64 data URIs are supported. Re-encode as base64.");
            return;
        }
    }
}

Try / catch

try
{
    var (stream, contentType) = ImageSource.Resolve(source);
}
catch (ArgumentException ex) when (ex.Message.Contains("base64"))
{
    // Re-encode the payload as base64 and retry
}

Prevention

When it happens

Trigger: Passing a data URI like 'data:image/png,%89PNG...' (URL-encoded, no ;base64 flag) or 'data:text/plain,Hello' (plain text, no ;base64). Any data URI where the header between 'data:' and ',' does not contain the substring 'base64' (case-insensitive) triggers this error.

Common situations: A data URI generated by a browser or tool that defaults to URL-encoding rather than base64. A data URI copied from an HTML source attribute that used percent-encoding. A user constructing a data URI manually without adding the ';base64' parameter.

Related errors

AI-assisted analysis of iOfficeAI/OfficeCLI@1ced45e900 (2026-08-13). Data as JSON: /api/errors/046643a3b9cc08de. Report an issue: GitHub.