sgl-project/sglang · error · ValueError

{b64_format_hint} (missing ;base64 marker)

Error message

{b64_format_hint} (missing ;base64 marker)

What it means

The input matched the generic data-URI shape but lacked the ;base64 marker, e.g. data:image/png,.... Only base64-encoded payloads are supported, so the parser rejects URL-encoded or plain data URIs.

Source

Thrown at python/sglang/multimodal_gen/runtime/utils/image_io.py:19

# SPDX-License-Identifier: Apache-2.0
import base64
import os
import re


def save_base64_image_to_path(base64_data: str, target_path: str) -> str:
    b64_format_hint = (
        "Failed to decode base64 image. "
        "Expected format: `data:[<media-type>];base64,<data>`"
    )

    match = re.match(r"data:(.*?)(;base64)?,(.*)", base64_data)
    if not match:
        raise ValueError(b64_format_hint)
    media_type = match.group(1)
    is_base64 = match.group(2)
    if not is_base64:
        raise ValueError(f"{b64_format_hint} (missing ;base64 marker)")
    data = match.group(3)
    if not data:
        raise ValueError(f"{b64_format_hint} (empty data payload)")

    if media_type.startswith("image/"):
        ext = media_type.split("/")[-1].lower()
        if ext == "jpeg":
            ext = "jpg"
    else:
        ext = "jpg"
    target_path = f"{target_path}.{ext}"
    os.makedirs(os.path.dirname(target_path), exist_ok=True)

    try:
        image_data = base64.b64decode(data)
    except Exception as exc:
        raise Exception(f"Failed to decode base64 image: {str(exc)}") from exc

View on GitHub (pinned to 0132848349)

Solutions

1. Base64-encode the bytes and include the marker: data:image/png;base64,...
2. Ensure client libraries (e.g. canvas toDataURL) are used unchanged — they already emit ;base64

Example fix

// before
save_base64_image_to_path("data:image/png,%89PNG%0D%0A", "/tmp/img")
// after
save_base64_image_to_path("data:image/png;base64,iVBORw0KGgo...", "/tmp/img")

def has_base64_marker(s: str) -> bool:
    return ";base64," in s

def is_base64_data_uri(s: str) -> bool:
    return isinstance(s, str) and s.startswith("data:") and ";base64," in s

try:
    save_base64_image_to_path(uri, path)
except ValueError as e:
    if "missing ;base64 marker" in str(e):
        uri = uri.replace(",", ";base64,", 1) if uri.startswith("data:") else uri

When it happens

Trigger: Supplying a data URI without ;base64 such as data:image/png,%89PNG... to _maybe_url_image / prepare_warmup_image_path.

Common situations: Clients URL-encoding image bytes instead of base64-encoding them; hand-built data URIs that omit the marker.

Related errors

• Failed to decode base64 image. Expected format: `data:[<medi
• {b64_format_hint} (empty data payload)
• No base64 image data found
• Cosmos3 action input accepts one image field; use a list or
• data URI must use ;base64 encoding

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/1e435cac7bce36b1. Report an issue: GitHub.