Which Safari version first introduced fetch duplex support, and can we detect it programmatically before concurrent uploads?

Answer

The first Safari release that accepted duplex: 'half' with a ReadableStream body was Safari 16.1 (released in 2023). Prior to 16.1, the call throws a TypeError immediately, preventing concurrent uploads.

Confirmed Facts

• Safari 16.1 and newer support request‑streaming via duplex: 'half'.
• There is no public navigator property, feature flag, or API that exposes this capability.
• Detection must be performed by attempting the fetch and catching any synchronous exception.
• The exception is thrown at request initiation, before any network activity, so the test can safely precede concurrent uploads.

How to Detect Support

async function supportsDuplexHalf() {
  // Create a minimal ReadableStream that yields nothing
  const stream = new ReadableStream({ start(controller) { controller.close(); } });
  try {
    await fetch('https://example.com/placeholder', {
      method: 'POST',
      body: stream,
      duplex: 'half',
    });
    return true; // No exception => Safari 16.1+ (or other browsers that support it)
  } catch (e) {
    // Safari 16.0 or earlier will throw a TypeError
    return false;
  }
}

supportsDuplexHalf().then(isSupported => {
  console.log('duplex: half supported:', isSupported);
});

Edge Cases & Caveats

• If the stream is empty or closed before the fetch call, Safari may throw unrelated errors; keep the stream minimal.
• The test only confirms support for duplex: 'half' with a ReadableStream; it does not guarantee success under all network conditions.
• Beta or developer builds older than 16.1 may behave differently; always test against the released version you target.
• Relying on the exact error message string is fragile; the try/catch approach is the most robust.

Recommendation

Use the supportsDuplexHalf helper before initiating any concurrent uploads. If it returns false, fall back to a non‑streaming upload strategy (e.g., FormData or Blob).

Missing Diagnostic Detail?

If you’re unsure about the Safari version your users are on, consider logging navigator.userAgent during the detection step. That extra data can help troubleshoot unexpected failures.