The on-device translation is not available at page A while working at page B.

[Victor Zhang]

Hi, I'm building a Chrome Extension with Chrome Translator API, I added warmup polling and retry method to check `availability()` and increase the chance of successful `create()` translator, but it will still failed at some page. while working at other page on the same Chrome profile browser.

Sometime it will recovered after I relaunch the Chrome browser, but now it wouldn't work now.

How can I fix this issue? thanks!

[Victor Zhang]

is there any maximum translator limitation for each Shadow DOM or the whole browser?

[François Beaufort]

Which pair of languages did you use?
Could you share the code by any chance?

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[Victor Zhang]

Hi François,

Thanks for your reply, my main language pair is en->zh-Hans, I'm product manager, and has no technical background, just vibe coding with the best thought I can.

last week, I encountered two type of error messages on the console logs, "udp multicast:1 The translation service count exceeded the limitation." and "The on-device translation is not available.", I come up with a idea that designing a backend coordinator to control the maximum number of the concurrent translator, set the max number to 3. this approach works well for a week, but this week I still encountered the same error.

While I post here for help, I have code agent to fix for me, I tested and it has fix the issue. here are the design changes:

Problem: On iframe-heavy sites (X.com has 10+), concurrent Translator.create() calls hit "The on-device translation is not available" or "service count exceeded" despite having coordination logic.

 

Root Cause: The background registry tracked translators after creation succeeded. By the time it detected the limit and evicted, Chrome had already rejected the new create() call.

 

Solution: Gate creation before calling Translator.create(), not after.

 

Protocol:

1. RESERVE → Background checks limit, evicts if needed, waits for DESTROY_ACK
2. CREATE → Content script calls Translator.create() (slot guaranteed)
3. CONFIRM/RELEASE → Report success or failure

Key difference: Eviction is now synchronous. the content.js wait for the old translator to be destroyed (confirmed via ACK) before approving a new creation. This ensures Chrome's internal count decrements first.

 

Config: Max 3 concurrent translators, 5s ACK timeout, LRU eviction preferring idle instances.

 

Here are my questions:

1. Recommended max concurrent Translator instances per extension?
2. Does translator.destroy() release the slot synchronously?
3. Any API to query remaining capacity?

Best regards,

Victor Zhang

[François Beaufort]

The following code which creates 100 Translator instances does not throw for me.

const options = { sourceLanguage: "en", targetLanguage: "zh-Hans" };
const promises = [];
for (let i = 0; i < 10; i++) promises.push(Translator.create(options));
await Promise.all(promises);

This other code which creates 100 Translator instance per iframe does not throw for me either.

const options = { sourceLanguage: "en", targetLanguage: "zh-Hans" };
const promises = [];
for (let i = 0; i < 10; i++) {
  const iframe = document.createElement("iframe");
  document.body.appendChild(iframe);
  promises.push(iframe.contentWindow.Translator.create(options));
}
await Promise.all(promises);

Having said that, https://source.chromium.org/chromium/chromium/src/+/main:components/on_device_translation/features.h;l=33;drc=8c7bd73f24f4936795272eece99089a6d75651cf suggests there's a limit of 10 translator instances per browsing context (profile). Could that be it?

[Victor Zhang]

hi ,

thank you for your support, I changed the max limit to 8, changed the flag(chrome://flags/#translation-api) from "default" to "Enabled without language pack limit" and relaunch, it works, it seems the translator is not stable at the default flag, even thought I have downloaded the en->zh language pair.

Best regards,
Victor Zhang