Chrome heavy ad intervention for VAST video: 4 MB and 60 s limits

Microsoft's video guidelines warn that since September 2020 Chrome can redirect an ad iframe to an error page if it uses too many resources, and video ads can be caught. A creative is heavy if it uses the main thread for more than 60 seconds in total, more than 15 seconds in any 30 second window, or more than 4 MB of network bandwidth. A plain video file is a network cost, so file weight is the lever you hold.

The three triggers

The page says iframes that load more than 4 MB of data are the ones affected, and recommends staying within its technical specifications. It does not say how much of the 4 MB a streamed video counts for, so test in your ad server's preview.

What you can do with Sume

Shorter files weigh less. Trim duration cuts a 30 second master to 15, and the output width and height, from 256 to 2160, let you drop to a smaller frame. The trim docs reject codec and crf, so you cannot tune bitrate directly; shorter and smaller are the two controls.

Microsoft also recommends a 500 kbps rendition for mobile on cellular, which your ad server or transcoder produces from the master.

import os
import requests

key = os.environ["SUME_API_KEY"]
resp = requests.post(
    "https://api.sume.com/v1/video-trim",
    headers={"Authorization": f"Bearer {key}", "Idempotency-Key": "heavy-cut-15"},
    json={
        "video_url": "https://media.sume.com/artifacts/artf_demo/master.mp4",
        "start": 0,
        "duration": 15,
        "mode": "sync",
    },
    timeout=60,
)
resp.raise_for_status()
print(resp.json())

Limits

Main-thread use comes from ad scripts and players, not from the MP4 that Sume produces. Sume can not measure it.

The guidance is dated: the page metadata shows a 2025 update. Chrome's current thresholds may differ, so check Chrome's own documentation before relying on these numbers.