SynthID not detected in Gemini: that does not mean the video is real

If the Gemini verification tool finds no SynthID watermark, that only means Google's own marks were not found. Google's help page says the tool can currently recognize only content created by Google AI tools, so a clip from another generator, or one that has been heavily changed, can pass as unmarked.

What the checker says and does not say

Per Google, a SynthID detection means all or part of the image or video was created or edited by Google's AI models. The absence of a watermark means the content was not created or edited by Google AI, but it could have been made by other AI systems. Google also says Omni videos include an imperceptible SynthID watermark that you can verify in the Gemini app, Gemini in Chrome and Google Search.

Pre-flight before you spend a check

Each check uses a slice of a small daily quota, so reject files that cannot pass. The function below applies the documented limits.

import os

MAX_BYTES = 100 * 1024 * 1024
MAX_SECONDS = 90  # help page: videos less than 90 seconds each

def can_check(path, seconds):
    size = os.path.getsize(path)
    if size > MAX_BYTES:
        return False, "file is over 100 MB"
    if seconds >= MAX_SECONDS:
        return False, "video must be under 90 seconds"
    return True, "ok"

with open("sample.bin", "wb") as f:
    f.write(b"0" * 1024)
print(can_check("sample.bin", 30))
print(can_check("sample.bin", 120))

What this means for clips you generate

Google's claim is about its own tools. Sume's docs make no claim about whether a clip that you receive through the Video Router keeps a provider watermark, and they do not say how re-encoding by tools such as video trim changes it. Treat that as unknown and test one clip in the Gemini checker before you build a workflow on it.

A better evidence stack

No single check is proof. A claim that a clip is real needs positive evidence, such as the source file, and not just a failed search for a watermark.

• A detector result, with the date, the file hash and the tool.
• The generation record, including the job id and the model.
• The original camera file, if one exists, with its metadata.
• A Content Credentials check, where the file format supports it.

Keep your own record

A checker result is a weak, one-sided signal. Your own record is stronger: store the Sume job id, the model id, the prompt and usage.cost from the poll response next to each file. Google's C2PA Content Credentials check is the other signal, and it is described in the same announcement as rolling out in the Gemini app with Search and Chrome to follow.