Open Finder, press Cmd + Shift + G, paste ~/Library/Application Support/Google/Chrome/ and look for a folder called OptGuideOnDeviceModel. If it’s there, it’s probably around 4GB. Chrome put it there without asking.
The file inside is named weights.bin — the weights for Gemini Nano, Google’s on-device large language model. Before any AI feature surfaces in the UI, Chrome quietly reads your GPU and unified memory to decide if your hardware qualifies, then downloads the full payload in the background. No prompt. No notification. No opt-in.
What It Actually Does
Google’s official response: Gemini Nano powers scam detection and developer APIs without sending data to the cloud. That framing is technically accurate but misses the point. The AI Mode pill in Chrome’s omnibox routes queries to cloud servers — the local model has nothing to do with that. It powers features like “Help me write.” So most users have 4GB sitting on their drive for a writing assistant they have probably never opened.
Security researcher Alexander Hanff documented the behaviour in detail on his blog, running a controlled test on a fresh Chrome profile on macOS using the OS’s own filesystem event logs. The browser created the model directory and downloaded the full payload during idle browsing time, completing in just over fourteen minutes with no human interaction.
How to Check If It’s There
Mac:
- Open Finder →
Cmd + Shift + G - Paste:
~/Library/Application Support/Google/Chrome/ - Look for
OptGuideOnDeviceModel
Windows:
Win + R- Paste:
%LOCALAPPDATA%\Google\Chrome\User Data\ - Look for
OptGuideOnDeviceModel
How to Delete It and Stop It Coming Back
Deleting the folder directly does nothing permanent — Chrome reinstalls it the next chance it gets. You need to disable the feature before touching the files.
Quickest method — Mac and Windows:
Go to chrome://settings/system and toggle On-device AI off. On Mac, weights.bin disappears immediately. Then delete OptGuideOnDeviceModel to recover the space. PCWorld confirmed this works on both platforms.
If the settings toggle is not visible:
Go to chrome://flags and disable both flags:
#optimization-guide-on-device-model#prompt-api-for-gemini-nano
Click Relaunch, then delete OptGuideOnDeviceModel with Chrome fully closed. Full steps with screenshots at Android Authority.
Windows only — permanent registry policy:
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome, create a DWORD called GenAILocalFoundationalModelSettings and set it to 1. Chrome respects enterprise policies and will not override this with updates, making it the most reliable long-term fix on Windows.
The Bigger Picture
This is not new behaviour. Reports of Chrome downloading the model appeared as early as April 2025 when it weighed around 3GB. By November 2025 it had grown to 4GB. The direction is clear.
The on-device argument — that local processing is better for privacy — is not wrong in principle. But a privacy benefit does not waive the requirement to ask. If keeping AI processing local was meant to earn user trust, silently consuming 4GB of someone’s disk is a strange way to go about it.