If you use the AI Studio UI, you might be walking into a massive bill without a single warning.

Here is how it happened, and the UK/EU privacy "pro-tip" I found in the fine print.

In AI Studio, you start on the Free Tier. You upload your codebase (say, 700k tokens) and work for free until you hit the daily quota. At that point, the UI suggests adding an API key to "continue the conversation."

The Trap: Most users (myself included) assume that after adding the key, they will be billed for incremental usage (the 10-100 tokens they just typed).

The Reality: The AI Studio UI does not use Context Caching by default. Instead, it re-submits the entire 700k token history for every single "turn." Crucially, even though that history was built up for "free," you are now "back-taxed" for it at the Paid Tier rate on every subsequent message.

Message 1: (700k history + 10 new tokens) = Billed for 700,010 tokens. Message 2: (700k history + 20 new tokens) = Billed for 700,020 tokens.

Within 10-15 "turns," I was billed for 170,000,000 tokens. The total? £121.29.

The AI Studio UI is, frankly, deceptive about this, and subsequent communication with Google was not helpful. Instead, they updated the terms.

The Token Counter: The counter at the top of the page remains incremental (e.g., it goes from 700,000 to 700,010). It never warns: "Your next message will cost $5.00."

Batch Billing Lag: Google batches these charges and reports them hours later. You don't see the "bleeding" in real-time. By the time you get the notification, you’re already £100 in the hole. The Evidence Gap: In my support case, Google’s own itemized "evidence" showed only £0.25 of SKU-level usage, but a total subtotal of £121.29. They couldn't explain where 99% of the bill came from.

A Bonus Gimmick: The UK/EU Privacy Loophole While fighting this bill, I dug into the Gemini API Additional Terms of Service (Updated Dec 18, 2025, just after I submitted my dispute). I found a fascinating "Pro-tip" for those of us in the UK or EEA.

Under the new terms, Google has decoupled "Paid Service" status from spending money. The Clause instead reads: If you have an active Cloud Billing account linked to your project, your AI Studio usage is legally classified as a "Paid Service," even when you are using the free quota. The Benefit: "Paid Services" have a strict non-training policy. Google does not use your prompts to improve its models if you are in the Paid Tier.

The Loophole: If you are in the UK/EEA and link a billing account to your project, you get Enterprise-level privacy (No training) on your free-tier usage by default.

My Recommendation:

Link your account for privacy, but NEVER use the API key in the UI to extend an existing chat which reaches the free quota limit. If you hit the free quota limit, stop. Do not "upgrade" the existing session. If you must continue, start a fresh session to avoid the "Context Tax" on your history.

Use Context Caching via API: If you actually need to work on a 1M token codebase commercially, avoid the AI Studio UI entirely. Use the API with explicit context caching to avoid paying for the same 700k tokens 100 times over.

I am currently disputing the bill based on the lack of "Informed Consent" for the transaction and the retroactive application of the Dec 18 terms.

TL;DR: Google AI Studio's UI is a "Financial Biohazard" for long-context developers. It back-taxes your free history the moment you plug in a key, with zero real-time cost transparency.