If you are using Google AI Studio and seeing direct charges on your credit card despite having a $300 Google Cloud credit balance, you are likely stuck in “AI Studio Postpay” mode.
The fix is to “promote” your project to Vertex AI billing. Here is the exact workflow to stop the card charges and start utilising your credits:
The Problem
By default, AI Studio API keys often bypass Google Cloud promotional credits. If your card is getting hit for Gemini 3 Flash usage, the system isn’t “seeing” your Cloud trial balance.
The 3-Step Solution
1. Link AI Studio to a specific GCP Project Don’t use the “Default” project.
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In Google AI Studio, go to Settings (Gear Icon) > Plan.
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Choose “Link a paid API key”.
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Select an existing Google Cloud Project (or create a new one like “Production-Gemini”). This forces the billing to follow GCP rules, not AI Studio’s standalone rules.
2. Enable the Vertex AI API
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Go to the Google Cloud Console.
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Select your linked project.
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Search for “Vertex AI API” and click Enable.
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Note: This is the “magic switch” that allows your usage to be deducted from your $300 promotional credit bucket.
3. Fix “API Keys are Disallowed” (If needed) If you get a security error when creating a key in your Cloud project:
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Go to IAM & Admin > Organization Policies.
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Search for
constraints/iam.disableApiKeyCreation. -
Set to Override and turn the enforcement Off.
The Result
Once linked, your usage is billed as “Vertex AI” instead of “Gemini API (AI Studio).” Google Cloud billing always exhausts Promotional Credits first before touching your credit card.
Pro-Tip: Check your “Billing Reports” after 24 hours. You should see the usage cost followed by a negative “Credit” line item, bringing your out-of-pocket cost to ₹0.