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CAD: Cost Anomaly Detection or Create Astounding Debt?

The world of AI is exciting, but there are plenty of expensive pitfalls ready to catch out the unwary, as one Register reader found when taking Anthropic's Claude Opus for a spin courtesy of Amazon Bedrock.

Our reader managed to run up Bedrock charges totaling $30,141.33 in April 2026, despite using AWS Cost Anomaly Detection (CAD) to avoid any nasty surprises. Thirty-three days before our reader's first use of Bedrock, the threshold in CAD was set to "Absolute ≥ $100 AND Relative ≥ 40%" so alerts should have fired if things got too spendy.

As for which services to monitor, our reader chose "AWS Services," which Amazon says "tracks all AWS services automatically." Except it apparently doesn't, at least not in the way our reader expected. The problem is that AWS Marketplace isn't supported by CAD, so costs incurred wouldn't trigger an alert. 

And how are Anthropic Claude models billed? Through the AWS Marketplace.

After burning through our reader's AWS Activate credits (totaling $8,026.54 in this case), Amazon started charging for model inference on the Bedrock Marketplace, racking up $30,141.33, plus another $675.07 in AWS infrastructure charges, without a peep from the CAD service.

"The credits masking made it worse," our reader told us. "AWS Activate credits did cover the first ~$8k of charges, which meant the Marketplace billing was silently working for weeks before the credits ran out. There was no notification when credits were exhausted – the charges simply started accumulating as invoiced amounts."

The first warning that things were mounting up came in the form of a surprisingly large invoice.

Corey Quinn, a cloud economist at the Duckbill Group and occasional contributor to this publication, told The Register: "It's unintuitive that Bedrock model spend is Marketplace unless you're entirely too familiar with AWS."

Quinn told us he does most of his Claude inference directly with Anthropic to take advantage of the company's real-time billing, alerts, cutoffs, per-key limits, and so on. The approach has avoided some potentially expensive mistakes.

As far as AWS is concerned, the lack of CAD support for AWS Marketplace charges makes it all too easy to run up a big bill without realizing it, particularly when it comes to AI usage.

This could be regarded as a cautionary tale. If one digs deeply enough into the AWS documentation on CAD, there is a line that warns that AWS Marketplace is an unsupported service. However, it isn't clear that Claude on Bedrock is billed through the AWS Marketplace. The fact that Marketplace billing bypasses the monitoring tools compounds the issue, and could easily leave a customer getting an unpleasant surprise at invoice time. 

An AWS spokesperson told The Register: "AWS offers multiple tools to help customers manage spend, including AWS Budgets, which covers Amazon Bedrock spend on AWS Marketplace and other services. As noted in our documentation, AWS Marketplace charges are not currently supported by Cost Anomaly Detection. Customers with questions should reach out to AWS Support." ®