How Claude's Extra Usage Billing Works
You'll see this called two different things depending on where you look: Anthropic's own interface now calls it usage credits, but most people (and CapMeter's own popup) still say extra usage, since that's what it was called when it launched and what everyone searches for. Same feature either way.
What it actually is
It's a prepaid balance that lets you keep working after your plan's session limit runs out, billed at standard API rates instead of being included in your subscription. It's off by default — you have to turn it on yourself in Settings → Usage on claude.ai, add funds with a payment method, and only then does it kick in automatically once your regular plan limit hits zero.
The one setting that actually prevents a surprise
When you enable it, there are two options: "Adjust limit", which sets a monthly spending cap you choose, or "Set to unlimited," which removes that cap entirely. This is the single decision that determines whether "extra usage" can ever surprise you. A cap means the absolute worst case is a number you picked yourself; unlimited means a long unattended agent session really can run up a bill you didn't plan for, since API-rate billing per token doesn't feel like it's adding up the way a message count does. There's also a system-wide ceiling regardless of your own cap — credits can't be redeemed faster than $2,000 in a single day.
What happens at zero
This part is more reassuring than people expect: if your usage credit balance runs out — or you never funded it in the first place — Claude simply stops letting you continue past your plan limit until the next reset. It does not overdraft, and it does not bill you for going over. The only way to spend more than you intended is to have funded the balance and left the cap high (or unlimited) while running something heavier than you were watching.
Watching it while it's happening
The interface shows a running balance, but reading a dollar figure mid-session doesn't tell you whether it's about to become a problem — $8 spent could mean "nearly done" or "just getting started," depending on how fast it's moving. That's the specific gap CapMeter's extra usage tracking is built around: a live $/hour rate next to the balance, so you can tell whether the current pace is fine or worth stopping to check, before the month's total surprises you.