At 9:28 AM (KST) on August 18, 2026, Anthropic's official developer account ClaudeDevs announced a performance improvement to the Claude Code CLI. Its p99 CPU usage dropped by half.
Why Did CPU Suddenly Spike?
The cause was the garbage collector (GC) in the Bun runtime. This GC ran on a fixed timer, so it cut in and took CPU even when work was at its busiest. One developer said in a reply, "A timer-driven GC stealing CPU in the middle of a turn is a really sneaky bug. From the outside, it would've looked like the model's responses were getting slower."
The Fix: Wait Until Idle
ClaudeDevs said this fix changes the GC to wait until the process goes idle. p99 is a metric for the delay experienced by the heaviest 1% of all requests. It shows how much CPU is used at the worst moments, not the average.
2x Lower, but Not Zero Usage
"2x lower" doesn't mean CPU usage went away. It means the figure at the p99 range, the moments of heaviest load, was cut in half, and how much you notice in everyday use may vary with your work patterns.
The Shipping Version Isn't Out Yet
The ClaudeDevs tweet has no specific version number. The official changelog page (code.claude.com/docs/en/changelog) also doesn't reflect this item as of 5 PM (KST) on August 18, 2026, when this was written. The announcement itself came from the official account, but the exact release timing remains not officially confirmed.
If you keep the Claude Code CLI running for long stretches, the most reliable approach is to update to the latest version and see the difference yourself. Separate cases related to Bun's GC, such as CPU staying high while idle, have also been reported several times before as GitHub issues.