Claude’s New addTools() Can Reuse 98.7% of Your Next Request. Editing tools[] Reuses None.
![Claude’s New addTools() Can Reuse 98.7% of Your Next Request. Editing tools[] Reuses None.](https://miro.medium.com/v2/resize:fit:700/1*PQPDhVBIYAzYTqEV_Tbd9Q.png)
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Author(s): Chew Loong Nian – AI ENGINEER Originally published on Towards AI. Anthropic’s SDK 0.128.0, released with Claude Opus 5.5, can hand the model a new tool mid-run without touching tools[]. It needs one beta flag the runner won’t add for you. If your Claude agent picks up a new tool halfway through a long conversation, the way you add it decides how much of the next request can still come from the prompt cache: 98.7% of it with the new runner.addTools(), none if you edit tools[]. After in
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