Simon Willison — Week of 2026-04-18 to 2026-05-01#

Highlight of the Week#

The alpha release of llm 0.32a0 marks a foundational architectural pivot for Simon’s ecosystem of CLI tools. By moving away from a simple text-in/text-out abstraction to one that natively models complex message sequences and typed streams, the library is now future-proofed to handle the realities of modern frontier models. This opens the door for seamless integration of server-side tool calls, multi-modal inputs, and reasoning tokens.

Simon Willison — 2026-04-18#

Highlight#

The deep dive into Anthropic’s Claude Opus 4.7 system prompt diff is today’s most insightful read, offering a rare glimpse into how AI labs tweak model behavior between point releases. It highlights the practical value of tracking system prompts to understand hidden tool capabilities, safety guardrails, and shifting knowledge cutoffs.

Posts#

Changes in the system prompt between Claude Opus 4.6 and 4.7 Anthropic recently released Opus 4.7, and Simon analyzed the hidden diffs in its system prompt compared to the February 4.6 release. The update reveals new integrations like “Claude in Powerpoint”, expanded child safety wrappers, and new instructions to make the model less pushy and less verbose. Interestingly, Anthropic removed a manual injection clarifying the 2025 US President, as the model’s native knowledge cutoff has been officially updated to January 2026. Simon also extracted the list of 23 hidden tools available to the Claude chat UI by directly prompting the model to list its own capabilities.