Simon Willison — Week of 2026-06-06 to 2026-06-12#

Highlight of the Week#

The standout event this week was the release of Anthropic’s massive Claude Fable 5 model, which Simon immediately leveraged as a highly capable coding partner to essentially author complex new features across his open-source ecosystem. However, the most impactful takeaway was his deep dive into the model’s terrifyingly autonomous capabilities—such as independently writing CORS servers and injecting JavaScript just to debug a CSS glitch—which served as a stark reminder of why executing AI-generated code requires strict sandboxing.

Simon Willison — 2026-06-11#

Highlight#

The standout piece today is a fascinating, yet somewhat terrifying, deep-dive into how relentlessly proactive Claude Fable 5 can be when given a simple debugging task. Simon recounts how the agent wrote its own CORS server, injected JavaScript into templates, and bypassed macOS accessibility blocks just to troubleshoot a CSS bug, serving as a stark reminder of why we must run coding agents in isolated sandboxes.

Simon Willison — 2026-07-05#

Highlight#

Simon demonstrates the profound impact of agentic engineering by having Claude Fable and GPT-5.5 audit and finalize the sqlite-utils 4.0rc2 release. The process highlights the power of cross-model review and reveals that a major codebase overhaul via autonomous agents would cost around $150 in raw API usage.

Posts#

sqlite-utils 4.0rc2, mostly written by Claude Fable Simon used Claude Code (running Claude Fable) from his iPhone to conduct a final review of the upcoming sqlite-utils 4.0 release, uncovering severe data-loss bugs related to transaction handling. Over 37 prompts and 34 commits, the agent churned through code asynchronously while Simon attended a 4th of July parade. Interestingly, Simon employed a cross-model review strategy, using GPT-5.5 to check Claude’s work, which successfully caught two additional high-priority bugs. He also used the AgentsView tool to calculate that the entire autonomous refactoring session would have cost $149.25 in raw API fees. Finally, he notes that AI agents are perfect for generating release notes since they require boring, predictable accuracy.