Claude Fable Demo: Relentlessly Proactive Bug Fixing with Browser Automation

Simon Willison's Claude Fable is relentlessly proactive post demonstrates how Claude Fable 5 autonomously diagnosed a horizontal scrollbar bug in Datasette Agent. After telling it to check dependencies, Fable took unexpected actions — opening browser windows, injecting JavaScript, and even writing a custom CORS web server — all without explicit instructions.

Key Behaviors Observed

• Browser window detection: Fable used uv run --with pyobjc-framework-Quartz to enumerate macOS windows, filtering for Safari windows containing expected strings (e.g., textarea). It then used screencapture -x -o -l [window_id] for screenshots.
• Template injection: To trigger a modal dialog (which normally requires a keyboard shortcut), Fable edited Datasette's templates to inject <script>window.addEventListener('load', function() { setTimeout(function() { document.dispatchEvent(new KeyboardEvent('keydown', {key: '/', bubbles:true})); },1200); });</script>. This simulated the / key 1.2 seconds after page load.
• Custom CORS web server: Fable wrote a Python HTTP server using http.server (standard library) running on 127.0.0.1:9999. It accepted POST requests with JSON, wrote to /tmp/diag.json, and set Access-Control-Allow-Origin: * headers — allowing JavaScript on the test page to send diagnostic data back.

How It Worked

Fable identified the scrollbar bug was likely in a dependency (Datasette itself). It edited a local template to auto-trigger the dialog, opened Safari with a freshly generated test HTML page, took a screenshot via screencapture, and ran JavaScript in the browser that posted measurements to its own local web server. All of this was done autonomously — Willison only gave it the initial prompt and a screenshot.

This showcases Fable's relentless proactivity: it invents workarounds, instruments browsers, and orchestrates multi-step debugging pipelines without being told to.