{"slug": "claude-fable-is-relentlessly-proactive", "title": "Claude Fable is relentlessly proactive", "summary": "Claude Fable 5 autonomously hacked its own browser automation and screenshot capabilities to debug a horizontal scrollbar bug in Datasette Agent. The AI edited Datasette's templates to inject JavaScript that simulated keyboard shortcuts, wrote custom HTML test pages, and created a Python web server to capture JavaScript measurements — all without being instructed to use any automation tools.", "body_md": "After two days of experience with [Claude Fable 5](https://simonwillison.net/2026/Jun/9/claude-fable-5/) I think the best way to describe it is **relentlessly proactive**. It knows a whole lot of tricks and it will deploy pretty much any of them to get to its goal.\n\nI'll illustrate this with an example. I was hacking on [Datasette Agent](https://agent.datasette.io/) today when I noticed a glitch: a horizontal scrollbar that shouldn't be there in the jump menu chat prompt. I snapped this screenshot:\n\nThen I started a fresh `claude`\n\nsession in my `datasette-agent`\n\ncheckout, dragged in the screenshot and told it:\n\n`Look at dependencies to help figure out why there is a horizontal scrollbar here`\n\nI had a hunch the cause was in a dependency of Datasette Agent (likely Datasette itself) and I knew Fable was good at digging into dependency code, either by inspecting installed files in its own virtual environment `site-packages`\n\nor by referencing a local checkout on disk. Telling it to start with dependencies felt like a good bet.\n\nI got distracted by a domestic task and wandered away from my computer.\n\nWhen I came back a few minutes later I saw my machine *open a browser window* in my regular Firefox and then *navigate to the dialog in question*. I had not told Claude Code to use any browser automation, and I was pretty sure it wasn't possible for it to trigger mouse movements or keyboard shortcuts within a window, so how was it doing that?\n\nI watched in fascination as it continued with its explorations, then saw it open a Safari window instead of Firefox. I also grabbed this snapshot from the Claude terminal:\n\nWhat was it doing there with `uv run --with pyobjc-framework-Quartz`\n\n?\n\nIt turns out Fable had hacked up its own pattern for taking screenshots of browser windows. It was using Python to iterate through all available windows on my machine, then filtering for Safari windows with expected strings such as `\"textarea\"`\n\nin the window name. It used that to find their window number - an integer like 153551 - which it could then use with the `screencapture`\n\nCLI tool to grab a PNG.\n\nOK fine, that's a neat way of taking screenshots. But what was it taking screenshots of?\n\nTurns out it had been writing its own scratch HTML pages to try and recreate the bug, then opening Safari and grabbing screenshots.\n\nHere's that [/tmp/textarea-scrollbar-test.html](https://static.simonwillison.net/static/2026/textarea-scrollbar-test.html) page it created, and the screenshot it took with `screencapture -x -o -l 153551 /tmp/safari-cases.png`\n\n:\n\n(I have way too many open tabs!)\n\nOK, so I can see how it's opening test pages and taking screenshots, but how on earth was it triggering the modal dialog that was meant to be under test? That's only available via a click or a keyboard shortcut, and I couldn't see a mechanism for it to run those in Safari.\n\nI eventually figured out what it had done.\n\nClaude was running in a folder that contained the source code for the application. It knows enough about [Datasette](https://datasette.io/) to be able to run a local development server. It turns out it was editing Datasette's own templates to add JavaScript that would trigger the correct keyboard shortcut as soon as the window opened, adding code like this:\n\n```\n\n```\n\n1.2 seconds after the window opens, this code triggers a simulated `/`\n\nkey, which is the keyboard shortcut for opening the modal dialog.\n\nThere was one challenge left. In order to understand what was going on, Claude needed to run JavaScript on the page to take measurements for itself.\n\nIt wrote its own custom web application to capture information via CORS, then ran that as a local server and opened a page with JavaScript that would POST directly to it!\n\nHere's the Python web app it wrote, using the standard library [http.server](https://docs.python.org/3/library/http.server.html) package:\n\n``` python\nfrom http.server import HTTPServer, BaseHTTPRequestHandler\n\nclass H(BaseHTTPRequestHandler):\n def do_POST(self):\n n = int(self.headers.get(\"Content-Length\", 0))\n open(\"/tmp/diag.json\", \"w\").write(self.rfile.read(n).decode())\n self.send_response(200)\n self.send_header(\"Access-Control-Allow-Origin\", \"*\")\n self.end_headers()\n def do_OPTIONS(self):\n self.send_response(200)\n self.send_header(\"Access-Control-Allow-Origin\", \"*\")\n self.send_header(\"Access-Control-Allow-Headers\", \"*\")\n self.end_headers()\n def log_message(self, *a): # quiet\n pass\n\nHTTPServer((\"127.0.0.1\", 9999), H).serve_forever()\n```\n\nAll this does is accept a POST request full of JSON and write that to the `/tmp/diag.json`\n\nfile. It sends `Access-Control-Allow-Origin: *`\n\nheaders (including from `OPTIONS`\n\nrequests) so that code running on another domain can still communicate back to it.\n\nThen Claude injected this code into the template that it was loading in a browser:\n\n``` js\nconst host = document.querySelector(\"navigation-search\");\nconst ta = host.shadowRoot.querySelector(\"textarea\");\nconst cs = getComputedStyle(ta);\nfetch(\"http://127.0.0.1:9999/diag\", {\n method: \"POST\",\n body: JSON.stringify({\n dpr: window.devicePixelRatio,\n scrollWidth: ta.scrollWidth, clientWidth: ta.clientWidth,\n whiteSpace: cs.whiteSpace, width: cs.width,\n }),\n});\n```\n\nThis took measurements of the `