Simon Willison runs Claude Fable 5 through its paces, finding it slower and pricier than Opus 4.8 but far more knowledgeable thanks to its 1 million-token context. He tests it on real-world coding tasks—upgrading a MicroPython sandbox to full CPython in WASM and adding pause-resume hooks to Datasette Agent—showing it can build complex features end-to-end.

• Fable 5 costs twice as much as Opus 4.5–4.8 ($10/$50 per million tokens vs their pricing) and runs slower, but handles every complex prompt thrown at it.
• Its 1M-token context gives it dramatically deeper recall than Opus 4.8—listing a dozen detailed open-source projects with dates versus Opus's brief list of four.
• It successfully converted a MicroPython-wasm project to full CPython in WASM, ultimately producing a working 13.9 MB wheel runnable via uv, demonstrating real end-to-end coding capability.

Anthropic has restricted xAI's access to its Claude models used for coding, a move aimed at reducing competition. xAI cofounder Tony Wu acknowledged that while this will impact productivity, it will also drive their team to develop their own coding solutions.

• Anthropic revoked xAI's access to Claude models used for coding, reportedly to limit a competitor's capabilities.
• xAI cofounder acknowledged the cutoff will hurt productivity but frame it as motivation to build in-house coding tools.

Anthropic's new coding model, Opus 4.5, is praised as the most advanced tool for programming, capable of producing user-focused plans and reliable code without hitting limitations. While it excels in coding and writing, it has minor flaws in editing, highlighting the ongoing evolution in AI coding models.

• Opus 4.5 produces user-focused plans and reliable code without hitting the usual limitations seen in prior models
• Strongest performance is in coding and writing tasks
• Editing tasks reveal minor flaws, showing the model isn't uniformly perfect across all use cases