Claude Fable 5 arrives with a 1 million-token context window, a 128 000-token output cap and a knowledge cutoff of January 2026. Anthropic says it matches Claude Mythos 5 on raw performance but layers on strict safety classifiers; if a request trips a guardrail you’ll get a rejection notice or can auto-fallback to another model. At $10 per million input tokens and $50 per million output tokens, it’s twice the cost of Opus 4.5–4.8. It runs noticeably slower than Opus, but so far it’s handled every complex prompt thrown at it. Simon Willison ran a side-by-side on a prompt asking for all his open-source projects. Opus 4.8 gave a brief, cautious list of four main items. Fable 5 produced a dozen, complete with approximate release dates, from files-to-prompt (April 2024) and datasette-extract (2024) back through Django’s July 2005 open-sourcing. That depth of recall hints at a much larger parameter count, though Anthropic hasn’t published any model-size specs. In the Claude.ai environment—where every chat runs in a full container sandbox—you can install packages, clone GitHub repos and execute code. Willison tested Fable by converting his micropython-wasm project to full CPython in WASM. Fable identified Brett Cannon’s CPython-WASI builds, struggled with zip-based stdlib boots, then produced a Proof-of-Concept using a directory-preopen approach. After a few prompt tweaks it even generated a 13.9 MB wheel that you can run via “uv run --with <wheel-url> python -c…”, proving it can handle real-world coding workflows.