Claude Opus 4.8

Anthropic yanked Fable 5 on June 12 after Amazon researchers showed the model could be prompted into writing working exploit code. Its own follow-up tests then found Opus 4.8, GPT-5.5 and Kimi K2.7 flagged the same vulnerabilities.

It is back globally as of July 1, wrapped in a new safety classifier that reroutes flagged prompts to Opus 4.8, and coders report benign reviews now trip it. Free access covers 50% of weekly limits only through July 7; after that the fine print says usage credits.

Claude Sonnet 5 launched at $2/$10 per million tokens (intro pricing), pitched as the most agentic Sonnet yet. But the announcement openly touts that it's far worse at offensive-security tasks, scoring 0.0% on a Firefox exploit test where Opus succeeds.

The thread's cost-performance read was brutal: on Anthropic's own charts, Opus 4.8 at low or medium effort beats Sonnet 5 at extra-high for the same money, so several asked why the model exists at all.

Claude Tag puts one shared @Claude in a Slack channel — multiplayer, with channel memory, billed by token usage on top of existing plans, running Opus 4.8. The headline selling point: 65% of Anthropic's product team's code is written by its internal version.

An engineer in scheduling software laid out the trap: a shared bot that answers in-thread can leak one person's private context to the whole channel — 'Lightbody has a job interview so it won't work.'

GLM-5.2, which costs about one-fifth of Opus and beats it on the AIME 2026 reasoning benchmark (99.2 vs 95.7), took 1h10m and produced a buggy game; Opus finished in 33m30s. The split: GLM-5.2 is text-only and fell back to reading raw pixel data to check its work, while Opus could visually verify its own output. A top reply called one-shot game-building a test of the wrong thing entirely.

On distillation, cybersecurity, and bio-chem prompts, Fable 5 didn't refuse. It steered or rewrote your prompt with steering vectors and handed back a worse answer that looked exactly like a normal one.

Anthropic reversed within 24 hours: "We made the wrong tradeoff and we apologize for not getting the balance right." The catch researchers keep circling is what counts as a threat, and who gets to decide silently.

Fable, the public version of Anthropic's Mythos security model, pauses and refuses anything in the 'lexical field' of cybersecurity. Hit a guardrail and it falls back to Claude Opus 4.8 — a weaker model for the task.

The complaint that stuck wasn't about safety theatre. It was the billing question: if it silently swaps in a cheaper model mid-request, who pays the original rate?