Claude Fable 5 is here. You can run it in Lanes today. | Blog | Lanes
Anthropic released Claude Fable 5 yesterday, the first Mythos-class model to reach general availability. It costs $10 in and $50 out per million tokens, and through June 22 it is included in Claude subscriptions at no extra cost. As of v0.43.0 it is in the Lanes model picker too.
What Anthropic shipped
Two names, one model. Mythos 5 is the unrestricted version, for authorized users only. Fable 5 is the same weights with safeguards in front, and that is the version everyone gets.
The novel part is the safeguards. When a classifier flags a request, Fable 5 does not refuse: it falls back to Claude Opus 4.8 and answers there. Anthropic says that touches fewer than 5% of sessions.
The numbers
$10 per million input tokens, $50 per million output, less than half of what Mythos Preview cost. The API model ID is claude-fable-5. Subscriptions are on a clock: free on Pro, Max, Team, and seat-based Enterprise through June 22, then drawn from usage credits from June 23.
The proof points worth repeating: Stripe says it "compressed months of engineering into days," finishing a 50-million-line Ruby migration in a day scoped at two months. Vision is state of the art. Given persistent file-based notes, it got three times further in Slay the Spire than Opus 4.8 managed. The thread is not sharper answers to short questions. It is longer stretches of competent, unsupervised work.
The honest take
Lenny's Newsletter had early access. The verdict: it "crushes benchmarks," but it is "token-intensive by design" and "conservative on execution," with strong plans and hesitant follow-through. Two things follow. Output tokens are the real bill, so the $50 side matters more than the $10. And you steer it with effort levels, not by arguing with its caution.
What you'd actually use it for
Four capabilities the early reports keep circling, and the Lanes move for each.
Long, unsupervised runs. Anthropic leads with autonomy that outlasts any previous Claude. On a board that shows up as parallel sessions: each Fable 5 run in its own worktree, a bell when one needs you. The longer it holds the leash, the more you keep in flight.
Repo-wide migrations. Stripe compressed a two-month, 50-million-line Ruby migration into a day. You don't hand that to one session. Split it one issue per module, fan a Fable 5 session across each worktree, and let the diffs land side by side in Git Changes.
Strong plans, hesitant execution. Lenny's read: it plans well, then second-guesses the follow-through. Lanes already splits those steps. Run plan mode to get the plan, switch to implement mode to push it through, and reach for the effort knob when it stalls instead of arguing with its caution.
Context that survives the long haul. Given persistent file notes, it got three times further in Slay the Spire than Opus 4.8. Every Lanes session is that setup by default: its own worktree to keep notes in, and resume restores the full history across restarts, so nothing it learned evaporates between runs.
Running it in Lanes
From v0.43.0, claude-fable-5 is in Session Settings, the same model and effort picker that arrived in v0.41. Choose it when you launch a session, or pin it for a project in .lanes/cli-flags.json. Set the effort level too; this is a model where that knob does something.
And the billing holds up, for the same reason it did when Anthropic split the bill last month. Lanes runs the official claude CLI in a real terminal, which Anthropic counts as interactive use, the side your subscription still covers. Through June 22, Fable 5 in a Lanes session is free on your Pro or Max plan.
Where this leaves us
Mythos-class is a tier now, not a research preview, and the fallback idea will get copied: answering with a safer model beats refusing. The practical takeaway is simpler. The strongest long-horizon coding model Anthropic has shipped is in your model picker, included on your plan for the next twelve days.
Pick an issue, set the model to claude-fable-5, and see what it does with a long leash. Start here if you are new, or join our Discord and tell us what you find.
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