Claude Fable 5.1 vs Claude Opus 5.5

What each one accepts

Token prices

Charged from a prepaid USD balance, no taxes added. One key runs both, through /v1/chat/completions in the OpenAI format. A failed generation is $0.

My default here is Opus 5.5, with Fable 5.1 kept for the few jobs where Opus slips. Anthropic says Opus 5.5 reaches Fable 5.1 level on most work, and every token on it is much cheaper. Fable earns its rate only when a wrong answer would cost you more than the bigger invoice.

When to pick Claude Fable 5.1

• An agent has to run for hours without a human and juggle many tools in one session.
• You are doing research or analysis where a single bad conclusion is more expensive than the whole run.
• A large refactor stalled halfway when Opus 5.5 drove it on your own repository.
• You want a last, strict pass on a security review of your own code after cheaper models have done the first rounds.

When to pick Claude Opus 5.5

• You need a model for daily work in Claude Code and do not want to count every session.
• A migration spreads across hundreds of files and needs steady, long work.
• You write technical docs that must follow a company style guide.
• Traffic is high, and Fable's rate would turn into a painful monthly total.

Differences you will notice

• The bill. Claude Fable 5.1 is billed at $9.45 / $47.25 per 1M tokens and Claude Opus 5.5 at $3.78 / $18.90 per 1M tokens. Fable is the pricier one on input and on output, so the cost of 1,000 replies in the table above grows by the same factor.
• Limits are a tie. Each model reads up to 1,000,000 tokens and writes up to 128K tokens in one reply. Window size will not settle this choice.
• Thinking on Opus. Opus 5.5 always thinks and you cannot turn that off. Reasoning tokens count as output, so look at the usage field, not at how long the visible answer is.
• Lineage. Anthropic describes Fable 5.1 as the generally available version of Claude Mythos 5.1, with added safeguards around biology and cybersecurity. Opus 5.5 opens the Claude 5.5 family and knows events up to June 2026.
• Same plumbing. Both are closed models, and both answer on /v1/chat/completions and on the Anthropic-style /v1/messages. In Claude Code you swap them by changing ANTHROPIC_MODEL or typing /model.

How to decide

Collect 20-50 tasks from your real backlog and run each through both models. Move to Fable only the task types where Opus is clearly worse; in most teams that is a thin slice of traffic. If you work in a language other than English, include a dozen such texts in the test, because I have no measurements of this pair outside English. Setup steps are in Claude Code with a custom base URL, and the rates for the whole Claude line are in Claude Opus 5.5 API pricing.

Questions

Can I keep a Fable agent from spending too much?

Yes. Create a separate API key for that agent and give it a monthly spending limit in the dashboard. When the limit is hit the key stops working, and the rest of your balance stays safe.

Do I need two accounts to use both models?

No. One Kadraris account, one balance and one key cover Fable 5.1, Opus 5.5 and every other model in the catalog. There is no subscription, and the balance does not expire.

Does a long system prompt change the comparison?

It shifts the totals but rarely the ranking. Check the cached input row in the table above for each model: a stable prefix that is read from cache costs less than fresh input, and trimming the prompt helps on both.