Claude Fable 5.1 vs Claude Opus 5.5

Specs side by side

Price per token

Prices exclude taxes and are charged from your prepaid USD balance. Both models work through the same key, in the OpenAI-compatible /v1/chat/completions endpoint. Failed generations cost nothing.

Start with Claude Opus 5.5: Anthropic puts it at Fable 5.1 level on most work, and it costs half as much per token. Keep Claude Fable 5.1 for the tasks where Opus 5.5 gets things wrong in your tests and a mistake costs more than a doubled bill, such as unattended agents that run for hours or analysis where an error is expensive. Both have the same context window and output limit, and both run in Claude Code, so the extra money buys quality on the hardest cases and nothing else.

Key differences

• Price. Claude Fable 5.1 costs $10.80 / $54.00 per 1M tokens, Claude Opus 5.5 costs $5.40 / $27.00 per 1M tokens. Fable is 2 times more on both input and output, so the 1,000-reply row in the table above is 2 times higher too.
• Context. Both: 1,000,000 tokens of context and up to 128K output tokens. Limits do not decide this one.
• Caching. Neither has a separate cached-input rate on Jantaris, so a repeated prefix is billed as regular input. With a long, stable system prompt the 2x gap stays intact, so trimming it pays off on both.
• Claude Code. Both work on /v1/chat/completions and on the Anthropic-compatible /v1/messages, so both run in Claude Code. Switching is just a different name in ANTHROPIC_MODEL or the /model command.
• Open weights. Neither. Both are closed models available only through an API.
• Polish and other languages. I have not measured this pair on Polish text. If you write in a language other than English, run 10-20 of your own texts through both: at a 2x price gap, Opus has to lose clearly for Fable to pay off.
• Place in the family. According to Anthropic, Fable 5.1 is the generally available version of Claude Mythos 5.1, with extra safeguards around biology and cybersecurity. Opus 5.5 is the first model of the Claude 5.5 family, with knowledge up to June 2026. Thinking cannot be switched off on Opus 5.5, and thinking tokens are billed as output.

When to pick Claude Fable 5.1

• Agents that run unattended for hours and use many tools.
• Research and analysis where one error costs more than the whole token bill.
• Large refactors that Opus 5.5 only got halfway through in your tests.
• Security review of your own code as the last pass after cheaper models.

When to pick Claude Opus 5.5

• The default model for everyday work in Claude Code.
• Refactors and migrations that touch hundreds of files.
• Technical writing that has to follow a house style guide.
• High traffic, where Fable's 2x rate adds up fast.

My rule is simple: Opus 5.5 by default, and only the tasks Opus gets wrong on my own examples go to Fable 5.1. Both models sit behind one key and one balance that never expires, with no subscription. Claude Code setup is in Claude Code with a custom base URL, and Claude family pricing in Claude Opus 5.5 API pricing.

FAQ

Claude Fable 5.1 or Opus 5.5: which should I use?

Start with Opus 5.5. Take 20-50 real tasks, run them through both models and move to Fable only the task types where Opus clearly loses. That is usually a small share of traffic.

How much more expensive is Fable 5.1 than Opus 5.5?

2 times on every input and output token. Current rates: $10.80 / $54.00 per 1M tokens and $5.40 / $27.00 per 1M tokens.

Do both models work in Claude Code through Jantaris?

Yes. Set ANTHROPIC_BASE_URL to https://jantaris.com, put your sk-jt- key in ANTHROPIC_AUTH_TOKEN and choose claude-opus-5.5 or claude-fable-5.1. For a Fable agent, create a separate key with a monthly spending limit in the dashboard. Details are in the Claude Code docs.