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Official commercial channel

Claude Fable 5 API

Anthropic’s tier above Opus — the one you pick when the ceiling is the point.

  • Context

    1M

  • Max output

    128K

  • Serving precision

    Vendor-managed

  • Licence

    Proprietary

Claude Fable 5 is the generally available model in the Mythos class Anthropic introduced above Opus — the restricted Claude Mythos 5 in the same class is offered only to a limited set of cyber defenders. Its 1-million-token window is the API default rather than an opt-in beta, with no long-context price premium, and output tops out at 128,000 tokens across text, image and file input. Adaptive thinking is always on, raw thinking is never returned, and effort, task budgets, memory, context editing and compaction all shipped at launch. Vendor list pricing is $10 per million input and $50 per million output.

Proprietary, provided through the vendor’s official commercial licensing. Contract, invoice, audit trail — not an account pool and not a reverse-engineered client protocol.

Specifications

Model ID
claude-fable-5
Published by
Anthropic
How it is served
Official commercial channel
Licence
Proprietary
Context
1M
Max output
128K
Serving precision
Vendor-managed
Input modalities
Text · Image
Released
2026-06-01

What it is good at

A tier above Opus
Mythos is a class Anthropic created above Opus, aimed at autonomous knowledge work and coding. If the hardest judgement calls are the job and quality is worth paying for, it deserves one honest comparison on your own tasks.
A million tokens with no premium
The 1M window is the API default, not something to opt into, and long context is not priced separately. One fewer variable when you estimate the cost of very long inputs.
Task budgets and context compaction
Effort, task budgets, memory, context editing and compaction were all available at launch. On long autonomous runs these are the practical levers for controlling both cost and drift.

Where it fits

  • The hardest reasoning and judgement work you have
  • Long-running autonomous knowledge-work and coding agents
  • Capability and cost comparisons against Opus 5
  • Deep analysis and drafting over million-token documents

Three steps

Change one line of base_url, put the model ID below in the `model` field, and leave the rest of your code alone.

model: "claude-fable-5"

curl https://api.nexaix.net/v1/chat/completions \
  -H "Authorization: Bearer $NEXAIX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-fable-5",
    "stream": true,
    "messages": [
      { "role": "user", "content": "Summarise this contract." }
    ]
  }'

About this model

Fable 5 or Opus 5?

Usually Opus 5: it lists at half the price and lands within 0.5% of Fable 5’s peak on CursorBench at half the cost per task. Reach for Fable 5 where you have confirmed you need that last increment of ceiling. Run both with the same key against your real tasks first.

Any difference in data retention versus Opus 5?

Yes, and we would rather say it plainly: Opus 5 supports zero data retention while Fable 5 currently carries a 30-day retention requirement. Nothing is written to disk on our side for either, but the upstream difference is real. If you have hard compliance requirements, prefer Opus 5.

Do you offer Claude Mythos 5?

No. Per Anthropic, Mythos 5 is offered only to a restricted set of cyber defenders and is not generally available.

Also worth a look

Run your own eval on it

Take the trial credit, run the evals you already have against this model, and then decide whether it gets your production traffic.