Available on Julius

Anthropic: Claude Fable 5.1

Explore demanding reasoning tasks with explicit constraints, source material, and checks on the result.

Try Claude Fable 5.1 on Julius

Explore a worked example

Available on Pro, Max, Ultra, and Business. Compare plans

Reviewed October 13, 2026 · Sources

Production dashboard plotting feasible plans and the optimal mix of 60 Standard and 10 Deluxe units for $3,100 contribution. — Production planning dashboardIllustrative output · Fictional dataView full size

Best suited to: Difficult reasoning and complex, multi-step work
Published context: 1,000,000 tokens
Native inputs: Text and images
Released: September 1, 2026

Claude Fable 5.1 pricing

Compare the provider’s API rates with access through Julius.

Anthropic API

Input / 1M tokens: $10
Output / 1M tokens: $50

Standard API rates per million tokens. Cache reads cost $0.25 per million tokens. Other caching operations and optional tools have separate rates.

Check provider pricing

Use your Julius plan

Available on Pro, Max, Ultra, and Business. Credits cover the work performed with models and tools.

View Julius plans and credits

Prices in USD, reviewed October 13, 2026. API token charges are separate from Julius subscriptions and credits.

What is Claude Fable 5.1?

Claude Fable 5.1 is Anthropic’s model for demanding reasoning and extended agent tasks. It supports a one-million-token context window and work involving documents, spreadsheets, and slides.

Fable is worth considering when a task has several dependencies and an initial result needs deeper scrutiny. In Julius, give it an explicit goal, the source files, and the checks that a finished answer must pass.

Demanding reasoning

Anthropic positions Fable for difficult problems and longer agent tasks when Opus at higher effort is insufficient.

Work across deliverables

The release emphasizes research and work with documents, spreadsheets, and slides. Julius provides its own tools for creating deliverables.

Choose a production mix within two resource limits

A higher contribution per unit does not necessarily produce the best total contribution.

Worked example · Fictional sample data · Expected results calculated from the CSVs

1. Input
Products and resource limits
2. Check
Test feasible whole-unit plans
3. Deliver
A plan within both limits

Expected result

Make 60 Standard and 10 Deluxe units for $3,100 in contribution. This uses all 160 hours and 200 material units.

Standard units: 60
Deluxe units: 10

Total contribution · USD

All Deluxe: $2,800
Best feasible mix: $3,100

What the analysis needs to catch

Making only Deluxe units returns $2,800. Ranking products by contribution per unit misses the two resource constraints.

Make this deliverable

A verified production allocation and a resource-use table.

Build an interactive planning tool with editable capacity limits using the AI website builder.

Try this example in Julius

The prompt includes the sample data. Review it in chat before sending. Results may vary; compare your answer with the calculations shown here.

View the sample output (PNG)View the workflow and chart (SVG)Download capacity.csv Download products.csv Download all data and calculations

View the prompt and sample data

Use integer quantities to maximize total contribution, subject to the hours, material, and product-demand limits in these files. Enumerate the feasible plans or use an integer solver, verify both resource constraints, and compare the optimum with an all-Deluxe plan. Show the allocation and explain the tradeoff.

capacity.csv

resource,available
hours,160
material,200

products.csv

product,contribution_per_unit,hours_per_unit,material_per_unit,max_demand
Standard,40,2,3,70
Deluxe,70,4,2,40

These expected results come from the downloadable Python calculations. This example illustrates a workflow; it is not a measured benchmark of Claude Fable 5.1.

Using Claude Fable 5.1 in Julius

Start with your task

The try buttons carry Claude Fable 5.1 into a new chat when your plan includes it. Example buttons also fill the composer with the sample data. Review the prompt, or replace it with your own files and question, before sending.

If the model is unavailable on your plan, Julius explains the access requirement. You can choose another option in the model menu.

Work through an analysis in code Research a question Create a report

Know the boundaries

It has higher standard API token prices than Sonnet and Opus. Escalate based on the task, rather than assuming the largest model is needed for every question.
Fable and Mythos have different access arrangements. Julius’s Fable option does not imply access to Anthropic’s separate verification programs.

Model details and official research

Go to the original documentation for specifications, release details, and evaluation methods.

Claude Fable 5.1 documentation

Official specifications, capabilities, pricing, and links to the announcement.

Claude Fable 5.1: frequently asked questions

Is Claude Fable 5.1 available on Julius?

Yes. It appears as Claude Fable 5.1 in the model menu. Available on Pro, Max, Ultra, and Business plans.

How much does Claude Fable 5.1 cost?

Anthropic lists $10 per million input tokens and $50 per million output tokens. Standard API rates per million tokens. Cache reads cost $0.25 per million tokens. Other caching operations and optional tools have separate rates. Julius subscriptions and credits are separate.

Are Claude Fable 5.1 and Claude Mythos 5.1 the same offering?

Anthropic describes matching capabilities but different access arrangements: Mythos 5.1 is for organizations admitted through its verification programs. The model offered in Julius’s menu is Claude Fable 5.1.