The short answer: Opus by default, Fable when Opus falls short
Start with Claude Opus 5.5: Anthropic's own documentation recommends it for most workloads, it costs $4 input and $20 output per million tokens, and it is included on Claude Pro. Switch to Claude Fable 5.1 for demanding reasoning and long-horizon agent work only after Opus at higher effort still misses your requirements, accepting $10 and $50 per million tokens and a Slower latency class.
Scope and evidence
Task: deciding between Anthropic's two upper-tier models for long agent runs, research and document work. Prices and specs come from Anthropic's Fable 5.1 and Opus 5.5 overviews and the Claude pricing page; cost per task comes from Artificial Analysis Intelligence Index v5, all checked on October 28, 2026.
Figures here are published ones; AppFor.ai did not benchmark either model. Artificial Analysis gave Fable 5.1's cost per task but no overall index score for it in the v5 announcement.
Compare the same criteria
Each line takes Anthropic's documentation for both upper-tier models and turns the difference into a decision.
- Anthropic's guidance. Fable 5.1: for demanding reasoning and long-horizon agentic work. Opus 5.5: for long-running agentic coding and knowledge work, and the recommended starting point. What this means: Fable is the escalation path.
- API price per million tokens. Fable 5.1: $10 input, $50 output, $0.25 cache read. Opus 5.5: $4 input, $20 output, $0.20 cache read. What this means: 2.5 times the base price for Fable.
- Latency and default effort. Fable 5.1: Slower, default high. Opus 5.5: Moderate, default medium. What this means: Fable spends more time and tokens out of the box.
- Thinking. Both: adaptive thinking always on, 1M context, 128K output, June 2026 cutoff. What this means: no difference in limits.
- Cost per Intelligence Index task at max effort. Fable 5.1: $13.04, the highest among the top 10. Opus 5.5: $10.04 with the top score of 58. What this means: Fable's extra cost needs to show up in your own evaluations.
- Claude plans. Fable 5.1: not on Free, usage credits on Pro, 50% of weekly limits on Max. Opus 5.5: included on Pro and Max. What this means: Opus is the practical choice in the apps.
Which fits your real work?
A daily coding agent that fixes issues across a monorepo. Opus 5.5 fits as Anthropic's recommended default at lower cost.
A multi-day research synthesis where Opus at high effort keeps missing contradictions between sources. Fable 5.1 fits Anthropic's escalation rule.
A Claude Pro user drafting reports in the app. Opus 5.5 fits, because Fable draws on separate usage credits on Pro.
Cost of getting the work done
API: 2 million input and 200,000 output tokens cost $12 on Opus 5.5 and $30 on Fable 5.1 before caching, both with 50% off on the Batch API. Artificial Analysis measured $10.04 against $13.04 per index task at max effort.
Apps: Claude Pro is $20 billed monthly or $17 a month billed annually and includes Opus; Fable uses usage credits there. Max starts at $100 and allows Fable within half of the weekly limits.
Try both on the same small task
Suggested check: take one task where Opus already failed, then see whether Fable passes, which is the test Anthropic describes.
- Every claim in the notes appears in the table.
- Each disagreement is real when you reread the notes.
- The strongest-evidence judgement cites the note's own reasoning.
- Run Opus at high effort first; escalate to Fable only on the cases it got wrong.
Decision checklist
Choose by the lines that apply. Details are in our Claude Fable 5.1 profile and our Claude Opus 5.5 profile.
- If you are starting a new workload, begin with Opus 5.5.
- If Opus at high effort still fails your checks, test Fable 5.1.
- If latency matters, Opus is in the Moderate class and Fable in Slower.
- If you work in the Claude app on Pro, Opus is included and Fable uses credits.
- If cost per task matters, compare both on your own evaluations.
Go to the source
Sources & further reading
Based on the official documentation below. Features and access can change; check the provider before making a decision.
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