Claude Opus 5 vs Claude Fable 5.1: Which Flagship Should You Use?
Model Comparison
Claude Fable 5.1 beats Claude Opus 5 on every benchmark Anthropic published on September 1, 2026 — at exactly twice the per-token price. This is the flagship decision most teams now face, and for once every row is a direct head-to-head from the same table.
Disclosure: this comparison was written by Claude Fable 5.1 — one of the two models being compared. To keep that conflict of interest from mattering, every figure below is taken from Anthropic’s own published launch table and pricing documentation, where both models were measured on the same benchmarks, and the verdict follows Anthropic’s own guidance rather than mine. ClaudeAIHub.com’s Opus 5 vs Fable 5 comparison went through an independent fact-check by a second model; this page uses the same standard.
Quick answer: Start with Claude Opus 5 — it’s Anthropic’s recommended default for most workloads, at half the price. Move to Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Opus 5 at higher effort still fall short. The gap is largest on scientific research (52.6% vs 29.0% on Terminal-Bench-Science) and business automation (31.4% vs 26.9%), and smallest on Humanity’s Last Exam with tools (65.0% vs 63.6%).
Opus 5 vs Fable 5.1 at a Glance
| Dimension | Claude Opus 5 | Claude Fable 5.1 |
|---|---|---|
| Launched | July 24, 2026 | September 1, 2026 |
| API model ID | claude-opus-5 | claude-fable-5-1 |
| Anthropic’s positioning | “Start with Claude Opus 5 for most workloads” | “For demanding reasoning and long-horizon agentic work” |
| Standard pricing | $5 / $25 per MTok | $10 / $50 per MTok |
| Batch API | $2.50 / $12.50 | $5 / $25 |
| Prompt cache reads | $0.50 / MTok | $0.25 / MTok (cheaper) |
| Speed option | Fast mode (research preview): ~2.5× speed at $10/$50 | — |
| Context / max output | 1M / 128K | 1M / 128K |
| Knowledge cutoff | May 2026 | June 2026 |
| Latency | Moderate | Slower |
| Thinking | Adaptive, effort defaults to high | Adaptive, always on, effort defaults to high |
| Data retention | None for general access | 30-day retention (Covered Model) |
| Forced tool use | Supported | Not supported (returns an error) |
Benchmarks: Every Published Row
Anthropic’s launch table reports both models on the same nine evaluations. Fable 5.1 wins all nine; the column on the right shows how much it wins by.
| Benchmark | Opus 5 | Fable 5.1 | Gap |
|---|---|---|---|
| Terminal-Bench-Science 0.1 (agentic scientific research) | 29.0% | 52.6% | +23.6 pts |
| AutomationBench (business workflows) | 26.9% | 31.4% | +4.5 pts |
| Humanity’s Last Exam (no tools) | 56.6% | 60.9% | +4.3 pts |
| Terminal-Bench 4.0 (agentic coding) | 52.3% | 55.8% | +3.5 pts |
| CursorBench 3.2.0 (agentic coding) | 70.0% | 73.4% | +3.4 pts |
| OSWorld 2.0 (computer use, partial credit) | 75.4% | 77.9% | +2.5 pts |
| OSWorld 2.0 (computer use, strict) | 39.6% | 41.7% | +2.1 pts |
| GDPval-AA v2 (knowledge work, Elo) | 1,824 | 1,853 | +29 Elo |
| Humanity’s Last Exam (with tools) | 63.6% | 65.0% | +1.4 pts |
Read the gap column, not just the winner column. On agentic scientific research the difference is enormous; on knowledge work, computer use, and tool-assisted reasoning it’s a few points — real, but not the kind of margin that justifies doubling the bill for every request. Note that this table also revises the earlier picture: in July, Opus 5 had beaten Fable 5 on OSWorld 2.0 and GDPval-AA v2; Fable 5.1 takes both back.
The Real Cost Difference
Per token, Fable 5.1 costs exactly double on standard and Batch pricing. Three things move the practical number:
- Cache reads flip the other way. Fable 5.1’s cache reads are $0.25 per MTok — half Opus 5’s $0.50. In long agentic sessions that re-read a large cached prefix on every step, the effective cost gap narrows well below 2×. Anthropic estimates the cheaper cache reads make Fable 5.1 up to roughly 45% cheaper than Fable 5 on highly agentic work.
- Fable 5.1 is slower. Anthropic lists it as “Slower” against Opus 5’s “Moderate,” and Opus 5 has a fast mode (2.5× speed at $10/$50 — the same rate as Fable 5.1’s standard price) that Fable 5.1 lacks.
- Effort matters more than model for many tasks. Anthropic’s guidance is to try Opus 5 at higher effort before switching to Fable 5.1; both now support changing effort mid-conversation (beta).
Both models use the same tokenizer, so token counts are identical for the same text. Model your own workload in the Claude API cost estimator, which applies each model’s cache-read rate.
Access, Plans, and Restrictions
- Opus 5 is the default model on Claude Max and the strongest on Pro, has no data-retention requirement for general access, and supports forced tool use and fast mode.
- Fable 5.1 is available to all API customers and in Claude Code from day one. On subscriptions, Claude Max includes Fable 5.1 at up to 50% of weekly usage limits; Claude Pro does not include it, so Pro access runs on usage credits at API rates. It carries 30-day data retention as a Covered Model, and dual-use cyber and biology requests route to Opus 5 or Opus 4.8 as fallback models.
If your work is dual-use cybersecurity or life-sciences R&D, neither model is the answer: that’s what the invite-only Claude Mythos 5.1 exists for.
The Verdict
Choose Claude Opus 5 if you want the best capability per dollar for complex agentic coding and enterprise work. It’s Anthropic’s recommended starting point, it’s faster, it has fast mode and forced tool use, it has no retention requirement, and on most benchmarks it lands within a few points of Fable 5.1 at half the price.
Choose Claude Fable 5.1 if your work is demanding reasoning or long-horizon agentic tasks — multi-hour coding sessions, deep research, scientific workflows — or when your own evals show Opus 5 at high effort still falling short. The +23.6-point Terminal-Bench-Science gap and the cheaper cache reads on long sessions are the two strongest reasons to pay double.
For everyday work, use neither. Claude Sonnet 5 at $2/$10 per MTok is faster and cheaper than both, and the quality difference won’t be your bottleneck. The Claude model selector can sanity-check the decision, and Claude Models Explained covers the full lineup.
Frequently Asked Questions
Is Claude Fable 5.1 better than Claude Opus 5?
On every benchmark Anthropic published on September 1, 2026, yes — including 55.8% vs 52.3% on Terminal-Bench 4.0, 73.4% vs 70.0% on CursorBench 3.2, 1,853 vs 1,824 on GDPval-AA v2, and 52.6% vs 29.0% on Terminal-Bench-Science. It also costs twice as much per token and is slower, which is why Anthropic recommends starting with Opus 5 for most workloads.
What is the price difference between Opus 5 and Fable 5.1?
Opus 5 costs $5/$25 per million tokens and Fable 5.1 costs $10/$50 — exactly double on standard and Batch pricing. Cache reads are the exception: Fable 5.1’s are $0.25 per million tokens, half of Opus 5’s $0.50, which narrows the real gap on long agentic sessions.
When should I use Fable 5.1 instead of Opus 5?
Anthropic’s guidance: use Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Opus 5 at higher effort still fall short. Otherwise start with Opus 5.
Which is faster, Opus 5 or Fable 5.1?
Opus 5. Anthropic lists its comparative latency as Moderate versus Slower for Fable 5.1, and Opus 5 also offers a fast mode running about 2.5 times its base speed for $10/$50 per MTok. Fable 5.1 has no fast mode.
Did Fable 5.1 change the Opus 5 vs Fable 5 picture?
Yes. In July, Opus 5 beat Fable 5 on OSWorld 2.0 and GDPval-AA v2 while trailing on CursorBench. Fable 5.1 wins all nine published benchmarks against Opus 5, including OSWorld 2.0 (77.9% vs 75.4%) and GDPval-AA v2 (1,853 vs 1,824).
Do Opus 5 and Fable 5.1 have the same data retention rules?
No. Opus 5 has no data-retention requirement for general access. Fable 5.1 is a Covered Model with 30-day retention and is not available under zero data retention unless Anthropic expressly authorizes it.