Claude Opus 5.5 vs Claude Fable 5.1: Which Flagship Should You Use?
Model Comparison
Claude Opus 5.5 outscores Claude Fable 5.1 on every benchmark in Anthropic’s September 22, 2026 launch table — at 60% lower list prices. That flips the flagship decision: the cheaper model is now the stronger one on paper, and Fable 5.1’s case rests on the hardest, longest-running work.
Quick answer: Use Claude Opus 5.5 by default. It’s Anthropic’s recommended starting point, costs $4/$20 per million tokens against Fable 5.1’s $10/$50, runs faster, supports zero data retention, and leads Fable 5.1 on all nine published benchmarks — by 10.6 points on Terminal-Bench 4.0. Reach for Claude Fable 5.1 when Opus 5.5 at higher effort still falls short on demanding reasoning or long-horizon agentic work, which is exactly how Anthropic positions it.
Opus 5.5 vs Fable 5.1 at a Glance
| Dimension | Claude Opus 5.5 | Claude Fable 5.1 |
|---|---|---|
| Released | September 22, 2026 | September 1, 2026 |
| API model ID | claude-opus-5-5 | claude-fable-5-1 |
| Anthropic’s positioning | “Start with Claude Opus 5.5 for most workloads” | Demanding reasoning and long-horizon agentic work |
| Price / MTok (in / out) | $4 / $20 | $10 / $50 |
| Batch API | $2 / $10 | $5 / $25 |
| Cache reads / MTok | $0.20 | $0.25 |
| Cache writes (5m / 1h) | $5 / $8 | $12.50 / $20 |
| Fast mode | Yes — $8/$40, up to 2.5× speed (research preview) | No |
| Default effort | medium | high |
| Latency | Moderate | Slower |
| Context / max output | 1M / 128K | 1M / 128K |
| Knowledge cutoff | June 2026 | June 2026 |
| Data retention | Zero data retention available | 30-day retention (ZDR only if Anthropic authorizes) |
| Claude Code | Default model on every paid plan | Available via /model fable |
| Consumer plans | Pro, Max, Team, Enterprise | In-plan on Max; Pro via usage credits |
Benchmarks: Every Published Row
From Anthropic’s Opus 5.5 launch table, where both models ran side by side. Opus 5.5 used max effort except on Terminal-Bench 4.0 (xhigh).
| Benchmark | Opus 5.5 | Fable 5.1 | Gap |
|---|---|---|---|
| Terminal-Bench 4.0 (agentic coding) | 66.4% | 55.8% | +10.6 pts |
| AutomationBench (business workflows) | 40.0% | 31.4% | +8.6 pts |
| Terminal-Bench-Science 0.1 (scientific research) | 58.7% | 52.6% | +6.1 pts |
| CursorBench 4.0 (agentic coding) | 57.8% | 51.8% | +6.0 pts |
| FrontierCode v1.1 Main (agentic coding) | 54.4% | 50.3% | +4.1 pts |
| Humanity’s Last Exam (with tools) | 67.7% | 65.6% | +2.1 pts |
| OSWorld 2.1 (computer use, partial credit) | 81.8% | 80.7% | +1.1 pts |
| Chartography (chart recognition, with tools) | 89.0% | 88.4% | +0.6 pts |
| GDPval-AA v2.1 (knowledge work, Elo) | 1,846 | 1,735 | +111 Elo |
The gaps are largest on agentic coding and business automation and smallest on computer use and chart reading, where the two are within about a point. Two caveats keep this from being the whole story. Anthropic says benchmark margins “have become a less reliable guide to real-world differences” at this level of capability. And Opus 5.5’s scores come from max or xhigh effort, while its API default is medium — at the default, you’re trading some of that lead for lower cost and latency.
The Real Cost Difference
Opus 5.5’s list prices are 60% below Fable 5.1’s on input and output, and it wins on every other price line too: cache reads ($0.20 vs $0.25), cache writes, and Batch. Fable 5.1’s cheap cache reads were its cost advantage over Opus 5; that edge is gone against Opus 5.5.
Token efficiency widens the gap further. Anthropic reports Opus 5.5 uses fewer tokens to finish tasks than Opus 5, and customers measured large reductions — GitHub said it used among the fewest tokens and steps it measured. A job that costs $100 on Fable 5.1 at list price costs $40 on Opus 5.5 before any efficiency gains. Check your own numbers in the Claude API cost estimator.
Key Differences Beyond Benchmarks
- Data retention: Opus 5.5 supports zero data retention; Fable 5.1 is a Covered Model with 30-day retention. For regulated workloads this alone can decide it.
- Speed: Opus 5.5 is “Moderate” latency and offers fast mode; Fable 5.1 is “Slower” with no fast mode.
- Safeguards: both route sensitive requests elsewhere. Opus 5.5 sends most cybersecurity tasks to Opus 4.8 and uses Fable 5.1’s biology safeguards. Fable 5.1 falls back to Opus 5 or Opus 4.8 for dual-use cyber and biology work.
- API behavior: both have always-on adaptive thinking, reject forced tool use, and tie thinking blocks to the model and conversation — migration work is similar either way.
- Plan access: Opus 5.5 is the default in Claude Code and available on every paid plan. Fable 5.1 is included on Max at up to 50% of weekly limits and costs usage credits on Pro.
The Verdict
Choose Claude Opus 5.5 for almost everything that needs a frontier model: agentic coding, research, knowledge work, and enterprise deployments. It’s cheaper, faster, higher-scoring on every published benchmark, and it’s the model Anthropic recommends first.
Choose Claude Fable 5.1 when you’ve tested Opus 5.5 at high, xhigh, or max effort and it still falls short on your hardest, longest-running tasks. That’s Anthropic’s own framing, and it’s a narrower case than it was a month ago.
Consider Claude Sonnet 5.5 before either for everyday and high-volume work — at $2/$10 it comes within two points of Opus 5.5 on several benchmarks. See Sonnet 5.5 vs Opus 5.5, or get a recommendation from the model selector.
Looking for the previous matchup? Opus 5 vs Fable 5.1 is kept for reference.
Frequently Asked Questions
Is Claude Opus 5.5 better than Claude Fable 5.1?
On every benchmark Anthropic published at launch, yes — including 66.4% vs 55.8% on Terminal-Bench 4.0, 57.8% vs 51.8% on CursorBench 4.0, and 1,846 vs 1,735 on GDPval-AA v2.1. Anthropic still positions Fable 5.1 for demanding reasoning and long-horizon agentic work where Opus 5.5 at higher effort falls short, and cautions that benchmark margins are a less reliable guide at this level.
How much cheaper is Opus 5.5 than Fable 5.1?
Opus 5.5 costs $4/$20 per million input/output tokens versus Fable 5.1’s $10/$50 — 60% less on list price. It is also cheaper on cache reads ($0.20 vs $0.25), cache writes, and the Batch API, and Anthropic says it uses fewer tokens per task.
When should I still use Claude Fable 5.1?
When your own evaluations show Opus 5.5 at high, xhigh, or max effort still falling short on demanding reasoning or long-horizon agentic work. That is Anthropic’s guidance; for most workloads, it recommends starting with Opus 5.5.
Which is faster, Opus 5.5 or Fable 5.1?
Opus 5.5. Anthropic lists its latency as Moderate versus Slower for Fable 5.1, and Opus 5.5 offers a fast mode running up to 2.5 times faster for $8/$40 per million tokens. Fable 5.1 has no fast mode.
Do Opus 5.5 and Fable 5.1 have the same data retention rules?
No. Opus 5.5 is available with zero data retention. Fable 5.1 is a Covered Model with mandatory 30-day retention and is not available under zero data retention unless Anthropic expressly authorizes it.
Which model does Claude Code use by default?
Claude Opus 5.5, on Pro, Max, Team, and Enterprise plans and on the Anthropic API. You can switch to Fable 5.1 with /model fable; on Max it is included at up to 50% of weekly limits, and Pro users pay for it with usage credits.