Updated October 2026. If you build on Anthropic’s API, the real decision in late 2026 is no longer Claude versus GPT — it is Claude Fable 5.1 vs Opus 5.5. Fable 5.1 (September 1) is Anthropic’s flagship generally available model; Opus 5.5 (September 22) costs well under half as much and, according to Anthropic, performs at Fable 5.1’s level on most work. Here is how they compare on published numbers and real costs.
Table of Contents
Quick Comparison
| Claude Fable 5.1 | Claude Opus 5.5 | |
|---|---|---|
| Release | September 1, 2026 | September 22, 2026 |
| Input (per 1M tokens) | $10 | $4 |
| Output (per 1M tokens) | $50 | $20 |
| Cache reads (per 1M tokens) | $0.25 | $0.20 |
| Anthropic’s positioning | Hardest long-running, multi-step work | “Performs at the level of Claude Fable 5.1 on most work” |
| Best default for | Specialist tasks with a measured quality gain | Most production workloads |
Claude Fable 5.1 vs Opus 5.5: Benchmarks
These figures come from Anthropic’s Opus 5.5 announcement, which reports both models on the same benchmark versions:
| Benchmark | Opus 5.5 | Fable 5.1 | Opus 5 |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 55.8% | 52.3% |
| FrontierCode v1.1 (Main) | 54.4% | 50.3% | 48.0% |
| CursorBench 4.0 | 57.8% | 51.8% | 46.6% |
| GDPval-AA v2.1 (Elo) | 1846 | 1735 | 1708 |
| AutomationBench | 40.0% | 31.4% | 26.9% |
| Humanity’s Last Exam (with tools) | 67.7% | 65.6% | 63.6% |
| Terminal-Bench-Science 0.1 | 58.7% | 52.6% | 29.0% |
| OSWorld 2.1 (partial) | 81.8% | 80.7% | 74.0% |
| Chartography (with tools) | 89.0% | 88.4% | 83.4% |
Opus 5.5 leads on every row, with the biggest gaps in agentic coding (Terminal-Bench 4.0, CursorBench) and automation. Anthropic itself notes the gap is narrower in real use than these scores suggest, so the right reading is “roughly equal quality, very different price” rather than “Opus 5.5 is much smarter”.
On the independent Artificial Analysis Intelligence Index, Opus 5.5’s top score is 58 versus 53 for Fable 5.1 at max effort.
API Pricing: What the Gap Means in Dollars
Opus 5.5 is 60% cheaper on both input and output. On a workload of 20 million input and 5 million output tokens per month (before caching):
- Fable 5.1: 20 × $10 + 5 × $50 = $450/month
- Opus 5.5: 20 × $4 + 5 × $20 = $180/month
Both models get large savings from prompt caching ($0.25 vs $0.20 per million cached tokens read) and from lowering the effort level, so your real numbers will be lower — but the ratio between them stays about the same.
When Fable 5.1 Still Makes Sense
- You have tested both on your own prompts and Fable 5.1 produces measurably better results on a specific task type.
- Your workflow is already tuned around Fable 5.1’s behaviour and the cost difference is small at your volume.
- You need access to the Mythos 5.1 variant for vetted cybersecurity or life-sciences work — that model shares Fable 5.1’s weights.
When to Choose Opus 5.5
- Agentic coding, computer use and automation pipelines — where it leads on Anthropic’s benchmarks.
- High-volume production traffic where a 60% price cut matters.
- New projects with no existing dependency on Fable 5.1.
A Simple Routing Strategy
Make Opus 5.5 the default model in your app or n8n workflows, log failures or low-confidence outputs, and only retry those with Fable 5.1. Because both share the same API, switching is a one-line change to the model ID. Measure cost per successful output, not cost per token.
What about OpenAI? In Anthropic’s Fable 5.1 launch table, GPT-5.6 Sol trailed Fable 5.1 on every benchmark shown (for example 67.2% vs 73.4% on CursorBench 3.2.0). For the newer OpenAI lineup, see our GPT-6 Sol vs Luna vs Claude Opus 5.5 comparison.
Claude Fable 5.1 vs Opus 5.5: The Bottom Line
For most teams, Claude Opus 5.5 is the better buy: it matches or beats Fable 5.1 on Anthropic’s published benchmarks at 40% of the price. Keep Fable 5.1 for the narrow set of tasks where your own testing shows it wins. For a deeper look at Fable 5.1 on its own, read our Claude Fable 5.1 review.
Sources: Anthropic — Claude Opus 5.5, Anthropic — Claude Fable 5.1 and Mythos 5.1, Artificial Analysis.