Key Takeaways

• For most work, Claude Opus 5.5 is the better choice. It beats Claude Fable 5.1 on most of Anthropic’s published benchmarks, responds faster, and costs 60% less per token.
• Opus 5.5 costs $4 per million input tokens and $20 per million output tokens. Fable 5.1 costs $10 and $50.
• Anthropic’s own guidance is to start with Opus 5.5 and move to Fable 5.1 for the most demanding reasoning and long-horizon tasks, or when Opus at higher effort falls short.
• Key scores: Terminal-Bench 4.0 66.4 vs 55.8, GDPval-AA 1846 vs 1735, AutomationBench 40.0 vs 31.4, all in favor of Opus 5.5.
• Artificial Analysis gives Opus 5.5 an Intelligence Index of 58, against 53 for Fable 5.1.
• Both models offer a one-million-token context window, 128,000 output tokens and knowledge up to June 2026. They differ in default effort, speed and cybersecurity safeguards.

Claude Opus 5.5 is the model most people and teams should choose over Claude Fable 5.1. Released on September 22, 2026, three weeks after Fable 5.1, Opus 5.5 matches or beats Fable on most benchmarks Anthropic publishes, runs noticeably faster and costs $4 per million input tokens against Fable’s $10. Fable 5.1 still has a place: Anthropic recommends it for the hardest reasoning problems and the longest autonomous tasks, especially when Opus at a higher effort level does not get the job done.

The surprise is that the cheaper model often wins. Anthropic itself notes that, in its own use, the gap between the two is “narrower than these scores suggest,” which means Fable’s practical advantage on difficult work has shrunk to a small set of cases. For everything else, Opus 5.5 delivers similar or better results at a lower price.

Claude Opus 5.5 vs Fable 5.1 at a Glance

Feature	Claude Opus 5.5	Claude Fable 5.1
Release date	September 22, 2026	September 1, 2026
Input price per 1M tokens	$4	$10
Output price per 1M tokens	$20	$50
Cache read per 1M tokens	$0.20	$0.25
Cache write per 1M tokens	$5 (5-minute)	$12.50 (5-minute), $20 (1-hour)
Context window	1 million tokens	1 million tokens
Maximum output	128,000 tokens	128,000 tokens
Knowledge cutoff	June 2026	June 2026
Default effort	Medium	High in Claude Code; medium in the Claude apps
Thinking	Adaptive, always on	Adaptive, always on
Speed	Moderate	Slower
Artificial Analysis Intelligence Index	58	53

Benchmark Comparison

Anthropic’s launch page for Opus 5.5 compares it with Fable 5.1 and other leading models. Opus 5.5 leads Fable on every test in the selection below.

Benchmark	Claude Opus 5.5	Claude Fable 5.1	Claude Opus 5
Terminal-Bench 4.0 (agentic coding)	66.4	55.8	52.3
FrontierCode	54.4	50.3	48.0
CursorBench 4.0	57.8	51.8	46.6
GDPval-AA v2.1 (knowledge work, 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	58.7	52.6	29.0
OSWorld 2.1 (computer use)	81.8	80.7	74.0
Chartography	89.0	88.4	83.4

Some numbers need context. On Terminal-Bench 4.0, the Mythos 5.1 configuration of the same underlying model as Fable scored 60.9, still behind Opus 5.5. Anthropic’s earlier Fable 5.1 page used different test versions, listing 1853 on GDPval-AA v2, 65.0 on Humanity’s Last Exam with tools and 73.4 on CursorBench 3.2, so scores from the two pages cannot be mixed. Anthropic also cautions that at this capability level, benchmark margins have become a weaker guide to real-world differences.

Real-World Tests: Speed, Cost and Quality

Anthropic shared two internal comparisons that show where Opus 5.5 pulls ahead. In a project to port the HAProxy load balancer from C to Rust, Opus 5.5 finished in 9.5 hours against Fable 5.1’s 12 hours, at 51% lower cost. In a test of professional report writing, 16 of 18 reports by Opus 5.5 met the quality bar, while none from Fable 5.1 or Opus 5 did.

Independent testers found smaller gaps. Computing for Geeks gave both models the same DevOps tasks. Both produced valid Kubernetes manifests, with Fable costing $0.047 and Opus $0.037 for the set. Fable refused one Terraform task involving a security group that opened ports 22 and 443 when run under a strict system prompt, returning a refusal stop reason. The refusal depended on the prompt wording, but it shows that Fable’s stricter safeguards can occasionally block ordinary infrastructure work.

Both models handle long autonomous runs. Anthropic’s customer examples include an 18-hour unattended session with Opus 5.5 at legal software firm Clio and a rebase of 40 stacked pull requests at Stripe, while Ramp ran Fable 5.1 on a 38-hour machine learning task.

Pricing: How Much You Save With Opus 5.5

Opus 5.5’s list price is 60% lower than Fable 5.1’s on both input and output. For a job with 1 million input tokens and 200,000 output tokens, Opus 5.5 costs $8 and Fable 5.1 costs $20. Batch processing halves either figure, and Opus 5.5 offers a fast mode at $8 input and $40 output for latency-sensitive work.

Opus 5.5 is also cheaper than its predecessor. Opus 5 cost $5 input and $25 output, and Anthropic says Opus 5.5 is about 40% cheaper on typical workloads while producing output more than 30% faster. Fable 5.1 cut its own costs too: its cache read price fell by 75% compared with Fable 5, which makes typical workloads about 25% cheaper and agentic workloads up to about 45% cheaper. Even so, Fable remains well above Opus in cost per task.

Fable 5.1, Mythos 5.1 and Safeguards

Fable 5.1 and Mythos 5.1 launched together on September 1, 2026. They are the same model with different safety controls. Fable 5.1 is the generally available version. Mythos 5.1 is offered only through trusted access programs, currently to vetted US organizations, with fewer restrictions on sensitive work. Our guide to Claude Fable vs Mythos for developers explains how to tell which one a team needs, and our overview of Claude Fable 5.1 covers its launch in detail.

Cybersecurity handling differs sharply between the two models compared here. Opus 5.5 routes cyber tasks to the older Claude Opus 4.8. Fable 5.1 allows vulnerability discovery but not exploit writing, and Anthropic says it triggers about 60% fewer cyber interventions than before. When Fable refuses, requests can fall back to Opus 4.8 or Opus 5.

On alignment, Anthropic says Opus 5.5 achieved its best score yet on automated behavioral audits and attempts to cross boundaries about 85% less often than Opus 5 and Mythos 5.1. Opus 5.5 also ties Fable 5.1 for the lowest prompt injection success rate on Gray Swan’s benchmark. For data handling, Opus 5.5 supports zero data retention, while Fable 5.1 uses 30-day retention with eligible customers able to request zero retention in the meantime.

Developer Notes Before Switching

A few technical details matter when moving between the models. Thinking blocks are tied to the model that produced them, so a conversation cannot simply switch from Fable to Opus mid-stream while carrying old thinking content. Requests that force the model to call a specific tool can now return errors, and Opus 5.5 uses a newer computer-use toolset version. Anthropic also introduced protections that preserve thinking content against distillation for accounts created from August 31, 2026. On consumer plans, Anthropic raised usage limits for Pro, Max and Team subscribers alongside the Opus 5.5 launch.

Which Claude Model Should You Choose?

Choose Claude Opus 5.5 for coding, agentic workflows, research, writing, data analysis and almost any business task. It is faster, cheaper and stronger on most measured work. Choose Claude Fable 5.1 when a problem demands the deepest available reasoning, when a task will run for many hours with little supervision, or when Opus 5.5 at high or maximum effort still falls short. Teams can also combine them, sending most traffic to Opus and escalating the rare hard case to Fable.

For how Fable compares with OpenAI’s flagship, see our test of Fable 5.1 vs GPT-6 Astra on task execution. Our comparison of GPT-6 Luna, Gemini 3.8 Flash and Claude Opus 5.5 shows how Opus stacks up against budget rivals, and our Opus 4.6 vs 4.7 vs 4.8 comparison traces how quickly the Opus line has improved.

If you are interested in this topic, we suggest you check our articles:

• Claude Fable 5.1
• Claude Fable vs Mythos for Developers
• GPT-6 Luna vs Gemini 3.8 Flash vs Claude Opus 5.5
• Fable 5.1 vs GPT-6 Astra: Task Execution Compared
• Claude Opus 4.6 vs 4.7 vs 4.8 Comparison

Sources: Anthropic – Claude Opus 5.5, Anthropic – Claude Fable and Mythos 5.1, Computing for Geeks, DeepLearning.AI – The Batch

Written by Alius Noreika