Home / Model Watch / Claude Opus 5.5 vs Fable 5.1
Claude Opus 5.5 vs Fable 5.1
Claude Opus 5.5 came out on September 22, 2026, three weeks after Claude Fable 5.1, and Anthropic says it performs at Fable 5.1’s level on most work. It costs 60% less per input and output token, runs faster and is included in Claude Pro. Fable 5.1 is still the model Anthropic points to for its hardest reasoning and longest agent tasks.
Checked October 18, 2026 against Anthropic’s models overview, pricing page and Claude plans
| Claude Opus 5.5 | Claude Fable 5.1 | |
|---|---|---|
| Released | September 22, 2026 | September 1, 2026 |
| What Anthropic says it is for | Long-running agentic coding and knowledge work; the one to start with for most workloads | Demanding reasoning and long-horizon agentic work |
| Input, per 1M tokens | $4.00 | $10.00 |
| Cached input | $0.20 | $0.25 |
| Cache writes (5 minutes) | $5.00 | $12.50 |
| Output | $20.00 | $50.00 |
| Batch input / output | $2.00 / $10.00 | $5.00 / $25.00 |
| Fast mode | $8 in, $40 out (research preview) | Not offered |
| Speed (Anthropic’s comparison) | Moderate | Slower |
| Default effort on the API | Medium | High |
| Context window / max output | 1M / 128,000 tokens | 1M / 128,000 tokens |
| In the Claude apps | Included from Pro ($20 a month) | Pro: paid usage credits only; Max: up to half the weekly limit |
| Retirement | Not sooner than September 22, 2027 | Not sooner than September 1, 2027 |
Which is better: Opus 5.5 or Fable 5.1?
On paper, Opus 5.5. Anthropic’s announcement puts it ahead of Fable 5.1 on every test it published, mostly with Opus 5.5 at its highest effort setting:
| Test | Opus 5.5 | Fable 5.1 |
|---|---|---|
| Terminal-Bench 4.0 (agentic coding) | 66.4% | 55.8% |
| GDPval-AA v2.1 (knowledge work, Elo) | 1846 | 1735 |
| Humanity’s Last Exam, with tools | 67.7% | 65.6% |
| OSWorld 2.1 (computer use, partial credit) | 81.8% | 80.7% |
Anthropic adds two cautions of its own: at this level, benchmark margins have become a less reliable guide to real differences, and in its own use the gap between the two models is narrower than the scores suggest. Its developer docs still say to start with Opus 5.5 and move up to Fable 5.1 for demanding reasoning and long agent tasks, or when Opus 5.5 at higher effort still falls short on your own tests.
Two of Anthropic’s internal tests show where the money goes. Asked to translate the HAProxy load balancer from C to Rust, both models passed nearly all of its regression tests, but Opus 5.5 finished in 9.5 hours against 12 for Fable 5.1 and cost 51% less. Asked to write company reports from a copy of the web, 16 of Opus 5.5’s 18 reports passed a check that failed any invented figure or quote; none of Fable 5.1’s passed.
Prices side by side
US dollars per 1 million tokens, standard tier, from each company’s pricing page. The month: 30,000 requests of 2,000 tokens in (half read from cache) and 500 out. Set your own numbers in the calculator below.
What the difference costs
| Opus 5.5 | Fable 5.1 | Fable costs | |
|---|---|---|---|
| A chat app for a month: 30,000 chats of 2,000 tokens in (half cached) and 500 out | $426.00 | $1,057.50 | 2.5× as much |
| One agent step: 200,000 tokens of context from the cache, 2,000 new and 1,000 out | $0.068 | $0.12 | 1.8× as much |
Fable 5.1’s cached input costs only a quarter more than Opus 5.5’s, so the gap shrinks when most of a request is context the model has already read, as in coding agents. Fable 5.1 also thinks at high effort by default against medium for Opus 5.5, and thinking is billed as output, so the same task can use more tokens on Fable.
Opus 5.5 vs Fable 5.1 for your use
Choose models
- Turn on JavaScript to see the costs. The price tables below work without it.
Opus 5.5 and Fable 5.1 in the Claude apps
On Claude’s individual plans, Opus is included from Pro at $20 a month. Fable is not part of Pro’s usage: you can turn on usage credits and pay for it at API rates. On Max (from $100 a month), Fable can use up to half of your weekly limit; Team premium seats get the same, and Enterprise plans include it. Neither model is on the free plan.
On claude.ai and in Cowork, Fable 5.1 runs at medium effort by default; in Claude Code, at high. With the Opus 5.5 launch, Anthropic also raised the five-hour usage limits on Pro, Max, Team and seat-based Enterprise plans, and added a rate-limit reset that subscribers can save and use when they choose.
What the two have in common
- The same limits: a 1M-token context window, 128,000 tokens of output and a reliable knowledge cutoff of June 2026.
- The same kind of safeguards on cybersecurity, biology and distillation. When they step in, another Claude model answers instead; on Opus 5.5, most cybersecurity tasks go to Claude Opus 4.8.
- Thinking always on. Neither model can run with thinking switched off; you steer how much it thinks with the effort setting.
- The same places to use them: Anthropic’s API and the Amazon, Google and Microsoft clouds.
Fable 5.1 is the same model as Claude Mythos 5.1, whose safeguards are built for cybersecurity and life-science work; Mythos is open only to vetted organizations through Anthropic’s access programs. More on Fable’s prices and specs: Claude Fable 5.1.
What people say
Both launches drew big discussions on Hacker News: nearly 1,400 comments for Fable 5.1 and more than 1,100 for Opus 5.5.
- Price. Developers welcomed Opus 5.5’s lower prices, and one asked what Fable is for now that Opus matches it. One user with $20 plans at both companies said GPT-6 Astra used up their five-hour allowance in about an hour, while Opus 5.5 lasted nearly all day.
- Usage limits. At Fable 5.1’s launch, limits were a common complaint; one subscriber said it hit the five-hour limit on two accounts before finishing the first task.
- Writing. An Anthropic employee said Fable 5.1’s writing style was a big step up; at the Opus 5.5 launch, several commenters said Claude’s wordy style had pushed them to other tools and hoped Opus 5.5’s clearer writing would fix it.
- Side-by-side tests. One developer who builds the same games with each model found Opus 5.5 neck and neck with Fable 5.1 and GPT-6 Astra. Simon Willison’s test drawing of a pelican on a bicycle took Fable 5.1 almost 14 minutes and $3.30 at max effort; Opus 5.5 at max effort hit the 128,000-token output limit before finishing.
- Safeguards. Some users said everyday requests, such as firmware work or hardening a Dockerfile, were blocked or handed to an older model.
Questions
Is Claude Opus 5.5 better than Fable 5.1?
In Anthropic’s published scores, yes, on every test listed. Anthropic says the real-world gap is narrower than the scores suggest and still recommends Fable 5.1 for the most demanding reasoning and long-running agent work. For most work, it says, Opus 5.5 performs at Fable 5.1’s level for much less.
How much cheaper is Opus 5.5 than Fable 5.1?
Input and output tokens cost 60% less: $4.00 against $10.00 per million input tokens and $20.00 against $50.00 output. Cached input is closer, $0.20 against $0.25. In our chat-app example, a month costs $426.00 on Opus 5.5 and $1,057.50 on Fable 5.1.
Can I use Fable 5.1 on Claude Pro?
Only with usage credits, paid at API rates. Opus 5.5 is included in Pro. On Max, Fable can use up to half of the weekly limit.
Is there a Fable 5.5?
Not as of October 18, 2026. Anthropic called Opus 5.5 the first model of its Claude 5.5 family, followed by Sonnet 5.5 and Haiku 5.5; Fable 5.1 is still its top model open to everyone.
Sources
- Anthropic: Introducing Claude Opus 5.5 (scores, internal tests, usage limits, safeguards)
- Anthropic: Claude Fable 5.1 and Claude Mythos 5.1 (default effort in the apps, Mythos)
- Anthropic: Models overview (what each is for, speed, default effort, retirement)
- Anthropic: Pricing (token, batch and fast mode prices)
- Claude plans and pricing (model access by plan)
- Hacker News: Fable 5.1 launch and Opus 5.5 launch discussions
Capability for the price
Epoch Capabilities Index (up) against the blended price per 1M tokens, 3 input to 1 output (right, log scale). Dots on the line have no model that is both cheaper and higher scoring. Hover or tap a dot for its numbers.
Show the numbers (16 models)
| # | Model | Company | Score | Blended per 1M | Value line |
|---|---|---|---|---|---|
| 1 | Claude Opus 5.5 | Anthropic | 166.8 | $8.00 | Yes |
| 2 | GPT-6 Astra | OpenAI | 166.4 | $20.00 | |
| 3 | GPT-6.1 Sol | OpenAI | 166.3 | $4.00 | Yes |
| 4 | Claude Fable 5.1 | Anthropic | 164.7 | $20.00 | |
| 5 | Gemini 4 Argon | 164.6 | $4.00 | ||
| 6 | Claude Sonnet 5.5 | Anthropic | 164.4 | $4.00 | |
| 7 | GPT-6 Sol | OpenAI | 162.6 | $4.00 | |
| 8 | Claude Opus 5 | Anthropic | 162.6 | $10.00 | |
| 9 | GPT-5.6 Sol | OpenAI | 161.7 | $8.00 | |
| 10 | GPT-5.6 Terra | OpenAI | 159.4 | $4.50 | |
| 11 | Claude Haiku 5.5 | Anthropic | 159.0 | $0.20 | Yes |
| 12 | Gemini 3.8 Flash | 157.2 | $1.50 | ||
| 13 | GPT-6 Luna | OpenAI | 156.3 | $0.20 | |
| 14 | DeepSeek Flash | DeepSeek | 155.6 | $0.525 | |
| 15 | DeepSeek V4 Pro | DeepSeek | 155.3 | $1.98 | |
| 16 | Grok 4.7 | xAI | 154.4 | $3.00 |
Scores: Epoch AI, Capabilities & benchmarking (Epoch Capabilities Index), CC BY 4.0, checked October 18, 2026. Each score carries a margin of error of about ±3 points, so models a few points apart may be level. Prices: each company’s pricing page, standard tier.