Plain-English Explainer
Anthropic Model Names Explained: What Fable, Opus, Sonnet, and Haiku Actually Mean
Every name has a job. Here is what they are.
Anthropic's model names have two parts: a family name and a version number. The family (Haiku, Sonnet, Opus, or Fable) tells you where a model sits on the capability-versus-speed spectrum, and the number tells you which release it is. This page decodes both, covers the current lineup as of October 2026, explains where Mythos fits, and shows what you are actually running when you open the Claude app.
Claude model names pair a family with a version number. Haiku is the fastest family, Sonnet balances speed and intelligence, Opus is built for long-running agentic and knowledge work, and Fable is Anthropic's most capable model open to all customers. Anthropic calls models above the Opus class Mythos-class. As of October 2026 the current models are Claude Fable 5.1, Opus 5.5, Sonnet 5.5, and Haiku 5.5, plus Claude Mythos 5.1 for verified organizations.
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Free lesson · no account neededMeet ClaudeWhy Claude?6 minStart the free lessonKey Takeaways
- A Claude model name is a family plus a version number, for example Claude Sonnet 5.5.
- Haiku is the fastest family, Sonnet balances speed and intelligence, Opus is built for long-running agentic and knowledge work, and Fable is the most capable model open to all customers.
- Mythos-class is Anthropic's term for models above the Opus class; Claude Mythos 5.1 is offered only to verified organizations.
- As of October 2026 the current models are Claude Fable 5.1, Opus 5.5, Sonnet 5.5, and Haiku 5.5.
Current Claude model names at a glance
| Model | Family | Released | Anthropic's positioning | Who can use it |
|---|---|---|---|---|
| Claude Fable 5.1 | Fable | September 1, 2026 | Demanding reasoning and long-horizon agentic work | All customers |
| Claude Mythos 5.1 | Mythos | September 1, 2026 | Same capabilities and specifications as Claude Fable 5.1 | Organizations verified through Anthropic's verification programs |
| Claude Opus 5.5 | Opus | September 22, 2026 | Long-running agentic coding and knowledge work | All customers |
| Claude Sonnet 5.5 | Sonnet | September 28, 2026 | The best combination of speed and intelligence | All customers |
| Claude Haiku 5.5 | Haiku | October 7, 2026 | High-volume, latency-sensitive tasks such as classification, extraction, and routing | All customers |
The Families
Four Families, One Spectrum
Every generally available Claude model belongs to one of four families. The family name tells you the capability-versus-speed trade-off before you read anything else.
Haiku: the fast family
Claude Haiku 5.5 is the current Haiku model, released October 7, 2026. Anthropic positions it for high-volume, latency-sensitive tasks such as classification, extraction, and routing.
Sonnet: the balanced family
Claude Sonnet 5.5, released September 28, 2026, is the current Sonnet model. Anthropic describes it as the best combination of speed and intelligence, suited to everyday coding, analysis, and writing.
Opus: the agentic workhorse
Claude Opus 5.5, released September 22, 2026, is the current Opus model. Anthropic's docs suggest starting most workloads with it, especially long-running agentic coding and knowledge work.
Fable: the top general-use tier
Claude Fable 5.1, released September 1, 2026, is Anthropic's most capable model open to all customers. It is meant for demanding reasoning and long-horizon agentic work. It succeeded Fable 5, released June 9, 2026.
Mythos: the class above Opus
Anthropic calls models that sit above its Opus class in capability Mythos-class. Fable is the Mythos-class line made safe for general use. Models named Mythos are the same underlying model with some safeguards lifted, offered only to verified organizations.
How the Names Work Together
Reading a Model Name End to End
A full Claude model name has two parts: the family name and a version number. 'Sonnet 5.5' means the Sonnet family, version 5.5. The version counter goes up when Anthropic ships a new release in that family. It is not a separate product line.
Fable and Mythos are the one pairing that confuses people. Claude Fable 5.1 and Claude Mythos 5.1 are the same model with different levels of safeguards. Fable 5.1 is open to all customers. Mythos 5.1 is offered only to organizations verified through Anthropic's verification programs, such as its Cyber Verification Program. The same was true of Fable 5 and Mythos 5 in June 2026.
The first Mythos-class model was Claude Mythos Preview, released in April 2026 only to Project Glasswing partners for defensive cybersecurity work. Anthropic has said it does not plan to make Mythos Preview generally available. Fable 5 was the first Mythos-class model Anthropic made safe for general use.
When Anthropic announces a new model, you can orient quickly: find the family to know where it sits on capability versus speed, then read the version number to know how recent it is. For every Claude release with its date, see the Claude Model Timeline page on Nightschool AI.
What You Run in the App
Which Model Are You Actually Using?
When you open Claude.ai, the model selector in the interface shows which model you are using, and the options available depend on your plan. If you are not sure, check the selector before starting important work.
Fast mode is worth calling out explicitly. Anthropic's docs describe it as a research preview on supported Opus models, including Opus 5.5, that delivers faster output. It does not downgrade you to Sonnet or Haiku; the underlying model is still Opus.
API users choose a model by its exact ID, for example claude-sonnet-5-5 or claude-opus-5-5. Anthropic's docs note that every Claude model ID is a pinned snapshot, and from the 4.6 generation on the IDs carry no date. Older models such as Claude Opus 4.8, Sonnet 4.6, and Haiku 4.5 are still available as legacy models, but each has a published retirement window, so check Anthropic's model deprecations page before building on one.
Common Confusion Points
Questions That Trip People Up
A few naming details consistently confuse people. Here are the most common ones.
Is Fable 5.1 better than Opus 5.5?
Fable 5.1 is the more capable model, built for demanding reasoning and long-horizon agentic work. Anthropic's docs still suggest starting most workloads with Opus 5.5, which is faster, and moving to Fable 5.1 when Opus at higher effort falls short.
What does the version number mean?
It is a release counter. Sonnet 5.5 came after Sonnet 5, which came after Sonnet 4.6. Higher numbers within the same family are newer and generally more capable.
Does fast mode use a smaller model?
No. Fast mode runs supported Opus models with faster output. The model is not downgraded; output speed is optimized instead.
Why did the name Mythos appear?
Anthropic introduced Claude Mythos Preview in April 2026 as part of Project Glasswing, a defensive cybersecurity effort with partner organizations. Fable 5 followed in June 2026 as the Mythos-class model made safe for general use.
Go from knowing the names to using the models well
Understanding the naming system is step one. Getting good at choosing the right tier for a task, writing prompts that get the most out of each model, and building applications that use the API effectively takes practice. The Nightschool AI curriculum is hands-on from the first lesson.
Frequently Asked Questions
What is the Mythos class?
Anthropic uses Mythos-class for models that sit above its Opus class in capability. The first was Claude Mythos Preview, released in April 2026 only to Project Glasswing partners. Fable 5 (June 9, 2026) was the first Mythos-class model made safe for general use, and Fable 5.1 (September 1, 2026) is the current one. Models named Mythos, such as Mythos 5.1, are the same model with some safeguards lifted, offered only to verified organizations.
What is the difference between Opus, Sonnet, and Haiku?
They are families on a capability-versus-speed spectrum. Opus (current: 5.5) is built for long-running agentic coding and knowledge work. Sonnet (current: 5.5) is the balance of speed and intelligence. Haiku (current: 5.5) is the fastest, for high-volume, latency-sensitive tasks. Above all three sits Fable 5.1, Anthropic's most capable model open to all customers.
What is the difference between Fable and Mythos?
They share the same underlying model. Anthropic describes Claude Fable 5.1 and Claude Mythos 5.1 as the same model with different levels of safeguards. Fable 5.1 is available to all customers. Mythos 5.1 is offered only to organizations verified through Anthropic's verification programs, such as the Cyber Verification Program.
What model does fast mode use?
Fast mode, a research preview, runs supported Opus models, including Opus 5.5, with faster output. It does not switch to a smaller or lower-tier model. The model is not downgraded; only output speed changes.
How do I know which version of Claude I am talking to?
In Claude.ai, the model selector in the interface shows your current model. Via the API, the model ID in your request determines what you run, for example claude-opus-5-5 or claude-sonnet-5-5. Anthropic's docs note that every model ID is a pinned snapshot, so your application stays on that model until you change the ID.
Are older models like Opus 4.8 and Haiku 4.5 still available?
Yes, as legacy models. Anthropic lists Fable 5, Opus 5, Opus 4.8, Sonnet 5, Sonnet 4.6, Haiku 4.5, and others as still available, each with a retirement window. Haiku 4.5, for example, has a tentative retirement date of no sooner than October 15, 2026. The Claude Model Timeline on Nightschool AI lists every release with its date.
What does the number after the model name mean?
It is a version counter within a family. Sonnet 5.5 is a newer release in the Sonnet family than Sonnet 4.6 was. A higher number within the same family generally means improved capability or reliability. In the current lineup, Opus, Sonnet, and Haiku all share the 5.5 number, while Fable is at 5.1.
Which Claude model should I use for coding?
Anthropic's docs suggest starting most workloads, including complex agentic coding, with Opus 5.5. Sonnet 5.5 suits everyday code generation and agentic tool use where speed matters. Haiku 5.5 fits high-volume or sub-agent tasks. Move to Fable 5.1 when Opus at higher effort still falls short on demanding, long-horizon work.
Now put the right model to work
Knowing the names is the easy part. Using them well is the skill.