---
title: 'Claude Opus'
description: "Anthropic's Claude model line aimed at complex coding, analysis, and knowledge work requiring sustained judgment."
canonical_url: 'https://darkfactory.dev/glossary/opus'
markdown_url: 'https://darkfactory.dev/glossary/opus.md'
collection: glossary
date_published: '2026-10-07T00:00:00-04:00'
date_modified: '2026-10-07T00:00:00-04:00'
---

# Claude Opus


## Definition

Claude Opus is Anthropic's model line aimed at complex coding, analysis, and knowledge work requiring sustained judgment. Opus identifies a line of Claude releases. The full name, such as Claude Opus 5.5, identifies a particular release with its own capabilities and operating requirements.

## Origin and attribution

Anthropic introduced Opus with the Claude 3 family on March 4, 2024. At that launch it positioned Opus as the most capable of the three announced lines, above Sonnet and Haiku. Newer releases and the addition of Fable and Mythos have changed the range since that announcement.

Anthropic released Claude Opus 5.5 on September 22, 2026 for long-running agentic coding and knowledge work. Its Claude API identifier is `claude-opus-5-5`. Product attribution belongs to Anthropic; the launch sources do not identify a sole creator of the Opus name or line.

## Scope and limits

Performance depends on the release, task, effort setting, and harness. A cheaper model can meet the quality threshold for a bounded task, while a difficult task may justify a more expensive one. The Opus name alone cannot settle that choice.

Capabilities also change within the line. Opus 5.5 requires adaptive thinking and rejects some tool configurations accepted by older releases. The word Opus alone therefore cannot establish API compatibility, tool support, latency, or a context limit.

## Operational significance

Compare complete tasks at the effort settings you would deploy. Count failed attempts, tool work, and review time alongside token charges. Before a migration, check the exact release's guide and rerun the evaluations that measure unauthorized actions and incomplete work.

An Opus model can propose changes or tool calls. The application still decides which operations are permitted and verifies the resulting artifacts.

## Distinguish it from nearby terms

- Claude is the parent family. Opus is one line within it.
- Sonnet emphasizes a different balance of response speed and capability; Haiku targets high-volume, latency-sensitive work. These are product positions, not guarantees about every task.
- Fable is a separate Claude line intended for demanding reasoning and long-horizon work. It should not be treated as another spelling or version of Opus.
- A reasoning model describes behavior and inference methods. Opus names a product line whose supported thinking modes depend on the release.

## Check your understanding

An Opus upgrade improves a coding benchmark but changes a tool option used by your agent. What compatibility checks and task evaluations would justify changing the production model?

## Also called

Opus

## Related terms

- [Claude](https://darkfactory.dev/glossary/claude)
- [Claude Sonnet](https://darkfactory.dev/glossary/sonnet)
- [Claude Haiku](https://darkfactory.dev/glossary/haiku)
- [Claude Fable](https://darkfactory.dev/glossary/fable)
- [Reasoning model](https://darkfactory.dev/glossary/reasoning-model)
- [Agent harness](https://darkfactory.dev/glossary/agent-harness)
- [Model routing](https://darkfactory.dev/glossary/model-routing)
- [Evaluation (eval)](https://darkfactory.dev/glossary/evaluation)

## Related factory areas

- [Model selection, routing & budgets](https://darkfactory.dev/factory/model-routing-budgets)
- [Verification, evaluation & quality truth](https://darkfactory.dev/factory/verification)

## Evidence and further reading

- [Anthropic: Introducing the Claude 3 model family](https://www.anthropic.com/news/claude-3-family)
- [Anthropic: Introducing Claude Opus 5.5](https://www.anthropic.com/claude-opus-5-5)
- [Anthropic: Claude Opus 5.5 model documentation](https://platform.claude.com/docs/en/models/opus-5-5/overview)
- [Anthropic: Claude models overview](https://platform.claude.com/docs/en/models/overview)
- [Anthropic: Claude model IDs and versioning](https://platform.claude.com/docs/en/about-claude/models/model-ids-and-versions)