Claude Opus 4.6: Features, Benchmarks, Pricing and What Changed (July 2026)

Anthropic released Claude Opus 4.6 on 5 February 2026, and it is comfortably the most capable AI model the company has ever shipped. This is not a minor version bump. Opus 4.6 brings a 1 million token context window, a completely reworked thinking system, doubled output limits, and benchmark results that put clear daylight between it and everything else on the market.

If you are building with the Claude API, using Claude Code professionally, or just trying to understand where the frontier sits right now, this is the model that defines it.

Key takeaway: Opus 4.6 scores highest on Terminal-Bench 2.0 (agentic coding), Humanity's Last Exam (reasoning), and GDPval-AA (professional knowledge work). It has a 14.5 hour task horizon, 1M token context, and 128K max output.

The 1M Token Context Window

Opus 4.6 moves beyond the standard 200,000 token limit to a massive 1M token context window. This is roughly 700,000 words, enough to hold an entire corporate document library or a complete codebase in memory.

Qualitative Shift in Accuracy

On MRCR v2 (extreme scale needle-in-a-haystack), Opus 4.6 scores 76%, compared to 18.5% for Sonnet 4.5. This effectively eliminates "context rot" for long-running sessions.

Adaptive Thinking Replaces Extended Thinking

Opus 4.6 introduces adaptive thinking, replacing the old manual budget system. The model now dynamically decides when and how much to think based on task complexity.

Old MethodManual budget_tokens. Wasted tokens or constrained reasoning.

New Adaptive ThinkingModel chooses reasoning depth. Controlled by effort levels (Low to Max).

Fast Mode: 2.5x Speed at a Premium

Available as a research preview, Fast Mode delivers up to 2.5x faster output generation. It's the same frontier intelligence running on optimized inference infrastructure.

Usage & Cost

Enable via speed: "fast" parameter. Ideal for real-time agent workflows and interactive coding.

Note: Fast Mode carries a premium price point ($30/$150 per MTok).

Benchmark Performance

Benchmark	What It Measures	Opus 4.6	GPT-5.2
Terminal-Bench 2.0	Agentic coding	65.4%	64.7%
GDPval-AA (Elo)	Knowledge work	1606	1462
ARC AGI 2	Problem solving	68.8%	54.2%

Breaking Changes and Migration

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Prefilling is gone: Assistant message prefilling is not supported on Opus 4.6. Migrating to structured outputs is required.
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Output Tokens: Maximum output tokens doubled to 128K. SDKs require streaming for large requests to avoid timeouts.

Claude Opus 4.6 Pricing

Tier	Input (per MTok)	Output (per MTok)
Standard	$5 (£4)	$25 (£20)
Fast Mode	$30 (£24)	$150 (£120)

Frequently Asked Questions

What is the significance of Claude Opus 4.6's 1M token context window?

Claude Opus 4.6 features a massive 1 million token context window, equivalent to about 700,000 words. This allows it to process entire corporate document libraries or large codebases, significantly reducing 'context rot' and improving accuracy on extreme-scale tasks like MRCR v2.

How does Claude Opus 4.6's new adaptive thinking system work?

Opus 4.6 introduces an adaptive thinking system that replaces the old manual budget_tokens method. The model now dynamically determines the necessary depth of reasoning based on task complexity, controlled by configurable effort levels (Low to Max), leading to more efficient and effective problem-solving.

What is Fast Mode in Claude Opus 4.6 and how is it enabled?

Fast Mode is a research preview feature in Claude Opus 4.6 that delivers up to 2.5 times faster output generation. It can be enabled by setting the speed parameter to 'fast' (e.g. speed: 'fast') in API calls, making it ideal for real-time agent workflows and interactive coding, though it comes at a premium price.

What are the key breaking changes developers should be aware of when migrating to Claude Opus 4.6?

Developers migrating to Claude Opus 4.6 must note two main breaking changes: prefilling of assistant messages is no longer supported, requiring a shift to structured outputs. Additionally, while max output tokens doubled to 128K, SDKs now require streaming for large requests to prevent timeouts.

What is the pricing structure for Claude Opus 4.6, including Fast Mode?

The standard pricing for Claude Opus 4.6 is $5 per MTok for input and $25 per MTok for output. Fast Mode, a premium option, costs $30 per MTok for input and $150 per MTok for output, reflecting its accelerated performance.