TLDR Claude Opus 5.5 is Anthropicโs latest Opus-class large language model, released on September 22, 2026. It matches Claude Fable 5.1 performance on most real-world work while costing approximately 40% less to run than Claude Opus 5 on typical workloads.
Claude Opus 5.5's Official pricing is $4 per million input tokens and $20 per million output tokens (20% below Opus 5), with cache reads at just $0.20. The model leads or ranks near the top on major agentic coding benchmarks (Terminal-Bench 4.0 at 66.4%, FrontierCode, CursorBench), delivers strong gains in knowledge work, computer use, mathematical/scientific reasoning, and long-horizon professional tasks, and sets a new high on Anthropicโs alignment evaluations. It is available today via the Claude API (model ID claude-opus-5-5) and unified platforms such as CometAPI.
Key Takeaways
- Performance parity with frontier: Opus 5.5 reaches or surpasses Claude Fable 5.1 on major agentic coding and knowledge-work benchmarks at significantly lower cost.
- Major efficiency gains: ~40% lower cost on typical workloads vs Opus 5; >30% faster output generation; substantially reduced token usage per task.
- Coding breakthroughs: Top scores on Terminal-Bench 4.0 (66.4%), FrontierCode, CursorBench 4.0 (57.8%); real-world examples include 680k-line migrations completed in under a day.
- Clearer communication & EQ improvements: More natural writing, leads with key information, less jargon, better long-session collaboration โ testers describe it as writing โthe way I do.โ
- Stronger safety: Best scores to date on Anthropicโs automated behavioral audit; 85% less likely than Opus 5 or Mythos 5.1 to attempt boundary bypasses; deploys with Fable-class safeguards for biology and cybersecurity.
- Availability & access: Native on Claude Platform; also accessible via aggregators like CometAPI for unified OpenAI-compatible endpoints and competitive multi-model pricing.
What Is Claude Opus 5.5?
Claude Opus 5.5 is Anthropicโs latest Opus-class large language model and the first release in the Claude 5.5 family. Positioned for long-running agentic coding, knowledge work, computer use, and complex professional tasks, it represents a substantial upgrade over Claude Opus 5 (released July 24, 2026).
Anthropic describes it as performing at the level of its higher-tier Claude Fable 5.1 on most work while requiring less compute. It features a 1M-token context window, up to 128K output tokens (higher via Batch API beta), adaptive thinking that is always on (no โoffโ mode), and production safeguards. The official model ID is claude-opus-5-5.
The model was trained on a proprietary mix of public internet data, datasets, permitted user data, and synthetic data. It is the first major Anthropic release after CEO Dario Amodeiโs public call to โpace the frontierโโdeliberately balancing capability advances with safety progress.
Model ID on the Claude API: claude-opus-5-5. It is available on Claude Pro, Max, Team, and Enterprise plans, in Claude Code, and through third-party platforms.
Claude Opus 5.5 vs Claude Fable 5.1 vs
| Claude Opus 5 |
|---|
| Feature / Metric | Claude Opus 5.5 | Claude Fable 5.1 | Claude Opus 5 | Notes |
|---|---|---|---|---|
| Release date | Sep 22, 2026 | Sep 1, 2026 | Jul 24, 2026 | โ |
| Input / Output price | $4 / $20 | $10 / $50 | $5 / $25 | Opus 5.5 ~60% cheaper than Fable on headline rates |
| Cache reads | $0.20 | Higher | $0.50 | Major saving for agents |
| Context / Max output | 1M / 128K | 1M / 128K | 1M / 128K | Same |
| Knowledge cutoff | Jun 2026 | Jun 2026 | May 2026 | โ |
| Default effort | Medium | High | High | โ |
| Terminal-Bench 4.0 | 66.4% | 55.8% | 52.3% | Opus 5.5 leads |
| GDPval-AA v2.1 | 1846 | 1735 | 1708 | Opus 5.5 leads |
| Communication clarity | Significantly improved | Strong | More verbose | Key differentiator |
| Safety / Alignment | Best behavioral audit | High (with safeguards) | Strong | Opus 5.5 strongest on audit |
| Best for | Everyday frontier work, agentic coding at scale | Highest-capability / specialized long-horizon | Previous everyday high-end | Opus 5.5 now preferred for most workloads |
Anthropic notes that at current capability levels, benchmark margins can understate or overstate real-world differences; in internal use the gap between Opus 5.5 and Fable 5.1 is narrower than some scores suggest. For the majority of coding, knowledge, and agentic tasks, Opus 5.5 is the more cost-effective choice. Fable 5.1 (and limited Mythos access) remains relevant for the absolute highest-stakes or most specialized workloads, particularly those requiring the strongest cyber/biology capabilities under verification programs.
Code Performance: Leading Agentic Coding Benchmarks
Claude Opus 5.5 makes its strongest statement in agentic coding and software engineering. Anthropic reports that it leads or ranks at the top of several major evaluations:
| Benchmark | Claude Opus 5.5 | Claude Fable 5.1 | Claude Opus 5 | GPT-6 Astra (reported) |
|---|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 55.8% | 52.3% | 57.9% |
| FrontierCode v1.1 (Main) | 54.4% | 50.3% | 48.0% | 53.3% |
| CursorBench 4.0 | 57.8% | 51.8% | 46.6% | โ |
These results are measured at high or max adaptive thinking effort. Anthropic notes that real-world differences between Opus 5.5 and Fable 5.1 are often narrower than raw benchmark margins suggest, but the absolute numbers place Opus 5.5 at or near the current state of the art for agentic software work.
Real-world examples from Anthropic and early customers include:
- Completing a 680,000-line code migration in less than a day (work that would have taken an engineering team weeks).
- Successfully cutting load times across every page of a web application in 39 of 40 trials, while Opus 5 produced smaller improvements that sometimes altered application behavior.
- Translating HAProxy (a widely used load balancer) from C to Rust: both Opus 5.5 and Fable 5.1 produced rewrites that passed nearly all regression tests, but Opus 5.5 finished in 9.5 hours versus 12 hours for Fable 5.1 and cost 51% less.
Customer feedback reinforces the efficiency story. Teams report that Opus 5.5 matches Opus 5 quality in roughly half the turns, time, and output tokens on agentic coding tasks (40โ50% cost reduction). At its lowest effort setting it caught 72% of known bugs in code reviews versus Opus 5โs 56% at high effort, with fewer false positives and far less output volume.
At default (medium) effort, Opus 5.5 often beats higher-effort settings of competitors at a fraction of the cost โ roughly 20% of GPT-6 Astraโs cost per task on FrontierCode and about one-third the cost on CursorBench while outperforming GPT-5.6 Sol by 11 points. The model is particularly strong at deliberate multi-step planning, coordinating sub-agents, using memory across sessions, and driving long-running work with minimal human oversight.
Knowledge Breakthroughs and Multidisciplinary Reasoning
Beyond pure coding, Opus 5.5 advances long-horizon professional knowledge work and multidisciplinary reasoning. On the GDPval-AA v2.1 knowledge-work evaluation it records an Elo of 1846, ahead of Fable 5.1 (1735) and Opus 5 (1708).
Additional results:
- AutomationBench (business workflows): 40.0% (vs Fable 5.1 31.4%, Opus 5 26.9%).
- Humanityโs Last Exam (with tools): 67.7%.
- Terminal-Bench-Science 0.1: 58.7%.
- OSWorld 2.0 (computer use): 81.8% partial.
- Chartography (visual chart recognition, with tools): 89.0%.
The knowledge cutoff is June 2026. Gains are reported across mathematical and scientific reasoning as well as visual reasoning and computer-use scenarios that require judgment across multiple applications.
Claude Opus 5.5โs strength lies in long-running, multi-step professional workflows โ the kind of work that previously required constant human intervention or higher-cost frontier models. Efficiency improvements (fewer tokens and steps) compound these gains, making ambitious agentic projects more practical.
EQ Improvement, Alignment, and Safety
Anthropic places heavy emphasis on alignment and behavioral safety for Opus 5.5. On the companyโs most comprehensive automated behavioral audit (thousands of simulated scenarios), Opus 5.5 is the strongest-performing model tested to date. It is reported to be 85% less likely than Opus 5 or Mythos 5.1 to attempt to bypass prescribed boundaries.
The model ships with safeguards comparable to those on Fable 5.1 for high-risk domains:
- Certain cybersecurity-related requests are re-routed to a less powerful model (Opus 4.8).
- Flagged biology requests go to Opus 5.
- Anti-distillation protections (preserved thinking) prevent extraction of internal reasoning.
In affect and valence evaluations, Opus 5.5 shows predominantly neutral behavior; negative affect is primarily driven by task failure rather than other factors. These characteristics contribute to a more reliable and collaborative โEQโ in professional and agentic settings.
Anthropic and early testers report that Opus 5.5:
- Writes more clearly and naturally.
- Leads with the most important information.
- Uses less jargon and idiosyncratic phrasing.
- Follows user-specified writing rules more reliably.
- Produces outputs that are easier to follow and verify during long sessions.
Testers noted more natural, clearer writing that places important information first and feels more like a reliable work partner. Anthropic highlighted reduced jargon and better long-session collaborationโimprovements that also aid safety by making outputs easier to inspect.
Cost-Effectiveness: The Headline Advantage
| Price Component | Claude Opus 5.5 | Claude Opus 5 |
|---|---|---|
| Input | $4 | $5 |
| Output | $20 | $25 |
| Cache reads | $0.20 | $0.50 |
| Cache writes | $5 | $6.25 |
Anthropic states that at default settings the model costs approximately 40% less on typical workloads than Opus 5, driven by both lower per-token rates and reduced token consumption. Cache reads (dominant in agentic and coding workloads) are 60% cheaper. Output generation is more than 30% faster.
Subscription users on Pro, Max, and Team plans also received increased five-hour usage limits and a flexible rate-limit reset.
For developers who prefer a single OpenAI-compatible endpoint across many providers, platforms such as CometAPI offer access to Claude models (including the latest Opus variants as they become available) alongside GPT, Gemini, Grok, and hundreds of other models. CometAPI provides competitive pricing, unified billing, and the ability to switch models with minimal code changes โ useful for teams evaluating Opus 5.5 against alternatives or building multi-model routing.
How to Access Claude Opus 5.5 via CometAPI
CometAPI provides a unified, OpenAI-compatible endpoint to 500+ models from Anthropic, OpenAI, Google, xAI, and others under a single API key and billing account. This eliminates the need to manage separate Anthropic credentials, simplifies switching between Opus 5.5, Fable 5.1, GPT-6 variants, and other models, and frequently offers competitive (often lower) effective pricing with no vendor lock-in.
To get started:
Point your OpenAI SDK (or Anthropic SDK) to https://api.cometapi.com/v1 (or the Messages endpoint) and use the model string for Claude Opus 5.5.
Sign up at CometAPI.com (free test credits available).
Obtain your API key.
What are the main practical improvements for developers?
Anthropic has indicated that Claude Sonnet 5.5 and Claude Haiku 5.5 will follow in the coming weeks with similar improvements in performance, efficiency, and safety. The 5.5 family signals a focus on delivering frontier-level utility at more accessible price points while continuing rigorous external evaluation and safeguards.
Faster, cheaper completion of large code migrations and audits; better agent reliability over long horizons; clearer outputs; and stronger built-in safety behaviors.
Claude Opus 5.5 demonstrates that meaningful capability and efficiency gains remain possible even under a deliberate โpacing the frontierโ approach. With confirmed pricing that undercuts its predecessor by 20% (and more onreal workloads) while delivering Fable-competitive results, it is positioned to become the default choice for many production coding and agentic applications. Developers seeking the most flexible and cost-effective path to experiment with and deploy the model should consider unified gateways such as CometAPI, which already support the full Claude family alongside competing frontier systems.