Claude Opus 3.5 / 4 / 4.5 – version comparison
Technical scope Major labs & flagship models, anthropic, vendor-analysis
Opus is Anthropic's flagship tier: the largest, most capable and most expensive models in the Claude family, reserved for work where reasoning depth matters more than latency or cost. If you are trying to untangle which Opus you are actually looking at – 3.5, 4 or 4.5 – the short version is this: each number marks a generational jump in reasoning, coding and agentic capability, with 4.5 the current top of the line as of late 2025, Opus 4 the previous flagship, and Opus 3.5 a step that Anthropic effectively folded into the 4 series rather than shipping under its own name. Prices and exact benchmark figures move; everything specific below should be checked against Anthropic's own model documentation before you commit a production budget to it.
The confusion is understandable. Anthropic's naming has shifted under its feet, the "3.5" label means different things on different tiers, and the company has at times jumped straight from one major version to the next without the intermediate release people expected. This piece is a disambiguation reference: what each Opus version is, when it arrived, how capability and price have moved, and when an older version is still the correct choice. For the wider context of how Opus sits above Sonnet and Haiku, see our guide to the Claude model family.
Claude Opus versions in one paragraph
Claude Opus is the top rung of Anthropic's three-tier ladder, sitting above Sonnet (the balanced mid-tier) and Haiku (the fast, cheap small model). Opus is tuned for the hardest problems: multi-step reasoning, large-codebase engineering, long-horizon agentic tasks and analysis that benefits from the model's full capacity. The lineage runs Opus 3 (early 2024), through the Claude 3.5 generation, into Claude Opus 4 and Claude Opus 4.5. Each step raised performance on coding and reasoning benchmarks, extended how long the model can work autonomously, and – in the 4 series – introduced "extended thinking," where the model can spend additional inference time reasoning before it answers. The practical catch is cost: Opus has always been several times more expensive per token than Sonnet, which is why most teams reserve it for the tasks that genuinely need it.
Version timeline
Claude 3 Opus launched in March 2024 as part of the original Claude 3 family (Haiku, Sonnet, Opus). It was Anthropic's first model to seriously compete with the top of OpenAI's lineup on reasoning and knowledge benchmarks, and it carried a 200,000-token context window that was generous for the time.
The Claude 3.5 generation is where naming gets slippery. Anthropic shipped Claude 3.5 Sonnet in mid-2024 and refreshed it later that year, and it shipped Claude 3.5 Haiku. A separately branded "Claude 3.5 Opus" as a shipped, generally available model is the part most people misremember: Anthropic signalled an Opus-class 3.5 was coming, then effectively rolled those gains forward into the Claude 4 generation rather than releasing a standalone 3.5 Opus that outlived the moment. If you see "Opus 3.5" referenced, treat it as either the 3.5-era expectation or a loose shorthand, and verify against the model list before you write code against a model ID. The real continuity on the Opus tier runs from Claude 3 Opus to Claude Opus 4.
Claude Opus 4 arrived in 2025 alongside Claude Sonnet 4, repositioning Opus as a frontier coding and agentic model. This generation introduced the extended-thinking mode and was marketed heavily around sustained, hours-long autonomous work on software tasks. Anthropic later shipped a Claude Opus 4.1 point release that sharpened coding and agentic performance further without a full generational reset.
Claude Opus 4.5 is the current flagship as of late 2025, positioned by Anthropic as its strongest model for coding, agentic workflows and complex reasoning, with improved efficiency relative to Opus 4 and 4.1. Because this is the fastest-moving part of the lineup, confirm the precise release date, model identifier and status on Anthropic's models overview rather than trusting any single date you read elsewhere.
Was there ever a Claude Opus 3.5?
There was no long-lived, generally available model officially branded "Claude 3.5 Opus" in the way Claude 3.5 Sonnet existed. Anthropic shipped 3.5-generation Sonnet and Haiku models, and the Opus-tier improvements expected in that cycle were carried into Claude Opus 4. When you encounter "Opus 3.5" in blog posts or forum threads, read it as either a reference to that unshipped expectation or imprecise shorthand, and pin any production decision to an exact model ID from Anthropic's documentation.
Capability progression
Across the Opus line the direction of travel is consistent even where exact numbers shift: each version improved on software engineering benchmarks such as SWE-bench, on graduate-level reasoning benchmarks such as GPQA, and on agentic tool-use evaluations. If those benchmark names are unfamiliar, our explainer on AI benchmarks – MMLU, GPQA and LMSYS Arena covers what each measures and where the methodology traps lie.
Claude 3 Opus was a strong generalist for its era: good at long-document analysis, nuanced writing and knowledge-heavy reasoning, but it predated the explicit "thinking" machinery and the agentic tooling that later defined the tier. The Claude 4 generation reframed Opus around two things: coding at the level of a serious engineering collaborator, and long-horizon autonomy, where the model plans, executes tool calls, checks its own work and continues over extended sessions. Anthropic's headline claims for Opus 4 centred on SWE-bench coding performance and on the ability to sustain multi-hour agentic tasks without losing the thread. Opus 4.1 pushed those coding and agentic numbers higher as an incremental release.
Claude Opus 4.5 extends the same arc: Anthropic presents it as the best Claude yet for coding and agentic work, with better results per token than its predecessors. The caution that always applies to flagship claims applies here too. Vendor-reported benchmark figures are measured under the vendor's chosen configuration – prompt format, tool scaffolding, number of attempts – and independent reproductions often land lower or tell a more mixed story. Treat any single SWE-bench or GPQA percentage you see as a point-in-time, methodology-dependent figure, not a fixed property of the model. For how Opus compares against the mid-tier on this same axis, see our analysis of Claude Sonnet 3.5 and 4.
Pricing per version (date-stamped)
Opus has consistently been the priciest tier in the Claude family, charged per million tokens with separate input and output rates, and with output tokens costing several times more than input. As of late 2025, Anthropic's published Opus pricing has generally sat in the high single digits to low double digits of dollars per million input tokens and substantially higher for output – an order of magnitude more expensive than Sonnet on a like-for-like basis. Anthropic has also, notably, adjusted Opus pricing across generations rather than holding it flat, and Opus 4.5 was positioned partly on improved cost-efficiency.
Because these rates change and because prompt caching and batch processing can materially cut the effective cost, do not treat any figure in a third-party article as current. Check the live numbers on Anthropic's pricing page before you model a budget. Two durable truths survive the price churn: Opus always costs several times more than Sonnet per token, and the gap is usually only worth paying when the task genuinely exceeds what Sonnet can do reliably. For high-volume, latency-sensitive work the right answer is often Haiku, which we cover in our Claude Haiku analysis.
Use case fit per version
The newest Opus is not automatically the right Opus. Older versions remain defensible in several situations. If you have a validated pipeline built and tested against Claude 3 Opus or Claude Opus 4, and that model still meets your quality bar, there is real value in stability: you avoid re-running your evaluation suite, re-tuning prompts and re-checking edge cases for a marginal capability gain you may not need. Regulated and audit-heavy environments often prefer a frozen, well-characterised model over the latest release for exactly this reason.
Opus 4.5 and Opus 4.1 earn their premium on frontier coding, large-codebase refactoring, and long-running agentic systems that chain many tool calls and need to recover gracefully from errors. If you are building that class of system, our coverage of AI agents news and developments tracks how the agentic frontier is moving across vendors. For straightforward summarisation, classification, routine drafting or structured extraction, Opus is overkill at any version – Sonnet or Haiku will do the job at a fraction of the cost, and the quality difference on easy tasks is often imperceptible.
Migration considerations
Anthropic maintains a published deprecation and retirement policy, and older Claude models do get retired on announced timelines. The practical discipline is to treat model IDs as dependencies you must monitor, not set-and-forget constants. When a model is slated for retirement, Anthropic typically provides advance notice and a recommended successor, but the successor will not be byte-for-byte identical in behaviour – prompts that were tuned against one Opus can drift when pointed at the next.
API compatibility across the Opus line is generally good at the request level: the Messages API shape is stable, and moving between Opus versions is usually a matter of changing the model string. The behavioural migration is the harder part. Extended-thinking modes, tool-use formatting and system-prompt sensitivity have all evolved across generations, so a safe migration means re-running your evaluation set against the new version, watching for regressions in tone, refusal behaviour and structured-output adherence, and budgeting for the cost shift in either direction. Build an abstraction layer so swapping the model ID is a one-line change, and keep your eval harness current. If you hit rate-limit friction during migration load-testing, the patterns in our piece on 429 rate-limit errors generalise across providers.
For builders: which Opus to use
Default to the current flagship – Claude Opus 4.5 as of late 2025 – when you are starting fresh and the task genuinely needs Opus-class reasoning or coding. You get the best capability and, by Anthropic's account, better efficiency than earlier Opus versions, which narrows the cost penalty.
Stay on an older Opus when you have a validated, stable pipeline that already meets your quality bar and the cost of re-validation outweighs the capability gain, or when change-control requirements favour a frozen model. Drop down to Sonnet or Haiku whenever the task does not clearly need Opus – which, honestly, is most tasks. The most common and expensive mistake is running easy workloads on the flagship tier out of habit. Benchmark your actual workload across tiers on your own data, measure quality against cost, and let that evidence pick the version. Confirm every price, model ID and benchmark figure against Anthropic's primary documentation at the moment you decide, because on this beat the specifics change faster than any article can.