In the matchup of Claude Opus 5.5 vs Claude Opus 5, the newer model wins on nearly every line Anthropic publishes: it costs less at 4 dollars per million input tokens and 20 dollars per million output tokens against 5 and 25, its cache reads are less than half the price at 0.20 dollars per million tokens against 0.50, its knowledge runs a month fresher to June 2026, and Anthropic now labels Claude Opus 5 a legacy model with an explicit recommendation to migrate. Every figure here comes from Anthropic's official model documentation, fetched on 23 September 2026, and the Claude Code changelog for version 2.1.280.
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The context for the comparison: Claude Opus 5.5 arrived in late September 2026 and immediately became the default Opus model in Claude Code v2.1.280, whose changelog reads "Added Claude Opus 5.5 (claude-opus-5-5), now the default Opus model — 1M context, $4/$20 per Mtok with $0.20/Mtok cache reads." Claude Opus 5, released on 24 July 2026, moved to Anthropic's legacy list the same week. Two months is a short reign — here is exactly what changed between the two, and what did not.
Claude Opus 5.5 vs Claude Opus 5: The Official Numbers Side By Side
| Spec (per Anthropic docs) | Claude Opus 5.5 | Claude Opus 5 |
|---|---|---|
| Status | Current Opus model | Active, legacy |
| Model ID | claude-opus-5-5 | claude-opus-5 |
| Input price / MTok | 4 dollars | 5 dollars |
| Output price / MTok | 20 dollars | 25 dollars |
| Cache read / MTok | 0.20 dollars | 0.50 dollars |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens | 128K tokens |
| Thinking | Adaptive, always on | Adaptive |
| Default effort | medium | high |
| Knowledge cutoff | June 2026 | May 2026 |
| Released | September 2026 | 24 July 2026 |
| Retirement | Not sooner than 22 September 2027 | Not sooner than 24 July 2027 |
Anthropic describes Claude Opus 5.5 as built for long-running agentic coding and knowledge work, and its models overview now points newcomers to Opus 5.5 as the starting choice for most workloads. Claude Opus 5's own page carries the legacy banner and a direct line: although Claude Opus 5 is still available, you should consider migrating to Claude Opus 5.5 for improved performance.
Where Claude Opus 5.5 Beats Claude Opus 5 Outright
Price is the cleanest win. A 20 percent cut on input and output tokens is significant on its own; the cache-read cut from 0.50 to 0.20 dollars per million tokens is the bigger deal for agent workloads, because long-running agents burn most of their spend re-reading cached context between turns. An agent session that loops over the same project files hundreds of times pays that cache rate on every loop — dropping it by 60 percent changes what always-on agents cost to operate. Freshness is the second win: a June 2026 reliable knowledge cutoff against May 2026 means Opus 5.5 natively knows about another month of tools, model releases and framework changes — a real difference in a space where the tools change monthly, as the pace tracked in the Kimi 2.6 benchmark write-up makes obvious.
Defaults are the third, subtler shift. Claude Opus 5.5 runs adaptive thinking always on with a default effort of medium, where Claude Opus 5 defaulted to high effort. Anthropic's documentation frames effort as the dial that steers how much the model thinks — so out of the box, the two models spend differently on deliberation, and a fair head-to-head on your own tasks means setting effort explicitly rather than trusting defaults. What the documentation does not publish is a benchmark table pitting the two directly against each other, so any specific quality delta you read elsewhere is somebody's testing, not an official number.
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Two shared cost levers survive the comparison unchanged, and both matter to anyone running Claude Opus 5.5 or Claude Opus 5 at volume. The Batch API gives a 50 percent discount on input and output on both models, and both support up to 300K output tokens per request on the Message Batches API under Anthropic's extended-output beta header — so non-urgent bulk work like content generation, dataset processing or overnight agent runs costs half price on either model. Prompt caching is where they part ways again: Anthropic's pricing pages put cache reads at 10 percent of base input price as the standard rate, with Claude Opus 5.5 at a preferential 5 percent — which is how the 0.20-dollar figure against Opus 5's 0.50 comes about, and why the gap widens rather than narrows as your sessions get longer.
Where Claude Opus 5 Still Holds Its Ground
Claude Opus 5 is legacy, not dead. Its status in Anthropic's documentation is active, it remains available on the Claude API, Amazon Bedrock, Google Cloud, Microsoft Foundry and the Claude Platform on AWS, and its retirement commitment runs no sooner than 24 July 2027 on Anthropic-operated platforms. Teams with validated pipelines pinned to claude-opus-5 have at least ten months of runway and no forced migration. There are legitimate reasons to stay temporarily: regulated workflows that revalidate slowly, prompt suites tuned against Opus 5's high-effort default, or simple change-freeze discipline mid-project.
Capability-wise, the floor remains high. Anthropic's Opus 5 page describes it as a step-change improvement over Claude Opus 4.8, with the largest gains in deep reasoning, agentic and long-horizon tasks, and test-time compute scaling — that description did not stop being true when Opus 5.5 shipped. And both models share the specs that define what is buildable: the 1M-token context window and 128K output. Nothing you architected around context length changes either way, which is exactly the situation covered in the what are AI agents explainer — the harness and workflow design usually outlive any single model swap.
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What The Comparison Means If You Build Agents
The decision logic is short. New build: start on Claude Opus 5.5 — cheaper, fresher, and it is what Anthropic's own tooling now defaults to, including Claude Code as of v2.1.280 on 22 September 2026. Existing build on Opus 5: plan the migration rather than rushing it — run your evals on 5.5, mind the effort-default difference, and bank the price cut once outputs verify. Cost-sensitive always-on agents: the cache-read pricing alone justifies the test, because that line dominates long-session economics.
Model choice is also only one layer of an agent stack, and often not the decisive one. A well-structured system — persistent instructions, skills, memory, the pattern documented in the Agent OS guide — routinely gets more from a mid-tier model than a bare prompt gets from a frontier one. Cross-vendor comparisons make the same point from another angle: the DeepSeek harness vs Claude Code comparison shows how much of real-world performance lives in the harness rather than the weights, and the Hermes bot vs Grok bot matchup reaches a similar conclusion for assistant-style bots. For hands-on results across the current crop of model brains, the Goldie Bench write-up covers how they compare on the tests that matter for actual agent work.
Claude Opus 5.5 vs Claude Opus 5: Common Questions
Is Claude Opus 5 being switched off? No — legacy status means it is no longer the recommended current model, but it stays available with retirement not sooner than 24 July 2027. Does Claude Code still let you pick Opus 5? The v2.1.280 changelog changes the default Opus model only; pinned model choices in your configuration continue to apply. Is Opus 5.5 the top model overall? Not by price tier — Anthropic's lineup lists Claude Fable 5.1 above it at 10 and 50 dollars per million tokens for demanding reasoning, with Opus 5.5 positioned as the starting point for most workloads. Same context, same output limit, so the choice between those two is about capability ceiling and budget, not fit.
The verdict stands as it opened: Claude Opus 5.5 wins this comparison on the published numbers, and the only good reasons to stay on Claude Opus 5 are operational, not technical. Run your own comparison before moving production work — on documented pricing and cutoffs alone, the newer model is the default answer, which is precisely why Anthropic made it the default. If you are choosing where to learn this stack alongside other builders, the best Claude community roundup covers where the sharpest discussion happens.
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