Opus vs Sonnet: Opus 5.5 vs Sonnet 5.5 — Which Claude Model Should You Use?

Short answer: Claude Opus 5.5 is built for long-running agentic coding and judgment-heavy knowledge work, while Claude Sonnet 5.5 fits most everyday tasks where speed and quality both matter. Since Sonnet 5.5 launched on September 28, 2026, the performance gap has narrowed, but Opus still costs exactly twice as much. This guide is for developers using the API and for people choosing a model in the Claude.ai app, based on the official docs as of October 26, 2026.

What are the key differences between Opus 5.5 and Sonnet 5.5?

The official docs describe Opus 5.5 as "for long-running agentic coding and knowledge work" and Sonnet 5.5 as "the best combination of speed and intelligence". Context, max output and knowledge cutoff are now identical; the differences are price, latency, whether thinking can be turned off, and default effort.

Source: the official models overview and model pages (checked October 26, 2026).

How big is the performance gap?

In the benchmarks Anthropic published with Sonnet 5.5, Sonnet 5.5 comes close to Opus 5.5 on several measures and beats it on one. All figures are Anthropic's own, and test settings (such as effort) vary by row.

Anthropic also says Opus 5.5 "remains clearly stronger at complex, open-ended work requiring sustained judgment." Well-defined tasks often run fine on Sonnet 5.5; ambiguous, long-horizon work is where Opus pulls ahead.

How much more does Opus cost?

Opus 5.5 is $4 input / $20 output per million tokens and Sonnet 5.5 is $2 / $10, so Opus costs exactly twice as much. Sonnet 5.5 keeps Sonnet 5's input and output prices and halves prompt cache reads from $0.20 to $0.10 (official "What's new"). Anthropic says Sonnet 5.5 cost up to 30% less per task than its predecessor in its own testing. Prices can change — check the official pricing page, or compare monthly costs in the Claude API cost calculator.

When should you use Opus, and when Sonnet?

• Opus 5.5: large multi-file refactors, agents that plan and execute for hours, ambiguous analysis or planning that needs sustained judgment, accuracy-critical work
• Sonnet 5.5: everyday code writing and review, drafting and summarizing, data cleanup, chat responses — most work where speed and cost both matter
• Haiku 5.5: classification, extraction, routing and subagents at high volume (cheapest with prompts at or under 100K tokens)

A cost-efficient habit is to start with Sonnet 5.5 and step up to Opus 5.5 only when results are shallow or you keep getting stuck. Opus 5.5 also offers Fast mode, about 2.5x faster at double the price ($8/$40).

Switching models mid-conversation: per the official docs, on the Claude API and Google-managed Google Cloud, Opus 5.5 can read Sonnet 5.5's thinking blocks, so moving a conversation up from Sonnet 5.5 to Opus 5.5 keeps its reasoning. Sonnet 5.5 can't read Opus 5.5's blocks, so stepping down runs later turns without it (the request still succeeds; unreadable blocks are dropped and not billed).

How do you switch models in the API?

Change the model string in your request — for example, from claude-opus-5-5 to claude-sonnet-5-5.

{
  "model": "claude-sonnet-5-5",
  "max_tokens": 1024,
  "messages": [
    {"role": "user", "content": "Hello"}
  ]
}

If you're moving from Sonnet 5 to Sonnet 5.5, a model ID change alone can break some requests:

• thinking: {"type": "disabled"} returns a 400 → send {"type": "between_tools"} instead (not allowed with xhigh or max effort).
• Forced tool use (tool_choice any/tool) returns a 400 → keep auto and use strict tool use or structured outputs.
• On the Claude API, the older computer_20251124 tool is rejected → use computer_toolset_20260801.
• Longer text between tool calls now comes back in thinking blocks, which are empty at the default display setting.

See the official What's new in Claude Sonnet 5.5 for the full list. If you're still on Sonnet 4.5 (claude-sonnet-4-5-20250929), it is scheduled to retire on November 30, 2026, with claude-sonnet-5-5 as the recommended replacement (official model deprecations page).

How do you choose in the Claude.ai app and Claude Code?

In the app, use the model picker. Per claude.com/pricing (checked October 26, 2026), Opus is available on Pro, Max, Team and Enterprise but not Free; the Free plan lists Sonnet and Haiku. Which version is the app default isn't stated in the official docs, so check the picker. Anthropic says Sonnet 5.5's default effort is medium in the apps and Claude Code and high on the API. In Claude Code, switch models with /model.

Frequently asked questions

Q. Which is better for coding, Opus 5.5 or Sonnet 5.5?
Everyday writing, review and debugging usually run fine on Sonnet 5.5 — in Anthropic's numbers it even scored higher on Terminal-Bench 4.0. Long sessions on large codebases with many design decisions suit Opus 5.5 better.

Q. Is Sonnet 5.5 more expensive than Sonnet 5?
No. Input and output stay at $2/$10, cache reads drop to $0.10, and it uses the same tokenizer, so the same text counts as the same number of tokens.

Q. What is Fast mode?
An Opus 5.5 option that responds about 2.5x faster at double the price ($8/$40).

Q. Can I still use Sonnet 5?
Yes. It's listed as legacy (still available), with retirement not sooner than June 30, 2027. Anthropic says higher-risk cybersecurity requests may be routed from Sonnet 5.5 back to Sonnet 5.

Q. What changed from Opus 5 to Opus 5.5?
Opus 5.5 performs, in Anthropic's words, "at the level of Fable 5.1 on most work", costs 40% less than Opus 5 on typical workloads and is 30%+ faster, with API pricing down from $5/$25 to $4/$20. See the Opus 5.5 guide.

In short: Opus 5.5 for complex, open-ended judgment; Sonnet 5.5 for the speed–cost balance; Haiku 5.5 for high-volume, low-latency jobs — and manage your request code around the API model IDs in the table above.

Note: Prices, specs and benchmarks are from the official model docs and the Sonnet 5.5 announcement as of October 26, 2026, and may change. Benchmarks are Anthropic's own figures. This site is not affiliated with Anthropic.

Related articles

• Claude Models Compared: Haiku vs Sonnet vs Opus 5.5
• Claude Opus 5.5 guide
• Claude API Model IDs and Versioning: Aliases, Pinned Snapshots, and Migration
• Claude API Context Windows and Tokens: Limits and Management
• Claude API Pricing by Model — Batch and Prompt Caching
• Claude vs ChatGPT: What's the Difference?