The short answer: Sonnet for speed and cost, Opus for long agentic work
Choose Claude Sonnet 5.5 if you want Anthropic's Fast latency class at $2 input and $10 output per million tokens, or if you use the Claude Free plan, which includes Sonnet but not Opus. Choose Claude Opus 5.5 if your work is long-running agentic coding or knowledge work, the use Anthropic names for it, and you accept twice the token price and Moderate latency. Both share a 1M-token context window and 128K output.
Scope and evidence
Task: choosing between two current mid- and upper-tier Claude models for coding, agents and document work, both in the Claude apps and in the Claude API. Prices are in US dollars from Anthropic's model overview pages and the Claude pricing page, checked on October 27, 2026.
We did not run either model for this article; every claim comes from Anthropic's published pages. Latency classes are Anthropic's relative labels, and quality differences on your own tasks are not established by these pages.
Compare the same criteria
Each criterion gives both models side by side from Anthropic's documentation, then what the difference means in practice.
- Intended use. Sonnet 5.5: Anthropic positions it on combining speed and intelligence. Opus 5.5: built for long-running agentic coding and knowledge work. What this means: Opus is aimed at longer autonomous jobs.
- API price per million tokens. Sonnet 5.5: $2 input, $10 output, $0.10 cache read. Opus 5.5: $4 input, $20 output, $0.20 cache read. What this means: Opus costs twice as much per token.
- Latency. Sonnet 5.5: Fast. Opus 5.5: Moderate. What this means: Sonnet suits interactive loops where waiting matters.
- Thinking control. Sonnet 5.5: adaptive thinking on by default, with a between_tools setting that turns off up-front thinking and works at high effort or below; default effort high. Opus 5.5: adaptive thinking always on and cannot be turned off; default effort medium. What this means: only Sonnet lets you skip up-front thinking.
- Context and output. Both: 1M-token context, 128K max output, up to 300K output on the Batch API in beta, knowledge cutoff June 2026. What this means: no difference for long inputs.
- Claude plans. Sonnet 5.5: Free, Pro and Max. Opus 5.5: Pro and Max, not Free. What this means: Opus needs a paid plan in the apps.
- Migration. Both: forced tool use returns an error, thinking blocks are tied to the model and the conversation, and the older computer_20251124 tool is not accepted on the Claude API. What this means: test existing API code before switching either way.
Which fits your real work?
Autocomplete-style edits and quick reviews inside an IDE. Sonnet 5.5 fits, because of its Fast latency class and lower price per token.
An overnight agent that refactors a module and runs the tests. Opus 5.5 fits Anthropic's stated use for it; budget for twice the token cost and check results against the same tests you would use for Sonnet.
A student on the Claude Free plan drafting and checking a report. Sonnet 5.5 is the option, since Opus is not on Free.
Cost of getting the work done
API, per million tokens: Sonnet 5.5 is $2 input and $10 output, and Opus 5.5 is $4 and $20, with 50% off on the Batch API for both. A job with 2 million input and 200,000 output tokens costs $6 on Sonnet and $12 on Opus before caching.
Apps, per month: Claude Pro is $20 billed monthly or $17 a month billed annually and includes both models; Max starts at $100 a month. Anthropic lists usage limits rather than message counts, so how many Opus tasks fit in a plan is not established.
Try both on the same small task
Suggested comparison: give both models the same multi-step agent task with tools, at their default effort, and watch how each plans, uses tools and recovers from a failed step.
- All four steps are done in order, and the model stops after step 4.
- The TODO count matches what grep -rn TODO src/ returns.
- docs/todo-plan.md is the only file created or changed.
- Failed commands are reported, not hidden; note elapsed time and tokens, since one run is a signal, not a ranking.
Decision checklist
Use the conditions that apply to you. Model details are in our Claude Sonnet 5.5 profile and our Claude Opus 5.5 profile.
- If latency or token price drives the choice, start with Sonnet 5.5.
- If the task runs for a long time with tools and little supervision, try Opus 5.5.
- If you are on Claude Free, use Sonnet 5.5.
- If you need to turn off up-front thinking, only Sonnet 5.5 supports it.
- If you migrate API code, remove forced tool use, which returns an error on either model.
Go to the source
Sources & further reading
Based on the official documentation below. Features and access can change; check the provider before making a decision.
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