Claude Opus vs Sonnet: Which 5.5 Model Fits Your Work?

Claude Opus vs Sonnet: Which Should You Pick?

Make Sonnet your everyday default if you can describe what a good answer looks like before the model starts. Make Opus your default if discovering the right approach is much of the job.

For a founder turning approved notes into customer emails, Sonnet is the sensible starting point. For a senior operator producing a recurring report from a known template, the same applies. For a technical builder fixing a reproducible bug with a failing test, start with Sonnet and a clear completion check.

Choose Opus earlier when a CTO needs a migration plan across interdependent systems, an agent must investigate an unfamiliar repository, or a research task contains conflicting evidence. An agent is a model working through tools and intermediate steps toward a result. In those jobs, a plausible first step can be wrong enough to waste the rest of the run.

These are editorial recommendations based on Anthropic's documented positioning, published evaluations, and the cost calculation below. They are not results from a hands-on trial. Model specifications, access, and API prices were verified against Anthropic's live pages on 18 October 2026.

Sources: Anthropic pricing, Opus specifications, Sonnet specifications, and Anthropic's comparison of their intended work. API maximum output is a model limit, not a guaranteed Claude app response length.

The rule that flips the choice is simple: move to Opus when the missing ingredient is judgment, not when the prompt merely needs clearer instructions. A vague request with no source material is still a vague request on the more expensive model.

Claude Sonnet vs Opus: What Anthropic Builds Each For

Anthropic positions these models as complements, with a distinction between executing a bounded task and steering a difficult project. Their shared context capacity does not make their judgment interchangeable.

Claude Opus 5.5: The Choice for Sustained Judgment

Claude Opus 5.5 is Anthropic's model for “long-running agentic coding and knowledge work.” The useful part of that description is the sustained work: investigating a problem, deciding which evidence matters, and maintaining a coherent approach through several tools or files. Anthropic's Opus model documentation

For example, a platform lead might ask for a plan to replace an authentication service while preserving existing customer sessions. The difficult work is identifying hidden dependencies and failure modes. Opus is the stronger starting choice for that investigation. Its disadvantage is paying the higher rate even when the next task is a simple rewrite.

Our Opus 5.5 guide for agents and coding goes deeper into that operating role. Skip Opus as an automatic default when most of your queue has clear inputs, a familiar format, and an inexpensive acceptance check.

Claude Sonnet 5.5: The Choice for Bounded Daily Work

Claude Sonnet 5.5 is strongest at “well-scoped everyday tasks,” in Anthropic's words. Its examples include fixing bugs and producing documents, slides, and spreadsheets. That makes it a strong candidate for work where you supply the evidence and desired shape of the result. Anthropic's Sonnet announcement

A senior operator can give it approved account notes, a house template, and an instruction to flag missing facts instead of filling gaps. That is a bounded transformation. Sonnet's weakness becomes more relevant when the brief asks it to decide which contradictory account of events is reliable and what the business should do about it.

The Sonnet 5.5 setup guide covers getting started. The decision here is whether your job needs its economical execution or Opus's stronger handling of open-ended work.

Claude Opus vs Sonnet for Writing

Winner for daily writing: Sonnet. Winner for a document whose argument needs to be discovered: Opus.

Use Sonnet for rewriting an email, turning meeting notes into an update, adapting an approved article into a customer announcement, or drafting slides from supplied evidence. Those jobs reward fidelity to the brief and efficient revision. Anthropic reports clearer writing in both 5.5 models and specifically positions Sonnet for polished everyday deliverables. Sonnet release, Opus release

A useful Sonnet brief says who will read the document, what they must understand or do, which sources are allowed, and which claims must remain unchanged. For a founder's investor update, that might mean: use the approved metrics, preserve the risks, and shorten the explanation without adding a growth claim. Check the output against those constraints before deciding that a different model is necessary.

Choose Opus when the writing task is also an analysis task. A strategy memo may need to reconcile customer interviews, operational constraints, and a weak financial assumption before there is a defensible recommendation to write. The premium buys a better candidate for that reasoning process; it does not excuse checking the sources.

For creative writing, start with Sonnet when you are exploring voices or revising a passage. Try Opus when the challenge is maintaining a complicated argument, character motivation, or structure across the work. Treat this as a selection rule to evaluate against your own editorial bar, not an objective ranking of literary taste.

Claude Opus vs Sonnet for Coding

Winner for scoped implementation: Sonnet. Winner for architecture, unfamiliar failures, and sprawling changes: Opus.

Give Sonnet a reproducible bug, the relevant files, and a test that defines success. Use it for a small feature whose behavior is already specified. Give Opus the investigation when nobody yet knows why the service fails or which components must change. Anthropic explicitly distinguishes Sonnet's bug-fixing strength from Opus's advantage on complex, open-ended work. Anthropic's comparison

The distinction is not the number of files alone. Applying a known API rename across many files can be tightly scoped. Changing a few lines in a payment calculation can demand substantial reasoning. Match the model to uncertainty and the cost of a wrong decision.

What Anthropic's Benchmarks Do and Do Not Establish

Anthropic's published figures do not show an Opus win on every coding task. Sonnet scores 70.6% on Terminal-Bench 4.0, compared with 66.4% for Opus at Xhigh, Opus's highest-scoring effort setting there. On FrontierCode 1.1 Main, Opus scores 54.4%, while Sonnet scores 52.1% at Xhigh and 46.2% at Max. These are figures published by Anthropic, not benchmarks run for this article. Anthropic's benchmark table and footnotes

The lower Sonnet score at Max has a useful explanation: additional review could lead to timeouts or changes outside the requested scope, which FrontierCode penalizes. More activity is not automatically a better finished change. Opus's launch methodology also states that production safeguards were enabled and fallback models could complete flagged tasks. Read the configuration alongside the score. Opus evaluation methodology

For your own evaluation, judge a change by whether it passes the relevant tests, stays within scope, and can be reviewed without reconstructing the whole solution. A benchmark margin cannot tell you how much review your repository needs.

Claude Code Opus vs Sonnet: Set the Model Deliberately

Claude Code currently defaults to Opus 5.5 for direct Anthropic use and the listed paid Claude account types, unless settings override it. Choosing Sonnet for routine tasks is therefore a deliberate configuration choice, not a description of the product's default. Provider aliases can resolve differently. Claude Code model configuration

Use the full model name when version precision matters. These are Claude Code commands, entered inside its session:

/model claude-sonnet-5-5
/effort medium

For a difficult investigation, select /model claude-opus-5-5 and choose the effort the task warrants. Both 5.5 models default to Medium in Claude Code when no override applies. A typed /model choice is saved for future sessions; use the picker and its session-only option when you want a temporary change. Sonnet 5.5 requires Claude Code v2.1.284 or later; run claude update in your terminal if you need to update. Configuration and version requirements

The documented opusplan mode uses Opus for planning and Sonnet for execution. It fits a design that can be written down clearly before implementation. Keep Opus through execution when the implementation itself keeps exposing architectural decisions.

Long Agent Runs and Research: Where Opus Earns Its Place

Winner for open-ended agent work: Opus. A long but predictable queue can still belong on Sonnet.

Elapsed time is a weak proxy for difficulty. An agent processing many documents against a fixed checklist may have little judgment to exercise. An agent investigating an inconsistent production failure must repeatedly decide what to inspect and whether its previous hypothesis was wrong. Anthropic says Opus remains stronger on complex work requiring sustained judgment. Sonnet-versus-Opus positioning

For a mid-market CTO, a useful boundary is to put Opus on diagnosis and design, then give Sonnet implementation units with explicit checks. If the units cannot be specified without losing important context, keep the job with Opus. In either case, define the files or systems it may change and the evidence required before calling the work complete.

Claude Research Uses Opus Regardless of Your Chat Selection

The Claude app's Research feature is not a Sonnet-versus-Opus toggle. Anthropic's help center says Research runs on Opus 5.5 and falls back to Opus 4.8 when needed, regardless of the model selected for the conversation. Research is available on Pro, Max, Team, and Enterprise and requires web search to be enabled. Claude Research help

That matters for a founder comparing vendors or an operator preparing a market brief. Selecting Sonnet before enabling Research does not make the research run a Sonnet workload. For a normal chat about supplied sources, Sonnet remains a reasonable starting point; for a difficult synthesis, Opus is the stronger choice.

Context Windows, Output Limits, and Thinking Settings

Context capacity is a tie; effort and thinking behavior are the meaningful differences.

The context window is the material a model can work with during a request, including the conversation and supporting content. Both APIs list 1M tokens of context and 128K tokens of standard maximum output. Both also support up to 300K output tokens on the Message Batches API with the documented beta header. That exception does not describe a normal chat response. Opus limits, Sonnet limits

For paid Claude plans, Anthropic's help center lists 1M context for both models in chat and Claude Code. A portion is reserved for the response. A large context window means room to supply more evidence; it is not a guarantee that every relevant detail will be used correctly. Paid-plan context limits, usage versus length limits

Match Effort to the Job

Both models support Low, Medium, High, Extra high (xhigh), and Max. Effort controls how much work the model puts into a response, including thinking and tool calls. On the API, Opus defaults to Medium and Sonnet to High. In Claude Code, both default to Medium; Sonnet also defaults to Medium in the Claude apps. API effort documentation, Code settings, Sonnet defaults

Start routine work at the default or a lower setting that meets your quality bar. Raise effort for a difficult reasoning task, then judge the result. If Sonnet needs its most expensive setting and still requires extensive correction, compare it with Opus instead of assuming the lower-priced model must be economical.

In the Claude app, thinking cannot be turned off for either 5.5 model. On the API, Opus thinking is always on. Sonnet permits between_tools at High effort or below: this disables thinking before the first response while retaining thinking between tool calls. It is not equivalent to disabling every form of reasoning. Thinking controls, Sonnet API settings

For API evaluations, set output_config.effort explicitly and provide enough max_tokens, the cap covering thinking plus response text. Otherwise a comparison may pit different default effort levels against each other or truncate one model before it can finish. Effort and output budgeting

Claude Opus vs Sonnet Pricing: When the Cheaper Rate Wins

Sonnet wins on equal token usage. Opus becomes cheaper only if its total billable workload falls far enough, or more valuable if it reduces expensive human correction.

The standard input and output rates in the top table were rechecked on claude.com/pricing on 18 October 2026. Prompt-cache reads, which reuse previously processed input, are $0.10 per million tokens for Sonnet and $0.20 for Opus. Five-minute cache writes are $2.50 and $5, respectively. These are API rates; they are not a conversion table for subscription allowances. Anthropic pricing

Here is a normalized example calculated for this comparison. Assume each attempt uses 100,000 uncached input tokens and 10,000 billed output tokens, including any billed thinking. Exclude tools, caching, Batch discounts, Fast mode, and residency surcharges. Sonnet costs $0.30 per attempt; Opus costs $0.60.

Two identical Sonnet attempts therefore equal one Opus attempt at $0.60. Three Sonnet attempts cost $0.90. This is a modeled crossover using published rates, not a claim that either model usually needs a particular number of attempts. The values are calculated as input millions multiplied by the input rate, plus output millions multiplied by the output rate. Rate inputs

With the same mix of token types, Sonnet's bill must involve more than twice Opus's billable workload before Opus becomes cheaper on tokens alone. Real runs change the mix: they may reread cached material, generate different amounts of thinking, and call different tools. Record the whole completed job rather than extrapolating from one response.

Opus vs Sonnet Token Usage

There is no reliable universal multiplier for how many tokens your job will use on each model. Anthropic says Sonnet complements Opus best at lower effort settings and can approach comparable performance at similar cost at higher settings. That supports testing the model and effort together, not promising that every Sonnet task costs half as much. Anthropic's cost-versus-effort discussion

For a solo builder, include review time in the decision even if API charges are small. For a high-volume product, evaluate cost per accepted result, counting failures and retries. Keep those measurements separate from subscription limits: an API price ratio does not establish a message allowance ratio.

Which Claude Plans Can Use Each Model?

Sonnet is available on Free; accessing both models starts with a paid Claude plan. Anthropic lists Opus and Sonnet on Pro, Max, Team, and Enterprise. Pro is $20 billed monthly, with Claude Code included. Enterprise administrators may restrict available models or effort levels. Plans and access, administrator restrictions

For subscription limits, Anthropic's help center describes a session limit that resets every five hours and weekly limits on paid plans. Usage across Claude's app surfaces and Claude Code draws on the same allowance. Consumption depends on conversation length and complexity, features, model, and effort. Usage-based Enterprise plans are billed by consumption. Anthropic's usage-limit explanation

The help center explicitly says higher effort uses more tokens and can bring you to a limit faster. It does not provide a fixed Sonnet-to-Opus allowance conversion in that guidance. Do not turn the API's price ratio into a promise of twice as many subscription messages. Effort and usage

For someone deciding whether to upgrade, first identify the constraint: model access, session allowance, or output quality. Moving to a higher plan addresses access or capacity; selecting a different model addresses a different problem.

Switching Between Opus and Sonnet

Change your default at a task boundary, and make the handoff explicit when a job spans models. In the Claude app, changing the model applies from the next response. In Claude Code, the model picker can save a new default or make a session-only change. App controls, Code controls

The important API limitation is that conversation history and reusable thinking are not the same thing. Current Anthropic documentation says Opus 5.5 can read Sonnet 5.5 thinking blocks on the Claude API and Google Cloud. Sonnet 5.5 cannot read Opus 5.5 thinking blocks. Incompatible blocks are dropped for the request without an error. Keep the original history rather than deleting blocks yourself. Sonnet's thinking blocks are also tied to the account that produced them. Preserved-thinking rules

That asymmetry makes an explicit written plan valuable. When Opus designs a change and Sonnet implements it, pass the decisions, constraints, evidence, and acceptance checks in ordinary text. Do not assume the cheaper model inherits the previous model's internal reasoning. These provider-specific rules should not be generalized into a claim about every Claude app conversation.

An API migration from older models also needs a compatibility check. Both 5.5 models reject forced tool use; an integration that assumes it can mandate a particular tool must adapt. Progress text between tool calls can also move into thinking blocks and be hidden at the default display setting. Those are workflow changes, not defects that a larger context window fixes. Opus API changes, Sonnet API changes

Do not switch a successful routine workflow merely because Opus is positioned higher. Keep Sonnet when its results pass your checks. Equally, do not move an effective Opus investigation to Sonnet midstream just to lower the next request's rate. Compare the cost and quality of the finished work.

Claude Opus vs Sonnet vs Haiku

Claude Haiku 5.5, Anthropic's model for high-volume, cost-sensitive work, deserves a look for bounded extraction, classification, and summaries; see our Haiku 5.5 guide and Claude Haiku vs Sonnet comparison. Anthropic's Haiku positioning

Claude Fable 5.1 is Anthropic's option for ambitious, long-running projects beyond this everyday default decision; our Fable 5.1 explainer covers that separate choice. Anthropic's Fable positioning

Frequently Asked Questions

Set Your Default This Week

Use Sonnet as the baseline for routine work, then identify the specific jobs that justify Opus. A founder can use a customer update, an operator a recurring report, and a builder a small bug fix. Add one ambiguous task from your real queue so the comparison includes the work where judgment matters.

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