Claude Sonnet 5.5 vs Opus 5.5 (2026): Is Half the Price Enough?

Anthropic shipped Claude Opus 5.5 on September 22, 2026 and followed it six days later, on September 28, with Claude Sonnet 5.5, which it calls "a faster, lower-cost complement to Claude Opus 5.5." Both models share a 1M-token context window, 128K max output, a June 2026 knowledge cutoff and the same five effort levels. The only headline difference on the price sheet is that Sonnet costs exactly half as much per token.

That makes the question simple to ask and harder to answer: is half the price enough? This comparison puts Anthropic's own benchmark table next to Artificial Analysis's independent numbers, works through what a task actually costs once you account for token usage (the part most comparisons skip), and ends with a task-by-task table of when to pick Sonnet and when to pay for Opus. If you want the full background on either model first, see our Claude Opus 5.5 complete guide and Claude Sonnet 5.5 complete guide.

What is the difference between Claude Sonnet 5.5 and Opus 5.5?

On paper, very little apart from price and speed. Anthropic's models overview lists them side by side with identical context, output limits and knowledge cutoff. The practical differences are positioning, default effort, latency and how many tokens each model spends to finish a job.

Anthropic's own framing is clear: Sonnet 5.5 is "strongest at well-scoped everyday tasks, fixing bugs, and creating polished documents, slides, and spreadsheets," while "Opus 5.5 remains clearly stronger at complex, open-ended work requiring sustained judgment." Anthropic's docs still say to start with Opus 5.5 for most workloads when you are unsure.

Two details are worth noticing. First, cache reads cost the same $0.20 per million on both, because Opus 5.5 gets a steeper cache discount (5% of input instead of 10%). If your workload is dominated by cached context, such as a coding agent re-reading a large repo, the price gap on that portion disappears. Second, the API default effort differs: a request with no effort setting runs Sonnet at high and Opus at medium, so an untuned side-by-side test is not apples to apples.

Sonnet 5.5 vs Opus 5.5 benchmarks: how close is it?

Anthropic published both models in one table in the Sonnet 5.5 launch post. Opus leads on seven of the eight rows, but by small margins, and Sonnet wins Terminal-Bench 4.0.

*Anthropic's footnote: the Opus 5.5 Terminal-Bench figure was run at xhigh effort, not the same setting as Sonnet, so treat that row with care. FrontierCode is the one coding benchmark with a real gap, and it is also the one closest to hard, open-ended engineering work. Note that Sonnet's 46.2% is its max-effort score; Anthropic reports 52.1% at xhigh, which shrinks the best-vs-best gap to 2.3 points.

What does independent testing say?

Artificial Analysis (AA) tested both at max effort. Its numbers broadly agree with Anthropic's, and add the cost and accuracy data that vendor tables leave out.

The interesting split is on knowledge versus honesty. Opus knows more (66% vs 54% factual accuracy), but Sonnet is more willing to say it does not know, with a lower hallucination rate (47% vs 59%). For retrieval-backed apps where the facts come from your own documents, Sonnet's profile is arguably the safer one. For questions answered from the model's own memory, Opus is better.

How much does Sonnet 5.5 really save per task?

This is where "half the price" gets complicated. Price per token is half. Tokens per task are not the same. AA measured Sonnet 5.5 at roughly 193K output tokens per Intelligence Index task, about 60% more than Opus 5.5 at max effort (roughly 119K).

Worked example 1: AA's measured numbers at max effort

• Output only. Sonnet: 193K x $10/M = $1.93. Opus: 119K x $20/M = $2.38. Sonnet is about 19% cheaper on output, not 50%.
• Whole task, including input. AA's measured all-in cost is $7.60 for Sonnet and $5.98 for Opus. At max effort on agentic tasks, Sonnet was about 27% more expensive per task. Longer runs mean more turns, and every turn re-sends context as input.

So at max effort, Sonnet 5.5 is not the budget option. Its advantage shows up at lower effort and on shorter, well-scoped jobs.

Worked example 2: a typical coding-agent bug fix

Assume an Opus 5.5 run that reads 400K input tokens (300K served from cache, 100K fresh) and writes 40K output tokens. Cache-write charges are left out to keep the arithmetic readable.

The realistic saving sits somewhere between 16% and 48% depending on how much extra work Sonnet does on your tasks, and it can flip negative at max effort, as AA's figures show. The rule of thumb: budget for Sonnet at roughly 55-85% of Opus cost, not 50%, and measure on your own traces before committing. Sonnet also finishes faster in wall-clock terms per token (139 vs 92 tokens/s), which partly offsets the longer runs.

One more caveat on the cost claims circulating online: Anthropic's "up to 30% less per task" and "30%+ faster" figures compare Sonnet 5.5 with its predecessor Sonnet 5, not with Opus 5.5. Some coverage has blurred that. For the older generation's trade-off, see our Claude Opus 5 vs Sonnet 5 comparison and the Sonnet 5 launch guide.

Claude Sonnet 5.5 vs Opus 5.5 for coding: which is better?

For day-to-day coding, the benchmarks say Sonnet 5.5 is close enough. It is 2.3 points behind on CursorBench 4.0 and ahead on Terminal-Bench 4.0 in both Anthropic's and AA's runs. For open-ended engineering, Opus pulls ahead: the 8.2-point FrontierCode gap (Sonnet at max effort) is the largest in Anthropic's table, though Sonnet's xhigh score of 52.1% narrows it to 2.3.

CodeRabbit's independent code-review test lines up with that. On its 13-case Signal benchmark of known issues:

• Sonnet 5.5 caught 6 of 13 (46.2%), with 41.2% actionable precision, at about $0.46-0.47 per review and a 5:27 mean review time.
• Opus 5.5 at standard effort caught 8 of 13 (61.5%) with 66.7% precision.
• Opus 5.5 at max effort caught 10 of 13 (76.9%) with 52.0% precision. Opus lists at twice Sonnet's per-token price.

CodeRabbit's verdict was that Sonnet 5.5 is fit for routine PR review but "not an Opus 5.5 replacement," with Opus kept for high-risk changes. Thirteen cases is a small sample, so read it as directional.

Sonnet 5.5 vs Opus 5.5 in Claude Code

Claude Code 2.1.280 made Opus 5.5 the default Opus model, and 2.1.284 made Sonnet 5.5 the default Sonnet model on the Anthropic API. Both default to medium effort in Claude Code. Switching is one command:

# start a session on either model
claude --model sonnet
claude --model opus

# switch mid-session, and set effort
/model sonnet
/effort high

# plan with Opus, execute with Sonnet
claude --model opusplan

The opusplan alias is the most practical answer to this whole comparison for many teams: Opus handles plan mode and architecture decisions, then Claude Code switches to Sonnet for code generation. One caution from the docs: the sonnet alias only resolves to Sonnet 5.5 on the Anthropic API. On Bedrock and Google Cloud it currently resolves to Sonnet 4.5, and on Claude Platform on AWS to Sonnet 4.6, so pin the full model ID there.

How to call Sonnet 5.5 and Opus 5.5 via the API

Both use the Messages API. Effort is set with output_config.effort. Because the defaults differ, set it explicitly when you A/B the two models.

import anthropic

client = anthropic.Anthropic()

for model in ["claude-sonnet-5-5", "claude-opus-5-5"]:
    response = client.messages.create(
        model=model,
        max_tokens=4096,
        messages=[{"role": "user", "content": "Find the bug in this function: ..."}],
        output_config={"effort": "medium"},
    )
    print(model, response.usage)

Logging response.usage per model is the cheapest way to get your own version of worked example 2. Anthropic's docs recommend starting Sonnet 5.5 at medium for well-specified agentic coding and multistep tool use, high for harder or longer tasks, and medium or low for chat, and setting max_tokens to 128,000 with streaming for agentic coding. Sonnet's effort levels are recalibrated versus Sonnet 5, so do not carry old settings over. Our Opus 5.5 migration guide covers the same effort-sweep process for Opus.

Is Sonnet 5.5 good enough? Task-by-task recommendations

Verdict: when is half the price enough?

Half the price is enough when the task is well defined, you can check the result, and you run it at low, medium or high effort. That covers most everyday coding, terminal automation, routine review, document work and grounded Q&A. For those jobs, default to Sonnet 5.5 and expect to pay somewhere around 55-85% of what Opus would cost, while getting output faster.

It is not enough when the work is open-ended, when a missed issue is expensive, when the model has to answer from its own knowledge, or when you would reach for max effort anyway. At max effort Sonnet burns enough extra tokens that Opus 5.5 was both cheaper and stronger in AA's testing. For those cases pay for Opus, or use opusplan to spend Opus tokens only on the planning step. If you are also weighing OpenAI, our Sonnet 5.5 vs GPT-6.1 Sol comparison and Opus 5.5 vs GPT-6 Sol cover that side.

FAQ

Is Claude Sonnet 5.5 better than Opus 5.5?

No, not overall. Opus 5.5 leads on seven of the eight benchmarks in Anthropic's own table and scores 58 vs 56 on the Artificial Analysis Intelligence Index. Sonnet 5.5 wins on Terminal-Bench 4.0, output speed, per-token price and hallucination rate.

How much cheaper is Sonnet 5.5 than Opus 5.5?

Per token, exactly half: $2/$10 vs $4/$20 per million input/output tokens. Cache reads cost $0.20 per million on both. Per task, the saving is smaller because Sonnet used about 60% more output tokens in AA's testing, and at max effort AA measured Sonnet at $7.60 per task vs $5.98 for Opus.

Is Sonnet 5.5 good enough for coding?

For well-scoped coding, yes. It scores 55.5% on CursorBench 4.0 vs 57.8% for Opus and beats Opus on Terminal-Bench 4.0. For open-ended engineering it trails on FrontierCode 1.1 (by 8.2 points at max effort, 2.3 at xhigh), so keep Opus for architecture work and large refactors.

Which model does Claude Code use by default?

The opus alias resolves to Opus 5.5 (since Claude Code 2.1.280) and sonnet to Sonnet 5.5 on the Anthropic API (since 2.1.284). Both default to medium effort in Claude Code. The session's starting model depends on your account and settings; check it with /model.

Do Sonnet 5.5 and Opus 5.5 have the same context window?

Yes. Both have a 1M-token context window, 128K max output on the Messages API (up to 300K on the Batch API with a beta header) and a June 2026 reliable knowledge cutoff.

Is Sonnet 5.5 faster than Opus 5.5?

Yes. Artificial Analysis measured about 139 output tokens per second for Sonnet 5.5 vs about 92 for Opus 5.5, and Anthropic lists Sonnet as "Fast" and Opus as "Moderate" latency. On long agentic runs Sonnet may produce more tokens, which narrows the wall-clock gap.

Does Sonnet 5.5 hallucinate less than Opus 5.5?

In Artificial Analysis's testing, yes: a 47% hallucination rate vs 59% for Opus 5.5. Opus is more accurate on factual knowledge (66% vs 54% on AA-Omniscience), so it knows more but also guesses more when it does not know.

Sources

• Anthropic: Claude Sonnet 5.5 launch post and benchmark table
• Anthropic docs: Models overview (IDs, pricing, context, default effort)
• Anthropic docs: Effort parameter and per-model recommendations
• Claude release notes (Opus 5.5 and Sonnet 5.5 dates)
• Claude Code changelog (2.1.280, 2.1.284)
• Claude Code docs: Model configuration, aliases and effort defaults
• Artificial Analysis: Claude Sonnet 5.5 analysis
• Artificial Analysis: Claude Sonnet 5.5 model page
• Artificial Analysis: Claude Opus 5.5 model page
• Artificial Analysis: Claude Opus 5.5 takes the top spot
• CodeRabbit: Sonnet 5.5 vs Opus 5.5 code review benchmarks
• The Decoder: Sonnet 5.5 coverage

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