Claude Opus vs Sonnet (5.5): Pricing, Benchmarks & Which to Use

Claude Opus 5.5 runs $4 per million input tokens and $20 per million output. Claude Sonnet 5.5, released September 28, 2026, runs $2 and $10, the same price Sonnet 5 carried. That works out to exactly 2x on both sides of the meter, and it makes the arithmetic easy: every uncached token you route to Opus instead of Sonnet costs twice as much, whatever the prompt length.

The per-token ratio is the only part of this comparison that is genuinely settled. Anthropic's own documentation tells you to start with Opus 5.5 for most workloads and to reach for Fable 5.1 when your evals on Opus at higher effort still fall short. That runs close to the opposite of the more common advice, which is to default to Sonnet and escalate.

Both defaults are defensible, but cost-per-task data complicates the case for starting on Sonnet. On the Artificial Analysis Intelligence Index, Sonnet 5.5 costs 2.2x what Opus 5.5 does to reach the same score, because it has to run at a higher effort level and spend more tokens to get there.

Per token, Sonnet is the cheaper model. Per task at a given level of quality, Opus often is, so in practice Claude Opus vs Sonnet comes down to which steps in your pipeline actually need that quality.

TL;DR: When to Pick Opus vs Sonnet

Anthropic's guidance points at Opus 5.5 as the starting point, so start there for open-ended work where a wrong step is expensive to reverse, and move steps down to Sonnet 5.5 when they have a clear specification, run at high volume, or have someone waiting on the response.

Opus costs 2x Sonnet per token, but per task the ratio runs the other way. On the Artificial Analysis Intelligence Index, Opus 5.5 reaches the same score as Sonnet 5.5 for less than half the cost, because Sonnet needs a higher effort level to get there. The quality gap itself is small, with Sonnet trailing Opus by a narrow margin on most of Anthropic's benchmarks and scoring higher on Terminal-Bench.

Claude Opus vs Sonnet: Pricing and Specs

Opus 5.5 costs 2x Sonnet 5.5 per token, but per task the ratio runs the other way. On the Artificial Analysis Intelligence Index, Opus 5.5 at xhigh effort and Sonnet 5.5 at max reach the same score, and the Opus run costs $3.46 per task against $7.60 for Sonnet. At every score both models reach, Opus gets there for less, and Sonnet only comes out cheaper at the bottom of the curve, where its low setting costs less per task than any Opus setting.

Anthropic's current lineup runs four tiers, and Opus is no longer the top of it. Rates and specs below are from the Claude pricing page and the models overview.

Opus 5.5 and Sonnet 5.5 share the same limits. Both carry a 1M-token context window at standard pricing with no beta header, and both cap output at 128K tokens in a single request.

Where they differ is thinking. Opus 5.5 thinks on every request and defaults to medium effort, while Sonnet 5.5 defaults to high and can drop up-front thinking with thinking: {"type": "between_tools"}. Neither accepts thinking: {"type": "disabled"}, which returns a 400 on both, so effort is the setting that decides how many tokens each tier actually spends.

That is also the arithmetic behind the chart. Because Sonnet's input and output rates are both exactly half of Opus's, the only way Sonnet can cost 2.2x per task is by spending several times as many tokens on it, which is what running at a higher effort level does.

Legacy rates and what is actually still callable

The older versions are a mixed bag, and their status is easy to get wrong. Every Opus from 4.5 up and every Sonnet from 4.5 up are still Active on Anthropic's deprecations page, not legacy, with retirement dates that mostly sit in 2027.

A few rows carry near-term pressure. Opus 4.1 is already gone from the Claude API, Sonnet 4.5 was deprecated on September 30, 2026 and retires on November 30, 2026, and Opus 4.5's retirement window opens on November 24. Anthropic commits to at least 60 days' notice before retiring a publicly released model, so nothing disappears without warning, but a "not sooner than" date is a floor rather than a promise.

The pricing detail that matters most in this table is that every 5-series model is cheaper or equal to its predecessor. Sonnet 5.5 and Sonnet 5 at $2/$10 undercut Sonnet 4.6 and Sonnet 4.5 at $3/$15, and Opus 5.5 at $4/$20 undercuts every Opus back to 4.5 at $5/$25. Staying on an older version to save money does not work in either tier.

The tokenizer changes the ratio on cross-generation comparisons

Anthropic puts the boundary at Sonnet 4.6. Models from Claude 4.7 onward use a newer tokenizer that produces roughly 30% more tokens for the same text, and Sonnet 4.6 and earlier use the previous one. The exact increase depends on the content.

This does not touch the headline comparison, because Opus 5.5 and Sonnet 5.5 both sit on the newer tokenizer and the 2x holds. However, it matters a great deal if you are budgeting a move from Sonnet 4.6 up to any 5-series model, since the list price understates what the same corpus of text will cost you once it is tokenized differently. Measure the token count on the model you plan to deploy rather than the one you are leaving.

Batch keeps the 2x, caching narrows it

The Batch API takes 50% off input and output on every model, so Opus 5.5 drops to $2/$10 and Sonnet 5.5 to $1/$5, and the 2x stays intact.

Caching is where the two tiers converge. Cache reads on Opus 5.5 are priced at 0.05x base input rather than the standard 0.1x, which puts them at $0.20 per MTok, the same as Sonnet 5.5. Cache writes still follow base input, at 1.25x for the five-minute TTL and 2x for the one-hour TTL, so a five-minute write costs $5 per MTok on Opus 5.5 against $2.50 on Sonnet 5.5.

As a result, the more of a workload's input comes from cache hits, the closer its Opus bill gets to its Sonnet bill on the input side, and the 2x only applies in full to uncached input and to output. Opus 5 did not work this way, since its cache reads follow the standard 0.1x at $0.50 per MTok. The cache pricing math and TTL breakpoints get more involved once you are choosing between the two durations, and the Message Batches API guide covers the 24-hour completion window that comes with the batch discount.

One asymmetry runs the other way. Fast mode, in research preview, delivers significantly faster output for Opus 5.5 at $8/$40 and for Opus 5 and Opus 4.8 at $10/$50, and it has no Sonnet equivalent. It is only available on the first-party Claude API. Opus 4.7 returns an error on speed: "fast", and Opus 4.6 accepts the parameter but runs at standard speed and standard rates, which is the kind of silent no-op worth catching before it ends up in a benchmark.

Claude Sonnet 5.5

Claude Sonnet 5.5 succeeds Sonnet 5 as the current model in the Sonnet tier. Most of what changed is in how it thinks and how fast it answers, while the price stayed where it was.

Sonnet 5.5 Release Date and Availability

Anthropic released Sonnet 5.5 on September 28, 2026, across the Claude API, the Claude apps, Amazon Web Services, Google Cloud, and Microsoft Azure, according to its launch announcement. The API ID is claude-sonnet-5-5, and its tentative retirement date is not sooner than September 28, 2027.

It keeps the 1M-token context window and 128K output cap from Sonnet 5, and its reliable knowledge cutoff moves up to June 2026, matching Opus 5.5.

Sonnet 5.5 Pricing

Sonnet 5.5 costs $2 per million input tokens and $10 per million output, and Anthropic's Sonnet 5.5 release notes confirm the same prices as Sonnet 5, including prompt caching and batch processing. Cache reads run $0.20 per MTok, five-minute cache writes $2.50, and batch requests $1/$5.

The $2/$10 rate is worth a note because it moved. It launched on Sonnet 5 as introductory pricing scheduled to expire August 31, 2026, and Anthropic has since confirmed it as the standard price, cancelling the planned increase to $3/$15. Sonnet 5.5 launched at that same standard rate, so if you sized a budget against the increase, the money is still there.

What Changed From Sonnet 5

Anthropic says Sonnet 5.5 generates output 30%+ faster than Sonnet 5 and costs up to 30% less per task at the same per-token price, which points at fewer tokens per task rather than cheaper tokens. On Anthropic's benchmarks, the gains over Sonnet 5 are largest in agentic terminal work, where Terminal-Bench 4.0 goes from 10.3% to 70.6%, and in computer use, where OSWorld 2.1 goes from 57.0% to 80.1%.

The changes that need attention before you switch a workload over are in the API rather than the benchmarks, per the migration notes:

• Effort is recalibrated - The same effort level does not produce the same amount of thinking it did on Sonnet 5, so settings carried over from Sonnet 5 need a fresh sweep against your evals.
• Thinking can't be disabled - thinking: {"type": "disabled"} returns a 400. The lowest setting is between_tools, which skips up-front thinking.
• Forced tool use is not supported - tool_choice set to any or to a named tool returns a 400.
• Computer use needs the new toolset - On the Claude API and Google Cloud, only computer_toolset_20260801 works, and the earlier computer_20251124 tool returns a 400.

Of these, forced tool use is the one to search your codebase for first, because any request that forces a tool call will fail on Sonnet 5.5 until it is rewritten.

Sonnet 5.5 vs Opus 5.5

On list price, Opus 5.5 is exactly 2x Sonnet 5.5 on uncached input and output. On Anthropic's own benchmarks, the gap between them is much narrower than that.

The scores below are from Anthropic's Sonnet 5.5 announcement, with Sonnet 5 included for reference.

Opus 5.5's Terminal-Bench score was run at xhigh effort. The Sonnet 5.5 FrontierCode figure is also at xhigh, and Anthropic reports a lower 46.2% at max, which it attributes to timeouts and scope. Anthropic also expects the GDPval-AA and AA-Briefcase scores to be slightly understated because of a pre-release bug affecting structured outputs.

Sonnet 5.5 trails Opus 5.5 by a small margin on most rows and scores higher on Terminal-Bench. Half the per-token price makes that margin look cheap to give up, but closing it takes Sonnet a higher effort level, which is where the per-task cost in the pricing section's chart comes from. Anthropic's positioning follows the same split, describing Opus 5.5 as built for "complex work requiring careful judgment" and Sonnet 5.5 as "strongest at well-scoped everyday tasks, fixing bugs, and creating polished documents, slides, and spreadsheets."

The per-token ratio is fixed, but the per-task ratio is not, because the two models spend tokens differently by default. Opus 5.5 thinks on every request at medium effort, while Sonnet 5.5 defaults to high, and Anthropic's guidance suggests starting Sonnet 5.5 at medium for well-specified agentic coding and at medium or low for chat and latency-sensitive work. Running both at their defaults means the bill ratio you see can drift well away from 2x, and on the Intelligence Index chart in the pricing section it inverts at matched scores.

Thinking tokens bill as output, so what thinking adds to an Opus 5.5 bill moves with the effort setting, and the comparison worth running is cost per completed task at the effort level you plan to ship. If you plan to move a conversation between the two tiers, Sonnet 5.5 cannot read thinking blocks produced by Opus 5.5, so an Opus-to-Sonnet handoff mid-thread is worth testing before it ships.

When to use Opus vs Sonnet

The docs are direct about the starting point: if you are unsure which model to use, start with Opus 5.5 for most workloads. Fable 5.1 is positioned for demanding reasoning and long-horizon agentic work, or for when evals on Opus 5.5 at higher effort still fall short.

The common alternative runs the other way, defaulting to Sonnet and escalating to Opus only when Sonnet visibly fails. That advice is not wrong, and it comes from a real place, since the cheaper tier clears the bar on a large share of production work and the savings compound at volume. The two defaults conflict because they optimize different things. Anthropic's guidance minimizes the chance you ship something worse than you could have, while the escalate-from-Sonnet guidance minimizes the per-token rate, which stops being the same as minimizing spend once Sonnet has to run at a higher effort level to match Opus.

Four things decide which of those you should be optimizing for:

• Task shape - Well-specified work is where the tiers converge. When the input is clear, the expected output is clear, and correctness is checkable, you are asking the model to generate rather than to decide, and the extra reasoning headroom has less to work with. Open-ended work is where they separate, because the model has to infer what the right problem is before solving it. The signal to watch for is iteration count: if you keep re-prompting the same request because the output misses something that required judgment rather than execution, that is the ceiling talking rather than the prompt.
• Error cost - The 2x per-token gap is fixed and small in absolute terms on any single request. The cost of a wrong answer is neither. A wrong answer that a test catches or a reviewer sends back costs you a retry. A wrong answer that reaches a customer, mutates a record, or ships into a release costs something else entirely, and the gap between $0.02 and $0.04 on the call that produced it stops being the interesting number. Because of that, the tier decision tends to follow the blast radius of the step rather than its difficulty.
• Latency budget - Anthropic rates Opus 5.5 at moderate comparative latency and Sonnet 5.5 as fast, with Haiku 5.5 fastest and Fable 5.1 slower than all of them. For anything running asynchronously that ordering rarely changes a decision. It changes decisions for interactive work, where time to first token is what people actually perceive as speed in a streaming interface. High-concurrency pipelines feel it differently, since a slower per-call model stretches the tail latency of the whole batch.
• Effort floor - Opus 5.5 thinks on every request, so low effort is as far down as its thinking goes. Sonnet 5.5 can drop below that to between_tools, which skips up-front thinking entirely. The bottom of the effort range is where Sonnet's half-price tokens actually turn into a lower bill, so high-volume steps that clear the bar at low effort are the ones that belong on Sonnet.

The error-cost point is also the argument for splitting a pipeline rather than picking one model for it. Planning steps and irreversible actions can sit on Opus while the bounded, checkable steps around them sit on Sonnet or Haiku, so the bill tracks the shape of the work instead of the ceiling of it.

What none of this gives you is a number for your workload. Anthropic publishes prices, context limits, output ceilings, latency ordering, and benchmark scores, and the Intelligence Index puts the per-task ratio at 2.2x in Opus's favor at matched scores, but both measure someone else's task sets rather than your requests. The per-token ratio is known to two decimal places and the quality ratio stays unknown until someone measures it on your own traffic, which means the honest answer to "which tier do I need" is a number you produce rather than one you look up.

Claude Sonnet vs Opus vs Haiku

Moving between Opus 5.5 and Sonnet 5.5 changes how much reasoning you are paying for and how the model spends it. The limits are identical, so a workload that fits one fits the other.

Haiku 5.5 now shares those limits too, with a 1M-token context window, a 128K output ceiling, adaptive thinking, and the effort parameter, defaulting to medium like Opus. A workload no longer has to be trimmed to fit before it can be priced against Haiku, and the latest Claude models now differ mainly in how much reasoning each call buys and what it costs.

Where Haiku 5.5 differs is pricing, since it is the only current model whose rate depends on prompt length. Up to 100K input tokens it costs a twentieth of Sonnet 5.5, but past that threshold every request pays $0.50/$2.50, even when part of the prompt is a cache hit, so a long-context step on Haiku costs a quarter of Sonnet rather than a twentieth.

Capability is the other gap. Anthropic positions Haiku 5.5 for high-volume, cost-sensitive work like summarization, compaction, and subagent tasks, and keeps Sonnet 5.5 and Opus 5.5 as the better choices for complex agentic coding, where Haiku 5.5 scores 39.2% on Terminal-Bench 4.0 against 70.6% for Sonnet 5.5. That makes it the right home for bounded steps that clear your evals, rather than a cheaper drop-in for Sonnet.

Claude Model Version Comparisons

Claude Opus 5 vs Sonnet 5

This is the previous-generation pairing, and both models remain Active. The specs are identical apart from price: both run 1M context, 128K max output, adaptive thinking, and a default effort of high. The API IDs are claude-opus-5 and claude-sonnet-5.

Opus 5 costs $5/$25 against Sonnet 5's $2/$10, exactly 2.5x on both meters, and its cache reads follow the standard 0.1x at $0.50 per MTok, so caching does not narrow the gap the way it does on the 5.5 pair. Opus 5 also supports Fast mode at $10/$50, in research preview, where Sonnet 5 has no equivalent. Anyone still on Opus 5 is paying more than Opus 5.5 costs on input, output, and cache reads.

Claude Opus 5 vs Fable 5

Fable 5.1 is Anthropic's frontier tier, and it sits above Opus at $10 input and $50 output per MTok, double Opus 5 on both. Fable 5 remains available at the same rates, so the version choice inside the tier is about the cache read price rather than the base rate: cache hits cost 0.025x base input on Fable 5.1 against the standard 0.1x on Fable 5, which is $0.25 per MTok versus $1.

The specs otherwise track Opus 5 closely, with 1M context and 128K max output on both. The differences are that Fable 5.1 runs adaptive thinking always on rather than optionally, Anthropic rates its comparative latency as slower than Opus, and its reliable knowledge cutoff is June 2026 against Opus 5's May 2026.

Anthropic frames the escalation condition narrowly, which is useful. Fable is positioned for demanding reasoning and long-horizon agentic work, or for when your evals on Opus 5.5 at higher effort still come up short. That second clause is doing real work: raising effort on Opus is the cheaper move and it should be exhausted before doubling the rate. Mythos 5.1 is the same rate and limited availability, so it is not a routing option for most teams.

Claude Sonnet 5 vs Opus 4.8

This pairing comes up because Opus 4.8 was the flagship immediately before Opus 5, and plenty of production systems are still pointed at it. Both models sit on the newer tokenizer, so a token-count comparison between them is apples to apples.

Opus 4.8 costs $5/$25 against Sonnet 5's $2/$10, the same 2.5x ratio that Opus 5 carries, and it supports Fast mode at $10/$50. It remains Active with a retirement date not sooner than May 28, 2027, so there is no urgency to move.

The reason to move anyway is that Opus 5.5 costs less than Opus 4.8, at $4/$20 against $5/$25. If you are already paying flagship rates, staying on 4.8 costs you money, and Anthropic publishes a migration guide for the jump. The live question is usually whether the workload needs a flagship at all, in which case Sonnet 5.5 at half the Opus 5.5 per-token rate is the comparison that matters.

Claude Sonnet 4.6 vs Opus 4.6

Both are still Active, with retirements not sooner than February 2027, and both sit on the older tokenizer, which makes this the one pairing in this section where cross-generation token inflation is not a factor.

Opus 4.6 runs $5/$25 against Sonnet 4.6's $3/$15, a ratio of roughly 1.67x rather than the 2x that the 5.5 pair carries. Both include the full 1M context window at standard pricing. Fast mode is not available on either, and Opus 4.6 will accept speed: "fast" without applying it or charging the premium.

Anyone weighing this pair should weigh Sonnet 5.5 alongside it, since Sonnet 5.5 costs less than Sonnet 4.6 at $2/$10. Be aware that the tokenizer change means a straight rate comparison flatters the newer model, so measure your own token counts before assuming the saving is the full third that the list prices suggest.

Claude Opus 4.1 vs Sonnet 4.5

Opus 4.1 no longer runs on the Claude API. Anthropic deprecated it on June 5, 2026 and retired it on August 5, 2026, and requests to it now fail. It survives on Amazon Bedrock and Google Cloud, which set their own retirement schedules, so a workload calling it through Bedrock rather than the first-party API may still be running.

Sonnet 4.5 is still Active, but it comes with two constraints worth knowing. Its context window is 200K, not 1M: the context-1m-2025-08-07 beta was retired for Sonnet 4.5 and Sonnet 4 on April 30, 2026, and requests above 200K now return an error. Sonnet 4.5 itself was deprecated on September 30, 2026 and retires on November 30, 2026.

For anyone on either model, Anthropic's recommended replacement for Opus 4.1 is Opus 4.8, though Opus 5.5 costs less and is the better target. Sonnet 4.5 should move to Sonnet 5.5, which is cheaper at $2/$10 and carries the 1M window that Sonnet 4.5 lost. One migration detail catches people: temperature, top_p, and top_k return a 400 error on Claude 4.7 and later when set to a non-default value, and the Python SDK from v1.0 removes them outright, so passing them raises a TypeError. Prompt for the behavior instead.

How to Route, Observe, and Evaluate Every Claude Call

Anthropic publishes the price ratio and leaves the quality ratio to you. Respan's AI evaluation platform is where you produce that second number, on your own traffic, next to what each tier costs to run.

• Measure quality with scores, not vibes - An LLM judge, a deterministic code check, or a human reviewer compose into one evaluator that turns any output into a single score, so "Opus felt better" becomes a number you can defend in a budget conversation.
• Run both models against the same dataset - Build a test set from real production requests by filter and sampling rate, then run Opus and Sonnet through your evaluators at scale. Experiments return per-row and average scores with side-by-side distributions, and any row clicks straight into the full trace behind it.
• Keep scoring after you decide - Deploy the same evaluator on live production spans, filtered by status, customer, or thread and sampled to control cost, so a tier that quietly stops clearing the bar after a model update surfaces as a score change instead of a support ticket.
• Put cost next to quality - Every request is logged as a span automatically with latency and cost attached, broken down by model, by request, and by end customer. The quality delta and the cost of each run sit in the same platform rather than in two different tabs, and soft and hard spend caps stop a runaway Opus experiment at a ceiling you set.
• Switch models by changing a single word - One endpoint reaches 1,000+ models across every major provider, so moving a workload from claude-sonnet-5-5 to claude-opus-5-5 is a config change rather than a deploy. Setup is OpenAI-compatible and the gateway typically adds 30–40ms of latency.
• Stay up when a provider does not - Set a fallback chain once, at org level or per request, and traffic reroutes automatically when a model errors or rate-limits. Failed requests retry on your rules, and load balancing distributes across providers, which keeps a two-tier setup from becoming a single point of failure.

Octolane AI cut its LLM debugging time by 90% with Respan, and its CEO described model switching based on use case as "a breeze."

Frequently Asked Questions

What is the difference between Opus and Sonnet?

Opus 5.5 sits one tier above Sonnet 5.5 in Anthropic's lineup and costs 2x per token. It also thinks on every request, while Sonnet 5.5 can skip up-front thinking and defaults to a higher effort level. Both share a 1M-token context window and a 128K output cap, and Anthropic positions Opus 5.5 for "complex work requiring careful judgment" and Sonnet 5.5 for "well-scoped everyday tasks."

How much worse is Sonnet than Opus?

Not by much on Anthropic's benchmarks. Sonnet 5.5 trails Opus 5.5 by roughly 2 to 3 points on most rows, such as 52.1% vs 54.4% on FrontierCode and 80.1% vs 81.8% on OSWorld, and it scores higher on Terminal-Bench at 70.6% vs 66.4%. Closing that gap takes Sonnet a higher effort level, which is why it can cost more per task than Opus despite the lower per-token price.

Should I use Opus or Sonnet for coding?

It depends on what the coding task asks the model to decide. Sonnet 5.5 handles work with a clear specification and a checkable result well, and it costs half of Opus 5.5 per token, which matters when a coding agent fires many calls per task. Opus 5.5 earns the difference on architecture decisions, comprehension across a large codebase, and anything where a wrong change is expensive to unwind.

Anthropic positions Opus 5.5 for long-running agentic coding and knowledge work, and its guidance is to start there rather than escalate to it. The reliable way to settle it is to run both tiers against a dataset of your own coding requests, score the output, and compare the score gap to the cost per task on each. Respan runs both sides of that comparison in one place.

How much more does Opus cost than Sonnet?

Exactly 2x on uncached input and output. Opus 5.5 is $4 per million input tokens and $20 per million output; Sonnet 5.5 is $2 and $10. Batch processing takes 50% off both and keeps the 2x, but cache reads cost $0.20 per MTok on both models, so the more of your input comes from cache, the smaller the gap gets. Per task, the ratio can invert: on the Artificial Analysis Intelligence Index, Sonnet 5.5 at max effort costs 2.2x what Opus 5.5 at xhigh does to reach the same score.

How much does Sonnet 5.5 cost?

Claude Sonnet 5.5 costs $2 per million input tokens and $10 per million output, the same as Sonnet 5. Batch requests run $1/$5, cache reads $0.20 per MTok, and five-minute cache writes $2.50. At those rates it is half the price of Opus 5.5 on uncached input and output, though that advantage narrows or reverses per task whenever Sonnet needs a higher effort level to match Opus's output.

When was Sonnet 5.5 released?

Anthropic released Claude Sonnet 5.5 on September 28, 2026. It is available through the Claude API as claude-sonnet-5-5, in the Claude apps, and on Amazon Web Services, Google Cloud, and Microsoft Azure.

Does Opus have a longer context window than Sonnet?

No. Opus 5.5 and Sonnet 5.5 both carry a 1M-token context window at standard pricing with no beta header, and both cap a single response at 128K output tokens. The same holds across the current lineup, since Fable 5.1 and Haiku 5.5 share those limits, though Haiku 5.5 charges a higher rate once a prompt passes 100K tokens. Among older versions, Haiku 4.5 is limited to 200K context and 64K output, and Sonnet 4.5 to 200K after Anthropic retired its 1M beta in April 2026.

Is there a model above Opus?

Yes. Claude Fable 5.1 sits above Opus 5.5 at $10 input and $50 output per MTok, 2.5x Opus 5.5 on both meters, and Anthropic positions it for demanding reasoning and long-horizon agentic work. Its documented escalation condition is that your evals on Opus 5.5 at higher effort still fall short, so raising effort on Opus is the cheaper step to try first. Claude Mythos 5.1 shares the same rates with limited availability.

Should I use Opus or Sonnet for agents?

Agent workloads are where a single-tier answer serves you worst, because one run mixes steps with very different requirements. Planning, ambiguous decisions, and actions that cannot be undone are the steps that justify Opus 5.5. Tool result summarization, formatting, and bounded lookups usually clear on Sonnet 5.5 or Haiku 5.5.

The number to watch is cost per completed task rather than cost per call, since a cheaper model that needs extra turns can erase its own advantage. Tracing every call in an agent run as a span with cost attached is how that number becomes visible, which is what Respan's agent observability is built to do.

Can I use Opus and Sonnet in the same application?

Yes. Anthropic exposes both through the same API, so the model string is the main difference between a call to one and a call to the other. On the 5.5 pair, Sonnet rejects forced tool use and cannot read thinking blocks produced by Opus, so a conversation that moves from Opus to Sonnet mid-thread should be tested before it ships. Running them behind one gateway endpoint makes the switch a configuration change, keeps cost and latency for both in the same set of traces, and gives you fallback chains if one provider errors or rate-limits. Respan handles the routing, the logging, and the evaluation in a single platform, and it is free to start.