Claude Sonnet 5.5 vs Opus 5.5: Half the Price, Enough for Most

Claude Sonnet 5.5 and Opus 5.5 share the same 1-million-token context window, the same 128K maximum output, and the same tool calling and structured output capabilities.

Sonnet costs $2 per million input tokens. Opus costs $4. Half the price, identical specs. Here is when to use which.

At a glance

	Sonnet 5.5	Opus 5.5
Input (official)	$2	$4
Output (official)	$10	$20
Context	1M	1M
Max output	128K	128K
Protocols	Messages + Chat	Messages + Chat
Release	Sep 28	Sep 22

Beyond the price gap the two are identical on paper. The difference is at the ceiling: Opus handles complex tasks Sonnet cannot finish.

When to use Sonnet 5.5

Sonnet is the default. Most work does not need Opus:

• Daily coding and code review
• Document analysis and technical writing
• General Q&A
• Batch processing

Sonnet handles these. Opus is wasteful here.

When to switch to Opus 5.5

Long-running agent tasks. Sonnet drifts or degrades after several tool-calling rounds. Opus holds course.

Tasks where mistakes are expensive. Production code changes, security reviews, compliance checks. The model cost savings are not worth one wrong call.

Multi-step deep reasoning. Opus catches edge conditions Sonnet misses.

The simplest strategy

Default to Sonnet. Switch up only when it fails. Most of the time, it will not.

Same protocol, same parsing code, same endpoint. Changing models is editing one field.

The math

100 coding tasks per day (5K input + 3K output each)

	Sonnet 5.5	Opus 5.5
Daily	$4	$8
Monthly	$120	$240

A 90% Sonnet 10% Opus split lands between them, getting you 90% of the savings and 10% of the insurance.

On HiAPI

Opus 5.5 runs at $2.02 input on HiAPI, half the official $4. Sonnet 5.5 is coming. Swapping models is one field with one key (see the Claude Code setup guide).

FAQ

How much does Sonnet 5.5 lose to Opus 5.5 on coding?

Most coding tasks are close. The gap shows on long multi-step work with complex context.

Can I mix them?

Yes. Run 90% on Sonnet, 10% on Opus. Same parsing code, separate billing.

Does caching help?

Both support caching. Opus's cache read is 2.5% of input, making long conversations on Opus much cheaper through the cache.

Should I still use Sonnet 4.6?

No. Sonnet 5.5 is 33% cheaper and stronger. The upgrade is a one-field change.

Bottom line

Sonnet 5.5 and Opus 5.5 are identical on specs and 50% apart on price. Default to Sonnet. Escalate to Opus only when it cannot finish. That strategy saves roughly half your monthly model spend.