Claude Models Compared: Haiku vs Sonnet vs Opus 5.5 — Which Should You Use? (September 2026)

Claude comes in three model tiers — Haiku, Sonnet, and Opus. The answer to "which should I use?" is simple: Sonnet for most things, Haiku for simple/bulk work, Opus only for truly hard problems. Here's how they differ and how to choose.

Reading the model names

Each name is tier + version.

• Haiku = the lightest, fastest tier.
• Sonnet = the balanced performance/cost tier.
• Opus = the most capable tier.
• The number (4.5/5/5.5) is the version — higher is newer. As of September 27, 2026 the recommended lineup is Haiku 4.5 / Sonnet 5 / Opus 5.5 (Opus 5.5 shipped September 22; Opus 5 still works but is marked legacy in the official docs). Above these three sits a top-tier model, Fable 5.1 (the most powerful, but with higher price and latency).

How the three differ

• Haiku 4.5 — fastest and cheapest. Great for simple, high-volume work like classification, summarization, and extraction. Context (how much it handles at once) is about 200K tokens.
• Sonnet 5 — the best balance of quality and cost, so it's the recommended default for most tasks, from everyday chat to coding. Context ~1M tokens. Per Anthropic's official announcement, Sonnet 5 is a major step forward for agentic tasks — work that requires planning, tool use, and running multiple steps autonomously — and is now the default model on Free and Pro plans. API pricing is $2 per million input tokens and $10 per million output tokens — originally announced as introductory pricing through August 31, 2026, but Anthropic cancelled the planned September 1 increase to $3/$15 and made $2/$10 the standard price (official pricing docs, checked September 2, 2026).
• Opus 5.5 — the current Opus, released September 22, 2026. Anthropic says it performs "at the level of Fable 5.1 on most work", costs 40% less to run than Opus 5 on typical workloads and generates output more than 30% faster. API price $4 per million input tokens / $20 output (Opus 5: $5/$25), 1M context, thinking always on with a default effort of medium. Suited to complex reasoning, hard coding and long agentic tasks. See the Opus 5.5 guide. (Benchmark figures are Anthropic's own and vary by source and version.)

Choosing in chat (claude.ai)

When picking a model in chat, Sonnet is enough for most things. According to Anthropic's announcement, Sonnet 5 is now the default model on Free and Pro plans, so you're already using the latest Sonnet without any extra steps. Step up to Opus only when you're stuck or the problem is very complex. The free plan may limit which models you can pick; Pro/Max let you use higher tiers (plans/limits change — see the plan comparison).

Choosing in the API (developers)

In the API — the way developers connect Claude directly to apps or services — mixing models by task difficulty is the most cost-effective approach.

• Default to Sonnet 5. API model ID: claude-sonnet-5.
• Explicitly call Haiku 4.5 for routine jobs (commit messages, log summaries, simple classification). API model ID: claude-haiku-4-5.
• Reserve Opus 5.5 for the hard cases Sonnet can't solve. API model ID: claude-opus-5-5 (Opus 5 is claude-opus-5, legacy). Anthropic's own model-selection guidance is "start with Opus 5.5 for most workloads, and use Fable 5.1 for demanding reasoning and long-horizon agentic work" — follow that when quality comes first, and the Sonnet-first strategy above when cost does. The five breaking changes when moving from Opus 5 are in the Opus 5.5 guide.

Many analyses show this cuts cost significantly versus using Opus for everything. Claude Code switches models automatically by task. For pricing and keys, see the API key guide. Always confirm the exact model ID on the official models page before coding — IDs change with new releases.

Heads up when migrating to Sonnet 5: According to the official documentation, Sonnet 5 has adaptive thinking — a built-in reasoning step before answering — turned on by default. This means the same request can use more tokens (and therefore cost slightly more) than on Sonnet 4.6. If you set a max_tokens budget tuned for Sonnet 4.6, revisit it when switching to Sonnet 5 to avoid unexpected truncation.

Frequently asked questions

Which Claude model should I use?

Sonnet is the safest default for most tasks. Simple or high-volume work runs faster and cheaper on Haiku, and it's most efficient to reserve Opus for genuinely hard reasoning or complex coding. Per Anthropic's announcement, Sonnet 5 now approaches Opus 4.8's capability on agentic tasks at a lower price.

What's the difference between Opus and Sonnet?

Opus is stronger on the hardest problems but slower and more expensive. Sonnet balances performance, speed, and cost well, fitting most daily and work tasks. When the difference is small, Sonnet is often enough. As of September 2026 the API prices are Sonnet 5 $2/$10 and Opus 5.5 $4/$20 (input/output per million tokens), so Opus costs exactly twice as much; Opus 5.5 is 20% cheaper than Opus 5 but the gap to Sonnet is unchanged. Details: Opus vs Sonnet.

When should I use Haiku?

It suits classification, quick summaries/replies, and bulk repetitive processing — anywhere speed and cost matter more than deep reasoning. Its strength is handling a lot, fast and light.

Can I switch models in claude.ai?

Yes, via the model picker near the input box. The available models depend on your plan, and paid plans unlock more models and features than free.

Can I use the latest model on the free plan?

Per Anthropic's announcement, Sonnet 5 is currently the default model on the Free plan. The highest-end model and larger usage are on paid plans. The exact composition changes often, so check claude.ai. For per-plan usage limits, checking, and reset, see the usage limits guide.

One-line summary

When in doubt, use Sonnet. Haiku when you need fast/cheap, Opus when it's truly hard.

Related: Claude vs Gemini, Claude Fable 5 & Mythos 5 guide.

Disclaimer: Model lineup, versions, pricing, context, and benchmarks change often. Figures here (context 1M/200K, Sonnet 5 API $2/$10 — now the standard price, the planned Sep 1, 2026 increase to $3/$15 was cancelled, Haiku $1/$5, Opus $5/$25) are as of August 2026, and performance comparisons are general tendencies, not an absolute ranking. Check the official model page for the latest. This site is not affiliated with Anthropic.

Which Claude Model Should You Actually Use? (Decision Guide)

If you're new to Claude, the three-tier naming can feel abstract. This section turns it into a concrete checklist you can run through in 30 seconds before starting any task.

Step 1 — Describe your task in one sentence, then match it below

1. Is the output short and the task repetitive or high-volume? (e.g., classifying 500 support tickets, summarising short emails, translating a product name list)
   → Use Haiku. Fastest response, lowest cost per call.
2. Is this an everyday writing, coding, or research task? (e.g., drafting a blog post, debugging a Python function, analysing a PDF report)
   → Use Sonnet. This is the right default for the vast majority of work.
3. Is the problem genuinely hard — multi-step reasoning, ambiguous trade-offs, or requires synthesising many sources? (e.g., architectural decisions for a codebase, complex legal document review, advanced maths proofs)
   → Use Opus. Highest reasoning quality; use only when Sonnet falls short.

Confirmation signal: You picked the right tier when you're not re-running the prompt because the output was too shallow or too slow.

Step 2 — Check your plan and access

• Claude.ai chat (Free plan): Check the model selector in the chat interface — Sonnet 5 is currently the default, but available tiers depend on your plan. Upgrade to Pro if your target model is greyed out.
• API: Pass the model parameter in your request. Confirm the exact model string (e.g., claude-sonnet-5) on the official models page before coding — model strings change with releases.

The Three Mistakes Beginners Make When Picking a Model

Mistake 1 — Always defaulting to Opus

Opus costs significantly more per token than Sonnet. For routine tasks, Sonnet produces output that is indistinguishable in quality. Reserve Opus for tasks where you have already tried Sonnet and found it insufficient. Check current rates at claude.com/pricing.

Mistake 2 — Using Haiku for nuanced work to save money

Haiku trades reasoning depth for speed. If a task requires weighing trade-offs or producing polished long-form text, Haiku will require so many re-runs that you lose both time and money. Start with Sonnet.

Mistake 3 — Ignoring context window differences

Context window — the amount of text you can feed in one request — differs across models. Haiku 4.5 supports 200K tokens; Sonnet 5 and Opus 5.5 support 1M tokens. If you need to paste a very long document, confirm the context limit for your chosen model on the official docs before pasting — you may get an error or silent truncation otherwise.

Quick self-test: After your first response, ask yourself: "Was this too slow, too expensive, or not smart enough?" If yes, shift one tier. Most tasks land on Sonnet and stay there.

For the newly released Sonnet 5, the Claude Sonnet 5 guide covers its new features, pricing, and migration caveats in detail.

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