Claude Sonnet vs Opus: which model to use

Sonnet 5.5 vs Opus 5.5 at a glance

On price, Sonnet 5.5 costs half as much as Opus 5.5 for input, output, cache reads and cache writes. See Claude API pricing for Fable, Haiku and earlier models.

Benchmark scores

These scores come from Anthropic's Sonnet 5.5 announcement. Opus 5.5's Terminal-Bench 4.0 score and Sonnet 5.5's FrontierCode 1.1 score were both run at xhigh effort.

Speed and cost per task

• Sonnet 5.5 generates output 30%+ faster than Sonnet 5 and is Anthropic's fastest Sonnet model.
• Opus 5.5 generates output more than 30% faster than Opus 5, though Anthropic still rates its speed as moderate, next to Sonnet 5.5's fast rating.
• Fast mode for Opus 5.5 in Claude Code and on the Claude Platform runs up to 2.5x faster at $8 per million input tokens and $40 per million output tokens.
• Per task, Sonnet 5.5 costs up to 30% less than Sonnet 5, while Opus 5.5 costs 40% less than Opus 5 on typical workloads at default settings.
• Both earlier versions stay available on the API, with Sonnet 5 priced at $2 input and $10 output per million tokens ($0.20 cache read) and Opus 5 at $5 and $25 ($0.50 cache read).

When to use Sonnet and when to use Opus

Pick Sonnet 5.5 when your workload fits these patterns:

• You're handling well-scoped everyday tasks and bug fixes.
• You need to generate polished documents, slides, or spreadsheets.
• You're running fast iteration loops or high-volume requests where paying half the token price matters.

Pick Claude Opus 5.5 when the job looks like this:

• The problem is complex, open-ended, and needs sustained, careful judgment across steps.
• You're running long, sprawling jobs like codebase-wide migrations and audits.
• You want the higher reported scores on FrontierCode 1.1 (54.4% vs 52.1%) and CursorBench 4.0 (57.8 vs 55.5).

Anthropic's models overview recommends starting with Opus 5.5 for most workloads and moving up to Fable 5.1 for the most demanding reasoning and long-horizon agentic work.

Sonnet and Opus on Claude plans

If you use Claude through a subscription instead of API billing, the Claude Free plan gives you access to Sonnet and Haiku, but not Opus. Paid tiers unlock all three model families: Claude Pro at $20 a month and Claude Max at $100 or $200 a month both include Opus, Sonnet, and Haiku. Inside Claude Code and the Claude apps, Sonnet 5.5 defaults to medium effort, whereas on the Claude Platform it defaults to high effort (Opus 5.5 defaults to medium effort).

Sonnet and Opus model ids on Plan2API

Once you connect a subscription from a Claude Pro or Max account, Plan2API exposes Anthropic's models through a single OpenAI-compatible p2a_ key at https://api.plan2api.com/v1 using POST /v1/chat/completions. GET /v1/models lists every model id Plan2API serves. Plan2API's Sonnet ids are claude/claude-sonnet-5 and claude/claude-sonnet-4-6; its newest Opus id is claude/claude-opus-5-5. The anthropic/ prefix and dotted names such as claude-opus-5.5 also work.

curl https://api.plan2api.com/v1/chat/completions \
  -H "Authorization: Bearer $PLAN2API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude/claude-sonnet-5",
    "messages": [{"role": "user", "content": "Fix the failing test in this file."}]
  }'