Claude Opus 5.5 vs Claude Sonnet 5.5

Claude Opus 5.5
$1.34 / task
Wins 2 of 5 benchmarks
Claude Sonnet 5.5
$0.88 / task
Wins 0 of 5 benchmarks
Different efforts: medium · high · Compare all at high
Arena Text · creative writing Arena Text · creative writing: Human pairwise preferences in the stated text vote pool
Opus 5.5 · high1,519
Sonnet 5.5 · xhigh1,465
Harvey LAB-AA 1.1 Harvey LAB-AA 1.1: Legal agents work from case documents; every rubric criterion must pass and material hallucinations receive no credit
Opus 5.5 · max4.2
Sonnet 5.5 · max2.8
EQ-Bench Longform Creative Writing 1.11 Longform Creative Writing 1.11: Models plan, revise and write a story across eight turns, graded with a writing rubric
Opus 5.584.6
Sonnet 5.5—
EQ-Bench Creative Writing v3 Creative Writing v3 · Elo: Judge-rated creative writing under the v3 prompt set and pairwise ranking
Opus 5.52,050.1
Sonnet 5.5—
Lech Mazur Short-Story Lech Mazur Short-Story · comparison score: Judges compare stories written to ten required elements; the comparison score can be negative
Opus 5.5 · high3.7
Sonnet 5.5—

Specs

SpecOpus 5.5Sonnet 5.5
Input, per 1M$4$2
Output, per 1M$20$10
Cost per task?$1.34$0.88
Speed, tok/s81106
Context1M1M
WeightsClosedClosed
ReleasedSep 2026Sep 2026
See images✓✓
Read PDFs✓✓
Hear audio——
Watch video——
Call tools✓✓
Force a tool call——
Return strict JSON✓✓
Search the web✓✓
Run code✓✓
Use a computer✓✓

What breaks when you switch

Opus 5.5 → Sonnet 5.5
No forced tool use›

Forced tool_choice any or tool returns HTTP 400; remove the forced choice.

Replace disabled thinking›

Disabled thinking returns HTTP 400; use between_tools for the lowest thinking setting.

No manual thinking budgets›

Manual thinking budgets and extra between_tools fields return HTTP 400; remove manual budgets and send between_tools without display, budget_tokens or block_binding.

Remove sampling parameters›

Non-default temperature, top_p or top_k returns HTTP 400; remove these parameters.

No assistant prefill›

A prefilled final assistant turn returns HTTP 400; remove that prefill.

Old computer tool rejected · Claude API / Google Cloud›

The computer_20251124 tool returns HTTP 400 on the Claude API and Google Cloud; use computer_toolset_20260801 there.

Sonnet 5.5 → Opus 5.5
No forced tool use›

Forced tool_choice any or tool returns HTTP 400; remove the forced choice.

Thinking is always on›

Disabled thinking or a manual budget returns HTTP 400; omit thinking or use adaptive thinking.

Remove sampling parameters›

Non-default temperature, top_p or top_k returns HTTP 400; omit these parameters and use prompting.

No assistant prefill›

Assistant prefill returns HTTP 400; use structured outputs or system instructions, with system instructions on Bedrock.

Old computer tool rejected · Claude API / Google Cloud›

The computer_20251124 tool returns HTTP 400 on the Claude API and Google Cloud; use computer_toolset_20260801 there.

Return thinking blocks unmodified›

Edited, reordered or partially dropped thinking blocks return HTTP 400; echo assistant messages complete and unmodified with tool results.