Model comparison

Claude Sonnet 5.5 vs Step 3.5 Flash

Claude Sonnet 5.5 is the stronger model overall, scoring 61.9 to 42.3 on the Noometry Index. Step 3.5 Flash costs 27× less per token, which makes it the better buy when Claude Sonnet 5.5's lead doesn't matter for your workload.

Last verified October 11, 2026. 13 shared benchmarks.

Claude Sonnet 5.5 Anthropic

61.9

Rank #10 Confirmed

Step 3.5 Flash

StepFun

42.3

Rank #116 Confirmed

Summary

They share 13 benchmarks with published results for both. Claude Sonnet 5.5 scores higher in 8 categories and Step 3.5 Flash in 0 categories; 8 gaps are clear of the uncertainty.
The widest gap is in math, where Claude Sonnet 5.5 leads 87.9 to 42.6.
The biggest single-benchmark swing is NYT Connections (extended): 80.5% for Claude Sonnet 5.5 and 28.4% for Step 3.5 Flash.
Step 3.5 Flash is cheaper at $0.10 / $0.30 per million input/output tokens, against $2 / $10 for Claude Sonnet 5.5.
Claude Sonnet 5.5 accepts more context: 1M tokens versus 256K.
Step 3.5 Flash has downloadable open weights; the other is API-only.

Side by side

Claude Sonnet 5.5 and Step 3.5 Flash specifications
	Claude Sonnet 5.5	Step 3.5 Flash
Provider	Anthropic	StepFun
Noometry Index	61.9	42.3
Released	2026-09-28	2026-01-29
Weights	Proprietary	Open
Context window	1M	256K
Max output	128K	256K
Input $ / M tokens	$2	$0.10
Output $ / M tokens	$10	$0.30
Results tracked	32	19

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Category by category

Coding Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 67.3 (#6), Step 3.5 Flash: 42.4 (#105)

Coding benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Coding	1513	1436
FrontierCode	52.1%	—
CursorBench	55.5%	—
LMArena WebDev	1774	—
FrontierSWE	61.9%	—
SciCode	61%	—
ALE-Bench	1,819	—

Agentic & Tool Use Not comparable

Claude Sonnet 5.5: 45.0 (#16), Step 3.5 Flash: —

Agentic & Tool Use benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
APEX-Agents	75.5%	—

Reasoning Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 54.0 (#28), Step 3.5 Flash: 22.2 (#202)

Reasoning benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
NYT Connections (extended)	80.5%	28.4%
LMArena Hard Prompts	1495	1411
CritPt	31.4%	—
Mystery Game Puzzles	65%	—
Epoch Capabilities Index	165.03	—

Math Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 87.9 (#6), Step 3.5 Flash: 42.6 (#84)

Math benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Math	1510	1408
FrontierMath (Tiers 1-3)	88.8%	—
FrontierMath Tier 4	80.5%	—
MathArena Final-Answer Competitions	—	66.8%
OTIS Mock AIME 2024-2025	100%	—
ProofBench	100%	—
FrontierMath Erdős	2.9%	—

Knowledge Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 66.0 (#12), Step 3.5 Flash: 39.6 (#132)

Knowledge benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Expert	1540	1421
GPQA Diamond	95.6%	—
SimpleQA Verified	46.5%	—

Multimodal Not comparable

Claude Sonnet 5.5: 51.5 (#6), Step 3.5 Flash: —

Multimodal benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Vision	1289	—
Furniture Assembly	75%	—

Multilingual Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 55.3 (#30), Step 3.5 Flash: 50.5 (#119)

Multilingual benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Non-English	1452	1385
LMArena Chinese	1522	1447
LMArena Russian	1451	1385
LMArena French	—	1421
LMArena German	—	1405
LMArena Japanese	—	1354
LMArena Korean	—	1352
LMArena Spanish	—	1419

Instruction Following Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 78.3 (#11), Step 3.5 Flash: 73.1 (#124)

Instruction Following benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Instruction Following	1495	1385

Long Context Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 45.9 (#28), Step 3.5 Flash: 42.8 (#117)

Long Context benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Longer Query	1498	1402

Writing & Preference Claude Sonnet 5.5 leads

Claude Sonnet 5.5: 66.0 (#40), Step 3.5 Flash: 58.8 (#113)

Writing & Preference benchmarks
Benchmark	Claude Sonnet 5.5	Step 3.5 Flash
LMArena Text	1471	1403
LMArena Creative Writing	1465	1357
LMArena Multi-Turn	1474	1405

Frequently asked questions

Is Claude Sonnet 5.5 better than Step 3.5 Flash?

Which is cheaper, Claude Sonnet 5.5 or Step 3.5 Flash?

Step 3.5 Flash is cheaper. It lists at $0.10 per million input tokens and $0.30 per million output tokens; Claude Sonnet 5.5 lists at $2 and $10.

Is Claude Sonnet 5.5 or Step 3.5 Flash better for coding?

Claude Sonnet 5.5 scores higher on coding benchmarks: 67.3 versus 42.4 in the Noometry coding category.

Which has the bigger context window?

Claude Sonnet 5.5 does, with 1M tokens against 256K.

How many benchmarks do Claude Sonnet 5.5 and Step 3.5 Flash share?

13 benchmarks have published results for both models. Claude Sonnet 5.5 has 32 scored results on Noometry and Step 3.5 Flash has 19.