Claude Sonnet 4.5 vs Step 3.5 Flash
Claude Sonnet 4.5 (2025) and Step 3.5 Flash (2026) are frontier-tier reasoning models from Anthropic and StepFun. Claude Sonnet 4.5 ships a 200K-token context window, while Step 3.5 Flash ships a 256K-token context window. On pricing, Step 3.5 Flash costs $0.1/1M input tokens versus $3/1M for the alternative. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit.
Step 3.5 Flash is ~2900% cheaper at $0.1/1M; pay for Claude Sonnet 4.5 only for vision-heavy evaluation.
Specs
| Released | 2025-09-29 | 2026-01-29 |
| Context window | 200K | 256K |
| Parameters | — | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Proprietary | Open Source |
| Knowledge cutoff | 2025-12 | - |
Pricing and availability
| Claude Sonnet 4.5 | Step 3.5 Flash | |
|---|---|---|
| Input price | $3/1M tokens | $0.1/1M tokens |
| Output price | $15/1M tokens | $0.3/1M tokens |
| Providers |
Capabilities
| Claude Sonnet 4.5 | Step 3.5 Flash | |
|---|---|---|
| Vision | ||
| Multimodal | ||
| Reasoning | ||
| Function calling | ||
| Tool use | ||
| Structured outputs | ||
| Code execution |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on vision: Claude Sonnet 4.5, multimodal input: Claude Sonnet 4.5, function calling: Claude Sonnet 4.5, tool use: Claude Sonnet 4.5, and structured outputs: Claude Sonnet 4.5. Both models share reasoning mode, so the practical split is not just feature count. Use those differences to decide whether the page is about raw model quality, agentic coding support, multimodal ingestion, or predictable structured API behavior.
For cost, Claude Sonnet 4.5 lists $3/1M input and $15/1M output tokens, while Step 3.5 Flash lists $0.1/1M input and $0.3/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Step 3.5 Flash lower by about $6.44 per million blended tokens. Availability is 8 providers versus 1, so concentration risk also matters.
Choose Claude Sonnet 4.5 when vision-heavy evaluation and broader provider choice are central to the workload. Choose Step 3.5 Flash when long-context analysis, larger context windows, and lower input-token cost are more important. For production, rerun your own prompts through the exact provider, region, and tool stack you plan to ship. This keeps the decision grounded in measurable tradeoffs instead of brand-level assumptions.
FAQ
Which has a larger context window, Claude Sonnet 4.5 or Step 3.5 Flash?
Step 3.5 Flash supports 256K tokens, while Claude Sonnet 4.5 supports 200K tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Which is cheaper, Claude Sonnet 4.5 or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. Claude Sonnet 4.5 costs $3/1M input and $15/1M output tokens. Step 3.5 Flash costs $0.1/1M input and $0.3/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is Claude Sonnet 4.5 or Step 3.5 Flash open source?
Claude Sonnet 4.5 is listed under Proprietary. Step 3.5 Flash is listed under Open Source. License labels affect whether you can self-host, redistribute weights, or rely only on hosted APIs, so confirm the upstream license before deployment.
Which is better for vision, Claude Sonnet 4.5 or Step 3.5 Flash?
Claude Sonnet 4.5 has the clearer documented vision signal in this comparison. If vision is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Which is better for multimodal input, Claude Sonnet 4.5 or Step 3.5 Flash?
Claude Sonnet 4.5 has the clearer documented multimodal input signal in this comparison. If multimodal input is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run Claude Sonnet 4.5 and Step 3.5 Flash?
Claude Sonnet 4.5 is available on Microsoft Foundry, Anthropic, Snowflake Cortex, GCP Vertex AI, and AWS Bedrock. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
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Last reviewed: 2026-04-24. Data sourced from public model cards and provider documentation.