o3 vs Step 3.5 Flash
o3 (2025) and Step 3.5 Flash (2026) are frontier-tier reasoning models from OpenAI and StepFun. o3 ships a 128K-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 $1/1M for the alternative. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit. It focuses on practical selection signals rather than broad model-family marketing.
Step 3.5 Flash is ~900% cheaper at $0.1/1M; pay for o3 only for coding workflow support.
Specs
| Released | 2025-03-31 | 2026-01-29 |
| Context window | 128K | 256K |
| Parameters | — | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Unknown | Open Source |
| Knowledge cutoff | - | - |
Pricing and availability
| o3 | Step 3.5 Flash | |
|---|---|---|
| Input price | $1/1M tokens | $0.1/1M tokens |
| Output price | $4/1M tokens | $0.3/1M tokens |
| Providers |
Capabilities
| o3 | 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 structured outputs: o3 and code execution: o3. 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, o3 lists $1/1M input and $4/1M 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 $1.74 per million blended tokens. Availability is 3 providers versus 1, so concentration risk also matters.
Choose o3 when coding workflow support 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. It also helps separate model capability from provider packaging, which can change cost and latency.
FAQ
Which has a larger context window, o3 or Step 3.5 Flash?
Step 3.5 Flash supports 256K tokens, while o3 supports 128K tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Which is cheaper, o3 or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. o3 costs $1/1M input and $4/1M 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 o3 or Step 3.5 Flash open source?
o3 is listed under Unknown. 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 reasoning mode, o3 or Step 3.5 Flash?
Both o3 and Step 3.5 Flash expose reasoning mode. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Which is better for structured outputs, o3 or Step 3.5 Flash?
o3 has the clearer documented structured outputs signal in this comparison. If structured outputs is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run o3 and Step 3.5 Flash?
o3 is available on OpenAI API, OpenRouter, and OpenAI Batch API. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
Continue comparing
Last reviewed: 2026-04-24. Data sourced from public model cards and provider documentation.