o3 Mini vs Step 3.5 Flash
o3 Mini (2025) and Step 3.5 Flash (2026) are frontier-tier reasoning models from OpenAI and StepFun. o3 Mini ships a 200k-token context window, while Step 3.5 Flash ships a 256k-token context window. On SWE-bench Verified, Step 3.5 Flash leads by 25.1 pts. On pricing, Step 3.5 Flash costs $0.10/1M input tokens versus $1.10/1M for the alternative. This comparison covers specs, pricing, API access, capabilities, benchmarks, input and output token costs, and production fit for coding and agent workloads.
Step 3.5 Flash is ~1000% cheaper at $0.10/1M; pay for o3 Mini only for coding workflow support.
Decision scorecard
Local evidence first| Signal | o3 Mini | Step 3.5 Flash |
|---|---|---|
| Best for | reasoning-heavy apps, tool-calling agents, and provider-routed production | reasoning-heavy apps |
| Decision fit | Coding, RAG, and Agents | Coding, Agents, and Long context |
| Context window | 200k | 256k |
| Cheapest output | $4.40/1M tokens | $0.30/1M tokens |
| Provider routes | 4 tracked | 1 tracked |
| Shared benchmarks | 1 rows | SWE-bench Verified leader |
Decision tradeoffs
- o3 Mini has broader tracked provider coverage for fallback and procurement flexibility.
- o3 Mini uniquely exposes Function calling, Tool use, and Structured outputs in local model data.
- Local decision data tags o3 Mini for Coding, RAG, and Agents.
- Step 3.5 Flash holds a shared-benchmark lead on SWE-bench Verified, ahead by 25.1 points.
- Step 3.5 Flash has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Step 3.5 Flash has the lower cheapest tracked output price at $0.30/1M tokens.
- Local decision data tags Step 3.5 Flash for Coding, Agents, and Long context.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output route or tier on this page.
o3 Mini
$1,980
Cheapest tracked route/tier: OpenRouter
Step 3.5 Flash
$155
Cheapest tracked route/tier: OpenRouter
Estimated monthly gap: $1,825. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- Step 3.5 Flash is $4.10/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Function calling, Tool use, and Structured outputs before moving production traffic.
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- o3 Mini is $4.10/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- o3 Mini adds Function calling, Tool use, and Structured outputs in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-01-31 | 2026-01-29 |
| Context window | 200k | 256k |
| Parameters | — | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Proprietary | Proprietary |
| Openness | Proprietary | Proprietary |
| Commercial use | Commercial use with conditions | Commercial use with conditions |
| Knowledge cutoff | 2025-04 | - |
Pricing and availability
| Pricing attribute | o3 Mini | Step 3.5 Flash |
|---|---|---|
| Input price | $1.10/1M tokens | $0.10/1M tokens |
| Output price | $4.40/1M tokens | $0.30/1M tokens |
| Providers |
Capabilities
| Capability | o3 Mini | Step 3.5 Flash |
|---|---|---|
| Vision | No | No |
| Multimodal | No | No |
| Reasoning | Yes | Yes |
| Function calling | Yes | No |
| Tool use | Yes | No |
| Structured outputs | Yes | No |
| Code execution | Yes | No |
| IDE integration | No | No |
| Computer use | No | No |
| Parallel agents | No | No |
Benchmarks
| Benchmark | o3 Mini | Step 3.5 Flash |
|---|---|---|
| SWE-bench Verified | 49.3 | 74.4 |
Deep dive
On shared benchmark coverage, SWE-bench Verified has o3 Mini at 49.3 and Step 3.5 Flash at 74.4, with Step 3.5 Flash ahead by 25.1 points. The largest visible gap is 25.1 points on SWE-bench Verified, which matters most when that benchmark mirrors your workload. Treat isolated benchmark wins as directional, because provider routing, prompt style, and tool access can move real application results.
The capability footprint differs most on function calling: o3 Mini, tool use: o3 Mini, structured outputs: o3 Mini, and code execution: o3 Mini. 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 Mini lists $1.10/1M input and $4.40/1M output tokens on the cheapest tracked provider, while Step 3.5 Flash lists $0.10/1M input and $0.30/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Step 3.5 Flash lower by about $1.93 per million blended tokens. Availability is 4 providers versus 1, so concentration risk also matters.
Choose o3 Mini 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.
FAQ
Which has a larger context window, o3 Mini or Step 3.5 Flash?
Step 3.5 Flash supports 256k tokens, while o3 Mini 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, o3 Mini or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. o3 Mini costs $1.10/1M input and $4.40/1M output tokens. Step 3.5 Flash costs $0.10/1M input and $0.30/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is o3 Mini or Step 3.5 Flash open source?
o3 Mini is listed under Proprietary. Step 3.5 Flash is listed under Proprietary. 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 Mini or Step 3.5 Flash?
Both o3 Mini 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.
Which is better for function calling, o3 Mini or Step 3.5 Flash?
o3 Mini has the clearer documented function calling signal in this comparison. If function calling is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run o3 Mini and Step 3.5 Flash?
o3 Mini is available on OpenRouter, Azure OpenAI, OpenAI API, and Vercel AI Gateway. 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: 2026-06-04. Data sourced from public model cards and provider documentation.