Grok 4.1 Fast vs Step 3.5 Flash
Grok 4.1 Fast (2025) and Step 3.5 Flash (2026) are frontier-tier reasoning models from xAI and StepFun. Grok 4.1 Fast ships a 2m-token context window, while Step 3.5 Flash ships a 256k-token context window. On pricing, Step 3.5 Flash costs $0.10/1M input tokens versus $0.20/1M 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 ~100% cheaper at $0.10/1M; pay for Grok 4.1 Fast only for long-context analysis.
Decision scorecard
Local evidence first| Signal | Grok 4.1 Fast | Step 3.5 Flash |
|---|---|---|
| Best for | reasoning-heavy apps, multimodal apps, and tool-calling agents | reasoning-heavy apps |
| Decision fit | RAG, Agents, and Long context | Coding, Agents, and Long context |
| Context window | 2m | 256k |
| Cheapest output | $0.50/1M tokens | $0.30/1M tokens |
| Provider routes | 2 tracked | 1 tracked |
| Shared benchmarks | 0 rows | 0 rows |
Decision tradeoffs
- Grok 4.1 Fast has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Grok 4.1 Fast has broader tracked provider coverage for fallback and procurement flexibility.
- Grok 4.1 Fast uniquely exposes Vision, Multimodal, and Function calling in local model data.
- Local decision data tags Grok 4.1 Fast for RAG, Agents, and Long context.
- 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.
Grok 4.1 Fast
$285
Cheapest tracked route/tier: OpenRouter
Step 3.5 Flash
$155
Cheapest tracked route/tier: OpenRouter
Estimated monthly gap: $130. 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 $0.20/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Vision, Multimodal, and Function calling before moving production traffic.
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- Grok 4.1 Fast is $0.20/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Grok 4.1 Fast adds Vision, Multimodal, and Function calling in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-11-19 | 2026-01-29 |
| Context window | 2m | 256k |
| Parameters | — | 196B (11B active) |
| Architecture | - | mixture of experts |
| License | Proprietary | Proprietary |
| Openness | Proprietary | Proprietary |
| Commercial use | Commercial use with conditions | Commercial use with conditions |
| Knowledge cutoff | - | - |
Pricing and availability
| Pricing attribute | Grok 4.1 Fast | Step 3.5 Flash |
|---|---|---|
| Input price | $0.20/1M tokens | $0.10/1M tokens |
| Output price | $0.50/1M tokens | $0.30/1M tokens |
| Providers |
Capabilities
| Capability | Grok 4.1 Fast | Step 3.5 Flash |
|---|---|---|
| Vision | Yes | No |
| Multimodal | Yes | No |
| Reasoning | Yes | Yes |
| Function calling | Yes | No |
| Tool use | Yes | No |
| Structured outputs | Yes | No |
| Code execution | No | No |
| IDE integration | No | No |
| Computer use | No | No |
| Parallel agents | No | No |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on vision: Grok 4.1 Fast, multimodal input: Grok 4.1 Fast, function calling: Grok 4.1 Fast, tool use: Grok 4.1 Fast, and structured outputs: Grok 4.1 Fast. 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, Grok 4.1 Fast lists $0.20/1M input and $0.50/1M 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 $0.13 per million blended tokens. Availability is 2 providers versus 1, so concentration risk also matters.
Choose Grok 4.1 Fast when long-context analysis, larger context windows, and broader provider choice are central to the workload. Choose Step 3.5 Flash when provider fit 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, Grok 4.1 Fast or Step 3.5 Flash?
Grok 4.1 Fast supports 2m tokens, while Step 3.5 Flash supports 256k tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Which is cheaper, Grok 4.1 Fast or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. Grok 4.1 Fast costs $0.20/1M input and $0.50/1M 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 Grok 4.1 Fast or Step 3.5 Flash open source?
Grok 4.1 Fast 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 vision, Grok 4.1 Fast or Step 3.5 Flash?
Grok 4.1 Fast 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, Grok 4.1 Fast or Step 3.5 Flash?
Grok 4.1 Fast 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 Grok 4.1 Fast and Step 3.5 Flash?
Grok 4.1 Fast is available on OpenRouter and xAI Console. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Continue comparing
Last reviewed: 2026-05-29. Data sourced from public model cards and provider documentation.