Grok 4.1 Fast vs Phi 3.5 Mini Instruct
Grok 4.1 Fast (2025) and Phi 3.5 Mini Instruct (2024) are frontier reasoning models from xAI and Microsoft Research. Grok 4.1 Fast ships a 2m-token context window, while Phi 3.5 Mini Instruct ships a 128k-token context window. On pricing, Grok 4.1 Fast costs $0.20/1M input tokens versus $0.90/1M 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.
Grok 4.1 Fast is ~350% cheaper at $0.20/1M; pay for Phi 3.5 Mini Instruct only for provider fit.
Decision scorecard
Local evidence first| Signal | Grok 4.1 Fast | Phi 3.5 Mini Instruct |
|---|---|---|
| Best for | reasoning-heavy apps, multimodal apps, and tool-calling agents | provider-routed production |
| Decision fit | RAG, Agents, and Long context | Long context |
| Context window | 2m | 128k |
| Cheapest output | $0.50/1M tokens | $0.90/1M tokens |
| Provider routes | 2 tracked | 2 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 the lower cheapest tracked output price at $0.50/1M tokens.
- Grok 4.1 Fast uniquely exposes Vision, Multimodal, and Reasoning in local model data.
- Local decision data tags Grok 4.1 Fast for RAG, Agents, and Long context.
- Local decision data tags Phi 3.5 Mini Instruct for 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
Phi 3.5 Mini Instruct
$945
Cheapest tracked route/tier: Fireworks AI
Estimated monthly gap: $660. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- No overlapping tracked provider route is sourced for Grok 4.1 Fast and Phi 3.5 Mini Instruct; plan for SDK, billing, or endpoint changes.
- Phi 3.5 Mini Instruct is $0.40/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Check replacement coverage for Vision, Multimodal, and Reasoning before moving production traffic.
- No overlapping tracked provider route is sourced for Phi 3.5 Mini Instruct and Grok 4.1 Fast; plan for SDK, billing, or endpoint changes.
- Grok 4.1 Fast is $0.40/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Grok 4.1 Fast adds Vision, Multimodal, and Reasoning in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-11-19 | 2024-08-20 |
| Context window | 2m | 128k |
| Parameters | — | 3.8B |
| Architecture | - | decoder only |
| License | Proprietary | MIT(OSI) |
| Openness | Proprietary | Open source |
| Commercial use | Commercial use with conditions | Commercial use allowed |
| Knowledge cutoff | - | 2023-10 |
Pricing and availability
| Pricing attribute | Grok 4.1 Fast | Phi 3.5 Mini Instruct |
|---|---|---|
| Input price | $0.20/1M tokens | $0.90/1M tokens |
| Output price | $0.50/1M tokens | $0.90/1M tokens |
| Providers |
Capabilities
| Capability | Grok 4.1 Fast | Phi 3.5 Mini Instruct |
|---|---|---|
| Vision | Yes | No |
| Multimodal | Yes | No |
| Reasoning | Yes | No |
| 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, reasoning mode: 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 the core language-model surface, 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 Phi 3.5 Mini Instruct lists $0.90/1M input and $0.90/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Grok 4.1 Fast lower by about $0.61 per million blended tokens. Availability is 2 providers versus 2, so concentration risk also matters.
Choose Grok 4.1 Fast when reasoning depth, larger context windows, and lower input-token cost are central to the workload. Choose Phi 3.5 Mini Instruct when provider fit 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, Grok 4.1 Fast or Phi 3.5 Mini Instruct?
Grok 4.1 Fast supports 2m tokens, while Phi 3.5 Mini Instruct 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, Grok 4.1 Fast or Phi 3.5 Mini Instruct?
Grok 4.1 Fast is cheaper on tracked token pricing. Grok 4.1 Fast costs $0.20/1M input and $0.50/1M output tokens. Phi 3.5 Mini Instruct costs $0.90/1M input and $0.90/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is Grok 4.1 Fast or Phi 3.5 Mini Instruct open source?
Grok 4.1 Fast is listed under Proprietary. Phi 3.5 Mini Instruct is listed under MIT. 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 Phi 3.5 Mini Instruct?
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 Phi 3.5 Mini Instruct?
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 Phi 3.5 Mini Instruct?
Grok 4.1 Fast is available on OpenRouter and xAI Console. Phi 3.5 Mini Instruct is available on Fireworks AI and NVIDIA NIM. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
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Last reviewed: 2026-05-20. Data sourced from public model cards and provider documentation.