Grok 4 Fast Reasoning vs Phi 3.5 MoE Instruct
Grok 4 Fast Reasoning (2025) and Phi 3.5 MoE Instruct (2024) are frontier reasoning models from xAI and Microsoft Research. Grok 4 Fast Reasoning ships a 2m-token context window, while Phi 3.5 MoE Instruct ships a 128k-token context window. On pricing, Phi 3.5 MoE Instruct costs $0.50/1M input tokens versus $1.25/1M 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.
Phi 3.5 MoE Instruct is ~150% cheaper at $0.50/1M; pay for Grok 4 Fast Reasoning only for reasoning depth.
Decision scorecard
Local evidence first| Signal | Grok 4 Fast Reasoning | Phi 3.5 MoE Instruct |
|---|---|---|
| Best for | reasoning-heavy apps, multimodal apps, and long-context analysis | general production evaluation |
| Decision fit | Long context and Vision | Long context |
| Context window | 2m | 128k |
| Cheapest output | $2.50/1M tokens | $0.50/1M tokens |
| Provider routes | 2 tracked | 1 tracked |
| Shared benchmarks | 0 rows | 0 rows |
Decision tradeoffs
- Grok 4 Fast Reasoning has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Grok 4 Fast Reasoning has broader tracked provider coverage for fallback and procurement flexibility.
- Grok 4 Fast Reasoning uniquely exposes Vision, Multimodal, and Reasoning in local model data.
- Local decision data tags Grok 4 Fast Reasoning for Long context and Vision.
- Phi 3.5 MoE Instruct has the lower cheapest tracked output price at $0.50/1M tokens.
- Local decision data tags Phi 3.5 MoE 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 Fast Reasoning
$1,625
Cheapest tracked route/tier: xAI Console
Phi 3.5 MoE Instruct
$525
Cheapest tracked route/tier: Fireworks AI
Estimated monthly gap: $1,100. 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 Fast Reasoning and Phi 3.5 MoE Instruct; plan for SDK, billing, or endpoint changes.
- Phi 3.5 MoE Instruct is $2/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Vision, Multimodal, and Reasoning before moving production traffic.
- No overlapping tracked provider route is sourced for Phi 3.5 MoE Instruct and Grok 4 Fast Reasoning; plan for SDK, billing, or endpoint changes.
- Grok 4 Fast Reasoning is $2/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Grok 4 Fast Reasoning adds Vision, Multimodal, and Reasoning in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-09-01 | 2024-08-20 |
| Context window | 2m | 128k |
| Parameters | — | 16x3.8B (42B, 6.6B active) |
| Architecture | decoder only | 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 Fast Reasoning | Phi 3.5 MoE Instruct |
|---|---|---|
| Input price | $1.25/1M tokens | $0.50/1M tokens |
| Output price | $2.50/1M tokens | $0.50/1M tokens |
| Providers |
Capabilities
| Capability | Grok 4 Fast Reasoning | Phi 3.5 MoE Instruct |
|---|---|---|
| Vision | Yes | No |
| Multimodal | Yes | No |
| Reasoning | Yes | No |
| Function calling | No | No |
| Tool use | No | No |
| Structured outputs | No | 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 Fast Reasoning, multimodal input: Grok 4 Fast Reasoning, and reasoning mode: Grok 4 Fast Reasoning. 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 Fast Reasoning lists $1.25/1M input and $2.50/1M output tokens on the cheapest tracked provider, while Phi 3.5 MoE Instruct lists $0.50/1M input and $0.50/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Phi 3.5 MoE Instruct lower by about $1.13 per million blended tokens. Availability is 2 providers versus 1, so concentration risk also matters.
Choose Grok 4 Fast Reasoning when reasoning depth, larger context windows, and broader provider choice are central to the workload. Choose Phi 3.5 MoE Instruct 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 Fast Reasoning or Phi 3.5 MoE Instruct?
Grok 4 Fast Reasoning supports 2m tokens, while Phi 3.5 MoE 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 Fast Reasoning or Phi 3.5 MoE Instruct?
Phi 3.5 MoE Instruct is cheaper on tracked token pricing. Grok 4 Fast Reasoning costs $1.25/1M input and $2.50/1M output tokens. Phi 3.5 MoE Instruct costs $0.50/1M input and $0.50/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is Grok 4 Fast Reasoning or Phi 3.5 MoE Instruct open source?
Grok 4 Fast Reasoning is listed under Proprietary. Phi 3.5 MoE 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 Fast Reasoning or Phi 3.5 MoE Instruct?
Grok 4 Fast Reasoning 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 Fast Reasoning or Phi 3.5 MoE Instruct?
Grok 4 Fast Reasoning 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 Fast Reasoning and Phi 3.5 MoE Instruct?
Grok 4 Fast Reasoning is available on Microsoft Foundry and xAI Console. Phi 3.5 MoE Instruct is available on Fireworks AI. 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.