Grok 4 Fast Reasoning vs Phi-4 Reasoning Vision 15B
Grok 4 Fast Reasoning (2026) and Phi-4 Reasoning Vision 15B (2026) are frontier reasoning models from xAI and Microsoft Research. Grok 4 Fast Reasoning ships a not-yet-sourced context window, while Phi-4 Reasoning Vision 15B ships a not-yet-sourced context window. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit. It focuses on practical selection signals rather than broad model-family marketing.
Phi-4 Reasoning Vision 15B is safer overall; choose Grok 4 Fast Reasoning when reasoning depth matters.
Specs
| Specification | ||
|---|---|---|
| Released | 2026-03-01 | 2026-03-12 |
| Context window | — | — |
| Parameters | — | 15B |
| Architecture | decoder only | - |
| License | Proprietary | Microsoft Research |
| Knowledge cutoff | - | - |
Pricing and availability
| Pricing attribute | Grok 4 Fast Reasoning | Phi-4 Reasoning Vision 15B |
|---|---|---|
| Input price | - | - |
| Output price | - | - |
| Providers | - |
Pricing not yet sourced for either model.
Capabilities
| Capability | Grok 4 Fast Reasoning | Phi-4 Reasoning Vision 15B |
|---|---|---|
| Vision | No | No |
| Multimodal | No | Yes |
| Reasoning | Yes | No |
| Function calling | No | No |
| Tool use | No | No |
| Structured outputs | No | No |
| Code execution | No | No |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on multimodal input: Phi-4 Reasoning Vision 15B 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.
Pricing coverage is uneven: Grok 4 Fast Reasoning has no token price sourced yet and Phi-4 Reasoning Vision 15B has no token price sourced yet. Provider availability is 1 tracked routes versus 0. Treat unknown pricing as an integration gap, then verify the route you will actually call before estimating production spend.
Choose Grok 4 Fast Reasoning when reasoning depth and broader provider choice are central to the workload. Choose Phi-4 Reasoning Vision 15B when provider fit 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
Is Grok 4 Fast Reasoning or Phi-4 Reasoning Vision 15B open source?
Grok 4 Fast Reasoning is listed under Proprietary. Phi-4 Reasoning Vision 15B is listed under Microsoft Research. 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 multimodal input, Grok 4 Fast Reasoning or Phi-4 Reasoning Vision 15B?
Phi-4 Reasoning Vision 15B 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.
Which is better for reasoning mode, Grok 4 Fast Reasoning or Phi-4 Reasoning Vision 15B?
Grok 4 Fast Reasoning has the clearer documented reasoning mode signal in this comparison. If reasoning mode 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-4 Reasoning Vision 15B?
Grok 4 Fast Reasoning is available on Microsoft Foundry. Phi-4 Reasoning Vision 15B is available on the tracked providers still being sourced. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
When should I pick Grok 4 Fast Reasoning over Phi-4 Reasoning Vision 15B?
Phi-4 Reasoning Vision 15B is safer overall; choose Grok 4 Fast Reasoning when reasoning depth matters. If your workload also depends on reasoning depth, start with Grok 4 Fast Reasoning; if it depends on provider fit, run the same evaluation with Phi-4 Reasoning Vision 15B.
Continue comparing
Last reviewed: 2026-04-19. Data sourced from public model cards and provider documentation.