Grok 4 Fast Reasoning vs o3
Grok 4 Fast Reasoning (2025) and o3 (2025) are frontier-tier reasoning models from xAI and OpenAI. Grok 4 Fast Reasoning ships a 2m-token context window, while o3 ships a 200k-token context window. On pricing, Grok 4 Fast Reasoning costs $1.25/1M input tokens versus $2/1M 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 Fast Reasoning is ~60% cheaper at $1.25/1M; pay for o3 only for coding workflow support.
Decision scorecard
Local evidence first| Signal | Grok 4 Fast Reasoning | o3 |
|---|---|---|
| Best for | reasoning-heavy apps, multimodal apps, and long-context analysis | reasoning-heavy apps, multimodal apps, and tool-calling agents |
| Decision fit | Long context and Vision | Coding, RAG, and Agents |
| Context window | 2m | 200k |
| Cheapest output | $2.50/1M tokens | $8/1M tokens |
| Provider routes | 2 tracked | 3 tracked |
| Shared benchmarks | 0 shared | 0 shared |
Decision tradeoffs
- Grok 4 Fast Reasoning has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Grok 4 Fast Reasoning has the lower cheapest tracked output price at $2.50/1M tokens.
- Local decision data tags Grok 4 Fast Reasoning for Long context and Vision.
- o3 has broader tracked provider coverage for fallback and route flexibility.
- o3 uniquely exposes JSON / Tool use, Structured outputs, and Code execution in local model data.
- Local decision data tags o3 for Coding, RAG, and Agents.
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
o3
$3,600
Cheapest tracked route/tier: OpenAI API
Estimated monthly gap: $1,975. 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 o3; plan for SDK, billing, or endpoint changes.
- o3 is $5.50/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- o3 adds JSON / Tool use, Structured outputs, and Code execution in local capability data.
- No overlapping tracked provider route is sourced for o3 and Grok 4 Fast Reasoning; plan for SDK, billing, or endpoint changes.
- Grok 4 Fast Reasoning is $5.50/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for JSON / Tool use, Structured outputs, and Code execution before moving production traffic.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-09-01 | 2025-04-16 |
| Context window | 2m | 200k |
| Parameters | — | — |
| Architecture | Decoder Only | Decoder Only |
| License | Proprietary | Proprietary |
| Openness | Proprietary | Proprietary |
| Weights | Not released | Not released |
| Code | Unknown | Unknown |
| Commercial use | Commercial use: conditional | Commercial use: conditional |
| Knowledge cutoff | - | 2024-06 |
Pricing and availability
| Pricing attribute | Grok 4 Fast Reasoning | o3 |
|---|---|---|
| Input price | $1.25/1M tokens | $2/1M tokens |
| Output price | $2.50/1M tokens | $8/1M tokens |
| Providers |
Capabilities
| Capability | Grok 4 Fast Reasoning | o3 |
|---|---|---|
| Vision | Yes | Yes |
| Multimodal | Yes | Yes |
| Reasoning | Yes | Yes |
| JSON / Tool use | No | Yes |
| Structured outputs | No | Yes |
| Code execution | No | Yes |
| IDE integration | No | No |
| Computer use | No | No |
| Parallel agents | No | No |
Benchmarks
No shared benchmark scores are currently available for this pair.
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Last reviewed: 2026-07-10. Data sourced from public model cards and provider documentation.