Gemini 3.1 Pro Preview Custom Tools vs Phi 3.5 Mini Instruct
Gemini 3.1 Pro Preview Custom Tools (2026) and Phi 3.5 Mini Instruct (2024) are compact production models from Google DeepMind and Microsoft Research. Gemini 3.1 Pro Preview Custom Tools ships a 1m-token context window, while Phi 3.5 Mini Instruct ships a 128k-token context window. 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 Mini Instruct is ~122% cheaper at $0.90/1M; pay for Gemini 3.1 Pro Preview Custom Tools only for long-context analysis.
Decision scorecard
Local evidence first| Signal | Gemini 3.1 Pro Preview Custom Tools | Phi 3.5 Mini Instruct |
|---|---|---|
| Best for | multimodal apps and long-context analysis | provider-routed production |
| Decision fit | RAG, Long context, and Vision | Long context |
| Context window | 1m | 128k |
| Cheapest output | $12/1M tokens | $0.90/1M tokens |
| Provider routes | 1 tracked | 2 tracked |
| Shared benchmarks | 0 shared | 0 shared |
Decision tradeoffs
- Gemini 3.1 Pro Preview Custom Tools has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Gemini 3.1 Pro Preview Custom Tools uniquely exposes Vision, Multimodal, and Structured outputs in local model data.
- Local decision data tags Gemini 3.1 Pro Preview Custom Tools for RAG, Long context, and Vision.
- Phi 3.5 Mini Instruct has the lower cheapest tracked output price at $0.90/1M tokens.
- Phi 3.5 Mini Instruct has broader tracked provider coverage for fallback and procurement flexibility.
- 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.
Gemini 3.1 Pro Preview Custom Tools
$4,600
Cheapest tracked route/tier: OpenRouter
Phi 3.5 Mini Instruct
$945
Cheapest tracked route/tier: Fireworks AI
Estimated monthly gap: $3,655. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- No overlapping tracked provider route is sourced for Gemini 3.1 Pro Preview Custom Tools and Phi 3.5 Mini Instruct; plan for SDK, billing, or endpoint changes.
- Phi 3.5 Mini Instruct is $11.10/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Vision, Multimodal, and Structured outputs before moving production traffic.
- No overlapping tracked provider route is sourced for Phi 3.5 Mini Instruct and Gemini 3.1 Pro Preview Custom Tools; plan for SDK, billing, or endpoint changes.
- Gemini 3.1 Pro Preview Custom Tools is $11.10/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Gemini 3.1 Pro Preview Custom Tools adds Vision, Multimodal, and Structured outputs in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2026-01-01 | 2024-08-20 |
| Context window | 1m | 128k |
| Parameters | — | 3.8B |
| Architecture | Decoder Only | Decoder Only |
| License | Proprietary | MITOSI-approved |
| Openness | Proprietary | Open source |
| Commercial use | Commercial use: conditional | Commercial use: permitted |
| Knowledge cutoff | 2025-01 | 2023-10 |
Pricing and availability
| Pricing attribute | Gemini 3.1 Pro Preview Custom Tools | Phi 3.5 Mini Instruct |
|---|---|---|
| Input price | $2/1M tokens | $0.90/1M tokens |
| Output price | $12/1M tokens | $0.90/1M tokens |
| Providers |
Capabilities
| Capability | Gemini 3.1 Pro Preview Custom Tools | Phi 3.5 Mini Instruct |
|---|---|---|
| Vision | Yes | No |
| Multimodal | Yes | No |
| Reasoning | No | No |
| Function calling | No | No |
| Tool use | No | 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 scores are currently available for this pair.
Deep dive
The capability footprint differs most on vision: Gemini 3.1 Pro Preview Custom Tools, multimodal input: Gemini 3.1 Pro Preview Custom Tools, and structured outputs: Gemini 3.1 Pro Preview Custom Tools. 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, Gemini 3.1 Pro Preview Custom Tools lists $2/1M input and $12/1M 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 Phi 3.5 Mini Instruct lower by about $4.10 per million blended tokens. Availability is 1 providers versus 2, so concentration risk also matters.
Choose Gemini 3.1 Pro Preview Custom Tools when long-context analysis and larger context windows are central to the workload. Choose Phi 3.5 Mini Instruct when provider fit, lower input-token cost, and broader provider choice 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, Gemini 3.1 Pro Preview Custom Tools or Phi 3.5 Mini Instruct?
Gemini 3.1 Pro Preview Custom Tools supports 1m 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, Gemini 3.1 Pro Preview Custom Tools or Phi 3.5 Mini Instruct?
Phi 3.5 Mini Instruct is cheaper on tracked token pricing. Gemini 3.1 Pro Preview Custom Tools costs $2/1M input and $12/1M 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 Gemini 3.1 Pro Preview Custom Tools or Phi 3.5 Mini Instruct open source?
Gemini 3.1 Pro Preview Custom Tools 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, Gemini 3.1 Pro Preview Custom Tools or Phi 3.5 Mini Instruct?
Gemini 3.1 Pro Preview Custom Tools 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, Gemini 3.1 Pro Preview Custom Tools or Phi 3.5 Mini Instruct?
Gemini 3.1 Pro Preview Custom Tools 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 Gemini 3.1 Pro Preview Custom Tools and Phi 3.5 Mini Instruct?
Gemini 3.1 Pro Preview Custom Tools is available on OpenRouter. 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: 2026-06-15. Data sourced from public model cards and provider documentation.