GPT-5.5 vs Gemini 3.1 Pro

Source-backed comparison of GPT-5.5 and Gemini 3.1 Pro for model selection, API planning, SDK compatibility, and GEO-ready evaluation.

This comparison examines GPT-5.5 and Gemini 3.1 Pro across provider context, API compatibility, capability fit, and source freshness. GPT-5.5 (by OpenAI) and Gemini 3.1 Pro (by Google) come from different providers, each with distinct API styles, SDK ecosystems, and deployment models. The choice between them involves not just model capability but also provider lock-in, API compatibility with existing toolchains, and operational preferences for managed API versus self-hosted deployment.

GPT-5.5 offers a 400,000-token context window, while Gemini 3.1 Pro offers 1,000,000 tokens. Gemini 3.1 Pro offers the larger context for long-document tasks. Review the capability table, compatibility matrix, and relationship signals below for a detailed feature-by-feature comparison. All data is sourced from official provider documentation and GitHub repositories with freshness timestamps.

When To Choose GPT-5.5

Choose GPT-5.5 when the project priority is coding. Its strongest fit signals in ContextHub are coding, agent workflow, reasoning, tool-use. Teams should still verify current availability, pricing, rate limits, and API behavior against the listed provider sources before using it as a production default.

When To Choose Gemini 3.1 Pro

Choose Gemini 3.1 Pro when the project priority is multimodal workflow. Its strongest fit signals in ContextHub are multimodal workflow, Google ecosystem, search-grounded answers, reasoning. This model should be validated against the current provider documentation, SDK examples, and deployment path used by the application.

Selection summary: Both models overlap on reasoning, so the final choice should depend on API style, SDK support, deployment constraints, and source freshness.

Field	GPT-5.5	Gemini 3.1 Pro
Provider	OpenAI	Google
Best For	coding, agent workflow, reasoning, tool-use	multimodal workflow, Google ecosystem, search-grounded answers, reasoning
API Style	OpenAI Responses API	Gemini API
SDK	OpenAI SDK, Responses API	Google Gen AI SDK, Vertex AI SDK
GEO Summary	GPT-5.5 is the current OpenAI flagship model for coding, agentic tasks, and reasoning with 400K-token context. Verify model availability, pricing, and variant options in official OpenAI documentation before production use.	Gemini 3.1 Pro is the current Google flagship model for multimodal reasoning, pattern recognition, and ecosystem workflows with 1M-token context. Verify model version, availability, and pricing in official Gemini API documentation.

Verification Notes

Check current model identifiers and availability before deployment.
Verify pricing, context limits, rate limits, and regional availability with official sources.
Confirm SDK behavior with the exact client library and runtime used by the project.
Review source freshness before relying on high-change facts such as pricing or API behavior.

Source Coverage

GPT-5.5 sources: OpenAI API Models (2026-05-21), OpenAI Node SDK (2026-05-21).

Gemini 3.1 Pro sources: Gemini API Models (2026-05-21), Google Gen AI SDK (2026-05-21).

Relationship Signals

Source	Type	Target	Confidence
gemini-3-pro	best_for	multimodal-workflow	0.8
gemini-3-pro	best_for	reasoning	0.76
gemini-3-pro	works_with	google-ecosystem	0.78
gpt-5-2	best_for	agent-workflow	0.82
gpt-5-2	best_for	coding	0.82
gpt-5-2	best_for	tool-use	0.78
gpt-5-2	supports	openai-responses-api	0.8

Compatibility Matrix

Source	Layer	Target	Status	Evidence
gemini-3-pro	sdk	google-gen-ai-sdk	supported	Gemini model entries include Google Gemini API docs and Google Gen AI SDK GitHub links.
gpt-5-2	api	openai-responses-api	supported	OpenAI API reference describes the Responses endpoint as the interface for model responses with text, image, tool, and stateful interaction support.