Gemini 3.5 Flash vs GPT-5 — Mid-Tier AI API Comparison 2026

Requests per Day

Days per Month

Google

Gemini 3.5 Flash

$0.00

per month

Input cost

Output cost

Cost per request

Requests/month

OpenAI

GPT-5

$0.00

per month

Input cost

Output cost

Cost per request

Requests/month

Other Models to Consider

Claude Sonnet 4.6

Anthropic

$3.00 / $15.00 per 1M

1M context

Gemini 3.1 Pro

Google

$2.00 / $12.00 per 1M

1M context

Grok 4.3

xAI

$1.25 / $2.50 per 1M

1M context

Which Model for Which Use Case?

Long Document Processing

Gemini 3.5 Flash's 1M context window handles entire codebases, legal contracts, or research papers in a single request. GPT-5's 272K context may require chunking.

Better for long context: Gemini 3.5 Flash

High-Input Classification

For workloads with heavy input tokens (document classification, sentiment analysis), GPT-5's 17% cheaper input pricing ($1.25 vs $1.50/M) adds up at scale.

Better value: GPT-5

Content Generation

Content generation is output-heavy. Gemini 3.5 Flash is 10% cheaper on output ($9.00 vs $10.00/M), making it the better choice for blog posts, marketing copy, and creative writing.

Better value: Gemini 3.5 Flash

Function Calling & Agents

GPT-5 has mature function calling and the Assistants API for building AI agents. If you need tool use, OpenAI's ecosystem is more battle-tested.

Better ecosystem: GPT-5

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Frequently Asked Questions

Is GPT-5 cheaper than Gemini 3.5 Flash?

On input tokens, yes — GPT-5 costs $1.25/M vs Gemini 3.5 Flash at $1.50/M (17% cheaper). On output tokens, Gemini is cheaper at $9.00/M vs GPT-5's $10.00/M (10% cheaper). Overall cost depends on your input/output ratio.

Which has a larger context window?

Gemini 3.5 Flash has a 1M token context window — 3.7x larger than GPT-5's 272K context. For long documents, codebases, or extended conversations, Gemini handles significantly more context in a single request.

When should I choose GPT-5?

Choose GPT-5 if you need OpenAI's ecosystem (function calling, Assistants API, GPT plugins), prefer OpenAI's reasoning quality, or have high-input/low-output workloads where GPT-5's cheaper input pricing matters most.

When should I choose Gemini 3.5 Flash?

Choose Gemini 3.5 Flash if you need a 1M context window for long documents, want Google Cloud/Vertex AI integration, have high-output workloads where Gemini's cheaper output matters, or need multimodal capabilities.