Google Gemini API - Complete Guide

Version: 3.0.0 (14 Known Issues Added) Package: @google/[email protected] (⚠️ NOT @google/generative-ai) Last Updated: 2026-01-21

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⚠️ CRITICAL SDK MIGRATION WARNING

DEPRECATED SDK: @google/generative-ai (sunset November 30, 2025) CURRENT SDK: @google/genai v1.27+

If you see code using @google/generative-ai, it's outdated!

This skill uses the correct current SDK and provides a complete migration guide.

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Status

✅ Phase 1 Complete:

• ✅ Text Generation (basic + streaming)
• ✅ Multimodal Inputs (images, video, audio, PDFs)
• ✅ Function Calling (basic + parallel execution)
• ✅ System Instructions & Multi-turn Chat
• ✅ Thinking Mode Configuration
• ✅ Generation Parameters (temperature, top-p, top-k, stop sequences)
• ✅ Both Node.js SDK (@google/genai) and fetch approaches

✅ Phase 2 Complete:

• ✅ Context Caching (cost optimization with TTL-based caching)
• ✅ Code Execution (built-in Python interpreter and sandbox)
• ✅ Grounding with Google Search (real-time web information + citations)

📦 Separate Skills:

• Embeddings: See google-gemini-embeddings skill for text-embedding-004

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Table of Contents

Phase 1 - Core Features:

1. Quick Start
2. Current Models (2025)
3. SDK vs Fetch Approaches
4. Text Generation
5. Streaming
6. Multimodal Inputs
7. Function Calling
8. System Instructions
9. Multi-turn Chat
10. Thinking Mode
11. Generation Configuration

Phase 2 - Advanced Features: 12. Context Caching 13. Code Execution 14. Grounding with Google Search

Common Reference: 15. Known Issues Prevention 16. Error Handling 17. Rate Limits 18. SDK Migration Guide 19. Production Best Practices

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Quick Start

Installation

CORRECT SDK:

npm install @google/[email protected]

❌ WRONG (DEPRECATED):

npm install @google/generative-ai  # DO NOT USE!

Environment Setup

export GEMINI_API_KEY="..."

Or create .env file:

GEMINI_API_KEY=...

First Text Generation (Node.js SDK)

import { GoogleGenAI } from '@google/genai';

const ai = new GoogleGenAI({ apiKey: process.env.GEMINI_API_KEY });

const response = await ai.models.generateContent({
  model: 'gemini-2.5-flash',
  contents: 'Explain quantum computing in simple terms'
});

console.log(response.text);

First Text Generation (Fetch - Cloudflare Workers)

const response = await fetch(
  `https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent`,
  {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-goog-api-key': env.GEMINI_API_KEY,
    },
    body: JSON.stringify({
      contents: [{ parts: [{ text: 'Explain quantum computing in simple terms' }] }]
    }),
  }
);

const data = await response.json();
console.log(data.candidates[0].content.parts[0].text);

---

Current Models (2025)

Gemini 3 Series (December 2025)

gemini-3-flash

• Context: 1,048,576 input tokens / 65,536 output tokens
• Status: 🆕 Generally Available (December 2025)
• Description: Google's fastest and most efficient Gemini 3 model for production workloads
• Best for: High-throughput applications, low-latency responses, cost-sensitive production
• Features: Enhanced multimodal, function calling, streaming, thinking mode
• Benchmark Performance: Matches gemini-2.5-pro quality at gemini-2.5-flash speed/cost
• Recommended for: Production use cases requiring speed + quality balance

gemini-3-pro-preview

• Context: TBD (documentation pending)
• Status: Preview release (November 18, 2025)
• Description: Google's newest and most intelligent AI model with state-of-the-art reasoning
• Best for: Most complex reasoning tasks, advanced multimodal understanding, benchmark-critical applications
• Features: Enhanced multimodal (text, image, video, audio, PDF), function calling, streaming
• Benchmark Performance: Outperforms Gemini 2.5 Pro on every major AI benchmark
• ⚠️ Preview Models Warning: Preview models have NO SLAs and can change or be deprecated with little notice. Use GA (generally available) models for production. See Issue #13

Gemini 2.5 Series (General Availability - Stable)

gemini-2.5-pro

• Context: 1,048,576 input tokens / 65,536 output tokens
• Description: State-of-the-art thinking model for complex reasoning
• Best for: Code, math, STEM, complex problem-solving
• Features: Thinking mode (default on), function calling, multimodal, streaming
• Knowledge cutoff: January 2025

gemini-2.5-flash

• Context: 1,048,576 input tokens / 65,536 output tokens
• Description: Best price-performance workhorse model
• Best for: Large-scale processing, low-latency, high-volume, agentic use cases
• Features: Thinking mode (default on), function calling, multimodal, streaming
• Knowledge cutoff: January 2025

gemini-2.5-flash-lite

• Context: 1,048,576 input tokens / 65,536 output tokens
• Description: Cost-optimized, fastest 2.5 model
• Best for: High throughput, cost-sensitive applications
• Features: Thinking mode (default on), function calling, multimodal, streaming
• Knowledge cutoff: January 2025

Model Feature Matrix

Feature	3-Flash	3-Pro (Preview)	2.5-Pro	2.5-Flash	2.5-Flash-Lite
Thinking Mode	✅ Default ON	TBD	✅ Default ON	✅ Default ON	✅ Default ON
Function Calling	✅	✅	✅	✅	✅
Multimodal	✅ Enhanced	✅ Enhanced	✅	✅	✅
Streaming	✅	✅	✅	✅	✅
System Instructions	✅	✅	✅	✅	✅
Context Window	1,048,576 in	TBD	1,048,576 in	1,048,576 in	1,048,576 in
Output Tokens	65,536 max	TBD	65,536 max	65,536 max	65,536 max
Status	GA	Preview	Stable	Stable	Stable

⚠️ Context Window Correction

ACCURATE (Gemini 2.5): Gemini 2.5 models support 1,048,576 input tokens (NOT 2M!) OUTDATED: Only Gemini 1.5 Pro (previous generation) had 2M token context window GEMINI 3: Context window specifications pending official documentation

Common mistake: Claiming Gemini 2.5 has 2M tokens. It doesn't. This skill prevents this error.

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SDK vs Fetch Approaches

Node.js SDK (@google/genai)

Pros:

• Type-safe with TypeScript
• Easier API (simpler syntax)
• Built-in chat helpers
• Automatic SSE parsing for streaming
• Better error handling

Cons:

• Requires Node.js or compatible runtime
• Larger bundle size
• May not work in all edge runtimes

Use when: Building Node.js apps, Next.js Server Actions/Components, or any environment with Node.js compatibility

Fetch-based (Direct REST API)

Pros:

• Works in any JavaScript environment (Cloudflare Workers, Deno, Bun, browsers)
• Minimal dependencies
• Smaller bundle size
• Full control over requests

Cons:

• More verbose syntax
• Manual SSE parsing for streaming
• No built-in chat helpers
• Manual error handling

Use when: Deploying to Cloudflare Workers, browser clients, or lightweight edge runtimes

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Text Generation

Basic Text Generation (SDK)

import { GoogleGenAI } from '@google/genai';

const ai = new GoogleGenAI({ apiKey: process.env.GEMINI_API_KEY });

const response = await ai.models.generateContent({
  model: 'gemini-2.5-flash',
  contents: 'Write a haiku about artificial intelligence'
});

console.log(response.text);

Basic Text Generation (Fetch)

const response = await fetch(
  `https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent`,
  {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-goog-api-key': env.GEMINI_API_KEY,
    },
    body: JSON.stringify({
      contents: [
        {
          parts: [
            { text: 'Write a haiku about artificial intelligence' }
          ]
        }
      ]
    }),
  }
);

const data = await response.json();
console.log(data.candidates[0].content.parts[0].text);

Response Structure

{
  text: string,