Claude API: The Complete Guide for Developers in 2026
Everything you need to know about using Claude (Anthropic's API) in production — from basic calls to tool use, streaming, and multi-turn conversations.
Claude by Anthropic has rapidly become the preferred LLM for production AI applications. In 2026, Claude Opus 4.6 and Sonnet 4.6 lead benchmarks for reasoning, coding, and instruction-following. Here's how to use the API effectively.
Getting Started
Install the SDK:
```
npm install @anthropic-ai/sdk
```
Basic usage:
```typescript
import Anthropic from "@anthropic-ai/sdk";
const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY });
const message = await client.messages.create({
model: "claude-sonnet-4-6",
max_tokens: 1024,
messages: [{ role: "user", content: "Explain RAG in simple terms." }],
});
console.log(message.content[0].text);
```
Choosing the Right Model
System Prompts
System prompts define Claude's behaviour. Spend time here — it's the highest-leverage prompt engineering you can do.
```typescript
const message = await client.messages.create({
model: "claude-sonnet-4-6",
max_tokens: 1024,
system: "You are an expert AI consultant. Be concise, specific, and always give actionable advice.",
messages: [{ role: "user", content: "How should I approach AI implementation?" }],
});
```
Tool Use (Function Calling)
Claude's tool use is exceptional — it reliably decides when and how to use tools.
```typescript
const tools = [
{
name: "get_customer",
description: "Fetch customer data by ID",
input_schema: {
type: "object",
properties: { customer_id: { type: "string", description: "The customer ID" } },
required: ["customer_id"],
},
},
];
const message = await client.messages.create({
model: "claude-sonnet-4-6",
max_tokens: 1024,
tools,
messages: [{ role: "user", content: "Look up customer C123 and summarise their account." }],
});
```
Streaming
For long responses or real-time UX, use streaming:
```typescript
const stream = await client.messages.stream({
model: "claude-sonnet-4-6",
max_tokens: 2048,
messages: [{ role: "user", content: "Write a detailed business plan for an AI agency." }],
});
for await (const chunk of stream) {
if (chunk.type === "content_block_delta") {
process.stdout.write(chunk.delta.text);
}
}
```
Production Best Practices
Cost Optimisation
Claude Sonnet is 5x cheaper than Opus. For most production workloads, Sonnet delivers 95% of Opus quality at a fraction of the cost. Use Opus only for tasks where maximum intelligence is critical.
Need help integrating Claude into your product? Book a free call with our team.
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