How to Use Claude API for Developers — Complete Guide 2026

The Claude API lets you integrate Anthropic's models into your apps — chat, agents, RAG, and more. This guide walks through setup, the Messages API, streaming, tool use, and how to keep costs under control.

## Getting Started

**1. Get an API key.** Sign up at console.anthropic.com and create an API key. Store it in environment variables (e.g. ANTHROPIC_API_KEY) and never commit it.

**2. Choose an SDK or REST.** Anthropic provides Python and TypeScript/Node SDKs. You can also call the REST API directly.

**3. Pick a model.** Sonnet balances speed and quality; Opus for the hardest tasks; Haiku for low cost and latency. Model IDs and context windows are in the docs.

## Messages API Basics

The primary interface is the Messages API. You send a list of messages (user/assistant turns) and optional system prompt. The response includes content (text and/or tool use). Use max_tokens to cap output length.

**Streaming:** Use the streaming option to get tokens as they are generated. Essential for chat UIs and long answers.

## System Prompts and Tool Use

**System prompt:** Pass system to set behaviour (persona, rules, context). Keep it clear and within token limits.

**Tool use (function calling):** Define tools with names, descriptions, and input schemas. Claude can request tool calls; you run the function and append the result as a new turn, then continue the conversation.

## Pricing and Limits

Pricing is per token (input vs output). Haiku is cheapest; Opus is most expensive. Check the Anthropic pricing page. Use batch API for non-real-time workloads when available. Implement retries with backoff for rate limits.

## Best Practices

- **Security:** Never expose the API key. Use server-side or backend-only calls.
- **Streaming:** Use streaming for better UX and to start rendering sooner.
- **Token limits:** Respect max_tokens and context window; truncate or summarize if needed.
- **Cost:** Start with Haiku or Sonnet; move to Opus only where quality justifies it.

The Claude API is well suited to chat, coding assistants, and structured reasoning. Use the official docs and SDKs for the latest parameters and models.