Anthropic’s Claude API enables developers to integrate Claude’s language understanding directly into their applications. Here is a practical guide focused on what actually matters when building.
Getting Started
The API uses standard HTTP with JSON. The Python and TypeScript SDKs (pip install anthropic / npm install @anthropic-ai/sdk) simplify integration. Authentication is via API key in the header. The basic Messages API call:
import anthropic
client = anthropic.Anthropic()
message = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=1024,
messages=[{"role": "user", "content": "Hello, Claude"}]
)
print(message.content[0].text)System Prompts
The system parameter sets the context and persona for Claude in your application. This is where you define what Claude’s role is, what constraints it operates under, what it should and should not do, and what format it should use. A well-crafted system prompt is the primary lever for shaping Claude’s behaviour in your application. Keep it concise and specific — vague instructions produce inconsistent results.
Tool Use
Tool use (function calling) lets Claude decide when to use tools you define and return structured calls that your code executes. Define tools with a JSON schema; Claude decides when to call them and with what parameters. This is the foundation for agents — Claude can search the web, query databases, or call APIs based on its understanding of the user’s request.
Prompt Caching
For applications with long, repeated system prompts or context (RAG documents, large tool definitions), prompt caching reduces cost by 90% and latency significantly by caching the prompt prefix. Mark the portion of your prompt to cache with the cache_control parameter. Critical for production cost management.
Production Considerations
Rate limits: Claude’s API has input/output token-per-minute limits by tier. Handle 429 (rate limit) and 529 (API overload) errors with exponential backoff. Streaming: use the streaming API for applications where the user sees Claude’s response as it is generated — dramatically improves perceived latency. Costs: at Claude Sonnet 4.5 pricing (as of mid-2025), typical document processing is around $0.003–0.010 per 1,000-word document.
