Building Real Apps With the Claude API — Tool Use, RAG, and Agent Patterns Explained

Calling the Claude API is easy. Put a prompt in messages.create , get an answer back. But that alone doesn’t make a product. To build a real app, you need three more things: Tool Use, RAG, and Agent/Workflow patterns . Tool Use — Giving Claude Hands Base Claude only generates text. Tool Use lets it call external functions. [ { "name" : "get_apartment_price" , "description" : "Look up apartment prices for a district" , "input_schema" : { "type" : "object" , "properties" : { "district" : { "type" : "string" }, "year" : { "type" : "integer" } }, "required" : [ "district" ] } } ] Flow: user question → Claude picks tools → your code executes → return tool_result → Claude writes final response. RAG — Making Claude Know What It Doesn’t RAG (Retrieval-Augmented Generation) injects external data into prompts. Pipeline: Chunking Embedding Retrieval (BM25 + vector search) Re-ranking Context injection This turns vague model output into data-grounded answers. Agents and Workflows — Chaining Multipl