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

Source: Dev.to

Overview

Calling the Claude API is easy: put a prompt in messages.create and 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

1. User question →
2. Claude picks tools →
3. Your code executes →
4. Return tool_result →
5. 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 Multiple Steps

Three workflow patterns:

• Parallelization – run independent tasks at the same time
• Chaining – one step’s output feeds the next
• Routing – classify inputs and send them to specialized paths

Practical distinction

• Workflows are predictable and stable.
• Agents are flexible but less predictable.

Start with workflows. Add agent behavior after guardrails are in place.

How This Becomes a Real App

• Resume analysis service – run technical‑skills, career‑trajectory, and culture‑fit checks in parallel.
• Fortune‑telling app (saju) – connect deterministic calendar/element tools, retrieve interpretation knowledge via RAG, then chain base analysis → detailed interpretation → recommendations.
• Real‑estate analysis – combine price APIs (Tool Use), policy/news retrieval (RAG), and parallel analysis pipelines.

The API call is just the beginning. Tool Use gives Claude hands, RAG gives it memory, and workflows give it a brain.