09 Building Agents

Phase 2 - Agent Core
Stage 9 of 14. Previous: Stage 8 - Agent Architectures. Next: Stage 10 - Multi-Agent Systems.

Goal

Build agents from direct APIs first, then learn when frameworks add value.

Learn

• Building an agent loop from scratch
• Direct LLM API calls
• Parsing model output
• Error and rate-limit handling
• Provider-native function calling and tool use
• LangChain, LangGraph, LlamaIndex, CrewAI, AutoGen, and the OpenAI Agents SDK
• How to choose a framework without losing debuggability

Build

Build the same tool-using agent twice: once from scratch and once with a framework. Compare transparency, speed, debugging, and maintainability.

Exit Criteria

• You understand the agent loop beneath framework abstractions.
• You can decide when a framework is worth using.
• You can handle tool failures, rate limits, and malformed model output.

Checkpoint

Use the Stage 9 checkpoint before moving on.