Agency comes from the model. An agent product = model + harness.

Agency Comes from the Model

Before we talk about code, let’s get one thing straight.

Agency — the ability to perceive, reason, and act — comes from model training, not from external code orchestration. A working agent product needs both the model and the harness. The model is the driver, the harness is the vehicle.

This course teaches you to build the vehicle.

What a Harness Is

Harness = Tools + Knowledge + Observation + Action Interfaces + Permissions

    Tools:          file I/O, shell, network, database, browser
    Knowledge:      product docs, domain references, API specs, style guides
    Observation:    git diff, error logs, browser state, sensor data
    Action:         CLI commands, API calls, UI interactions
    Permissions:    sandboxing, approval workflows, trust boundaries

The model decides. The harness executes. The model reasons. The harness provides context.

The Agent Pattern

    User --> messages[] --> LLM --> response
                                      |
                            stop_reason == "tool_use"?
                           /                          \
                         yes                           no
                          |                             |
                    execute tools                    return text
                    append results
                    loop back -----------------> messages[]

That’s the minimal loop. Every AI agent needs this loop. The MODEL decides when to call tools and when to stop. The CODE just executes what the model asks for.

What You’ll Build

Phase 1: Core Loop — The agent loop, tools, planning, subagents, skills, context compaction

Phase 2: System Hardening — Permissions, hooks, memory, system prompts, error recovery

Phase 3: Task Runtime — Task graphs, background execution, cron scheduling

Phase 4: Multi-Agent Platform — Teams, protocols, autonomous agents, worktree isolation, MCP

Key Takeaway

Agency comes from the model. The harness makes agency real. Build great harnesses. The model will do the rest. Bash is all you need.

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