Agent harnesses

What you'll learn

Harness foundations · 2 lessons, about 1.5 hours

1. What an agent harness is FreeThe software around a model that turns it into an agent.
2. The core loop, built properlyStop reasons, parallel tool calls, errors and turn budgets.

Tools & environment · 3 lessons, about 2.5 hours

1. A tool registry for agentsSchemas from functions, validation, output limits and helpful errors.
2. File and search toolsRead, write, edit by exact replacement, glob and grep inside one folder.
3. Running code safelyExecution tools, step limits, output caps and real sandboxes.

Context & control · 4 lessons, about 3 hours

1. Permissions and approvalsRules, modes and approval prompts that keep a person in control.
2. Hooks and skillsExtend a harness at lifecycle events, and load know-how on demand.
3. Managing the context windowBudgets, clearing old tool results, compaction and a cacheable prefix.
4. Memory, plans and long tasksTo-do lists, progress notes, memory files and checkpoints.

Scaling & shipping · 5 lessons, about 4 hours

1. SubagentsHand a focused task to a fresh context with fewer tools.
2. Streaming, interrupts and sessionsLive output, stopping safely, and saving and resuming transcripts.
3. Evaluation harnessesRun agents on tasks in fresh environments and grade what they leave behind.
4. Tracing, budgets and guardrailsSee what an agent did, cap what it spends, and stop it when it loops.
5. Capstone: build a coding agentA harness that edits a project, runs its tests and stops when they pass.

Before you start

Lessons here build on earlier phases:

• Phase 1: Building with LLMs
• Phase 6: Modern LLM engineering
• Phase 7: Software engineering
• Phase 9: Model Context Protocol

The placement test can mark off what you already know, so you only study what's new to you.

What you'll build

Build a coding agent

A harness that edits a project, runs its tests and stops when they pass. Read the tutorial.

• Combine the loop, tools, permissions, a stop check and an event log into one coding agent
• Give the agent file tools that stay inside the project and refuse to change the tests
• Make the harness, not the model, decide when the work is done: when the tests pass

When you finish

Finish every lesson in this phase, or show you know it in the placement test, and you earn a certificate of completion, "Agent harnesses (Phase 10)", with its own link you can share.