How agent harnesses actually work — built from scratch, one primitive at a time.
“What I cannot create, I do not understand.”
SDLC with Agents is a course, a channel, and a CLI. swa is a teaching harness: each chapter adds one internal (model call → tool loop → context window → caching → subagents → worktrees → loops → review gates) until you have a working Claude-Code-class agent you understand completely. The essays, the videos, and the weekly digest are built from the same source.
Now building chapter 0: bare model call
The five laws
Every harness lives under them. Each one forbids a sentence you have probably heard. Refer to them by number: “that’s a Law 4 violation.”
| # | Law | Forbids |
|---|---|---|
| 1 | Statelessness. Memory is harness replay, never a model property. | forbids |
| 2 | Finite attention. Every token in context is paid twice: money, and attention taken from every other token. | forbids |
| 3 | Tools. The model has no hands. Same model, different tool list = different agent. | forbids |
| 4 | Independence. A check is worth exactly its independence from what it checks. | forbids |
| 5 | Stochasticity. The same prompt is a distribution, not an answer. | forbids |
The method: only measurement knows.
The chapters
One primitive per chapter, in build order. Each starts from a mystery you have felt and ends with the code in swa that explains it.
| ch | Status | Primitive | The mystery |
|---|---|---|---|
| 0 | ◐ | Bare model call | What is the model actually receiving? |
| 1 | ○ | Tool loop | Same model, 30 points apart |
| 2 | ○ | File + shell tools, permission gate | Auto mode: who decided that was safe? |
| 3 | ○ | Context window | Why does my agent get dumber after an hour? |
| 4 | ○ | Prompt caching | Why did this session cost $14 and that one $2? |
| 5 | ○ | Instruction layering | Why did it forget the rule in my file? |
| 6 | ○ | Subagents | A huge band collapsing to a dot |
| 7 | ○ | Worktrees | Why did two agents clobber each other? |
| 8 | ○ | Loops | Why did it rewrite the same test three times? |
| 9 | ○ | Review gates | Why can't it check its own work? |
| 10 | ○ | Skills | On-demand instructions and what they cost |
| 11 | ○ | Cost, failure modes, evals | Harness Bench |
| 12 | ○ | Student mode | The harness makes you explain it |
The weekly digest
What changed in agent harnesses this week, with a take on each item. Collected and scored by agents, approved by a human before it ships. One email a week.
Confirmation opens in a new tab. Every email has an unsubscribe link.