Why Osy# · ten chapters, one reason each

Why, then how, then proof.

The reference answers what does this construct do. These answer why would I want this at all, and then how it works, and then what happens when you change it. Every example in them is a real file out of a real app, read at build time.

Chapter 01
Built for agents · Written by agents
A reader that cannot ask a colleague can ask this compiler.
An agent gets the app's real shape, real access rules and real trace from four commands, not from guessing.
Chapter 02
Agents as declarations · Agentic apps
The apps that are hard everywhere else are the ordinary kind here.
Declare one; its answer streams into a page a token at a time; it can park on a person for two days.
Chapter 03
Workflows that wait · Workflows
A process that waits three days for a person is still just source.
The state is a column on your entity; a run outlives the deploy that started it.
Chapter 04
Runs exactly once · Durable execution, built in
Your database is transactional. The world is not.
A crash between charging a card and sending the confirmation cannot charge the card twice, and cannot send the mail twice.
Chapter 05
Secure by default · Security
Say who may read it, once. Every query obeys.
A caseworker may read a case if their unit handles it, they have not recused themselves, and they are senior enough for a restricted one — four rules, named once, one statement.
Chapter 06
Nothing to mock · Testing
An agent can prove its code works. Not say so.
The agent writes the test beside the feature, runs one command, and hands you a green that ran real functions against real rules on a real database, and looked at the screen.
Chapter 07
The editor is the compiler · The editor
Your editor is never wrong about your app.
Hover, rename, go-to-definition and the debugger come from the resolver that compiles your app.
Chapter 08
UI in the language · The UI model
The screen is written in the same language as the rules. There is no second stack.
Reactivity, layout and theming are parts of one language, not three libraries.
Chapter 09
Documents are data · Markdown
A document a person and an agent can edit at the same time.
One type: an editor kit, nine agent tools, streaming and a memory that cites its sources follow from it — nothing to integrate.
Chapter 10
One program · One program
You do not place code. The compiler does.
Where each line runs follows from what it touches, and the compiler prints where.