The JVM has bytecode and .NET has IL. Over the last couple of weeks Screenplay became that for the business: one .play model that people, AI assistants, Studio and your C# code all write, and that VS Code, your MCP host, your running Arc application and the Cratis CLI all draw as the same event model board.
Stage renders a narrow slice of a Screenplay model into C# on Arc and Chronicle, boots a throwaway sandbox and runs the model's examples, and refuses with a code whatever it can't express. Scene is the UI model between a screen in the model and React.
Three places Arc's conventions make a decision you might not see: a validator that runs in the browser and on the server but not identically, a proxy folder the generator owns, and routes, event identities and mappings chosen by a name. What each one does, and the fix.
Chronicle erases a person by destroying their key. The events stay, their marked values read as empty, and the fence stops the key from coming back.
Chronicle has clients for .NET, Kotlin and Java, TypeScript and Elixir, and Arc has servers on ASP.NET Core, Spring Boot and, as a source preview, Node.js. What each one gives you today, which registry it comes from, and the defaults that still differ from .NET.
Most specs in a Cratis application can call a method and compare the event it returns. What the in-process scenarios add on top, the ways a green spec proves nothing, and the few tests that still need a real Chronicle and a running host.
AuthProxy signs the user in, picks a tenant and forwards it as Tenant-ID. Arc reads a header called x-cratis-tenant-id until you tell it otherwise, and Chronicle turns the tenant into a namespace. Every hop from the edge to storage, what a missing tenant becomes at each one, and what stays yours to check.
The author list and the register dialog from the cratis template, built with Cratis Components 4. Which parts come from Components and which from Arc, how a field binds to a generated command, how a taken name gets back to the Name field, theming and renderer adapters, and where the library stops.
In Arc, a query that loads once and a query that keeps pushing results to React differ by one return type, given data that notifies on change. The whole path from a write to the screen, the transport underneath, the hook and its states, command dialogs, view models and live tables, and which hop is live and which is a snapshot.
Every place a rule can live in a Cratis application, from the rules copied into the browser to Chronicle's append. What each layer can and can't guarantee, why a taken author name is caught in only one of them, and which rejections find their way back to a form field.
A command is a record with a Handle method and a query is a static method on the read model it returns. What Arc builds from those two shapes, in which order it checks a request, what reaches the generated TypeScript, and what stays the application's job.
Checking a read model before you append enforces nothing. Constraints, concurrency scopes, decision reads and Arc aggregates check at write time, and each reaches a different set of appends.
One attribute turns AuthorRegistered into a list on a screen. Model-bound, fluent and PDL projections, reducers for when attributes can't say it, where read models are stored, how a live list learns about changes, and what Chronicle does when a projection changes or has to be rebuilt.
An event records something that happened, and no event is taken out of the log afterwards. How to name and shape events that still read in five years, and what Chronicle stores with each one: a sequence number, its source, the operation it belonged to, who caused it and why.
The sticky-note feature followed from its model to production, into year two and to agents writing its code, through every Cratis product this post names, with 26 more posts that take each one all the way down.
Scaffold a full-stack Cratis application from the official .NET templates, model a small library with Arc commands and a Chronicle read model, and see it live in a React UI — every command in this post was executed against the exact versions it names.
