For agents: start with the agent guide. Every page is Markdown by default; add Accept: text/html for HTML.

rat-stack 🐀

An Effect stack so pure (aspirational) Kit Langton will blush.

Build an Effect app and its cloud as one typed program. Joel’s goal is the best Effect + Alchemy application we can build; VISION.md explains why. AGENTS.md sets exact pins, checks, and hooks. The public tree lets you steal ideas. It carries no product support promise.

The pnpm + Turborepo workspace includes a working Effect v4 CLI, tests, formatting, and type-aware linting. Vendored source mirrors cover Effect, effect-solutions, XState, and Alchemy.

Define a Contract with Effect input, output, and failure schemas plus annotations. Bind a server-side handler, then project the capability onto every agent surface²³. The same inspectFile capability is the stats command, POST /inspectFile with an OpenAPI document, and an MCP tool over stdio.

Create a repository

gh repo create <name> --template joelhooks/rat-stack

Clone the new repository, then install and verify it:

nvm use
pnpm install
pnpm vendor:agent-sources
pnpm turbo run check test build

Node 24.18.0 and pnpm 11.3.0 are required. The requirement is declared in .nvmrc, engines, devEngines, and packageManager.

Workspace layout

Try the example CLI

The included stats command counts bytes, Unicode characters, words, and lines in a file:

pnpm cli stats README.md
pnpm cli stats README.md --json

After pnpm build, you can run the built entrypoint directly:

node apps/cli/dist/cli.js stats README.md

The same capability on the other surfaces:

pnpm cli openapi                 # OpenAPI 3.1 document for the REST projection
pnpm cli serve --port 3000       # POST /inspectFile, GET /openapi.json, GET /docs
pnpm cli mcp                     # MCP server over stdio, one tool per capability
pnpm cli mcp --code-mode         # MCP server with two tools: search and execute
pnpm cli catalog --types         # the `tools` declarations a code-mode program sees

Code mode²¹ is the fourth projection. The model gets search for ranked matches with TypeScript signatures and execute for JavaScript with tools in scope²⁰. The program runs in a fresh Node subprocess under --permission. It cannot touch the file system or spawn processes. To call the host, it uses tools.<name>(input). The host validates the input schema and runs the same handler used by every other surface. Node’s permission model allows network egress. Put a Worker or Deno runtime behind the same Sandbox service to isolate the network too.

packages/capability/src is where a capability becomes a Command, an HttpApiEndpoint, a Tool, and a catalog entry. packages/core/src/inspect-file.ts supplies the CLI’s example capability. Hosted content has its own registry in apps/site/src/capabilities/index.ts. Provider adapters stay outside core (rat-stack-boundaries/no-core-adapters); apps provide them to core’s job-shaped ports at composition. Add another to capabilities²¹, then verify every projection you keep; CLI command registration lives in apps/cli/src/command.ts.

What is in the stack?

• pnpm workspaces + Turborepo 2.11.2: cached typecheck / test / build across packages from day one.
• Effect 4.0.0: typed runtime, errors, filesystem service, and the three surfaces the capabilities project onto: effect/cli, effect/http-api (with OpenApi.fromApi), and effect/ai (Toolkit + McpServer).
• @effect/platform-node 4.0.0: Node-backed services; keep adapter and core pins matched.
• TypeScript 7.0.2: strict module and index-access checks.
• XState 6.0.0-alpha.65: real lifecycle states; do not replace those with boolean soup.
• @xstate/effect 0.1.0-alpha.7: the official XState v6 to Effect 4 bridge: createEffectActor runs a machine as a scoped Effect, fromEffect makes Effects into actors with typed failures³⁶ and requirements. packages/core/src/inspect-machine.ts is the example.
• Alchemy 2.0.0-beta.83: Infrastructure as Effects: apps/infra/alchemy.run.ts is the Stack; pnpm infra:plan / infra:deploy / infra:destroy; auth via pnpm alchemy profile edit.
• Oxlint + Ultracite + Oxfmt: native lint and format.
• Vitest 5.0.3 + @effect/vitest 4.0.0: it.effect and it.layer for every Effect test; running Effects by hand in a test file is a lint error.
• @effect/tsgo 0.51.1: patches TypeScript 7 in prepare so Effect language-service diagnostics (leaked requirements, any/unknown in channels, global Date/fetch/console inside Effect, Node built-ins where Effect has a service) fail tsc.
• varlock 1.20.0: .env.schema declares every variable with @env-spec decorators; pnpm check runs varlock load, secrets stay in gitignored .env.local.
• Vendored agent sources: Effect, effect-solutions, XState, Alchemy, Better Auth, and Foldkit via ./scripts/vendor-agent-sources.sh, each at the ref its workspace pin names (not x-algorithm; that stays app-specific).
• Agent fence³⁹: lefthook pre-commit + Pi/Cursor/Claude hooks that block git … --no-verify. Cheating should be uncomfortable and obvious.

Every dependency is pinned exactly. Review pin upgrades as stack changes.

Commands

Agentic surface

Every clone includes AGENTS.md for repo law and commands, CLAUDE.md as Claude Code’s pointer to that law, and VISION.md for project intent. Source mirrors live under .agent_sources/ after you run the vendor script. Repo-local Pi extensions belong in .pi/extensions/.

Make it yours

Start with the template:

gh repo create <name> --template joelhooks/rat-stack

Clone the new repository, then install it:

pnpm install
pnpm sources:fetch

pnpm sources:fetch imports missing pinned source⁹ commits from the configured repository remotes once, including in template children. Generation reads only local Git objects and never fetches. Re-run the command when adding a new pin that is not present locally.

Rename the workspace package names before you build on the example. The current name fields are:

• package.json: rat-stack
• apps/cli/package.json: @rat-stack/cli
• apps/infra/package.json: @rat-stack/infra
• apps/site/package.json: @rat-stack/site
• packages/capability/package.json: @rat-stack/capability
• packages/core/package.json: @rat-stack/core

Rename the bin entry in apps/cli/package.json, the root command in apps/cli/src/command.ts, and the version in apps/cli/src/version.ts. Then change every matching import. The current workspace imports are @rat-stack/capability, @rat-stack/cli, @rat-stack/core, @rat-stack/infra, and @rat-stack/site. Find them with:

rg -n '@rat-stack/' --glob '*.json' --glob '*.ts' -l

A new scope changes line lengths, so run the formatter once after the rename:

pnpm fix

Replace inspectFile in packages/core with one useful capability. Add it to capabilities²¹, then verify each surface you keep. Rewrite the Project law, Architecture, and Boundaries sections of AGENTS.md and the top of .pi/APPEND_SYSTEM.md; they describe rat-stack until you do. Keep expected failures typed and map them to deliberate exit codes.

apps/site is the public site and Worker. apps/infra is its Alchemy Stack. Delete both if you do not want a public site. If you keep them, change the Cloudflare Zone name and the DNS names and targets derived from it in apps/infra/alchemy.run.ts. Change the Worker domain and redirects in apps/site/src/worker.ts. Choose an Alchemy stage for each plan or deploy. The stage is a command-line choice, for example --stage <stage>. Configure credentials with:

pnpm alchemy profile edit --add Cloudflare

Run the acceptance gate before the first push:

pnpm turbo run check test build

What it promises and how to check

• One capability reaches every kept surface. Proof: packages/capability/test/to-command.test.ts, packages/capability/test/to-http-api.test.ts, packages/capability/test/to-toolkit.test.ts, packages/capability/test/to-code-mode.test.ts, and apps/cli/test/cli.e2e.test.ts.
• Schemas are the contract. Proof: packages/capability/test/catalog.test.ts, packages/capability/test/to-http-api.test.ts, and packages/capability/test/to-toolkit.test.ts check derived JSON Schema and typed failures.
• A fresh sandbox isolates model code. The Node sandbox has a 10-second wall-clock limit and a 1-second limit per synchronous evaluation. Both run inside the child. Scope close and interruption send SIGTERM, then SIGKILL after 250 milliseconds. Stdin EOF, a lost parent, or a failed protocol write ends the child. Child-process imports remain blocked. Proof: packages/capability/test/sandbox.test.ts checks isolation and VM budgets; sandbox-lifetime.test.ts and sandbox-orphan.test.ts check deadlines, parent death, and process-group cleanup.
• Rate limits fail closed. Proof: apps/site/test/worker.test.ts checks 429 responses before another execute worker starts and when the global limit denies.
• Markdown is the default machine representation. Proof: apps/site/test/worker.test.ts checks Markdown without Accept: text/html and HTML only when requested.

Keep or cut

The template is a working project. Delete what you will not use on day one; the fence will identify dependent code that must go too. After each cut, trim packages/capability/src/index.ts and run pnpm install, pnpm fix, then pnpm turbo run check test build. Update the package table in AGENTS.md.

defineContract, implement, and toCommand are the minimum that keep stats working. The contract module has no dependency on the other projections.

License

MIT

Last changed 2026-10-12 in 2c5b496. Source on GitHub. Change log.