Once

Once turns repository automation into typed, cacheable actions that humans and coding agents can discover, run, and reuse.

Install

Install the current release with mise:

mise use -g "github:tuist/once@$(mise latest github:tuist/once)"
mise exec -- once --version

Use mise use github:tuist/once@... inside a repository when the project should pin Once in its own mise.toml.

Connect A Coding Harness

Once includes a Model Context Protocol (MCP) server. Add it to a coding harness that supports servers over standard input and output, and start it through mise so global installs and project pins both work:

{
  "mcpServers": {
    "once": {
      "command": "mise",
      "args": [
        "-C",
        "/absolute/path/to/your/project",
        "exec",
        "--",
        "once",
        "mcp",
        "--workspace",
        "/absolute/path/to/your/project",
        "--allow-run"
      ]
    }
  }
}

Remove --allow-run if the harness should not edit manifests, build, test, run, or start runtime sessions.

For a brand-new Rust project, also pin Rust in the project:

mise use rust@1.96.0

Then ask the harness:

Use Once to initialize this directory as a Rust binary package. Discover the
available target kinds, fetch the Rust binary starter, create the files, and
build the target.

The harness can discover rust_binary, fetch the rust-binary-with-crate starter with once_query_example, write the files, validate the complete graph with once_validate_workspace, and verify the result with once_build_target.

The same live discovery loop supports a request such as “build an Android app with Once.” See the coding harness guide for typed graphs, annotated scripts, result checks, and project memory.

Run A Script

Add a small contract to an existing script so Once knows the inputs, outputs, environment, and working directory that shape the action:

#!/usr/bin/env bash
# once input "../assets/**/*"
# once output "../dist/"
# once cwd ".."

npm run build-assets

Run it as a cached action:

once exec -- bash scripts/build-assets.sh
once exec --remote --compute microsandbox -- bash scripts/build-assets.sh

Scripts can also run directly with a Once shebang:

#!/usr/bin/env -S once exec -- bash

Documentation

Read the documentation at once.tuist.dev.

License

MIT License.