Choose by how the thing is called, not by language.

• Reading stdin, writing stdout — a filter, parser, formatter? → command

• A typed API the host or another component calls in-process? → component

• A bytes → bytes transform run in a tight loop (frames, samples)? → kernel

• A multi-step procedure with no compiled binary? → task

• An external system you want to query? → federation

• Something WASM genuinely can't do (native threads, GPU)? → posix

When unsure, start with command — it's the default, the simplest, and the most portable. You can always promote to a richer shape later.

#+TOOLKIT:    my-tool
#+EXEC:       command
#+TRUST:      first-party
#+BUILD_LANG: rust
#+BUILD_SRC:  path:src

• The shapes in depth: The six shapes

• Reach the host: The Dock SDK