Commit yarn 4 in the tree and install with --immutable (closes #149)

Follows sneak's ruling at
#149 (comment). This
repository now carries yarn 4.18.1 under .yarn/releases, and whichever
yarn the distribution packages hands the work to it. yarn.lock is
converted once, and script/bootstrap installs with --immutable instead
of stopping on yarn's major version. Node on Alpine is nodejs-current.
Every Dockerfile stage that needs yarn also copies .yarnrc.yml and
.yarn/releases/. The policies and both checklists say so, with the
steps for converting and for a new repository.

Model: opus-5-5

.dockerignore

@@ -55,8 +55,11 @@
 **/[iI][dD]_[eE][dD]25519
 **/[iI][dD]_[eE][dD]25519_[sS][kK]
 
-# Dependencies: restored inside the image, never copied in.
+# Dependencies: restored inside the image, never copied in. yarn's install
+# state and cache stay out, and its release file stays in.
 **/node_modules
+.yarn/*
+!.yarn/releases
 
 # OS metadata.
 **/.DS_Store

.gitattributes

@@ -2,3 +2,6 @@
 # both sides instead of conflicting. Git never reports a conflict here: read
 # the merged entries after every merge or rebase.
 TODO.md merge=union
+
+# yarn's release file is one opaque file in diffs and merges.
+.yarn/releases/** binary

.gitignore

@@ -19,6 +19,8 @@ Thumbs.db
 
 # Node
 node_modules/
+.yarn/*
+!.yarn/releases
 
 # Secrets. Unanchored like every entry above, so each matches at every
 # depth. Matching is case-sensitive on Linux, so names use character

.prettierignore

@@ -1,2 +1,3 @@
 node_modules/
+.yarn/
 yarn.lock

.yarnrc.yml

@@ -0,0 +1,5 @@
+enableTelemetry: false
+
+nodeLinker: node-modules
+
+yarnPath: .yarn/releases/yarn-4.18.1.cjs

Dockerfile

@@ -2,17 +2,24 @@
 # lint` or `script/lint`, which are themselves a docker build and would
 # recurse into a daemon that does not exist in a build step.
 #
-# Every stage declares ARG CACHEBUST first: script/ passes a new value on
-# every build, which reruns every RUN step after it.
+# In every stage, ARG CACHEBUST comes right after the step that installs
+# system packages. script/ passes a new value on every build. That value
+# reruns every RUN step below the ARG line, and the install step keeps
+# its layer. node and yarn come from the distribution, which is trusted,
+# so they need no version and no hash. That yarn hands the work to the
+# release file in .yarn/releases/, so every stage copies that file and
+# .yarnrc.yml before it runs script/bootstrap.
 #
-# node 22-alpine, 2026-02-22
-FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS lint
+# alpine 3.24.3, 2026-11-04
+FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS lint
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
 
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .
@@ -21,14 +28,16 @@ RUN yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 
 # Test phase, same shape and for the same reason.
 #
-# node 22-alpine, 2026-02-22
-FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS test
+# alpine 3.24.3, 2026-11-04
+FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS test
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
 
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .
@@ -41,8 +50,9 @@ RUN echo "No tests defined."
 # this image cannot be produced unless lint and test passed. A stage
 # appended after this one would drop all three out of a plain build.
 #
-# node 22-alpine, 2026-02-22
-FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34
+# alpine 3.24.3, 2026-11-04
+FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
@@ -50,10 +60,11 @@ WORKDIR /app
 COPY --from=lint /app/package.json /dev/null
 COPY --from=test /app/package.json /dev/null
 
-# script/bootstrap installs all prerequisites. Manifests are copied
-# first so that layer stays cached until dependencies change.
+# script/bootstrap finds the system packages above and installs the node
+# packages.
 COPY script/ script/
-COPY package.json yarn.lock ./
+COPY package.json yarn.lock .yarnrc.yml ./
+COPY .yarn/releases/ .yarn/releases/
 RUN script/bootstrap
 
 COPY . .

README.md

@@ -111,7 +111,7 @@ development workflow, and the Makefile targets are thin shims that call them.
 The scripts are POSIX sh (not bash) so they run in minimal containers such as
 alpine. We provide:
 
-- `script/bootstrap` — install all dependencies (yarn install)
+- `script/bootstrap` — install all dependencies (`yarn install --immutable`)
 - `script/setup` — set up the repo for development after a fresh clone: runs
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (our own extension); used by
@@ -156,9 +156,9 @@ A stage with none of these steps declares `ARG CACHEBUST` right after `FROM`.
 The `script/` builds pass a new `CACHEBUST` value on every build, which reruns
 every `RUN` step after that line and leaves the layers above it and the cache
 mounts in place; `--no-cache` would build those layers again and empty the
-mounts. This repo's own `Dockerfile` declares `ARG CACHEBUST` right after `FROM`
-in every stage: each stage installs system packages and node packages together
-in one `script/bootstrap` step, and check builds may keep system packages but
+mounts. This repo's own `Dockerfile` installs git, make, node and yarn right
+after `FROM` in every stage, above `ARG CACHEBUST`. `script/bootstrap` installs
+the node packages below that line, because check builds keep system packages but
 not node packages.
 
 ## Rationale

TODO.md

@@ -21,6 +21,38 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-11-04: Each repository now commits yarn 4 and installs with
+  `yarn install --immutable` (https://git.eeqj.de/sneak/prompts/issues/149), on
+  sneak's ruling at
+  https://git.eeqj.de/sneak/prompts/issues/149#issuecomment-167558. Product
+  repositories convert when they next have work. These changed:
+    - This repository carries `.yarn/releases/yarn-4.18.1.cjs`, and its
+      `yarn.lock` is in yarn 4's format.
+    - `script/bootstrap` no longer stops on yarn's major version.
+    - Node comes from the distribution's newest Node package, which on Alpine
+      3.24 is `nodejs-current`.
+    - A `Dockerfile` stage that needs yarn also copies `.yarnrc.yml` and
+      `.yarn/releases/`, and may start from any distribution that packages node
+      and yarn.
+    - Both checklists give the steps, and the ignore files keep yarn's install
+      state out of git and the build context.
+- 2026-11-04: node, yarn, npm, python, pip and venv now come from the
+  distribution with no version and no hash
+  (https://git.eeqj.de/sneak/prompts/issues/146), on sneak's ruling at
+  https://git.eeqj.de/sneak/prompts/issues/146#issuecomment-166033. Third-party
+  packages stay pinned by hash. These changed:
+    - `script/bootstrap` no longer uses nvm or corepack, and stops when yarn's
+      major version is not 1.
+    - `script/bootstrap` installs node, yarn and the node packages only when
+      `yarn.lock` is in the tree.
+    - `script/bootstrap`, `script/fmt` and `script/fmt-check` run `yarn` or
+      `yarnpkg`, whichever is present.
+    - This repository's `Dockerfile` starts from Alpine 3.24.
+    - The Python `test` phase example starts from `debian:trixie-slim`.
+    - A Python repository's `script/bootstrap` creates its virtual environment
+      at `$HOME/.pipenv`, so it works on a host and in a `Dockerfile`.
+    - `PIP_ONLY_BINARY=:all:` stops a package with no wheel, so pip downloads no
+      build tools without a hash check.
 - 2026-11-04: Rewrote `prompts/REPO_POLICIES.md` sentence by sentence
   (https://git.eeqj.de/sneak/prompts/issues/136), on sneak's yes at
   https://git.eeqj.de/sneak/prompts/issues/136#issuecomment-165193. Every rule

package.json

@@ -2,5 +2,6 @@
   "private": true,
   "devDependencies": {
     "prettier": "3.8.1"
-  }
+  },
+  "packageManager": "yarn@4.18.1"
 }

prompts/CODE_STYLEGUIDE_JS.md

@@ -1,6 +1,6 @@
 ---
 title: Code Styleguide — JavaScript
-last_modified: 2026-02-22
+last_modified: 2026-11-04
 ---
 
 1. Use `const` for everything. If you need to reassign, use `let`. Never use
@@ -8,7 +8,8 @@ last_modified: 2026-02-22
 
 1. Use yarn for package management, avoid using npm.
 
-1. Use LTS node versions.
+1. Install node from the distribution. If its version is too old, use a newer
+   distribution.
 
 1. Use `prettier` for code formatting, with four spaces for indentation.
 

prompts/EXISTING_REPO_CHECKLIST.md

@@ -81,11 +81,17 @@ with your task.
     - pipenv and its dependencies
     - the packages from `Pipfile.lock`
 
-    It installs pipenv and its dependencies by hash, with
+    It creates a virtual environment at `$HOME/.pipenv` with `python3 -m venv`
+    when it is missing, then uses that environment's `pip` and `pipenv`. On a
+    host, it first installs the distribution's python and venv packages when
+    they are missing. It installs pipenv and its dependencies by hash, with
     `pip install --require-hashes`. It installs the packages from `Pipfile.lock`
     with `pipenv install --deploy`, plus `--dev` when the tests need the
-    development packages. It installs no Python there, because the tests use the
-    base image's. The tests run after `ARG CACHEBUST`, and every other stage
+    development packages. It sets `PIP_ONLY_BINARY=:all:` for both installs, so
+    a package with no wheel for the image stops the install. In that step it
+    installs no Python, because a step above it already did. `Pipfile` names no
+    `python_version`, because `--deploy` stops when the distribution's Python
+    differs from it. The tests run after `ARG CACHEBUST`, and every other stage
     installs its Python packages after it, as in the Python `test` phase example
     in `REPO_POLICIES.md`
 
@@ -158,10 +164,36 @@ with your task.
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and,
           in the same commit, set the lint phase digest to the one named in the
           `.golangci.yml` paragraph of `REPO_POLICIES.md`)
-    - [ ] JS: `package.json`, `yarn.lock`, `.prettierrc`, `.prettierignore`
-          (fetch from
+    - [ ] JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
+          `.prettierrc`, `.prettierignore` (fetch the last two from
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierrc` and
-          `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierignore`)
+          `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierignore`).
+    - [ ] JS: `yarn.lock` is in yarn 4's format. If its second line says
+          `yarn lockfile v1`, convert it with the steps below. Do them in one
+          commit with the re-vendored scripts, because yarn 1 ignores
+          `--immutable` and exits 0 on a tree with no `.yarnrc.yml`. Run them
+          with any distribution's node and yarn, and use `yarnpkg` on Debian and
+          Ubuntu.
+        1. Read the version from `yarnPath` in
+           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.yarnrc.yml`.
+        2. Run `yarn set version <version>`. It writes
+           `.yarn/releases/yarn-<version>.cjs`, `yarnPath` in `.yarnrc.yml`, and
+           `packageManager` in `package.json`.
+        3. Add `nodeLinker: node-modules` and `enableTelemetry: false` to
+           `.yarnrc.yml`.
+        4. Run `yarn install` once. It converts `yarn.lock`, and each package
+           keeps its version and gains a `checksum`.
+        5. Delete `approvedGitRepositories`, `enableScripts` and
+           `npmMinimalAgeGate` from `.yarnrc.yml`. The conversion wrote them.
+           They allow install scripts, any git repository and packages of any
+           age, which yarn otherwise refuses.
+        6. Name any package that must run its install script in `package.json`,
+           as `"dependenciesMeta": { "<name>": { "built": true } }`, and run
+           `yarn install` again.
+        7. Add `.yarn/*` and `!.yarn/releases` to `.gitignore` and
+           `.dockerignore`, `.yarn/` to `.prettierignore`, and
+           `.yarn/releases/** binary` to `.gitattributes`.
+        8. Run `yarn install --immutable`. It must pass and change nothing.
     - [ ] Python: `pyproject.toml`
     - [ ] Docs/writing: `.prettierrc`, `.prettierignore` (same URLs as above)
 
@@ -196,13 +228,19 @@ with your task.
       `docker build` in `script/`. Without the bootstrap the CI run dies in
       `script/fmt-check`, which runs the formatter on the host and finds nothing
       installed.
-- [ ] `script/fmt` and `script/fmt-check` pick their `yarn` with the same test
-      as `script/bootstrap`: the `yarn` on `PATH` when the node on `PATH` has
-      the pinned major version, and otherwise yarn under the pinned node, with
-      nvm loaded through `$HOME/.nvm/nvm.sh`. `script/bootstrap` leaves the node
-      and yarn it installs under nvm off the `PATH` of the shell that called it,
-      so a bare `yarn` exits 127 on a runner carrying nothing but docker and
-      git, or runs under another node.
+- [ ] `script/bootstrap` installs node, yarn and the node packages only when
+      `yarn.lock` is in the tree. node and yarn come from the distribution when
+      they are missing. It installs the node packages with
+      `yarn install --immutable`. Whichever yarn the distribution packages hands
+      the work to the release file that `yarnPath` in `.yarnrc.yml` names.
+- [ ] A `Dockerfile` stage that needs yarn may start from any distribution that
+      packages node and yarn. It copies `package.json`, `yarn.lock`,
+      `.yarnrc.yml` and `.yarn/releases/` before it runs `script/bootstrap`. A
+      stage that needs no yarn copies none of them.
+- [ ] `script/bootstrap`, `script/fmt` and `script/fmt-check` run yarn by
+      whichever name is present: `yarn`, or `yarnpkg` on Debian and Ubuntu. With
+      neither name on `PATH`, `script/fmt` and `script/fmt-check` print one line
+      saying to run `script/bootstrap`, and exit 1.
 - [ ] `script/bootstrap` installs no linter of its own — delete the block, its
       version variables and its call site. A JS repo's `yarn install` stays; it
       brings a linter along with every other dependency, and no verdict is taken
@@ -230,8 +268,16 @@ with your task.
       is a package from the distribution's own package index, which needs no
       version and no hash. Pulling that index is also allowed. In a
       `Dockerfile`, the distribution is the pinned base image's. On a host, it
-      is the host's. A package source that a `Dockerfile` adds itself does not
-      get this exception
+      is the host's, and the host's package manager stands for it, nix and brew
+      included. A package source that a `Dockerfile` adds itself does not get
+      this exception
+- [ ] node, yarn, npm, python, pip and venv come from the distribution wherever
+      possible, never from nvm, pyenv, corepack or `npm install -g yarn`. If the
+      distribution's version is too old, use a newer distribution. node means
+      the distribution's newest Node package, such as `nodejs-current` on Alpine
+      3.24
+- [ ] yarn's release file is in git under `.yarn/releases/`, written once by
+      `yarn set version` with an exact version
 - [ ] Using `yarn`, not `npm` (JS projects)
 
 # Directory Structure
@@ -260,11 +306,12 @@ with your task.
 # Final
 
 - [ ] `make check` passes
-- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying what CI has
-      (the runner image `docker.gitea.com/runner-images:ubuntu-latest`: docker,
-      git and node 24, with no yarn on `PATH`), and demonstrably executed the
-      checks — a sub-second build, or `CACHED` on a step that lints or tests,
-      one of the `RUN` steps below `ARG CACHEBUST`, means nothing ran. Above
+- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying what CI has.
+      That host is the runner image
+      `docker.gitea.com/runner-images:ubuntu-latest`, which has docker, git and
+      node 24. Take any yarn off `PATH` first. The run must show that the checks
+      executed. A sub-second build means nothing ran. So does `CACHED` on a step
+      that lints or tests, one of the `RUN` steps below `ARG CACHEBUST`. Above
       that line, these steps are expected to show `CACHED`:
     - the steps that install system packages
     - the steps that download Go's modules

prompts/NEW_REPO_CHECKLIST.md

@@ -148,13 +148,20 @@ Template files can be fetched from:
         - pipenv and its dependencies
         - the packages from `Pipfile.lock`
 
-        It installs pipenv and its dependencies by hash, with
-        `pip install --require-hashes`. It installs the packages from
-        `Pipfile.lock` with `pipenv install --deploy`, plus `--dev` when the
-        tests need the development packages. It installs no Python there,
-        because the tests use the base image's. The tests run after
-        `ARG CACHEBUST`, and every other stage installs its Python packages
-        after it, as in the Python `test` phase example in `REPO_POLICIES.md`
+        It creates a virtual environment at `$HOME/.pipenv` with
+        `python3 -m venv` when it is missing, then uses that environment's `pip`
+        and `pipenv`. On a host, it first installs the distribution's python and
+        venv packages when they are missing. It installs pipenv and its
+        dependencies by hash, with `pip install --require-hashes`. It installs
+        the packages from `Pipfile.lock` with `pipenv install --deploy`, plus
+        `--dev` when the tests need the development packages. It sets
+        `PIP_ONLY_BINARY=:all:` for both installs, so a package with no wheel
+        for the image stops the install. In that step it installs no Python,
+        because a step above it already did. `Pipfile` names no
+        `python_version`, because `--deploy` stops when the distribution's
+        Python differs from it. The tests run after `ARG CACHEBUST`, and every
+        other stage installs its Python packages after it, as in the Python
+        `test` phase example in `REPO_POLICIES.md`
 
     - Server: the final stage builds and runs the application
     - Non-server: the final stage brings up the dev environment
@@ -172,7 +179,21 @@ Template files can be fetched from:
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and,
           in the same commit, set the lint phase digest to the one named in the
           `.golangci.yml` paragraph of `REPO_POLICIES.md`)
-    - [ ] JS: `yarn init`, `yarn add --dev prettier`
+    - [ ] JS: make the first `yarn.lock` with the steps below.
+          `script/bootstrap` installs no yarn until `yarn.lock` exists, so run
+          them with any distribution's node and yarn, such as Alpine's
+          (`apk add nodejs-current yarn`). Use `yarnpkg` on Debian and Ubuntu.
+        1. Read the version from `yarnPath` in
+           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.yarnrc.yml`.
+        2. Run `yarn set version <version>`. It writes
+           `.yarn/releases/yarn-<version>.cjs`, `.yarnrc.yml` and
+           `package.json`.
+        3. Add `nodeLinker: node-modules` and `enableTelemetry: false` to
+           `.yarnrc.yml`.
+        4. Add `"private": true` to `package.json`.
+        5. Run `yarn add --dev prettier`. It writes `yarn.lock`.
+        6. Add `.yarn/releases/** binary` to `.gitattributes`.
+        7. Run `yarn install --immutable`. It must pass and change nothing.
     - [ ] Python: `pyproject.toml`
 
 ## Configure script/ Entrypoints and Makefile
@@ -183,14 +204,23 @@ are thin shims calling them. Model scripts:
 
 - [ ] scripts are POSIX sh (`#!/bin/sh`, `set -eu`, no bashisms) so they run on
       alpine images without bash
-- [ ] `script/bootstrap` / `make bootstrap` — installs all dependencies,
-      idempotently, assuming nothing (pkg manager detection nix/apt/brew/apk;
-      node used if its major version is the pinned one, else pinned version via
-      nvm from a hash-verified archive, with yarn and the packages installed
-      under it; pinned yarn via corepack); a non-server repo's development
-      environment stage runs it instead of inline installs; a gate phase or the
-      build stage installs what its base image lacks either inline or by running
-      it
+- [ ] `script/bootstrap` / `make bootstrap` installs all dependencies,
+      idempotently, and assumes nothing is present:
+    - It detects the package manager: nix, apt, brew or apk.
+    - It installs node, yarn and the node packages only when `yarn.lock` is in
+      the tree. node and yarn come from that package manager when they are
+      missing.
+    - It installs the node packages with `yarn install --immutable`. Whichever
+      yarn the distribution packages hands the work to the release file that
+      `yarnPath` in `.yarnrc.yml` names.
+    - A non-server repo's development environment stage runs it, with only
+      system packages installed inline above it.
+    - A gate phase or the build stage installs what its base image lacks, either
+      inline or by running it.
+- [ ] A `Dockerfile` stage that needs yarn may start from any distribution that
+      packages node and yarn. It copies `package.json`, `yarn.lock`,
+      `.yarnrc.yml` and `.yarn/releases/` before it runs `script/bootstrap`. A
+      stage that needs no yarn copies none of them.
 - [ ] `script/setup` / `make setup` — readies a fresh clone: runs `bootstrap`,
       then `install-precommit`, plus repo-specific init
 - [ ] `script/test` / `make test` —
@@ -220,13 +250,10 @@ are thin shims calling them. Model scripts:
       from `script/projectname`, each assigned on its own line before the build.
       The bootstrap is required: CI checks out and runs this alone, and
       `script/fmt-check` runs the formatter on the host.
-- [ ] `script/fmt` and `script/fmt-check` pick their `yarn` with the same test
-      as `script/bootstrap`: the `yarn` on `PATH` when the node on `PATH` has
-      the pinned major version, and otherwise yarn under the pinned node, with
-      nvm loaded through `$HOME/.nvm/nvm.sh`. `script/bootstrap` leaves the node
-      and yarn it installs under nvm off the `PATH` of the shell that called it,
-      so a bare `yarn` exits 127 on a runner carrying nothing but docker and
-      git, or runs under another node.
+- [ ] `script/bootstrap`, `script/fmt` and `script/fmt-check` run yarn by
+      whichever name is present: `yarn`, or `yarnpkg` on Debian and Ubuntu. With
+      neither name on `PATH`, `script/fmt` and `script/fmt-check` print one line
+      saying to run `script/bootstrap`, and exit 1.
 - [ ] No `docker build` in `script/` leaves a dangling image behind:
       `script/lint` and `script/test` write no image, and `script/docker` and
       `script/cibuild` tag theirs
@@ -241,11 +268,12 @@ are thin shims calling them. Model scripts:
 
 - [ ] `make check` passes
 - [ ] `make docker` succeeds
-- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying what CI has
-      (the runner image `docker.gitea.com/runner-images:ubuntu-latest`: docker,
-      git and node 24, with no yarn on `PATH`), and demonstrably executed the
-      checks — a sub-second build, or `CACHED` on a step that lints or tests,
-      one of the `RUN` steps below `ARG CACHEBUST`, means nothing ran. Above
+- [ ] `script/cibuild` succeeds in a fresh clone on a host carrying what CI has.
+      That host is the runner image
+      `docker.gitea.com/runner-images:ubuntu-latest`, which has docker, git and
+      node 24. Take any yarn off `PATH` first. The run must show that the checks
+      executed. A sub-second build means nothing ran. So does `CACHED` on a step
+      that lints or tests, one of the `RUN` steps below `ARG CACHEBUST`. Above
       that line, these steps are expected to show `CACHED`:
     - the steps that install system packages
     - the steps that download Go's modules
@@ -259,8 +287,16 @@ are thin shims calling them. Model scripts:
 - [ ] No mutable image/package references. The one exception is a package from
       the distribution's own package index, which needs no version and no hash.
       Pulling that index is also allowed. In a `Dockerfile`, the distribution is
-      the pinned base image's. On a host, it is the host's. A package source
+      the pinned base image's. On a host, it is the host's, and the host's
+      package manager stands for it, nix and brew included. A package source
       that a `Dockerfile` adds itself does not get this exception
+- [ ] node, yarn, npm, python, pip and venv come from the distribution wherever
+      possible, never from nvm, pyenv, corepack or `npm install -g yarn`. If the
+      distribution's version is too old, use a newer distribution. node means
+      the distribution's newest Node package, such as `nodejs-current` on Alpine
+      3.24
+- [ ] yarn's release file is in git under `.yarn/releases/`, written once by
+      `yarn set version` with an exact version
 - [ ] No unnecessary files in repo root
 - [ ] All dates written as YYYY-MM-DD
 

prompts/REPO_POLICIES.md

@@ -25,6 +25,7 @@ style conventions are in separate documents:
   a remote source, including:
     - Docker base images
     - Go modules
+    - Python packages
     - npm packages
     - GitHub Actions
 
@@ -33,29 +34,49 @@ style conventions are in separate documents:
     dependency by its content hash:
     - Docker: `@sha256:...`
     - Go: the module hash in `go.sum`
-    - npm: the integrity hash in the lockfile
+    - Python: the package hash in `Pipfile.lock`
+    - npm: the `checksum` in `yarn.lock`
     - GitHub Actions: `@<commit-sha>`
 
-    This also means never `curl | bash` to install tools like pyenv, nvm,
-    rustup, etc. Instead, download a specific release archive from GitHub,
-    verify its hash, and only then install. The hash is hardcoded in the
-    Dockerfile or script. Unverified install scripts are arbitrary remote code
-    execution. Double-check every external reference in every file before
-    committing.
+    This also means never `curl | bash` to install a tool such as rustup.
+    Instead, download a specific release archive from GitHub, verify its hash,
+    and only then install. The hash is hardcoded in the Dockerfile or script.
+    Unverified install scripts are arbitrary remote code execution. Double-check
+    every external reference in every file before committing.
 
     There is one exception: a package from the distribution's own package index
     needs no version and no hash. Pulling that index is also allowed. The
     distribution is trusted. In a `Dockerfile`, it is the pinned base image's
-    distribution. On a host, it is the host's. These commands are examples:
+    distribution. On a host, it is the host's. The host's package manager stands
+    for its distribution, and that includes nix and brew. These commands are
+    examples:
     - `apt-get update`
     - `apt-get install`
     - `apk update`
     - `apk add`
 
+    These come from the distribution wherever possible:
+    - node, meaning the distribution's newest Node package, such as
+      `nodejs-current` on Alpine 3.24
+    - yarn
+    - npm
+    - python
+    - pip and venv, the installer tools bundled with python
+
+    If the distribution's version is too old, use a newer distribution, never a
+    download. That rules out all of these:
+    - nvm
+    - pyenv
+    - corepack
+    - `npm install -g yarn`
+
+    yarn's release file is third-party code, and git pins it. `yarn set version`
+    writes it to `.yarn/releases/` once, given an exact version.
+
     These are not trusted, so everything from them stays pinned by hash:
     - Docker Hub
     - PyPI
-    - npm
+    - the npm registry
     - `go get`
 
     A package source that a `Dockerfile` adds itself is not the distribution's
@@ -99,18 +120,58 @@ style conventions are in separate documents:
     initialization.
 
     `script/bootstrap` installs all dependencies idempotently and assumes
-    nothing is present. Base tools come from nix, apt, brew, or apk, detected in
-    that order; apt runs noninteractive. For node it uses the installed node
-    only when its major version is the pinned one. When node is missing or has
-    another major, it installs the PINNED node version via nvm, and installs
-    yarn and the packages under it. The CI runner image ships node 24, for
-    example. nvm itself is installed first if missing, from a hash-verified
-    GitHub release archive, never `curl | sh`. bash is installed as an explicit
-    prerequisite, since nvm requires bash. Only the major version is compared
-    because the `Dockerfile` stages start from a node image whose exact version
-    is not the pin, and nvm cannot install a prebuilt node on alpine. yarn is
-    pinned via `corepack prepare yarn@<version> --activate`. Never install
-    "latest" or "lts"; always exact versions.
+    nothing is present. Base tools, node and yarn come from nix, apt, brew, or
+    apk, detected in that order; apt runs noninteractive. Each one is installed
+    only when it is missing. A tool from outside the distribution is never
+    installed as "latest" or "lts"; it is always an exact version.
+
+    `script/bootstrap` installs node, yarn and the node packages only when
+    `yarn.lock` is in the tree. With no `yarn.lock`, it installs no node and no
+    yarn. It installs the node packages with `yarn install --immutable`. That
+    install fails when a package's `checksum` is wrong, or when `package.json`
+    is out of step with `yarn.lock`.
+
+    Each repository commits the yarn that runs. `yarnPath` in `.yarnrc.yml`
+    points at the release file in `.yarn/releases/`. Whichever yarn the
+    distribution packages hands the work to that file. Alpine packages yarn 1,
+    and Debian 13 packages yarn 4.1. The same committed yarn does the work on
+    both. Corepack never runs, and no yarn is downloaded.
+
+    `.yarnrc.yml` holds these settings:
+    - `yarnPath`
+    - `nodeLinker: node-modules`
+    - `enableTelemetry: false`, because yarn otherwise sends usage reports from
+      any host that is not CI
+
+    Under these settings yarn runs no package's install script. A package that
+    must run one is named in `package.json`, as
+    `"dependenciesMeta": { "<name>": { "built": true } }`.
+
+    `.gitattributes` carries `.yarn/releases/** binary`, so diffs and merges
+    treat the release file as one opaque file.
+
+    yarn 1 ignores `--immutable` and exits 0, so on a tree with no `.yarnrc.yml`
+    the install is not frozen. A repository therefore re-vendors
+    `script/bootstrap` and converts its `yarn.lock` in one commit.
+
+    Debian and Ubuntu name the yarn command `yarnpkg`. `script/bootstrap`,
+    `script/fmt` and `script/fmt-check` each carry the same small function that
+    runs whichever name is present. With neither name on `PATH`, `script/fmt`
+    and `script/fmt-check` print one line saying to run `script/bootstrap`, and
+    exit 1.
+
+    A `Dockerfile` stage that needs yarn may start from any distribution that
+    packages node and yarn. It copies all of these before it runs
+    `script/bootstrap`:
+    - `package.json`
+    - `yarn.lock`
+    - `.yarnrc.yml`
+    - `.yarn/releases/`
+
+    With only `package.json` and `yarn.lock`, the install fails. Alpine's yarn 1
+    then says to enable corepack; never do that, and copy the missing files
+    instead. The Python and Go stages below need no yarn and copy no
+    `yarn.lock`, so `script/bootstrap` installs no node and no yarn there.
 
     `script/cibuild` runs the CI build, and the Gitea workflow calls it. The
     script changes to the repo root, runs `script/bootstrap`, runs
@@ -118,15 +179,7 @@ style conventions are in separate documents:
     runs `script/bootstrap` first**, because the workflow checks out the repo
     and runs nothing else, while `script/fmt-check` runs the formatter on the
     host. On a pristine checkout with nothing installed, the run dies there,
-    after the containerised gates have passed. **The bootstrap alone is not
-    enough.** When `script/bootstrap` installs node and yarn under nvm, it
-    leaves neither on the `PATH` of the shell that called it. A bare `yarn` then
-    either exits 127 or runs under another node. The host entrypoints that need
-    yarn, `script/fmt` and `script/fmt-check`, therefore use the same test as
-    `script/bootstrap`. When the node on `PATH` has the pinned major version,
-    they run the `yarn` on `PATH`. Otherwise they load nvm through
-    `$HOME/.nvm/nvm.sh` and run yarn under the pinned node. A runner carrying
-    nothing but docker and git then gets through `script/check`.
+    after the containerised gates have passed.
 
     Four further scripts are our own extensions to the standard:
     - `script/check` runs `script/test`, `script/lint` and `script/fmt-check`.
@@ -175,10 +228,12 @@ style conventions are in separate documents:
       does for its yarn packages
 
     The development environment stage installs development prerequisites by
-    running `script/bootstrap` rather than duplicating its installs inline. A
-    stage that runs `script/bootstrap` COPYs `script/` and the dependency
-    manifests (`package.json` + `yarn.lock`, `go.mod` + `go.sum`, etc.) before
-    running it.
+    running `script/bootstrap`. Only system packages may be installed inline
+    above it, so that check builds keep them; the `prompts` repo's own
+    `Dockerfile` does this. A stage that runs `script/bootstrap` COPYs `script/`
+    and the dependency manifests it needs before running it. These are examples:
+    - `package.json`, `yarn.lock`, `.yarnrc.yml` and `.yarn/releases/`
+    - `go.mod` and `go.sum`
 
 - **Linting and testing run in Docker, as phases of the `Dockerfile`.** There is
   no separate lint file. `script/lint` and `script/test` each build one phase
@@ -491,10 +546,18 @@ style conventions are in separate documents:
         - pipenv and its dependencies
         - the packages from `Pipfile.lock`
 
-        `script/bootstrap` installs no Python there: the tests use the base
-        image's Python, which the image's hash pins. If it installs anything
-        else there, the step goes after `ARG CACHEBUST`.
+        `script/bootstrap` installs no Python there: the tests use the
+        distribution's Python, which a step above it installs. If it installs
+        anything else there, the step goes after `ARG CACHEBUST`.
 
+    - Debian's Python refuses a system-wide `pip install`, so pipenv goes in a
+      virtual environment at `$HOME/.pipenv`. `script/bootstrap` creates it with
+      `python3 -m venv` when it is missing, then uses that environment's `pip`
+      and `pipenv`. On a host, it first installs the distribution's python and
+      venv packages when they are missing. One script then works on a host and
+      in a `Dockerfile`. `Pipfile` names no `python_version`, because
+      `pipenv install --deploy` stops when the distribution's Python differs
+      from it.
     - PyPI is not trusted, so `script/bootstrap` installs pipenv and its
       dependencies by hash. It does so with `pip install --require-hashes -r`
       and a requirements file in `script/` that lists each of them with `==` and
@@ -504,37 +567,65 @@ style conventions are in separate documents:
         - installs only what `Pipfile.lock` names
         - checks every package against the lockfile's hashes
         - fails when the lockfile is out of date with `Pipfile`
+    - `script/bootstrap` sets `PIP_ONLY_BINARY=:all:` for both installs, and
+      pipenv passes it on to pip. A package with no ready-built wheel for the
+      image then stops the install. Without the setting, pip builds that package
+      from source and downloads the build tools without a hash check.
     - A kept layer holds nothing a new install would refuse: pip checked the
       hashes of pipenv and its dependencies, and pipenv checked the hashes of
       the lockfile's packages. pipenv creates its virtual environment with the
       `virtualenv` package. The packages that `virtualenv` adds come from wheels
-      inside that package, so the hash pip checked for it covers them. Two
-      things in the layer are not fixed by a hash:
-        - A package with no ready-built wheel for the image is built from
-          source. pip downloads the build tools without a hash check, so a kept
-          layer's build of such a package may differ from a new one.
-        - System packages keep the versions the package manager gave when the
-          layer was built; a new install may get newer ones.
+      inside that package, so the hash pip checked for it covers them. System
+      packages, Python among them, are not fixed by a hash. They keep the
+      versions the package manager gave when the layer was built; a new install
+      may get newer ones.
     - As with Go's modules, the packages are kept in a layer, not a cache mount.
       A cache mount is shared by every build on the host, and nothing checks its
       files again.
 
-    In the example below, `script/bootstrap` does all of these:
-    - finds `make` and `git` already installed
-    - installs pipenv and its dependencies by hash
-    - runs `pipenv install --dev --deploy`
+    A Python repository's `script/bootstrap` adds these lines in `main`, above
+    the last `echo`. The `ensurepip` test fails when python is missing, and also
+    when Debian's venv package is missing:
+
+    ```sh
+    if [ ! -x "$HOME/.pipenv/bin/pip" ]; then
+        if ! python3 -m ensurepip --version >/dev/null 2>&1; then
+            pkg_install python3 python3-venv python3 python3
+        fi
+        python3 -m venv "$HOME/.pipenv"
+    fi
+    export PIP_ONLY_BINARY=:all:
+    "$HOME/.pipenv/bin/pip" install --require-hashes -r script/requirements.txt
+    "$HOME/.pipenv/bin/pipenv" install --dev --deploy
+    ```
+
+    Make `script/requirements.txt` once, with
+    `echo pipenv | pip-compile --generate-hashes --allow-unsafe -o script/requirements.txt -`.
+    `pip-compile` comes from Alpine's `py3-pip-tools` package; Debian and Ubuntu
+    do not package `pip-tools`. Make the first `Pipfile.lock` with
+    `$HOME/.pipenv/bin/pipenv lock`, after the `pip install` line has installed
+    pipenv.
+
+    In the example below, `script/bootstrap` finds `make`, `git`, python and
+    venv already installed. The example copies no `yarn.lock`, so
+    `script/bootstrap` installs no node and no yarn.
 
     ```dockerfile
     # Test phase. Both pytest runs share one 90-second timeout, so a hang
     # uses it once and gets no rerun.
-    # python:3.x-slim, YYYY-MM-DD
-    FROM python@sha256:... AS test
-    # make and git are base tools that script/bootstrap installs when they
-    # are missing. Installed here, they stay in their own layer when
-    # script/ changes.
+    # debian:trixie-slim, YYYY-MM-DD
+    FROM debian@sha256:... AS test
+    # Python, pip and venv come from the distribution, which is trusted, so
+    # they need no version and no hash. make and git are base tools that
+    # script/bootstrap installs when they are missing. Installed here, they
+    # stay in their own layer when script/ changes.
     RUN apt-get update \
-        && apt-get install -y --no-install-recommends make git \
+        && apt-get install -y --no-install-recommends ca-certificates make git \
+            python3 python3-venv \
         && rm -rf /var/lib/apt/lists/*
+    # script/bootstrap installs pipenv in a virtual environment at
+    # $HOME/.pipenv, and the tests below run that pipenv.
+    ENV PATH=/root/.pipenv/bin:$PATH
     WORKDIR /app
     COPY script/ script/
     COPY Pipfile Pipfile.lock ./
@@ -669,16 +760,16 @@ style conventions are in separate documents:
 
     ```makefile
     test:
-    	@timeout 90 sh -c 'python -m pytest || { \
+    	@timeout 90 sh -c 'pipenv run pytest || { \
     		echo "--- Rerunning with -v for details ---"; \
-    		python -m pytest -v; exit 1; }'
+    		pipenv run pytest -v; exit 1; }'
     ```
 
     Both pytest runs share that one 90-second timeout, as in the `Dockerfile`
     example above, so a hang uses it once and gets no rerun. Ctrl-C does not
     stop a running `make test`, which goes on until the tests end or the 90
     seconds run out. On a Mac without `timeout`, the target fails until it is
-    installed.
+    installed. On a host, the target needs `$HOME/.pipenv/bin` on `PATH`.
 
     The `exit 1` ensures the target always fails after a rerun. The first run
     already proved the tests are broken, so the build must not pass even if a
@@ -702,6 +793,7 @@ style conventions are in separate documents:
     - editor files (`.swp`, `*~`)
     - in-repo agent scratch directories (`.claude/`)
     - `node_modules/`
+    - `.yarn/*`, with `!.yarn/releases` so that git keeps yarn's release file
     - the repo's own build outputs
 
     Fetch the standard `.gitignore` from
@@ -723,16 +815,16 @@ style conventions are in separate documents:
   Such a `.dockerignore` is more dangerous than a short file with no secret
   patterns at all, because it reads as solved and stops anyone looking. Give
   every depth-independent pattern the `**/` prefix, and leave only genuinely
-  root-anchored entries unprefixed. Those are `.claude`, and the repo's own
-  host-built binary. The binary is written `/myapp` and never `**/myapp`, which
-  would also match `cmd/myapp/` and delete the package directory from the
-  context. Matching is case-sensitive, and an all-caps copy of each pattern
-  still misses `Server.Key`. So secret names use character ranges:
-  `**/*.[kK][eE][yY]`, `**/*.[pP][eE][mM]`, and likewise for `.envrc` and the
-  extensionless SSH keys. Where such a pattern also catches something the build
-  needs, re-include it with a negation (`!docs/example.env`). Deleting the
-  pattern reopens the exposure for every other file it covers. Fetch the
-  standard `.dockerignore` from
+  root-anchored entries unprefixed. Those are `.claude`, `.yarn/*`,
+  `!.yarn/releases`, and the repo's own host-built binary. The binary is written
+  `/myapp` and never `**/myapp`, which would also match `cmd/myapp/` and delete
+  the package directory from the context. Matching is case-sensitive, and an
+  all-caps copy of each pattern still misses `Server.Key`. So secret names use
+  character ranges: `**/*.[kK][eE][yY]`, `**/*.[pP][eE][mM]`, and likewise for
+  `.envrc` and the extensionless SSH keys. Where such a pattern also catches
+  something the build needs, re-include it with a negation
+  (`!docs/example.env`). Deleting the pattern reopens the exposure for every
+  other file it covers. Fetch the standard `.dockerignore` from
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore` and extend
   it with the repo's own artifacts.
 
@@ -857,10 +949,8 @@ style conventions are in separate documents:
       function defined and never invoked has the same exit status and the same
       empty output as one that worked.
 
-    Node is the exception to the whole-token comparison. The canonical
-    `script/bootstrap` compares only node's major version. That is because the
-    `Dockerfile` stages start from a node 22 alpine image, which is not the
-    exact pin, and nvm cannot install a prebuilt node on alpine.
+    A tool from the distribution has no pin, so `script/bootstrap` installs it
+    when it is missing. make, git, node and yarn are such tools.
 
     Keep it POSIX sh: no arrays, no `[[`, no `grep -P`.
 
@@ -1038,7 +1128,8 @@ style conventions are in separate documents:
     - `Dockerfile`, `.dockerignore`
     - `.gitea/workflows/check.yml`
     - Go: `go.mod`, `go.sum`, `.golangci.yml`
-    - JS: `package.json`, `yarn.lock`, `.prettierrc`, `.prettierignore`
+    - JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
+      `.prettierrc`, `.prettierignore`
     - Python: `pyproject.toml`
 
 - Guidance for coding agents lives in one `AGENTS.md` at the repository root. It

script/bootstrap

@@ -1,24 +1,17 @@
 #!/bin/sh
 # script/bootstrap: install all dependencies needed to build and develop
 # this repo. Idempotent: every install is guarded by a check so already
-# installed tools are skipped. Base tooling comes from nix, apt, brew,
-# or apk (detected in that order); assumes nothing is present. The
-# installed node is used only when its major version is the pinned one;
-# otherwise (node missing, or another major) the pinned version is
-# installed via nvm (installing nvm itself first, from a hash-verified
-# release archive, never curl | sh), and yarn and the packages are
-# installed under it.
+# installed tools are skipped. Base tools, node and yarn come from nix,
+# apt, brew, or apk (detected in that order), with no version and no
+# hash, because the distribution is trusted. It installs node, yarn and
+# the node packages only when yarn.lock is in the tree. Whichever yarn
+# the distribution packages hands the work to the release file that
+# yarnPath in .yarnrc.yml names, so git pins the yarn that runs. Assumes
+# nothing is present.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-07-06
-NODE_VERSION="22.17.0"
-NVM_VERSION="0.40.3"
-# sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
-NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
-YARN_VERSION="1.22.22"
-
 PKGMGR=""
 SUDO=""
 APT_UPDATED=""
@@ -67,99 +60,33 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# verify_sha256 <file> <expected-hash>
-verify_sha256() {
-    if command -v sha256sum >/dev/null 2>&1; then
-        actual="$(sha256sum "$1" | cut -d' ' -f1)"
+# Debian and Ubuntu name the yarn command yarnpkg.
+run_yarn() {
+    if command -v yarn >/dev/null 2>&1; then
+        yarn "$@"
+    elif command -v yarnpkg >/dev/null 2>&1; then
+        yarnpkg "$@"
     else
-        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
-    fi
-    if [ "$actual" != "$2" ]; then
-        echo "bootstrap: sha256 mismatch for $1" >&2
-        echo "  expected: $2" >&2
-        echo "  actual:   $actual" >&2
+        echo "no yarn; run script/bootstrap first" >&2
         exit 1
     fi
 }
 
-# nvm is a bash script; run a command in a bash with nvm loaded
-nvm_sh() {
-    bash -c ". \"\$HOME/.nvm/nvm.sh\" && $*"
-}
-
-ensure_nvm() {
-    [ -s "$HOME/.nvm/nvm.sh" ] && return 0
-    # nvm prerequisites; nvm itself requires bash
-    if missing bash; then pkg_install bash bash bash bash; fi
-    if missing curl; then pkg_install curl curl curl curl; fi
-    if missing git; then pkg_install git git git git; fi
-    tmp="$(mktemp -d)"
-    curl -fsSL -o "$tmp/nvm.tar.gz" \
-        "https://github.com/nvm-sh/nvm/archive/refs/tags/v${NVM_VERSION}.tar.gz"
-    verify_sha256 "$tmp/nvm.tar.gz" "$NVM_SHA256"
-    mkdir -p "$HOME/.nvm"
-    tar -xzf "$tmp/nvm.tar.gz" -C "$HOME/.nvm" --strip-components=1
-    rm -rf "$tmp"
-}
-
-# True when the node on PATH has the pinned major version. Only the
-# major is compared: the Dockerfile stages start from a node 22 alpine
-# image whose exact version is not the pin, and nvm cannot install a
-# prebuilt node on alpine. Another major is not used: the pinned yarn 1
-# prints a deprecation warning under node 24. script/fmt and
-# script/fmt-check carry this function unchanged, to run the yarn
-# installed here.
-node_is_pinned_major() {
-    if ! command -v node >/dev/null 2>&1; then return 1; fi
-    installed="$(node --version)"
-    installed="${installed#v}"
-    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
-}
-
-ensure_node() {
-    if node_is_pinned_major; then
-        echo "bootstrap: using node $(node --version)"
-        return 0
-    fi
-    ensure_nvm
-    nvm_sh "nvm install $NODE_VERSION"
-    nvm_sh "nvm use $NODE_VERSION >/dev/null && \
-        echo \"bootstrap: using node \$(node --version)\""
-}
-
-ensure_yarn() {
-    if ! node_is_pinned_major; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
-            corepack prepare yarn@$YARN_VERSION --activate"
-        return 0
-    fi
-    if ! missing yarn; then return 0; fi
-    if ! missing corepack; then
-        corepack enable
-        corepack prepare "yarn@$YARN_VERSION" --activate
-    else
-        npm install -g "yarn@$YARN_VERSION"
-    fi
-}
-
-install_js_deps() {
-    if node_is_pinned_major; then
-        yarn install --frozen-lockfile
-    else
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
-            yarn install --frozen-lockfile"
-    fi
-}
-
 main() {
     cd "$ROOT"
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
 
-    ensure_node
-    ensure_yarn
-    install_js_deps
+    # A Dockerfile stage that needs no yarn copies no yarn.lock.
+    if [ -f yarn.lock ]; then
+        if missing node; then pkg_install nodejs nodejs node nodejs-current; fi
+        if missing yarn && missing yarnpkg; then
+            pkg_install yarn yarnpkg yarn yarn
+        fi
+
+        run_yarn install --immutable
+    fi
 
     echo "bootstrap complete"
 }

script/fmt

@@ -4,33 +4,16 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Must match the pin in script/bootstrap.
-NODE_VERSION="22.17.0"
-
-# True when the node on PATH has the pinned major version: the same
-# function as in script/bootstrap, which uses it to decide where it
-# installs yarn.
-node_is_pinned_major() {
-    if ! command -v node >/dev/null 2>&1; then return 1; fi
-    installed="$(node --version)"
-    installed="${installed#v}"
-    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
-}
-
-# Run the yarn script/bootstrap installed: the yarn on PATH when the
-# node on PATH has the pinned major version, and otherwise yarn under
-# the pinned node in nvm, which bootstrap leaves off the PATH of the
-# shell that called it. nvm is a bash script, hence the subshell.
+# Debian and Ubuntu name the yarn command yarnpkg.
 run_yarn() {
-    if node_is_pinned_major && command -v yarn >/dev/null 2>&1; then
-        exec yarn "$@"
-    fi
-    if node_is_pinned_major || [ ! -s "$HOME/.nvm/nvm.sh" ]; then
-        echo "fmt: no yarn; run script/bootstrap first" >&2
+    if command -v yarn >/dev/null 2>&1; then
+        yarn "$@"
+    elif command -v yarnpkg >/dev/null 2>&1; then
+        yarnpkg "$@"
+    else
+        echo "no yarn; run script/bootstrap first" >&2
         exit 1
     fi
-    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
-        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
 }
 
 main() {

script/fmt-check

@@ -4,33 +4,16 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Must match the pin in script/bootstrap.
-NODE_VERSION="22.17.0"
-
-# True when the node on PATH has the pinned major version: the same
-# function as in script/bootstrap, which uses it to decide where it
-# installs yarn.
-node_is_pinned_major() {
-    if ! command -v node >/dev/null 2>&1; then return 1; fi
-    installed="$(node --version)"
-    installed="${installed#v}"
-    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
-}
-
-# Run the yarn script/bootstrap installed: the yarn on PATH when the
-# node on PATH has the pinned major version, and otherwise yarn under
-# the pinned node in nvm, which bootstrap leaves off the PATH of the
-# shell that called it. nvm is a bash script, hence the subshell.
+# Debian and Ubuntu name the yarn command yarnpkg.
 run_yarn() {
-    if node_is_pinned_major && command -v yarn >/dev/null 2>&1; then
-        exec yarn "$@"
-    fi
-    if node_is_pinned_major || [ ! -s "$HOME/.nvm/nvm.sh" ]; then
-        echo "fmt-check: no yarn; run script/bootstrap first" >&2
+    if command -v yarn >/dev/null 2>&1; then
+        yarn "$@"
+    elif command -v yarnpkg >/dev/null 2>&1; then
+        yarnpkg "$@"
+    else
+        echo "no yarn; run script/bootstrap first" >&2
         exit 1
     fi
-    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
-        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
 }
 
 main() {

yarn.lock

@@ -1,8 +1,23 @@
-# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
-# yarn lockfile v1
+# This file is generated by running "yarn install" inside your project.
+# Manual changes might be lost - proceed with caution!
 
+__metadata:
+  version: 10
+  cacheKey: 10c0
 
-prettier@3.8.1:
-  version "3.8.1"
-  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
-  integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==
+"prettier@npm:3.8.1":
+  version: 3.8.1
+  resolution: "prettier@npm:3.8.1"
+  bin:
+    prettier: bin/prettier.cjs
+  checksum: 10c0/33169b594009e48f570471271be7eac7cdcf88a209eed39ac3b8d6d78984039bfa9132f82b7e6ba3b06711f3bfe0222a62a1bfb87c43f50c25a83df1b78a2c42
+  languageName: node
+  linkType: hard
+
+"root-workspace-0b6124@workspace:.":
+  version: 0.0.0-use.local
+  resolution: "root-workspace-0b6124@workspace:."
+  dependencies:
+    prettier: "npm:3.8.1"
+  languageName: unknown
+  linkType: soft