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. The
distribution's yarn only starts that file, and that file does the work.
yarn.lock is converted once, and script/bootstrap installs with
--immutable instead of stopping on yarn's major version. Node is the
distribution's newest Node package: nodejs-current on Alpine and
nodejs_latest on nix. 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

@@ -35,6 +35,12 @@
 # `services/api/.claude/` and must add its own anchored entry.
 .claude
 
+# yarn's install state and cache stay out, and its release file stays
+# in. Docker obeys the last matching line, so these two lines sit above
+# the secret patterns, which must win under .yarn/releases/.
+.yarn/*
+!.yarn/releases
+
 # Environment files. `*.env` covers bare `.env` and the `prod.env`
 # convention. Re-include a committed template with a negation if the
 # build needs one: `!docs/example.env`.

.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

@@ -19,10 +19,12 @@ 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
-# ranges rather than a lowercase form that misses `Server.Key`.
+# Secrets. Unanchored, so each matches at every depth. Matching is
+# case-sensitive on Linux, so names use character ranges rather than a
+# lowercase form that misses `Server.Key`.
 
 # Environment files. `*.env` covers bare `.env` and the `prod.env`
 # convention. Only the templates `example.env` and `sample.env` are

.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

@@ -6,17 +6,20 @@
 # 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.
+# so they need no version and no hash. That yarn only starts the release
+# file in .yarn/releases/, so every stage copies that file and
+# .yarnrc.yml before it runs script/bootstrap.
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS lint
-RUN apk add --no-cache git make nodejs yarn
+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 . .
@@ -27,13 +30,14 @@ RUN yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a AS test
-RUN apk add --no-cache git make nodejs yarn
+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 . .
@@ -48,7 +52,7 @@ RUN echo "No tests defined."
 #
 # alpine 3.24.3, 2026-11-04
 FROM alpine@sha256:be2656061b536c1883c4034790e21d0a09c5aaaf51968b89368a3c0371ea2c9a
-RUN apk add --no-cache git make nodejs yarn
+RUN apk add --no-cache git make nodejs-current yarn
 ARG CACHEBUST
 
 WORKDIR /app
@@ -59,7 +63,8 @@ COPY --from=test /app/package.json /dev/null
 # 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

TODO.md

@@ -21,6 +21,21 @@ 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: An agent whose work is confined to one package may run `go test`
   directly on that package's suite, or on individual tests in it, while
   iterating (https://git.eeqj.de/sneak/prompts/issues/51), on sneak's ruling at

package.json

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

prompts/EXISTING_REPO_CHECKLIST.md

@@ -164,12 +164,39 @@ 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
-          `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierrc` and
+    - [ ] JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
+          `.gitattributes`, `.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`).
-          Make the first `yarn.lock` with yarn 1, from a distribution that
-          packages it, such as Alpine (`apk add nodejs yarn`).
+    - [ ] Any repository with `yarn.lock`: that file 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. In `package.json`, name any package that must run its install script,
+           as `"dependenciesMeta": { "<name>": { "built": true } }`, and run
+           `yarn install` again.
+        7. Add `.yarn/*` and `!.yarn/releases` above the secret patterns in
+           `.gitignore` and `.dockerignore`. Docker obeys the last matching
+           line, and the secret patterns must win. Add `.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)
 
@@ -206,11 +233,14 @@ with your task.
       installed.
 - [ ] `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. With `yarn.lock` in the tree, `script/bootstrap` stops
-      with a one-line message when yarn's major version is not 1.
-- [ ] A `Dockerfile` stage copies `yarn.lock` before it runs `script/bootstrap`
-      only if that stage needs yarn. A stage that needs yarn starts from an
-      image whose distribution packages yarn 1, such as Alpine.
+      they are missing. It installs the node packages with
+      `yarn install --immutable`. The distribution's yarn only starts the
+      release file that `yarnPath` in `.yarnrc.yml` names, and that file does
+      the work.
+- [ ] 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
@@ -247,7 +277,11 @@ with your task.
       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
+      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 (repositories with `yarn.lock`)
 - [ ] Using `yarn`, not `npm` (JS projects)
 
 # Directory Structure

prompts/NEW_REPO_CHECKLIST.md

@@ -179,9 +179,22 @@ 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`. `script/bootstrap`
-          installs no yarn until `yarn.lock` exists, so use yarn 1 from a
-          distribution that packages it, such as Alpine (`apk add nodejs yarn`).
+    - [ ] JS, or any repository that runs prettier through yarn: 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
@@ -198,15 +211,17 @@ are thin shims calling them. Model scripts:
     - 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.
-    - With `yarn.lock` in the tree, it stops with a one-line message when yarn's
-      major version is not 1.
+    - It installs the node packages with `yarn install --immutable`. The
+      distribution's yarn only starts the release file that `yarnPath` in
+      `.yarnrc.yml` names, and that file does the work.
     - 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 copies `yarn.lock` before it runs `script/bootstrap`
-      only if that stage needs yarn. A stage that needs yarn starts from an
-      image whose distribution packages yarn 1, such as Alpine.
+- [ ] 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` —
@@ -278,7 +293,11 @@ are thin shims calling them. Model scripts:
       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
+      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 (repositories with `yarn.lock`)
 - [ ] No unnecessary files in repo root
 - [ ] All dates written as YYYY-MM-DD
 

prompts/REPO_POLICIES.md

@@ -35,7 +35,7 @@ style conventions are in separate documents:
     - Docker: `@sha256:...`
     - Go: the module hash in `go.sum`
     - Python: the package hash in `Pipfile.lock`
-    - npm: the integrity hash in the lockfile
+    - npm: the `checksum` in `yarn.lock`
     - GitHub Actions: `@<commit-sha>`
 
     This also means never `curl | bash` to install a tool such as rustup.
@@ -56,7 +56,8 @@ style conventions are in separate documents:
     - `apk add`
 
     These come from the distribution wherever possible:
-    - node
+    - node, meaning the distribution's newest Node package, such as
+      `nodejs-current` on Alpine 3.24
     - yarn
     - npm
     - python
@@ -69,6 +70,9 @@ style conventions are in separate documents:
     - 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
@@ -123,9 +127,32 @@ style conventions are in separate documents:
 
     `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, and it does not check yarn's version. Make a repository's first
-    `yarn.lock` with yarn 1, from a distribution that packages it, such as
-    Alpine (`apk add nodejs yarn`).
+    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/`. The distribution's yarn
+    only starts that file, and that file does the work. 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
+
+    By default, yarn runs no dependency's install script. A dependency 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
@@ -133,18 +160,18 @@ style conventions are in separate documents:
     and `script/fmt-check` print one line saying to run `script/bootstrap`, and
     exit 1.
 
-    With `yarn.lock` in the tree, `script/bootstrap` stops with a one-line
-    message when yarn's major version is not 1, before it installs any node
-    packages. Debian 13 and Ubuntu 26.04 package yarn 4. With
-    `--frozen-lockfile`, yarn 4 refuses a yarn 1 `yarn.lock`. Without that flag,
-    it rewrites the file in its own format, with yarn 4 hashes in place of the
-    yarn 1 ones. A host whose distribution packages yarn 4 moves to one that
-    packages yarn 1, such as Alpine or Ubuntu 24.04.
+    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/`
 
-    A `Dockerfile` stage copies `yarn.lock` before it runs `script/bootstrap`
-    only if that stage needs yarn. A stage that needs yarn starts from an image
-    whose distribution packages yarn 1, such as Alpine. The Python and Go stages
-    below need no yarn and copy no `yarn.lock`, so they never reach the stop.
+    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
@@ -210,8 +237,9 @@ style conventions are in separate documents:
     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 (`package.json` + `yarn.lock`,
-    `go.mod` + `go.sum`, etc.) before running it.
+    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`
 
 - **The lint and test gates run in Docker, as phases of the `Dockerfile`.**
   There is no separate lint file. `script/lint` and `script/test` each build one
@@ -771,6 +799,7 @@ 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
@@ -792,16 +821,16 @@ 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.
 
@@ -1105,7 +1134,8 @@ 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/`,
+      `.gitattributes`, `.prettierrc`, `.prettierignore`
     - Python: `pyproject.toml`
 
 - Guidance for coding agents lives in one `AGENTS.md` at the repository root. It

script/bootstrap

@@ -4,8 +4,10 @@
 # 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. Assumes nothing
-# is present.
+# the node packages only when yarn.lock is in the tree. The
+# distribution's yarn only starts the release file that yarnPath in
+# .yarnrc.yml names, and that file does the work. So git pins the yarn
+# that runs. Assumes nothing is present.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -78,21 +80,12 @@ main() {
 
     # 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; fi
+        if missing node; then pkg_install nodejs_latest nodejs node nodejs-current; fi
         if missing yarn && missing yarnpkg; then
             pkg_install yarn yarnpkg yarn yarn
         fi
 
-        # Debian 13 and Ubuntu 26.04 package yarn 4, which refuses a yarn 1
-        # yarn.lock, or rewrites it in its own format.
-        yarn_version="$(run_yarn --version)"
-        if [ "${yarn_version%%.*}" != "1" ]; then
-            echo "bootstrap: yarn $yarn_version refuses this yarn 1 yarn.lock;" \
-                "use a distribution that packages yarn 1" >&2
-            exit 1
-        fi
-
-        run_yarn install --frozen-lockfile
+        run_yarn install --immutable
     fi
 
     echo "bootstrap complete"

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