diff --git a/Makefile b/Makefile
index a71e286..5b04bad 100644
--- a/Makefile
+++ b/Makefile
@@ -26,10 +26,8 @@ check:
 build:
 	@script/build
 
-# Bundles the built dist/, so the binary reports the version script/build
-# stamped.
-build-bin: build
-	nix-shell -p bun --run "bun build dist/bin/quak.js --compile --outfile bin/quak"
+build-bin:
+	@script/build-bin
 
 install: build-bin
 	mkdir -p ~/bin
diff --git a/README.md b/README.md
index 5a4ff1e..f2e2b7b 100644
--- a/README.md
+++ b/README.md
@@ -135,10 +135,12 @@ requests. The "Library reference" below covers both.
 ## Running the backup from cron
 
 `quak backup` never prompts, so cron can run it. `make install` builds quak as
-one binary and copies it to `~/bin/quak`. Besides what Getting Started needs,
-`make install` needs `make` and [nix](https://nixos.org/download). It uses bun,
-a JavaScript runtime, to bundle the built `dist/` into one executable. It runs
-bun through `nix-shell -p bun`.
+one binary and copies it to `~/bin/quak`. It uses bun, a JavaScript runtime, to
+bundle the built `dist/` into one executable. Besides what Getting Started
+needs, `make install` needs `make`. On Linux it downloads bun from GitHub with
+`curl` and unpacks it with `unzip`. It checks the download against a SHA-256
+hash in the repo. On macOS it runs bun through `nix-shell -p bun`, so it needs
+[nix](https://nixos.org/download).
 
 Log in once with `~/bin/quak`, as the user the cron job will run as:
 
@@ -1771,6 +1773,8 @@ standard. The others are additions to it.
   `package.json` declares (`main`, `types`, `bin`) were written, and make the
   command line executable.
 - `script/version`: print the version `script/build` stamps (see "Version").
+- `script/build-bin`: run `script/build`, then bundle the built `dist/` into one
+  binary, `bin/quak`, with bun (see "Running the backup from cron").
 - `script/test`: run the test suite, by building the `test` phase of the
   `Dockerfile`. Requires Docker.
 - `script/lint`: run eslint and a prettier check, by building the `lint` phase
@@ -1799,8 +1803,7 @@ The Makefile targets are:
 - `make build`: runs `script/build`.
 - `make docker`: runs `script/docker`.
 - `make hooks`: runs `script/install-precommit`.
-- `make build-bin`: bundles the built `dist/` into one binary, `bin/quak`. It
-  runs bun through `nix-shell -p bun`.
+- `make build-bin`: runs `script/build-bin`.
 - `make install`: runs `make build-bin` and copies the binary to `~/bin/quak`.
 - `make dev`: runs the TypeScript compiler in watch mode.
 - `make clean`: removes `dist/`, `coverage/`, `.vitest-cache/` and the
@@ -1908,9 +1911,10 @@ These apply to people and to coding agents alike.
     - container images by digest;
     - the nvm archive `script/bootstrap` downloads, and the Python packages
       `bin/clip-keyword-vectors` installs, by SHA-256 hash;
+    - bun, which `script/build-bin` downloads on Linux, by SHA-256 hash;
     - Node and yarn, when `script/bootstrap` installs them, by version.
 - Not pinned:
-    - bun, which `make build-bin` takes from `nix-shell -p bun`;
+    - bun on macOS, which `script/build-bin` takes from `nix-shell -p bun`;
     - `make` and `git`, which `script/bootstrap` installs from the system
       package manager when they are missing, as every `Dockerfile` stage does;
     - `bash` and `curl`, which `script/bootstrap` installs the same way when it
diff --git a/script/build-bin b/script/build-bin
new file mode 100755
index 0000000..5c40f89
--- /dev/null
+++ b/script/build-bin
@@ -0,0 +1,56 @@
+#!/bin/sh
+# script/build-bin: run script/build, then bundle the built dist/ into one
+# executable, bin/quak, which reports the version script/build stamped. Our own
+# extension to scripts-to-rule-them-all.
+#
+# bun compiles the binary from a copy of its own executable. On Linux, nix's
+# bun loads through a C library in the nix store, and so does a binary compiled
+# from it. nix-collect-garbage can delete that library, and the binary then no
+# longer starts. On Linux this script therefore compiles with bun's own release
+# build, which loads through the system's C library. It downloads that build
+# and checks it against the hash below before running it.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# bun 1.3.13, 2026-11-11, and the SHA-256 of each release archive below. The
+# x86-64 archive is the baseline build, which also runs on CPUs without AVX2.
+BUN_VERSION="1.3.13"
+BUN_LINUX_X64_BASELINE_SHA256="9d8a24292a7068090205daac0a5a223f5f69736f5287e37bf88d3b4031edc750"
+BUN_LINUX_AARCH64_SHA256="70bae41b3908b0a120e1e58c5c8af30e74afae3b8d11b0d3fdd8e787ddfb4b22"
+
+# compile_with_downloaded_bun <archive-name> <sha256>
+compile_with_downloaded_bun() {
+    tmp="$(mktemp -d)"
+    trap 'rm -rf "$tmp"' EXIT
+    curl -fsSL -o "$tmp/$1.zip" \
+        "https://github.com/oven-sh/bun/releases/download/bun-v$BUN_VERSION/$1.zip"
+    echo "$2  $tmp/$1.zip" | sha256sum -c -
+    unzip -q "$tmp/$1.zip" -d "$tmp"
+    "$tmp/$1/bun" build dist/bin/quak.js --compile --outfile bin/quak
+}
+
+main() {
+    cd "$ROOT"
+    "$ROOT/script/build"
+    case "$(uname -s)-$(uname -m)" in
+        Linux-x86_64)
+            compile_with_downloaded_bun bun-linux-x64-baseline \
+                "$BUN_LINUX_X64_BASELINE_SHA256"
+            ;;
+        Linux-aarch64)
+            compile_with_downloaded_bun bun-linux-aarch64 \
+                "$BUN_LINUX_AARCH64_SHA256"
+            ;;
+        Linux-*)
+            echo "build-bin: bun has no release build for Linux on $(uname -m)" >&2
+            exit 1
+            ;;
+        *)
+            nix-shell -p bun --run \
+                "bun build dist/bin/quak.js --compile --outfile bin/quak"
+            ;;
+    esac
+}
+
+main "$@"