diff --git a/.gitignore b/.gitignore
index 39b0697..6cfe265 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 /vault
 /vault.exe
 .claude/
+/dist
diff --git a/CLAUDE.md b/CLAUDE.md
index b2e1e7d..ec3e57c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -14,6 +14,11 @@ root that builds a single `vault` binary. Module `github.com/andydixon/vault`.
 - `store.go` - on-disk vault: `records/` holds fixed-size encrypted catalogue
   records, `objects/` holds padded encrypted file bodies. Records a key cannot
   open are skipped, not treated as errors.
+- `lock*.go` - vault lock on `DIR/.lock`: fcntl on unix, LockFileEx on
+  Windows, an exclusive lock file elsewhere. Writers lock exclusively,
+  readers shared, taken after the passphrase prompt.
+- `build.sh` - cross-compiles every `go tool dist list` target except
+  iOS and android/386, amd64, arm (cgo-only).
 - `internal/pqcrypt` - file encryption (header + STREAM ChaCha20-Poly1305),
   suite registry, hybrid FrodoKEM-640 + ML-KEM-768 default suite.
 - `internal/format`, `internal/stream`, `internal/term` - header encoding,
@@ -24,6 +29,8 @@ root that builds a single `vault` binary. Module `github.com/andydixon/vault`.
 ```sh
 go test ./...
 go build .
+./build.sh                                  # all platforms, into ./dist
+TARGETS="linux/amd64" ./build.sh            # a subset
 ```
 
 End-to-end check:
@@ -43,5 +50,6 @@ export VAULT_PASSPHRASE=test VAULT_KEY=$PWD/k VAULT_DIR=$PWD/s
 - `get` publishes output only after the entire object has authenticated.
 - Decoy deniability: both key slots stay the same size and are both always
   tried; `init` always writes filler; foreign records are skipped silently.
-- Keep dependencies pure Go.
+- Every command that changes the vault holds the exclusive lock.
+- Keep dependencies pure Go and CGO_ENABLED=0 builds working.
 - Diagnostics go to standard error, lower-case, no trailing full stops.
diff --git a/README.md b/README.md
index 167f8cb..05f7dca 100644
--- a/README.md
+++ b/README.md
@@ -13,8 +13,14 @@ buckets.
 ```sh
 go build .        # produces ./vault
 go test ./...
+./build.sh        # every OS and CPU architecture, into ./dist
 ```
 
+`build.sh` cross-compiles static binaries for every target in
+`go tool dist list` except iOS and 32-bit or x86 Android, which need cgo. It builds 32-bit ARM Linux for
+both armv6 and armv7, and writes `dist/SHA256SUMS`. Set `TARGETS` to build a
+subset, for example `TARGETS="linux/amd64 darwin/arm64" ./build.sh`.
+
 ## Use
 
 ```sh
@@ -31,6 +37,13 @@ vault mv report.pdf old/report.pdf
 vault rm old/report.pdf
 ```
 
+Commands that change the vault take an exclusive lock on `DIR/.lock`, and
+commands that read it take a shared one. A second process waits, and says
+so, until the first finishes. The operating system releases the lock if a
+process dies. Plan 9 and WebAssembly have no file locks, so there the lock
+file itself is the lock. After a crash on those platforms you remove it by
+hand; the error message names it.
+
 Adding a name that already exists replaces it. `get` refuses to overwrite an
 existing file and writes nothing until the whole object has authenticated.
 
@@ -67,7 +80,6 @@ vault over time can still see it grow when real files are added.
 
 ## Limits
 
-- One writer at a time. Concurrent writes to the same vault are not locked.
 - The storage host can see how many files exist, their size buckets and when
   the vault changes.
 - The key file is the only way in. Losing it or its passphrase loses the data.
diff --git a/build.sh b/build.sh
new file mode 100755
index 0000000..9b09f7f
--- /dev/null
+++ b/build.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Build vault for every OS and CPU architecture Go can target without cgo.
+#
+#   ./build.sh                          # everything, into ./dist
+#   TARGETS="linux/amd64 darwin/arm64" ./build.sh
+#   OUT=/tmp/release ./build.sh
+#
+# Binaries are static, stripped and reproducible (CGO_ENABLED=0, -trimpath).
+# iOS and most Android targets are skipped (they need cgo); see below.
+# 32-bit ARM Linux is built twice: armv6 (Raspberry Pi Zero/1) and armv7.
+# SHA256SUMS is written next to the binaries.
+set -euo pipefail
+cd "$(dirname "$0")"
+
+out=${OUT:-dist}
+# iOS and 32-bit/x86 Android need cgo and a vendor SDK. android/arm64 builds
+# a static binary that runs in Termux.
+targets=${TARGETS:-$(go tool dist list | grep -vE '^ios/|^android/(386|amd64|arm)$')}
+jobs=${JOBS:-$(getconf _NPROCESSORS_ONLN 2>/dev/null || echo 4)}
+
+rm -rf "$out"
+mkdir -p "$out"
+
+build_one() {
+	local os=${1%/*} arch=${1#*/} variants=("") name ext=""
+	[[ $os == windows ]] && ext=.exe
+	[[ $arch == wasm ]] && ext=.wasm
+	[[ $os/$arch == linux/arm ]] && variants=(6 7)
+	for v in "${variants[@]}"; do
+		name=vault-$os-$arch${v:+v$v}$ext
+		if ! GOOS=$os GOARCH=$arch GOARM=${v:-} CGO_ENABLED=0 \
+			go build -trimpath -buildvcs=false -ldflags='-s -w -buildid=' -o "$OUT_DIR/$name" . 2>"$OUT_DIR/$name.log"; then
+			echo "FAILED $name" >&2
+			cat "$OUT_DIR/$name.log" >&2
+			return 1
+		fi
+		rm "$OUT_DIR/$name.log"
+		echo "built  $name"
+	done
+}
+export -f build_one
+export OUT_DIR=$out
+
+printf '%s\n' $targets | xargs -P "$jobs" -I{} bash -c 'build_one {}'
+
+cd "$out"
+if command -v sha256sum >/dev/null; then sha256sum vault-* >SHA256SUMS; else shasum -a 256 vault-* >SHA256SUMS; fi
+echo "$(ls vault-* | wc -l | tr -d ' ') binaries in $out"
diff --git a/lock.go b/lock.go
new file mode 100644
index 0000000..3a2663a
--- /dev/null
+++ b/lock.go
@@ -0,0 +1,28 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+const lockName = ".lock"
+
+// lockVault takes the vault lock: exclusive for commands that change the
+// vault, shared for commands that only read it. If another vault process
+// holds a conflicting lock it says so and waits. The returned function
+// releases the lock.
+func lockVault(root string, exclusive bool) (func(), error) {
+	release, err := acquireLock(filepath.Join(root, lockName), exclusive, func() {
+		fmt.Fprintln(os.Stderr, "waiting for another vault process to finish")
+	})
+	if err != nil && !exclusive {
+		// A reader on read-only storage cannot create the lock file, and
+		// nothing can be writing there either.
+		return func() {}, nil
+	}
+	return release, err
+}
diff --git a/lock_other.go b/lock_other.go
new file mode 100644
index 0000000..c482e56
--- /dev/null
+++ b/lock_other.go
@@ -0,0 +1,37 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !unix && !windows
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"time"
+)
+
+// acquireLock falls back to an exclusive lock file where the OS offers no
+// file locks. Every lock is exclusive here.
+// ponytail: a crash leaves the lock file behind; the error names it for manual removal.
+func acquireLock(path string, _ bool, waiting func()) (func(), error) {
+	for i := 0; ; i++ {
+		f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o600)
+		if err == nil {
+			f.Close()
+			return func() { os.Remove(path) }, nil
+		}
+		if !errors.Is(err, fs.ErrExist) {
+			return nil, err
+		}
+		if i == 0 {
+			waiting()
+		}
+		if i == 600 {
+			return nil, fmt.Errorf("vault still locked after a minute; if no other vault process is running, remove %s", path)
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
diff --git a/lock_unix.go b/lock_unix.go
new file mode 100644
index 0000000..07e69ad
--- /dev/null
+++ b/lock_unix.go
@@ -0,0 +1,39 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build unix
+
+package main
+
+import (
+	"errors"
+	"io"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// acquireLock uses fcntl record locks, which the kernel releases if the
+// process dies, and which also work on NFS.
+func acquireLock(path string, exclusive bool, waiting func()) (func(), error) {
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	lk := unix.Flock_t{Type: unix.F_RDLCK, Whence: io.SeekStart}
+	if exclusive {
+		lk.Type = unix.F_WRLCK
+	}
+	err = unix.FcntlFlock(f.Fd(), unix.F_SETLK, &lk)
+	if errors.Is(err, unix.EAGAIN) || errors.Is(err, unix.EACCES) {
+		waiting()
+		for err = unix.EINTR; errors.Is(err, unix.EINTR); {
+			err = unix.FcntlFlock(f.Fd(), unix.F_SETLKW, &lk)
+		}
+	}
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return func() { f.Close() }, nil
+}
diff --git a/lock_windows.go b/lock_windows.go
new file mode 100644
index 0000000..1b6cc47
--- /dev/null
+++ b/lock_windows.go
@@ -0,0 +1,36 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build windows
+
+package main
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+// acquireLock uses LockFileEx, which Windows releases if the process dies.
+func acquireLock(path string, exclusive bool, waiting func()) (func(), error) {
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	h := windows.Handle(f.Fd())
+	var flags uint32
+	if exclusive {
+		flags = windows.LOCKFILE_EXCLUSIVE_LOCK
+	}
+	err = windows.LockFileEx(h, flags|windows.LOCKFILE_FAIL_IMMEDIATELY, 0, 1, 0, new(windows.Overlapped))
+	if errors.Is(err, windows.ERROR_LOCK_VIOLATION) {
+		waiting()
+		err = windows.LockFileEx(h, flags, 0, 1, 0, new(windows.Overlapped))
+	}
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return func() { f.Close() }, nil
+}
diff --git a/main.go b/main.go
index 3ef7fa9..9da52df 100644
--- a/main.go
+++ b/main.go
@@ -46,7 +46,12 @@ const (
 	decoyEnv      = "VAULT_DECOY_PASSPHRASE"
 )
 
+// unlock releases the vault lock taken by open. fail calls it too, because
+// os.Exit skips deferred calls.
+var unlock = func() {}
+
 func main() {
+	defer func() { unlock() }()
 	if len(os.Args) < 2 {
 		fmt.Fprintln(os.Stderr, usage)
 		os.Exit(2)
@@ -90,7 +95,7 @@ func main() {
 		if len(pos) == 2 {
 			dest = strings.Trim(pos[1], "/")
 		}
-		s := open(*keyPath, *vaultDir)
+		s := open(*keyPath, *vaultDir, true)
 		cat, err := s.catalogue()
 		check(err)
 		check(addPath(s, cat, pos[0], dest))
@@ -100,7 +105,7 @@ func main() {
 		if len(pos) == 1 {
 			prefix = strings.Trim(pos[0], "/")
 		}
-		cat, err := open(*keyPath, *vaultDir).catalogue()
+		cat, err := open(*keyPath, *vaultDir, false).catalogue()
 		check(err)
 		list(cat, prefix)
 	case "get":
@@ -109,13 +114,13 @@ func main() {
 		if out == "" {
 			out = path.Base(pos[0])
 		}
-		check(open(*keyPath, *vaultDir).get(pos[0], out))
+		check(open(*keyPath, *vaultDir, false).get(pos[0], out))
 	case "mv", "rename":
 		want(2, 2)
-		check(open(*keyPath, *vaultDir).rename(pos[0], pos[1]))
+		check(open(*keyPath, *vaultDir, true).rename(pos[0], pos[1]))
 	case "rm", "delete":
 		want(1, 1)
-		check(open(*keyPath, *vaultDir).delete(pos[0]))
+		check(open(*keyPath, *vaultDir, true).delete(pos[0]))
 	case "help", "-h", "--help":
 		fmt.Println(usage)
 	default:
@@ -150,13 +155,17 @@ func defaultPath(env, name string) string {
 	return filepath.Join(home, ".vault", name)
 }
 
-func open(keyPath, vaultDir string) *store {
+// open unlocks the key and opens the vault, holding the vault lock
+// exclusively for writers and shared for readers.
+func open(keyPath, vaultDir string, exclusive bool) *store {
 	pass, err := term.ReadPassphrase("Enter passphrase:", passphraseEnv)
 	check(err)
 	id, err := unlockKeyFile(keyPath, pass)
 	check(err)
 	s, err := openStore(vaultDir, id)
 	check(err)
+	unlock, err = lockVault(vaultDir, exclusive)
+	check(err)
 	return s
 }
 
@@ -271,6 +280,7 @@ func check(err error) {
 }
 
 func fail(format string, args ...any) {
+	unlock()
 	fmt.Fprintf(os.Stderr, "vault: "+format+"\n", args...)
 	os.Exit(1)
 }
diff --git a/vault_test.go b/vault_test.go
index cd9a9cc..389bd2f 100644
--- a/vault_test.go
+++ b/vault_test.go
@@ -4,10 +4,14 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
+	"io"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"testing"
+	"time"
 )
 
 func TestKeyFile(t *testing.T) {
@@ -129,3 +133,45 @@ func TestDecoy(t *testing.T) {
 		t.Fatalf("only %d records; filler missing", len(entries))
 	}
 }
+
+// TestLockExcludesOtherProcess runs a copy of the test binary that holds the
+// exclusive lock, then checks this process has to wait for it.
+func TestLockExcludesOtherProcess(t *testing.T) {
+	if dir := os.Getenv("VAULT_TEST_LOCK_HOLDER"); dir != "" {
+		release, err := lockVault(dir, true)
+		if err != nil {
+			os.Exit(1)
+		}
+		os.Stdout.WriteString("locked\n")
+		io.Copy(io.Discard, os.Stdin) // hold until the parent closes stdin
+		release()
+		os.Exit(0)
+	}
+	dir := t.TempDir()
+	holder := exec.Command(os.Args[0], "-test.run=^TestLockExcludesOtherProcess$")
+	holder.Env = append(os.Environ(), "VAULT_TEST_LOCK_HOLDER="+dir)
+	stdin, _ := holder.StdinPipe()
+	stdout, _ := holder.StdoutPipe()
+	if err := holder.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer holder.Wait()
+	if line, _ := bufio.NewReader(stdout).ReadString('\n'); line != "locked\n" {
+		t.Fatalf("holder said %q", line)
+	}
+	waited := make(chan bool, 1)
+	go func() {
+		time.Sleep(200 * time.Millisecond)
+		stdin.Close() // let the holder go only after we have started waiting
+	}()
+	for _, exclusive := range []bool{false, true} {
+		release, err := acquireLock(filepath.Join(dir, lockName), exclusive, func() { waited <- true })
+		if err != nil {
+			t.Fatal(err)
+		}
+		release()
+		if !exclusive && len(waited) == 0 {
+			t.Fatal("shared lock did not wait for the exclusive holder")
+		}
+	}
+}