diff --git a/README.md b/README.md
index 3b1b18c..c91a100 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,11 @@
 # pyenv.github.io
 
 hi there
+
+## Tests
+
+Install [Bats](https://github.com/bats-core/bats-core), then run:
+
+```sh
+bats pythons/test/update.bats
+```
diff --git a/pythons/test/update.bats b/pythons/test/update.bats
new file mode 100644
index 0000000..d4ded4b
--- /dev/null
+++ b/pythons/test/update.bats
@@ -0,0 +1,122 @@
+#!/usr/bin/env bats
+
+setup() {
+  site="$BATS_TEST_TMPDIR/site"
+  mkdir -p "$site/binaries"
+  cp "$BATS_TEST_DIRNAME/../update.sh" "$BATS_TEST_DIRNAME/../index.html" "$site"
+  cd "$site" || return
+}
+
+write_meta() {
+  local name="$1"
+  local archive="$2"
+  cat > "binaries/$name.meta" <<EOF
+# pyenv-binary metadata
+version=${name%%-*}
+os=Linux
+arch=x86_64
+platform=linux-x86_64
+distro=ubuntu 24.04
+libc=glibc 2.39
+build_prefix=/tmp/pyenv/versions/$name
+archive=$archive
+EOF
+}
+
+write_binary() {
+  local name="$1"
+  local archive="$2"
+  local contents="$3"
+  printf '%s' "$contents" > "binaries/$archive"
+  printf definition > "binaries/$name"
+  write_meta "$name" "$archive"
+}
+
+write_archives() {
+  write_binary 3.14.0-ubuntu-24.04-x86_64 \
+    3.14.0-ubuntu-24.04-x86_64.tar.xz archive-data
+  write_binary 3.13.0-ubuntu-24.04-x86_64 \
+    3.13.0-ubuntu-24.04-x86_64.tar.gz gzip-archive-data
+}
+
+assert_success() {
+  [ "$status" -eq 0 ]
+}
+
+assert_failure() {
+  [ "$status" -ne 0 ]
+}
+
+@test "indexes xz and gzip prebuilt archives" {
+  write_archives
+
+  run bash ./update.sh
+  assert_success
+
+  xz_sha=8a6111c3ca752ed6d5f8e8a6daa3ba4b8c3b0bf55f00a87ac2b285024ef87e5f
+  [ binaries/3.14.0-ubuntu-24.04-x86_64.tar.xz -ef "$xz_sha" ]
+  xz_entry='<li><a href="binaries/3.14.0-ubuntu-24.04-x86_64.tar.xz">3.14.0-ubuntu-24.04-x86_64.tar.xz</a> (<a href="binaries/3.14.0-ubuntu-24.04-x86_64">definition</a>)</li>'
+  grep -Fqx "$xz_entry" index.html
+
+  gzip_sha=c809857f5fa564e1a2fdb3618161114bfb9a9c624ade6b580f6d79df0adfcc26
+  [ binaries/3.13.0-ubuntu-24.04-x86_64.tar.gz -ef "$gzip_sha" ]
+  gzip_entry='<li><a href="binaries/3.13.0-ubuntu-24.04-x86_64.tar.gz">3.13.0-ubuntu-24.04-x86_64.tar.gz</a> (<a href="binaries/3.13.0-ubuntu-24.04-x86_64">definition</a>)</li>'
+  grep -Fqx "$gzip_entry" index.html
+}
+
+@test "repeated updates do not duplicate entries" {
+  write_archives
+  bash ./update.sh
+  cp index.html index.before
+
+  run bash ./update.sh
+  assert_success
+
+  cmp -s index.before index.html
+  [ "$(grep -Fc '3.14.0-ubuntu-24.04-x86_64.tar.xz</a>' index.html)" -eq 1 ]
+  [ "$(grep -Fc '3.13.0-ubuntu-24.04-x86_64.tar.gz</a>' index.html)" -eq 1 ]
+}
+
+@test "rejects unsafe archive names before updating files" {
+  write_archives
+  printf 'archive=bad\narchive=name.tar.gz\n' > binaries/z-invalid.meta
+  printf definition > binaries/z-invalid
+  mkdir source
+  printf source-data > source/example.tar.gz
+  printf '<li><a href="">example.tar.gz</a></li>\n' >> index.html
+  cp -R . "$BATS_TEST_TMPDIR/site.before"
+
+  run bash ./update.sh
+  assert_failure
+
+  [[ "$output" == *"Invalid archive name in binaries/z-invalid.meta"* ]]
+  diff -r "$BATS_TEST_TMPDIR/site.before" .
+}
+
+@test "uses a safe archive name from metadata" {
+  write_binary custom-definition python-build.tar.xz archive-data
+
+  run bash ./update.sh
+  assert_success
+
+  entry='<li><a href="binaries/python-build.tar.xz">python-build.tar.xz</a> (<a href="binaries/custom-definition">definition</a>)</li>'
+  grep -Fqx "$entry" index.html
+}
+
+@test "rejects a missing definition" {
+  write_binary 3.14.0-ubuntu-24.04-x86_64 \
+    3.14.0-ubuntu-24.04-x86_64.tar.xz archive-data
+  rm binaries/3.14.0-ubuntu-24.04-x86_64
+
+  run bash ./update.sh
+  assert_failure
+}
+
+@test "rejects a missing archive" {
+  write_binary 3.14.0-ubuntu-24.04-x86_64 \
+    3.14.0-ubuntu-24.04-x86_64.tar.xz archive-data
+  rm binaries/3.14.0-ubuntu-24.04-x86_64.tar.xz
+
+  run bash ./update.sh
+  assert_failure
+}
diff --git a/pythons/test/update.sh b/pythons/test/update.sh
deleted file mode 100755
index 08fe0df..0000000
--- a/pythons/test/update.sh
+++ /dev/null
@@ -1,98 +0,0 @@
-#!/usr/bin/env bash
-
-set -e
-
-cd "${BASH_SOURCE%/*}/.."
-
-fail() {
-  echo "$1" >&2
-  exit 1
-}
-
-write_meta() {
-  cat > binaries/3.14.0-ubuntu-24.04-x86_64.meta <<EOF
-# pyenv-binary metadata
-version=3.14.0
-os=Linux
-arch=x86_64
-platform=linux-x86_64
-distro=ubuntu 24.04
-libc=glibc 2.39
-build_prefix=/tmp/pyenv/versions/3.14.0-ubuntu-24.04-x86_64
-archive=$1
-EOF
-}
-
-tmpdir="$(mktemp -d)"
-trap 'rm -rf "$tmpdir"' EXIT
-
-cp update.sh index.html "$tmpdir"
-mkdir "$tmpdir/binaries"
-printf archive-data > "$tmpdir/binaries/3.14.0-ubuntu-24.04-x86_64.tar.gz"
-printf definition > "$tmpdir/binaries/3.14.0-ubuntu-24.04-x86_64"
-
-cd "$tmpdir"
-write_meta 3.14.0-ubuntu-24.04-x86_64.tar.gz
-bash ./update.sh >/dev/null 2>&1 || fail "update failed"
-
-sha=8a6111c3ca752ed6d5f8e8a6daa3ba4b8c3b0bf55f00a87ac2b285024ef87e5f
-[ "binaries/3.14.0-ubuntu-24.04-x86_64.tar.gz" -ef "$sha" ] ||
-  fail "checksum path is not a hardlink to the archive"
-
-entry='<li><a href="binaries/3.14.0-ubuntu-24.04-x86_64.tar.gz">3.14.0-ubuntu-24.04-x86_64.tar.gz</a> (<a href="binaries/3.14.0-ubuntu-24.04-x86_64">definition</a>)</li>'
-grep -Fqx "$entry" index.html || fail "prebuilt archive is missing from index.html"
-
-cp index.html index.before
-bash ./update.sh >/dev/null 2>&1 || fail "second update failed"
-cmp -s index.before index.html || fail "second update changed index.html"
-[ "$(grep -Fxc "$entry" index.html)" -eq 1 ] || fail "prebuilt archive is listed more than once"
-
-bad_name=$'bad\nname'
-printf 'archive=bad\narchive=name.tar.gz\n' > "binaries/$bad_name.meta"
-printf archive-data > "binaries/$bad_name.tar.gz"
-printf definition > "binaries/$bad_name"
-rm "$sha"
-if output="$(bash ./update.sh 2>&1)"; then
-  fail "update accepted a control character in an archive name"
-fi
-case "$output" in
-*"Invalid archive name in binaries/"*) ;;
-*) fail "update did not reject the invalid archive name during validation" ;;
-esac
-[ ! -e "$sha" ] || fail "update created a checksum link before name validation completed"
-rm "binaries/$bad_name.meta" "binaries/$bad_name.tar.gz" "binaries/$bad_name"
-
-mkdir source
-printf source-data > source/example.tar.gz
-printf '<li><a href="">example.tar.gz</a></li>\n' >> index.html
-cp index.html index.before
-cp binaries/3.14.0-ubuntu-24.04-x86_64.meta binaries/z-invalid.meta
-sed -i 's/^archive=.*/archive=invalid.tar.gz/' binaries/z-invalid.meta
-if bash ./update.sh >/dev/null 2>&1; then
-  fail "update accepted invalid metadata"
-fi
-[ ! -e "$sha" ] || fail "update created a binary checksum link before validation completed"
-source_sha=6bb69d845f4a714ca982e2903d2c03fabeb2448b67185bca413d3efddea9397c
-[ ! -e "$source_sha" ] || fail "update created a source checksum link before validation completed"
-cmp -s index.before index.html || fail "update changed index.html before validation completed"
-rm -rf source binaries/z-invalid.meta
-
-write_meta ../outside.tar.gz
-printf outside > outside.tar.gz
-if bash ./update.sh >/dev/null 2>&1; then
-  fail "update accepted an archive outside binaries"
-fi
-
-write_meta 3.14.0-ubuntu-24.04-x86_64.tar.gz
-rm binaries/3.14.0-ubuntu-24.04-x86_64
-if bash ./update.sh >/dev/null 2>&1; then
-  fail "update accepted a missing definition"
-fi
-
-printf definition > binaries/3.14.0-ubuntu-24.04-x86_64
-rm binaries/3.14.0-ubuntu-24.04-x86_64.tar.gz
-if bash ./update.sh >/dev/null 2>&1; then
-  fail "update accepted a missing archive"
-fi
-
-echo "ok"
diff --git a/pythons/update.sh b/pythons/update.sh
index bfc49e3..3968883 100755
--- a/pythons/update.sh
+++ b/pythons/update.sh
@@ -67,10 +67,6 @@ for meta in binaries/*.meta; do
     exit 1
     ;;
   esac
-  if [ "$archive" != "$name.tar.gz" ]; then
-    echo "Invalid archive in $meta" >&2
-    exit 1
-  fi
   if [ ! -f "binaries/$archive" ] || [ ! -f "binaries/$name" ]; then
     echo "Missing archive or definition for $name" >&2
     exit 1