diff --git a/scripts/install-cli.sh b/scripts/install-cli.sh
index d1a05fb..4621d54 100755
--- a/scripts/install-cli.sh
+++ b/scripts/install-cli.sh
@@ -20,6 +20,11 @@
 #
 set -eu
 
+# Keep ASCII validation independent of the user's locale. On macOS, locale
+# collation can make shell ranges such as [A-Z] match lowercase characters.
+LC_ALL=C
+export LC_ALL
+
 if [ -z "${HOME:-}" ] && [ -z "${CHAT2DB_CLI_BIN_DIR:-}" ]; then
   echo "error: HOME is not set; set CHAT2DB_CLI_BIN_DIR to choose an install directory" >&2
   exit 1
diff --git a/tests/install_cli_script.rs b/tests/install_cli_script.rs
index 4fb76ff..4bfba22 100644
--- a/tests/install_cli_script.rs
+++ b/tests/install_cli_script.rs
@@ -373,6 +373,29 @@ mod unix_install_tests {
         )));
     }
 
+    #[test]
+    fn install_script_accepts_manifest_base_under_non_c_locale() {
+        let temp = tempfile::tempdir().unwrap();
+        let home = temp.path().join("home");
+        fs::create_dir_all(&home).unwrap();
+        let release = FakeRelease::start();
+        let lowercase_manifest_base = release.manifest_base_url.replace("127.0.0.1", "localhost");
+
+        let output = run_installer(
+            &home,
+            Some("/bin/zsh"),
+            &release,
+            &["--no-modify-path"],
+            &[
+                ("CHAT2DB_CLI_MANIFEST_BASE", &lowercase_manifest_base),
+                ("LC_ALL", "en_US.UTF-8"),
+            ],
+        );
+
+        assert_success(&output);
+        assert!(stdout(&output).contains("installed:"));
+    }
+
     #[test]
     fn install_script_rejects_noncanonical_manifest_bases() {
         for base in [