fix: fall back when Windows PowerShell shell startup fails

codex-rs/core/src/shell.rs

@@ -3,6 +3,10 @@ use crate::shell_snapshot::ShellSnapshot;
 use serde::Deserialize;
 use serde::Serialize;
 use std::path::PathBuf;
+#[cfg(windows)]
+use std::process::Command;
+#[cfg(windows)]
+use std::process::Stdio;
 use std::sync::Arc;
 use tokio::sync::watch;
 
@@ -315,7 +319,11 @@ pub fn default_user_shell() -> Shell {
 
 fn default_user_shell_from_path(user_shell_path: Option<PathBuf>) -> Shell {
     if cfg!(windows) {
-        get_shell(ShellType::PowerShell, /*path*/ None).unwrap_or(ultimate_fallback_shell())
+        default_windows_shell(
+            get_shell(ShellType::PowerShell, /*path*/ None),
+            get_shell(ShellType::Cmd, /*path*/ None),
+            shell_starts_successfully,
+        )
     } else {
         let user_default_shell = user_shell_path
             .and_then(|shell| detect_shell_type(&shell))
@@ -335,6 +343,38 @@ fn default_user_shell_from_path(user_shell_path: Option<PathBuf>) -> Shell {
     }
 }
 
+fn default_windows_shell(
+    powershell_shell: Option<Shell>,
+    cmd_shell: Option<Shell>,
+    powershell_probe: impl Fn(&Shell) -> bool,
+) -> Shell {
+    powershell_shell
+        .filter(powershell_probe)
+        .or(cmd_shell)
+        .unwrap_or_else(ultimate_fallback_shell)
+}
+
+#[cfg(windows)]
+fn shell_starts_successfully(shell: &Shell) -> bool {
+    let args = shell.derive_exec_args("$PSVersionTable.PSVersion | Out-Null", false);
+    let Some((program, args)) = args.split_first() else {
+        return false;
+    };
+
+    Command::new(program)
+        .args(args)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .status()
+        .is_ok_and(|status| status.success())
+}
+
+#[cfg(not(windows))]
+fn shell_starts_successfully(_shell: &Shell) -> bool {
+    true
+}
+
 #[cfg(test)]
 mod detect_shell_type_tests {
     use super::*;

codex-rs/core/src/shell_tests.rs

@@ -86,6 +86,44 @@ fn can_run_on_shell_test() {
     }
 }
 
+#[test]
+fn windows_default_shell_prefers_healthy_powershell() {
+    let powershell = Shell {
+        shell_type: ShellType::PowerShell,
+        shell_path: PathBuf::from("pwsh.exe"),
+        shell_snapshot: empty_shell_snapshot_receiver(),
+    };
+    let cmd = Shell {
+        shell_type: ShellType::Cmd,
+        shell_path: PathBuf::from("cmd.exe"),
+        shell_snapshot: empty_shell_snapshot_receiver(),
+    };
+
+    assert_eq!(
+        default_windows_shell(Some(powershell), Some(cmd), |_| true).shell_type,
+        ShellType::PowerShell
+    );
+}
+
+#[test]
+fn windows_default_shell_falls_back_to_cmd_when_powershell_fails_probe() {
+    let powershell = Shell {
+        shell_type: ShellType::PowerShell,
+        shell_path: PathBuf::from("pwsh.exe"),
+        shell_snapshot: empty_shell_snapshot_receiver(),
+    };
+    let cmd = Shell {
+        shell_type: ShellType::Cmd,
+        shell_path: PathBuf::from("cmd.exe"),
+        shell_snapshot: empty_shell_snapshot_receiver(),
+    };
+
+    assert_eq!(
+        default_windows_shell(Some(powershell), Some(cmd), |_| false).shell_type,
+        ShellType::Cmd
+    );
+}
+
 fn shell_works(shell: Option<Shell>, command: &str, required: bool) -> bool {
     if let Some(shell) = shell {
         let args = shell.derive_exec_args(command, /*use_login_shell*/ false);
@@ -173,10 +211,21 @@ async fn detects_powershell_as_default() {
         return;
     }
 
-    let powershell_shell = default_user_shell();
-    let shell_path = powershell_shell.shell_path;
+    let default_shell = default_user_shell();
 
-    assert!(shell_path.ends_with("pwsh.exe") || shell_path.ends_with("powershell.exe"));
+    match default_shell.shell_type {
+        ShellType::PowerShell => {
+            let shell_path = default_shell.shell_path;
+            assert!(shell_path.ends_with("pwsh.exe") || shell_path.ends_with("powershell.exe"));
+        }
+        ShellType::Cmd => {
+            assert!(
+                !get_shell(ShellType::PowerShell, /*path*/ None)
+                    .is_some_and(|shell| shell_starts_successfully(&shell))
+            );
+        }
+        other => panic!("unexpected Windows default shell: {other:?}"),
+    }
 }
 
 #[test]