diff --git a/codex-rs/core/src/tools/spec.rs b/codex-rs/core/src/tools/spec.rs
index ea445e87894..f5b630cd3b6 100644
--- a/codex-rs/core/src/tools/spec.rs
+++ b/codex-rs/core/src/tools/spec.rs
@@ -43,7 +43,12 @@ impl ToolsConfig {
         let shell_type = if !features.enabled(Feature::ShellTool) {
             ConfigShellToolType::Disabled
         } else if features.enabled(Feature::UnifiedExec) {
-            ConfigShellToolType::UnifiedExec
+            // If ConPTY not supported (for old Windows versions), fallback on ShellCommand.
+            if codex_utils_pty::conpty_supported() {
+                ConfigShellToolType::UnifiedExec
+            } else {
+                ConfigShellToolType::ShellCommand
+            }
         } else {
             model_family.shell_type
         };
diff --git a/codex-rs/utils/pty/src/lib.rs b/codex-rs/utils/pty/src/lib.rs
index 38e86bd2a6a..9e05416ed81 100644
--- a/codex-rs/utils/pty/src/lib.rs
+++ b/codex-rs/utils/pty/src/lib.rs
@@ -133,6 +133,15 @@ pub struct SpawnedPty {
     pub exit_rx: oneshot::Receiver<i32>,
 }
 
+#[allow(unreachable_code)]
+pub fn conpty_supported() -> bool {
+    // Annotation required because `win` can't be compiled on other OS.
+    #[cfg(windows)]
+    return win::conpty_supported();
+
+    true
+}
+
 #[cfg(windows)]
 fn platform_native_pty_system() -> Box<dyn portable_pty::PtySystem + Send> {
     Box::new(win::ConPtySystem::default())
diff --git a/codex-rs/utils/pty/src/win/mod.rs b/codex-rs/utils/pty/src/win/mod.rs
index 8206c9b890d..27057a36428 100644
--- a/codex-rs/utils/pty/src/win/mod.rs
+++ b/codex-rs/utils/pty/src/win/mod.rs
@@ -41,6 +41,7 @@ mod procthreadattr;
 mod psuedocon;
 
 pub use conpty::ConPtySystem;
+pub use psuedocon::conpty_supported;
 
 #[derive(Debug)]
 pub struct WinChild {
diff --git a/codex-rs/utils/pty/src/win/psuedocon.rs b/codex-rs/utils/pty/src/win/psuedocon.rs
index a8db98eefe2..4365180c738 100644
--- a/codex-rs/utils/pty/src/win/psuedocon.rs
+++ b/codex-rs/utils/pty/src/win/psuedocon.rs
@@ -42,6 +42,8 @@ use std::path::Path;
 use std::ptr;
 use std::sync::Mutex;
 use winapi::shared::minwindef::DWORD;
+use winapi::shared::ntdef::NTSTATUS;
+use winapi::shared::ntstatus::STATUS_SUCCESS;
 use winapi::shared::winerror::HRESULT;
 use winapi::shared::winerror::S_OK;
 use winapi::um::handleapi::*;
@@ -52,6 +54,7 @@ use winapi::um::winbase::STARTF_USESTDHANDLES;
 use winapi::um::winbase::STARTUPINFOEXW;
 use winapi::um::wincon::COORD;
 use winapi::um::winnt::HANDLE;
+use winapi::um::winnt::OSVERSIONINFOW;
 
 pub type HPCON = HANDLE;
 
@@ -59,6 +62,10 @@ pub const PSEUDOCONSOLE_RESIZE_QUIRK: DWORD = 0x2;
 #[allow(dead_code)]
 pub const PSEUDOCONSOLE_PASSTHROUGH_MODE: DWORD = 0x8;
 
+// https://learn.microsoft.com/en-gb/windows/console/createpseudoconsole
+// https://learn.microsoft.com/en-gb/windows/release-health/release-information
+const MIN_CONPTY_BUILD: u32 = 17_763;
+
 shared_library!(ConPtyFuncs,
     pub fn CreatePseudoConsole(
         size: COORD,
@@ -71,6 +78,12 @@ shared_library!(ConPtyFuncs,
     pub fn ClosePseudoConsole(hpc: HPCON),
 );
 
+shared_library!(Ntdll,
+    pub fn RtlGetVersion(
+        version_info: *mut OSVERSIONINFOW
+    ) -> NTSTATUS,
+);
+
 fn load_conpty() -> ConPtyFuncs {
     let kernel = ConPtyFuncs::open(Path::new("kernel32.dll")).expect(
         "this system does not support conpty.  Windows 10 October 2018 or newer is required",
@@ -87,6 +100,22 @@ lazy_static! {
     static ref CONPTY: ConPtyFuncs = load_conpty();
 }
 
+pub fn conpty_supported() -> bool {
+    windows_build_number().is_some_and(|build| build >= MIN_CONPTY_BUILD)
+}
+
+fn windows_build_number() -> Option<u32> {
+    let ntdll = Ntdll::open(Path::new("ntdll.dll")).ok()?;
+    let mut info: OSVERSIONINFOW = unsafe { mem::zeroed() };
+    info.dwOSVersionInfoSize = mem::size_of::<OSVERSIONINFOW>() as u32;
+    let status = unsafe { (ntdll.RtlGetVersion)(&mut info) };
+    if status == STATUS_SUCCESS {
+        Some(info.dwBuildNumber)
+    } else {
+        None
+    }
+}
+
 pub struct PsuedoCon {
     con: HPCON,
 }
@@ -320,3 +349,17 @@ fn append_quoted(arg: &OsStr, cmdline: &mut Vec<u16>) {
     }
     cmdline.push('"' as u16);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::windows_build_number;
+    use super::MIN_CONPTY_BUILD;
+
+    #[test]
+    fn windows_build_number_returns_value() {
+        // We can't stably check the version of the GH workers, but we can
+        // at least check that this.
+        let version = windows_build_number().unwrap();
+        assert!(version > MIN_CONPTY_BUILD);
+    }
+}