diff --git a/codex-rs/arg0/src/lib.rs b/codex-rs/arg0/src/lib.rs
index 9c455ddbb..956373783 100644
--- a/codex-rs/arg0/src/lib.rs
+++ b/codex-rs/arg0/src/lib.rs
@@ -12,6 +12,8 @@ const LINUX_SANDBOX_ARG0: &str = "codex-linux-sandbox";
 const APPLY_PATCH_ARG0: &str = "apply_patch";
 const MISSPELLED_APPLY_PATCH_ARG0: &str = "applypatch";
 const LOCK_FILENAME: &str = ".lock";
+#[cfg(target_os = "windows")]
+const WINDOWS_TOKIO_WORKER_STACK_SIZE_BYTES: usize = 16 * 1024 * 1024;
 
 /// Keeps the per-session PATH entry alive and locked for the process lifetime.
 pub struct Arg0PathEntryGuard {
@@ -112,7 +114,7 @@ where
 
     // Regular invocation – create a Tokio runtime and execute the provided
     // async entry-point.
-    let runtime = tokio::runtime::Runtime::new()?;
+    let runtime = build_runtime()?;
     runtime.block_on(async move {
         let codex_linux_sandbox_exe: Option<PathBuf> = if cfg!(target_os = "linux") {
             std::env::current_exe().ok()
@@ -124,6 +126,18 @@ where
     })
 }
 
+fn build_runtime() -> anyhow::Result<tokio::runtime::Runtime> {
+    let mut builder = tokio::runtime::Builder::new_multi_thread();
+    builder.enable_all();
+    #[cfg(target_os = "windows")]
+    {
+        // Defensive hardening: Windows worker threads have lower effective
+        // stack headroom, so use a larger stack for runtime workers.
+        builder.thread_stack_size(WINDOWS_TOKIO_WORKER_STACK_SIZE_BYTES);
+    }
+    Ok(builder.build()?)
+}
+
 const ILLEGAL_ENV_VAR_PREFIX: &str = "CODEX_";
 
 /// Load env vars from ~/.codex/.env.