diff --git a/codex-rs/windows-sandbox-rs/src/path_normalization.rs b/codex-rs/windows-sandbox-rs/src/path_normalization.rs
index fe6a932306..0d3fc9ddca 100644
--- a/codex-rs/windows-sandbox-rs/src/path_normalization.rs
+++ b/codex-rs/windows-sandbox-rs/src/path_normalization.rs
@@ -1,5 +1,10 @@
+use anyhow::Result;
 use std::path::Path;
 use std::path::PathBuf;
+use windows_sys::Win32::Storage::FileSystem::DRIVE_REMOTE;
+use windows_sys::Win32::Storage::FileSystem::GetDriveTypeW;
+
+use crate::winutil::to_wide;
 
 pub fn canonicalize_path(path: &Path) -> PathBuf {
     dunce::canonicalize(path).unwrap_or_else(|_| path.to_path_buf())
@@ -12,9 +17,41 @@ pub fn canonical_path_key(path: &Path) -> String {
         .to_ascii_lowercase()
 }
 
+pub fn ensure_windows_sandbox_local_path(path: &Path, context: &str) -> Result<()> {
+    let raw = path.to_string_lossy();
+    let normalized = normalize_windows_device_path(&raw).unwrap_or_else(|| raw.into_owned());
+    if normalized.starts_with(r"\\") {
+        anyhow::bail!(
+            "windows sandbox does not support {context} on UNC or mapped network paths: {}. Use a local drive workspace or run without the Windows sandbox for this session.",
+            path.display()
+        );
+    }
+
+    if let Some(root) = windows_drive_root(&normalized) {
+        let drive_type = unsafe { GetDriveTypeW(to_wide(&root).as_ptr()) };
+        if drive_type == DRIVE_REMOTE {
+            anyhow::bail!(
+                "windows sandbox does not support {context} on mapped network drives: {}. Use the underlying local drive path or run without the Windows sandbox for this session.",
+                path.display()
+            );
+        }
+    }
+
+    Ok(())
+}
+
+fn windows_drive_root(path: &str) -> Option<String> {
+    let bytes = path.as_bytes();
+    if bytes.len() >= 3 && bytes[0].is_ascii_alphabetic() && bytes[1] == b':' {
+        return Some(format!("{}:\\", path[..1].to_ascii_uppercase()));
+    }
+    None
+}
+
 #[cfg(test)]
 mod tests {
     use super::canonical_path_key;
+    use super::windows_drive_root;
     use pretty_assertions::assert_eq;
     use std::path::Path;
 
@@ -23,6 +60,16 @@ mod tests {
         let windows_style = Path::new(r"C:\Users\Dev\Repo");
         let slash_style = Path::new("c:/users/dev/repo");
 
-        assert_eq!(canonical_path_key(windows_style), canonical_path_key(slash_style));
+        assert_eq!(
+            canonical_path_key(windows_style),
+            canonical_path_key(slash_style)
+        );
+    }
+
+    #[test]
+    fn windows_drive_root_extracts_drive_prefix() {
+        assert_eq!(windows_drive_root(r"l:\cs-web"), Some(r"L:\".to_string()));
+        assert_eq!(windows_drive_root(r"C:/repo"), Some(r"C:\".to_string()));
+        assert_eq!(windows_drive_root(r"\\video1\node\cs-web"), None);
     }
 }
diff --git a/codex-rs/windows-sandbox-rs/src/setup_orchestrator.rs b/codex-rs/windows-sandbox-rs/src/setup_orchestrator.rs
index 8c334000ec..b608c5155b 100644
--- a/codex-rs/windows-sandbox-rs/src/setup_orchestrator.rs
+++ b/codex-rs/windows-sandbox-rs/src/setup_orchestrator.rs
@@ -15,6 +15,7 @@ use crate::allow::compute_allow_paths;
 use crate::helper_materialization::helper_bin_dir;
 use crate::logging::log_note;
 use crate::path_normalization::canonical_path_key;
+use crate::path_normalization::ensure_windows_sandbox_local_path;
 use crate::policy::SandboxPolicy;
 use crate::setup_error::SetupErrorCode;
 use crate::setup_error::SetupFailure;
@@ -167,6 +168,13 @@ fn run_setup_refresh_inner(
     }
     let (read_roots, write_roots) = build_payload_roots(&request, &overrides);
     let deny_write_paths = build_payload_deny_write_paths(&request, overrides.deny_write_paths);
+    ensure_windows_sandbox_local_path(request.command_cwd, "sandbox working directory")?;
+    for root in &write_roots {
+        ensure_windows_sandbox_local_path(root, "sandbox writable root")?;
+    }
+    for path in &deny_write_paths {
+        ensure_windows_sandbox_local_path(path, "sandbox protected path")?;
+    }
     let network_identity =
         SandboxNetworkIdentity::from_policy(request.policy, request.proxy_enforced);
     let offline_proxy_settings = offline_proxy_settings_from_env(request.env_map, network_identity);
diff --git a/codex-rs/windows-sandbox-rs/src/spawn_prep.rs b/codex-rs/windows-sandbox-rs/src/spawn_prep.rs
index aab2b54462..a106f4b0e8 100644
--- a/codex-rs/windows-sandbox-rs/src/spawn_prep.rs
+++ b/codex-rs/windows-sandbox-rs/src/spawn_prep.rs
@@ -13,6 +13,7 @@ use crate::identity::SandboxCreds;
 use crate::identity::require_logon_sandbox_creds;
 use crate::logging::log_start;
 use crate::path_normalization::canonicalize_path;
+use crate::path_normalization::ensure_windows_sandbox_local_path;
 use crate::policy::SandboxPolicy;
 use crate::policy::parse_policy;
 use crate::sandbox_utils::ensure_codex_home_exists;
@@ -103,6 +104,8 @@ fn prepare_spawn_context_common(
         anyhow::bail!("DangerFullAccess and ExternalSandbox are not supported for sandboxing")
     }
 
+    ensure_windows_sandbox_local_path(cwd, "sandbox working directory")?;
+
     normalize_null_device_env(env_map);
     ensure_non_interactive_pager(env_map);
     if inherit_path {
@@ -282,6 +285,12 @@ pub(crate) fn prepare_elevated_spawn_context(
     );
     let write_roots: Vec<PathBuf> = allow.into_iter().collect();
     let deny_write_paths: Vec<PathBuf> = deny.into_iter().collect();
+    for root in &write_roots {
+        ensure_windows_sandbox_local_path(root, "sandbox writable root")?;
+    }
+    for path in &deny_write_paths {
+        ensure_windows_sandbox_local_path(path, "sandbox protected path")?;
+    }
     let write_roots_override = if common.is_workspace_write {
         Some(write_roots.as_slice())
     } else {