diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 84500b5e55..38bce2ae32 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -682,7 +682,7 @@ Turns attach user input (text or images) to a thread and trigger Codex generatio
 - `{"type":"image","url":"https://…png"}`
 - `{"type":"localImage","path":"/tmp/screenshot.png"}`
 
-You can optionally specify config overrides on the new turn. If specified, these settings become the default for subsequent turns on the same thread. `outputSchema` applies only to the current turn. Experimental `environments` is turn-scoped: omit it to inherit the thread's sticky environments, pass `[]` to run the turn with no environments, or pass explicit environment ids to override the sticky selection for this turn only. Each environment `cwd` must be absolute and use that environment's native path convention, so a Windows environment uses a value such as `C:\\workspace` even when app-server runs on Linux. The path must also be portable to the selected executor; app-server rejects malformed paths and Windows device names before starting the thread or turn.
+You can optionally specify config overrides on the new turn. If specified, these settings become the default for subsequent turns on the same thread. `outputSchema` applies only to the current turn. Experimental `environments` is turn-scoped: omit it to inherit the thread's sticky environments, pass `[]` to run the turn with no environments, or pass explicit environment ids to override the sticky selection for this turn only. Each environment `cwd` must be absolute and use that environment's native path convention, so a Windows environment uses a value such as `C:\\workspace` even when app-server runs on Linux. The path must also be portable to the selected executor; app-server rejects malformed paths and Windows device names before starting the thread or turn. If a default environment uses a different path convention from app-server, clients must provide an explicit environment `cwd`.
 
 `approvalsReviewer` accepts:
 
diff --git a/codex-rs/core/src/environment_selection.rs b/codex-rs/core/src/environment_selection.rs
index f290aa6561..bb17cd9c98 100644
--- a/codex-rs/core/src/environment_selection.rs
+++ b/codex-rs/core/src/environment_selection.rs
@@ -212,6 +212,28 @@ url = "ws://127.0.0.1:8765"
         assert!(err.to_string().contains("duplicate"));
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn foreign_remote_environment_requires_explicit_native_cwd() {
+        let environment = Arc::new(
+            Environment::create_for_tests(Some("ws://127.0.0.1:8765".to_string()))
+                .expect("remote environment"),
+        );
+        let error = TurnEnvironment::new_with_uri(
+            REMOTE_ENVIRONMENT_ID.to_string(),
+            environment,
+            PathUri::parse("file:///workspace").expect("POSIX cwd URI"),
+            codex_utils_path_uri::PathConvention::Windows,
+            crate::shell::default_user_shell(),
+        )
+        .expect_err("host cwd must not be projected into a foreign environment");
+
+        assert_eq!(
+            error.to_string(),
+            "explicit environment cwd required for foreign environment `remote` using Windows path syntax"
+        );
+    }
+
     #[tokio::test]
     async fn resolved_environment_selections_use_first_selection_as_primary() {
         let cwd = AbsolutePathBuf::current_dir().expect("cwd");
diff --git a/codex-rs/core/src/session/turn_context.rs b/codex-rs/core/src/session/turn_context.rs
index 916a69930e..a724d3ea7f 100644
--- a/codex-rs/core/src/session/turn_context.rs
+++ b/codex-rs/core/src/session/turn_context.rs
@@ -73,11 +73,19 @@ impl TurnEnvironment {
         path_convention: PathConvention,
         shell: shell::Shell,
     ) -> CodexResult<Self> {
-        ApiPathString::from_path_uri(&cwd, path_convention).map_err(|err| {
-            CodexErr::InvalidRequest(format!(
-                "turn environment cwd `{cwd}` cannot be rendered for {path_convention}: {err}"
-            ))
-        })?;
+        match ApiPathString::from_path_uri(&cwd, path_convention) {
+            Ok(_) => {}
+            Err(_) if environment.is_remote() && path_convention != PathConvention::native() => {
+                return Err(CodexErr::InvalidRequest(format!(
+                    "explicit environment cwd required for foreign environment `{environment_id}` using {path_convention} path syntax"
+                )));
+            }
+            Err(err) => {
+                return Err(CodexErr::InvalidRequest(format!(
+                    "turn environment cwd `{cwd}` cannot be rendered for {path_convention}: {err}"
+                )));
+            }
+        }
         Ok(Self {
             environment_id,
             environment,
diff --git a/codex-rs/core/tests/remote_env_windows/wine_app_server_windows_exec_server_test.rs b/codex-rs/core/tests/remote_env_windows/wine_app_server_windows_exec_server_test.rs
index e2be06fb9d..1f1ade54c2 100644
--- a/codex-rs/core/tests/remote_env_windows/wine_app_server_windows_exec_server_test.rs
+++ b/codex-rs/core/tests/remote_env_windows/wine_app_server_windows_exec_server_test.rs
@@ -163,6 +163,30 @@ async fn exercise_app_server(websocket_url: String) -> Result<()> {
         "invalid environment cwd must fail before model inference"
     );
 
+    let default_request_id = app_server
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("mock-model".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let default_response: JSONRPCError = timeout(
+        APP_SERVER_TIMEOUT,
+        app_server.read_stream_until_error_message(RequestId::Integer(default_request_id)),
+    )
+    .await??;
+    assert_eq!(default_response.id, RequestId::Integer(default_request_id));
+    assert_eq!(default_response.error.code, -32600);
+    assert_eq!(
+        default_response.error.message,
+        format!(
+            "explicit environment cwd required for foreign environment `{REMOTE_ENVIRONMENT_ID}` using Windows path syntax"
+        )
+    );
+    assert!(
+        response_mock.requests().is_empty(),
+        "foreign default cwd must fail before model inference"
+    );
+
     let environment = remote_windows_environment();
     let thread_request_id = app_server
         .send_thread_start_request(ThreadStartParams {