diff --git a/codex-rs/core/src/mcp_connection_manager.rs b/codex-rs/core/src/mcp_connection_manager.rs
index 938d6d0b2b..eac8485048 100644
--- a/codex-rs/core/src/mcp_connection_manager.rs
+++ b/codex-rs/core/src/mcp_connection_manager.rs
@@ -1440,7 +1440,12 @@ async fn make_rmcp_client(
         } => {
             let command_os: OsString = command.into();
             let args_os: Vec<OsString> = args.into_iter().map(Into::into).collect();
-            RmcpClient::new_stdio_client(command_os, args_os, env, &env_vars, cwd)
+            let env_os = env.map(|env| {
+                env.into_iter()
+                    .map(|(key, value)| (key.into(), value.into()))
+                    .collect::<HashMap<_, _>>()
+            });
+            RmcpClient::new_stdio_client(command_os, args_os, env_os, &env_vars, cwd)
                 .await
                 .map_err(|err| StartupOutcomeError::from(anyhow!(err)))
         }
diff --git a/codex-rs/rmcp-client/src/rmcp_client.rs b/codex-rs/rmcp-client/src/rmcp_client.rs
index b073480775..4ccb329b8a 100644
--- a/codex-rs/rmcp-client/src/rmcp_client.rs
+++ b/codex-rs/rmcp-client/src/rmcp_client.rs
@@ -390,7 +390,7 @@ enum TransportRecipe {
     Stdio {
         program: OsString,
         args: Vec<OsString>,
-        env: Option<HashMap<String, String>>,
+        env: Option<HashMap<OsString, OsString>>,
         env_vars: Vec<String>,
         cwd: Option<PathBuf>,
     },
@@ -478,7 +478,7 @@ impl RmcpClient {
     pub async fn new_stdio_client(
         program: OsString,
         args: Vec<OsString>,
-        env: Option<HashMap<String, String>>,
+        env: Option<HashMap<OsString, OsString>>,
         env_vars: &[String],
         cwd: Option<PathBuf>,
     ) -> io::Result<Self> {
diff --git a/codex-rs/rmcp-client/tests/process_group_cleanup.rs b/codex-rs/rmcp-client/tests/process_group_cleanup.rs
index bb033af875..2d6c220ce8 100644
--- a/codex-rs/rmcp-client/tests/process_group_cleanup.rs
+++ b/codex-rs/rmcp-client/tests/process_group_cleanup.rs
@@ -73,8 +73,8 @@ async fn drop_kills_wrapper_process_group() -> Result<()> {
             ),
         ],
         Some(HashMap::from([(
-            "CHILD_PID_FILE".to_string(),
-            child_pid_file_str,
+            OsString::from("CHILD_PID_FILE"),
+            OsString::from(child_pid_file_str),
         )])),
         &[],
         None,