diff --git a/src/textual_webterm/session_manager.py b/src/textual_webterm/session_manager.py
index 0f243d6..ad1fe78 100644
--- a/src/textual_webterm/session_manager.py
+++ b/src/textual_webterm/session_manager.py
@@ -141,12 +141,15 @@ class SessionManager:
             )
             log.info("Created app session %s", session_id)
 
-        self.sessions[session_id] = session_process
-        self.routes[route_key] = session_id
-
+        # Open the session BEFORE registering it, so it's fully initialized
+        # when other code can access it via sessions/routes dicts
         await session_process.open(*size)
         log.debug("Session %s opened and ready", session_id)
 
+        # Now register the fully initialized session
+        self.sessions[session_id] = session_process
+        self.routes[route_key] = session_id
+
         return session_process
 
     async def close_session(self, session_id: SessionID) -> None:
diff --git a/src/textual_webterm/terminal_session.py b/src/textual_webterm/terminal_session.py
index 8795670..46aac48 100644
--- a/src/textual_webterm/terminal_session.py
+++ b/src/textual_webterm/terminal_session.py
@@ -64,9 +64,10 @@ class TerminalSession(Session):
 
     async def open(self, width: int = 80, height: int = 24) -> None:
         log.info("Opening terminal session %s with command: %s", self.session_id, self.command)
-        # Initialize pyte screen with the requested size
-        self._screen = pyte.Screen(width, height)
-        self._stream = pyte.Stream(self._screen)
+        # Initialize pyte screen with the requested size (under lock to prevent races)
+        async with self._screen_lock:
+            self._screen = pyte.Screen(width, height)
+            self._stream = pyte.Stream(self._screen)
 
         pid, master_fd = pty.fork()
         self.pid = pid
@@ -99,9 +100,10 @@ class TerminalSession(Session):
         fcntl.ioctl(self.master_fd, termios.TIOCSWINSZ, buf)
 
     async def set_terminal_size(self, width: int, height: int) -> None:
+        # First resize the PTY (blocking call in executor)
         loop = asyncio.get_running_loop()
         await loop.run_in_executor(None, self._set_terminal_size, width, height)
-        # Resize pyte screen to match
+        # Then resize pyte screen to match (after PTY resize completes)
         async with self._screen_lock:
             self._screen.resize(height, width)