diff --git a/CHANGELOG.md b/CHANGELOG.md
index 27ac5a3..5788b8a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.9.21 (2025-10-16)
+
+### Changes
+
+- Warn callers when they use the webstack client from different threads.
+
 ## 0.9.20 (2025-10-10)
 
 ### Changes
diff --git a/python/mujinwebstackclient/controllerwebclientraw.py b/python/mujinwebstackclient/controllerwebclientraw.py
index 6326264..0a861ba 100644
--- a/python/mujinwebstackclient/controllerwebclientraw.py
+++ b/python/mujinwebstackclient/controllerwebclientraw.py
@@ -157,7 +157,20 @@ class ControllerWebClientRaw(object):
     _subscriptionLock: threading.Lock  # Lock protecting _webSocket and _subscriptions
     _backgroundThread: BackgroundThread = None  # The background thread to handle async operations
 
-    def __init__(self, baseurl: str, username: str, password: str, locale: Optional[str] = None, author: Optional[str] = None, userAgent: Optional[str] = None, additionalHeaders: Optional[Dict[str, str]] = None, unixEndpoint: Optional[str] = None) -> None:
+    _threadName: Optional[str] = None  # The last thread this client was used in if we're warning on calls from different threads.
+
+    def __init__(
+        self,
+        baseurl: str,
+        username: str,
+        password: str,
+        locale: Optional[str] = None,
+        author: Optional[str] = None,
+        userAgent: Optional[str] = None,
+        additionalHeaders: Optional[Dict[str, str]] = None,
+        unixEndpoint: Optional[str] = None,
+        warnOnUseFromDifferentThreads: bool = False,
+    ) -> None:
         self._baseurl = baseurl
         self._username = username
         self._password = password
@@ -201,6 +214,11 @@ def __init__(self, baseurl: str, username: str, password: str, locale: Optional[
         # Set user agent header
         self.SetUserAgent(userAgent)
 
+        if warnOnUseFromDifferentThreads:
+            self._threadName = threading.current_thread().name
+            log.info('initialized client with warning on calls from different threads enabled and this may degrade performance')
+            log.info('set "warnOnUseFromDifferentThreads" to "False" to disable this if performance is poor')
+
     def __del__(self):
         self.Destroy()
 
@@ -262,6 +280,12 @@ def Request(
             # by default, disallow redirect since DELETE with redirection is too dangerous
             kwargs['allow_redirects'] = method in ('GET',)
 
+        if self._threadName is not None:
+            currentName = threading.current_thread().name
+            if currentName != self._threadName:
+                log.warning('client has been called across multiple threads, was "%s", now "%s"', self._threadName, currentName)
+                self._threadName = currentName
+
         response = self._session.request(method=method, url=url, timeout=timeout, headers=headers, **kwargs)
 
         # in verbose logging, log the caller
diff --git a/python/mujinwebstackclient/version.py b/python/mujinwebstackclient/version.py
index ff3b060..18d7d8f 100644
--- a/python/mujinwebstackclient/version.py
+++ b/python/mujinwebstackclient/version.py
@@ -1,3 +1,3 @@
-__version__ = '0.9.20'
+__version__ = '0.9.21'
 
 # Do not forget to update CHANGELOG.md
diff --git a/python/mujinwebstackclient/webstackclient.py b/python/mujinwebstackclient/webstackclient.py
index f4927a9..c44ecff 100644
--- a/python/mujinwebstackclient/webstackclient.py
+++ b/python/mujinwebstackclient/webstackclient.py
@@ -119,7 +119,17 @@ def offset(self):
     controllerIp = ''  # Hostname of the controller web server
     controllerPort = 80  # Port of the controller web server
 
-    def __init__(self, controllerurl='http://127.0.0.1', controllerusername='', controllerpassword='', author=None, userAgent=None, additionalHeaders=None, unixEndpoint=None):
+    def __init__(
+        self,
+        controllerurl='http://127.0.0.1',
+        controllerusername='',
+        controllerpassword='',
+        author=None,
+        userAgent=None,
+        additionalHeaders=None,
+        unixEndpoint=None,
+        warnOnUseFromDifferentThreads: bool = False,
+    ):
         """Logs into the Mujin controller.
 
         Args:
@@ -129,6 +139,8 @@ def __init__(self, controllerurl='http://127.0.0.1', controllerusername='', cont
             userAgent (str): User agent to be sent on each request
             additionalHeaders (dict): Additional HTTP headers to be included in requests
             unixEndpoint (str): Unix socket endpoint for communicating with HTTP server over unix socket
+            warnOnUseFromDifferentThreads (bool): Whether to warn callers if the client is used from different threads.
+                Defaults to not warning since checking the thread name on each call may significantly degrade performance.
         """
 
         # Parse controllerurl
@@ -155,7 +167,16 @@ def __init__(self, controllerurl='http://127.0.0.1', controllerusername='', cont
             'username': self.controllerusername,
             'locale': os.environ.get('LANG', ''),
         }
-        self._webclient = controllerwebclientraw.ControllerWebClientRaw(self.controllerurl, self.controllerusername, self.controllerpassword, author=author, userAgent=userAgent, additionalHeaders=additionalHeaders, unixEndpoint=unixEndpoint)
+        self._webclient = controllerwebclientraw.ControllerWebClientRaw(
+            self.controllerurl,
+            self.controllerusername,
+            self.controllerpassword,
+            author=author,
+            userAgent=userAgent,
+            additionalHeaders=additionalHeaders,
+            unixEndpoint=unixEndpoint,
+            warnOnUseFromDifferentThreads=warnOnUseFromDifferentThreads,
+        )
 
     def __del__(self):
         self.Destroy()
@@ -652,7 +673,7 @@ def DeleteJobs(self, timeout=5):
     #
 
     def CreateLogEntries(self, logEntries, timeout=5):
-        # type: (List[Tuple[str, Any, Dict[str, bytes]]], int) -> Any
+        # type: (List[Tuple[str, Any, Dict[str, bytes]]], float) -> Any
         files = []
         for logType, logEntry, attachments in logEntries:
             files.append(('logEntry/%s' % logType, ('', json.dumps(logEntry), 'application/json')))