diff --git a/UnleashClient/__init__.py b/UnleashClient/__init__.py
index a7435335..2d91ff42 100644
--- a/UnleashClient/__init__.py
+++ b/UnleashClient/__init__.py
@@ -23,6 +23,7 @@
     StreamingConnector,
 )
 from UnleashClient.constants import (
+    APPLICATION_HEADERS,
     DISABLED_VARIATION,
     ETAG,
     METRIC_LAST_SENT_TIME,
@@ -290,6 +291,7 @@ def initialize_client(self, fetch_toggles: bool = True) -> None:
                 start_scheduler = False
                 base_headers = {
                     **self.unleash_custom_headers,
+                    **APPLICATION_HEADERS,
                     "unleash-connection-id": self.connection_id,
                     "unleash-appname": self.unleash_app_name,
                     "unleash-instanceid": self.unleash_instance_id,
diff --git a/tests/unit_tests/test_client.py b/tests/unit_tests/test_client.py
index e63edece..2d331f1b 100644
--- a/tests/unit_tests/test_client.py
+++ b/tests/unit_tests/test_client.py
@@ -1,4 +1,5 @@
 import json
+import re
 import threading
 import time
 import uuid
@@ -1487,3 +1488,20 @@ def test_uc_bootstrap_initializes_offline_connector():
     assert unleash_client.is_enabled("testFlag")
 
     unleash_client.destroy()
+
+
+@responses.activate
+def test_spec_header_is_sent_when_fetching_features():
+    responses.add(
+        responses.GET, URL + FEATURES_URL, json=MOCK_FEATURE_RESPONSE, status=200
+    )
+
+    unleash_client = UnleashClient(
+        URL, APP_NAME, disable_metrics=True, disable_registration=True
+    )
+    unleash_client.initialize_client()
+    client_spec = responses.calls[0].request.headers["Unleash-Client-Spec"]
+
+    ## assert that the client spec looks like a semver string
+    semver_regex = r"^\d+\.\d+\.\d+(-[0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*)?(\+[0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*)?$"
+    assert re.match(semver_regex, client_spec)