Python: fix: handle case-insensitive User-Agent headers

python/packages/core/agent_framework/_telemetry.py

@@ -212,6 +212,13 @@ def prepend_agent_framework_to_user_agent(headers: dict[str, Any] | None = None)
     user_agent = get_user_agent()
     if not headers:
         return {USER_AGENT_KEY: user_agent}
-    headers[USER_AGENT_KEY] = f"{user_agent} {headers[USER_AGENT_KEY]}" if USER_AGENT_KEY in headers else user_agent
+    existing_user_agent_key = next(
+        (header_name for header_name in headers if header_name.lower() == USER_AGENT_KEY.lower()),
+        None,
+    )
+    if existing_user_agent_key is None:
+        headers[USER_AGENT_KEY] = user_agent
+    else:
+        headers[existing_user_agent_key] = f"{user_agent} {headers[existing_user_agent_key]}"
 
     return headers

python/packages/core/tests/core/test_telemetry.py

@@ -315,6 +315,17 @@ def test_prepend_to_existing_user_agent():
     assert "existing-agent/1.0" in result["User-Agent"]
 
 
+def test_prepend_to_existing_lowercase_user_agent():
+    """Test prepending to a case-insensitive User-Agent header."""
+    headers = {"user-agent": "existing-agent/1.0"}
+    result = prepend_agent_framework_to_user_agent(headers)
+
+    assert result is headers
+    assert [key for key in result if key.lower() == USER_AGENT_KEY.lower()] == ["user-agent"]
+    assert result["user-agent"].startswith("agent-framework-python/")
+    assert "existing-agent/1.0" in result["user-agent"]
+
+
 def test_prepend_to_empty_headers():
     """Test prepending to headers without User-Agent."""
     headers = {"Content-Type": "application/json"}