fix(proxy): case-insensitive User-Agent header lookup in _get_user_agent_tags

Header names are not case-sensitive, so a client sending "User-Agent" was
untagged while "user-agent" was tagged. Look the header up case-insensitively
and early-return on non-dict headers instead of nesting the whole body.

Test moved to tests/unit/litellm_core_utils/ after #43199.

litellm/litellm_core_utils/litellm_logging.py

@@ -7062,21 +7062,23 @@ class StandardLoggingPayloadSetup:
             return None
         user_agent_tags: list[str] | None = None
         headers: Final = proxy_server_request.get("headers", {})
-        if headers is not None and isinstance(headers, dict):
-            request_headers: Final = cast(  # cast-ok: request headers are untyped framework data
-                dict[str, str | None], headers
-            )
-            if "user-agent" in request_headers:
-                user_agent: Final = request_headers["user-agent"]
-                if user_agent is not None:
-                    if user_agent_tags is None:
-                        user_agent_tags = []
-                    user_agent_part: str | None = None
-                    if "/" in user_agent:
-                        user_agent_part = user_agent.split("/")[0]
-                    if user_agent_part is not None:
-                        user_agent_tags.append("User-Agent: " + user_agent_part)
-                    user_agent_tags.append("User-Agent: " + user_agent)
+        if not isinstance(headers, dict):
+            return None
+        # Header names are case-insensitive: a client that sends "User-Agent"
+        # or "USER-AGENT" must still be tagged (#40979).
+        user_agent: Final = next(
+            (v for k, v in headers.items() if str(k).lower() == "user-agent"),
+            None,
+        )
+        if user_agent is not None:
+            if user_agent_tags is None:
+                user_agent_tags = []
+            user_agent_part: str | None = None
+            if "/" in user_agent:
+                user_agent_part = user_agent.split("/")[0]
+            if user_agent_part is not None:
+                user_agent_tags.append("User-Agent: " + user_agent_part)
+            user_agent_tags.append("User-Agent: " + user_agent)
         return user_agent_tags
 
     @staticmethod

tests/unit/litellm_core_utils/test_litellm_logging.py

@@ -3386,6 +3386,24 @@ def test_get_user_agent_tags():
     assert "User-Agent: litellm" in tags
     assert "User-Agent: litellm/0.1.0" in tags
 
+    # Header names are case-insensitive ("User-Agent" vs "user-agent").
+    tags_capitalized = StandardLoggingPayloadSetup._get_user_agent_tags(
+        proxy_server_request={
+            "headers": {
+                "User-Agent": "curl/7.68.0",
+            }
+        }
+    )
+
+    assert tags_capitalized is not None
+    assert "User-Agent: curl" in tags_capitalized
+    assert "User-Agent: curl/7.68.0" in tags_capitalized
+
+    # Non-dict headers must not raise; the function early-returns.
+    assert StandardLoggingPayloadSetup._get_user_agent_tags(
+        proxy_server_request={"headers": None}
+    ) is None
+
 
 def test_get_request_tags():
     from litellm.litellm_core_utils.litellm_logging import StandardLoggingPayloadSetup