fix(sync-httpx): respect proxy env vars when force_ipv4 is enabled

When `force_ipv4: true` is set in config, the sync HTTPHandler creates a
custom `HTTPTransport(local_address="0.0.0.0")` which causes httpx to
ignore `HTTP_PROXY`/`HTTPS_PROXY` environment variables. This breaks
providers that use the sync client path (e.g. ChatGPT authenticator)
when running behind a proxy.

The fix detects proxy env vars at client creation time and skips the
custom transport so httpx auto-detects proxy settings. When no proxy is
configured, the IPv4-forcing transport is used as before.

Closes #24562
See also #24983

litellm/llms/custom_httpx/http_handler.py

@@ -1047,6 +1047,19 @@ class HTTPHandler:
         if client is None:
             transport = self._create_sync_transport()
 
+            # When proxy is configured via env vars, skip custom transport.
+            # httpx ignores proxy env vars when an explicit transport is
+            # provided, which breaks providers that use the sync client
+            # (e.g. ChatGPT authenticator) behind a proxy.
+            proxy_url = (
+                os.environ.get("HTTPS_PROXY")
+                or os.environ.get("https_proxy")
+                or os.environ.get("HTTP_PROXY")
+                or os.environ.get("http_proxy")
+            )
+            if proxy_url and transport is not None:
+                transport = None
+
             # Create a client with a connection pool
             self.client = httpx.Client(
                 transport=transport,

tests/test_litellm/llms/custom_httpx/test_http_handler.py

@@ -675,6 +675,64 @@ async def test_httpx_handler_uses_env_user_agent(monkeypatch):
         await handler.close()
 
 
+def test_sync_http_handler_skips_custom_transport_when_proxy_set(monkeypatch):
+    """
+    When HTTPS_PROXY is set and force_ipv4 creates a custom transport,
+    the sync HTTPHandler must skip the custom transport so that httpx
+    auto-detects proxy from environment variables.
+
+    httpx ignores proxy env vars when an explicit transport is provided,
+    which breaks sync-path providers (e.g. ChatGPT authenticator) behind
+    a proxy. See GitHub issue #24562.
+    """
+    original_force_ipv4 = litellm.force_ipv4
+    litellm.force_ipv4 = True
+
+    try:
+        # With proxy: custom transport should be dropped
+        monkeypatch.setenv("HTTPS_PROXY", "http://proxy:8080")
+        handler = HTTPHandler()
+        try:
+            transport = handler.client._transport
+            assert not isinstance(transport, httpx.HTTPTransport)
+        finally:
+            handler.close()
+
+        # Without proxy: custom transport should be applied (force_ipv4)
+        monkeypatch.delenv("HTTPS_PROXY", raising=False)
+        monkeypatch.delenv("HTTP_PROXY", raising=False)
+        handler2 = HTTPHandler()
+        try:
+            transport2 = handler2.client._transport
+            assert isinstance(transport2, httpx.HTTPTransport)
+        finally:
+            handler2.close()
+    finally:
+        litellm.force_ipv4 = original_force_ipv4
+
+
+def test_sync_http_handler_prefers_https_proxy_over_http_proxy(monkeypatch):
+    """
+    HTTPS_PROXY takes precedence over HTTP_PROXY, matching httpx convention.
+    """
+    original_force_ipv4 = litellm.force_ipv4
+    litellm.force_ipv4 = True
+
+    try:
+        monkeypatch.setenv("HTTPS_PROXY", "http://https-proxy:8080")
+        monkeypatch.setenv("HTTP_PROXY", "http://http-proxy:8080")
+
+        handler = HTTPHandler()
+        try:
+            # Should not have custom transport (proxy takes priority)
+            transport = handler.client._transport
+            assert not isinstance(transport, httpx.HTTPTransport)
+        finally:
+            handler.close()
+    finally:
+        litellm.force_ipv4 = original_force_ipv4
+
+
 def test_get_httpx_client_applies_float_timeout_without_mocking_handler():
     """
     Exercise real _get_httpx_client + HTTPHandler: params={'timeout': x} must reach httpx.Client(timeout=...).