From 3bcc4b68ed5ec75d0b895a404b5fa398c2a93d79 Mon Sep 17 00:00:00 2001
From: Sameer <121498481+sam7eer@users.noreply.github.com>
Date: Sat, 12 Sep 2026 18:45:41 +0530
Subject: [PATCH] fix(anthropic): match api_base by hostname and add
 request-path regression test

---
 .../messages/handler.py                       |  5 ++-
 ...erimental_pass_through_messages_handler.py | 32 ++++++++++++++++++-
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/handler.py b/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
index 9050a9405c8..fd6f9f1db07 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
@@ -77,7 +77,10 @@ def _points_at_openai_backend(api_base: str | None) -> bool:
     """
     if not api_base:
         return True
-    return "api.openai.com" in api_base
+    from urllib.parse import urlparse
+
+    parsed = urlparse(api_base if "://" in api_base else f"//{api_base}")
+    return (parsed.hostname or "").lower() == "api.openai.com"
 
 
 def _should_route_to_responses_api(
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
index 9f73fdfed15..1d1867d99d4 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
@@ -142,15 +142,21 @@ def test_anthropic_experimental_pass_through_messages_handler_dynamic_api_key_an
     [
         (None, True),
         ("https://api.openai.com/v1", True),
+        ("api.openai.com", True),
+        ("HTTPS://API.OPENAI.COM/v1", True),
         ("http://localhost:8000/v1", False),
         ("http://vllm-host:8000/v1", False),
+        ("https://api.openai.com.evil.example/v1", False),
+        ("https://not-api.openai.com.internal/v1", False),
     ],
 )
 def test_should_route_to_responses_api_considers_api_base_for_openai(api_base, expected):
     """Regression test for #40780. A self-hosted OpenAI-compatible backend declared as
     ``openai/<model>`` with a custom api_base must not be routed to the OpenAI Responses API
     (which reshapes images into ``input_image`` items the backend rejects); only real OpenAI
-    (api_base unset or api.openai.com) keeps the Responses API path."""
+    (api_base unset or the api.openai.com host) keeps the Responses API path. Hostname matching
+    is normalized, so case variants resolve to OpenAI while lookalike hosts that merely contain
+    the string do not."""
     from litellm.llms.anthropic.experimental_pass_through.messages.handler import (
         _should_route_to_responses_api,
     )
@@ -158,6 +164,30 @@ def test_should_route_to_responses_api_considers_api_base_for_openai(api_base, e
     assert _should_route_to_responses_api("openai", "openai/model", "model", api_base) is expected
 
 
+def test_openai_custom_api_base_forwards_messages_to_chat_completions():
+    """Regression test for #40780 at the request-path level: driving the real handler, an
+    ``openai/`` deployment with a custom api_base must forward /v1/messages to chat/completions
+    (``litellm.completion``) rather than the Responses API, guarding the api_base wiring at the
+    call site, not just the routing helper."""
+    from litellm.llms.anthropic.experimental_pass_through.messages.handler import (
+        anthropic_messages_handler,
+    )
+
+    with patch("litellm.completion") as mock_completion:  # test-quality-ok: routes via real handler; no injection seam
+        try:
+            anthropic_messages_handler(
+                max_tokens=100,
+                messages=[{"role": "user", "content": "Hello, how are you?"}],
+                model="openai/my-local-model",
+                api_base="http://localhost:8000/v1",
+                api_key="sk-noauth",
+            )
+        except (ValueError, TypeError, AttributeError) as e:
+            print(f"Error: {e}")
+        mock_completion.assert_called_once()
+        assert mock_completion.call_args.kwargs["api_base"] == "http://localhost:8000/v1"
+
+
 @pytest.mark.asyncio
 async def test_anthropic_messages_sanitizes_empty_text_blocks_before_dispatch():
     """Regression test for #22930.  The unified /v1/messages path must