From b04381957e1c9dd865d47983fe1ffb62a51e8d1e Mon Sep 17 00:00:00 2001
From: Yassin Kortam <yassin@berri.ai>
Date: Mon, 8 Jun 2026 15:45:05 -0700
Subject: [PATCH] perf(anthropic_messages): remove thread hop in
 aanthropic_messages dispatch

The async /messages entry point dispatched anthropic_messages_handler via
loop.run_in_executor(None, ...) wrapped in a copied contextvars context.
That handler is @client-decorated; with is_async=True its async wrapper
returns a coroutine immediately rather than blocking, so the executor only
ever ran a function that returned a coroutine which was then awaited on the
event loop. The thread hop did no blocking work; it was pure overhead and
contended for the default ThreadPoolExecutor that is shared with sync
logging callbacks and blocking SDK calls, hurting throughput under
concurrency.

Dispatch the handler directly on the running loop and await the result.
The asyncio.iscoroutine guard is retained because the mock_response
shortcut returns a materialized dict regardless of is_async, so a flat
await would crash that path.
---
 .../messages/handler.py                       | 19 ++++--------
 ...erimental_pass_through_messages_handler.py | 31 +++++++++++++++++++
 2 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/handler.py b/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
index a3ac465c463..8f82f37249a 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/handler.py
@@ -6,8 +6,6 @@
 """
 
 import asyncio
-import contextvars
-from functools import partial
 from typing import (
     Any,
     AsyncIterator,
@@ -300,11 +298,9 @@ async def anthropic_messages(
                 **kwargs,
             )
 
-    loop = asyncio.get_event_loop()
     kwargs["is_async"] = True
 
-    func = partial(
-        anthropic_messages_handler,
+    response = anthropic_messages_handler(
         max_tokens=max_tokens,
         messages=messages,
         model=model,
@@ -330,14 +326,11 @@ async def anthropic_messages(
         _litellm_messages_presanitized=True,
         **kwargs,
     )
-    ctx = contextvars.copy_context()
-    func_with_context = partial(ctx.run, func)
-    init_response = await loop.run_in_executor(None, func_with_context)
-
-    if asyncio.iscoroutine(init_response):
-        response = await init_response
-    else:
-        response = init_response
+    # ``anthropic_messages_handler`` returns a coroutine on the async backend
+    # paths but a materialized response on the ``mock_response`` shortcut, so
+    # only await when there is something to await.
+    if asyncio.iscoroutine(response):
+        return await response
     return response
 
 
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 b1e1d789d74..d0efc4ba261 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
@@ -1,3 +1,4 @@
+import asyncio
 import json
 import os
 import sys
@@ -673,3 +674,33 @@ async def test_async_wrapper_sets_presanitized_and_sanitizes_once():
     assert spy.call_count == 1
     assert captured["presanitized"] is True
     assert [b["type"] for b in captured["messages"][0]["content"]] == ["tool_use"]
+
+
+@pytest.mark.asyncio
+async def test_aanthropic_messages_does_not_use_thread_executor():
+    """The async Anthropic /messages entry must dispatch the handler directly on
+    the running event loop, never through ``loop.run_in_executor``.
+
+    Mutation control: revert handler.py to
+    ``await loop.run_in_executor(None, func_with_context)`` and this test must
+    fail with AssertionError('thread hop fired').
+    """
+    from litellm.llms.anthropic.experimental_pass_through.messages import handler
+
+    loop = asyncio.get_running_loop()
+
+    def fail_on_thread_hop(*args, **kwargs):
+        raise AssertionError("thread hop fired")
+
+    with patch.object(loop, "run_in_executor", side_effect=fail_on_thread_hop):
+        response = await handler.anthropic_messages(
+            max_tokens=8,
+            messages=[{"role": "user", "content": "hi"}],
+            model="anthropic/claude-3-5-haiku-20241022",
+            custom_llm_provider="anthropic",
+            api_key="sk-test",
+            stream=False,
+            mock_response="pong",
+        )
+
+    assert response["content"][0]["text"] == "pong"