From d041c0ffe17c71469af5163ffe3cca2087f5b0c9 Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Wed, 28 Oct 2026 04:58:07 -0700
Subject: [PATCH] fix(rate-limit): keep a request's parallel slot when its
 auto-executed MCP tool call fails

---
 .../hooks/parallel_request_limiter_v3.py      |  6 +-
 .../hooks/test_parallel_request_limiter_v3.py | 65 +++++++++++++++++++
 2 files changed, 68 insertions(+), 3 deletions(-)

diff --git a/litellm/proxy/hooks/parallel_request_limiter_v3.py b/litellm/proxy/hooks/parallel_request_limiter_v3.py
index 567b8452ae2..de958d532e0 100644
--- a/litellm/proxy/hooks/parallel_request_limiter_v3.py
+++ b/litellm/proxy/hooks/parallel_request_limiter_v3.py
@@ -5716,7 +5716,7 @@ class _PROXY_MaxParallelRequestsHandler_v3(CustomLogger):
     @with_service_target("rate_limits")
     async def async_post_call_failure_hook(
         self,
-        request_data: dict,
+        request_data: dict[str, object],
         original_exception: Exception,
         user_api_key_dict: UserAPIKeyAuth,
         traceback_str: str | None = None,
@@ -5739,7 +5739,7 @@ class _PROXY_MaxParallelRequestsHandler_v3(CustomLogger):
         that usage instead of refunding it.
         """
         try:
-            stash: Final = get_request_stash()
+            stash: Final = get_request_stash_for_call(_call_id_from_callback_kwargs(request_data))
             if stash is None:
                 return
             await self._release_stashed_parallel_slot(stash, user_api_key_dict.parent_otel_span)
@@ -5780,7 +5780,7 @@ class _PROXY_MaxParallelRequestsHandler_v3(CustomLogger):
             ptu_ops: Final = (
                 self._build_ptu_failure_settlement_ops(
                     stash,
-                    request_data,  # pyright: ignore[reportUnknownArgumentType]  # request_data is a bare dict
+                    request_data,
                     tpm_actual,
                 )
                 if reserved_tokens > 0
diff --git a/tests/unit/proxy/hooks/test_parallel_request_limiter_v3.py b/tests/unit/proxy/hooks/test_parallel_request_limiter_v3.py
index e056e723a04..4daf98fe6bf 100644
--- a/tests/unit/proxy/hooks/test_parallel_request_limiter_v3.py
+++ b/tests/unit/proxy/hooks/test_parallel_request_limiter_v3.py
@@ -4271,6 +4271,71 @@ async def test_mcp_tool_call_inside_admitted_request_reuses_its_parallel_slot()
     assert await in_flight() == 1
 
 
+@pytest.mark.asyncio
+async def test_failed_mcp_tool_call_keeps_its_requests_parallel_slot_and_reservation(monkeypatch) -> None:
+    monkeypatch.delenv("LITELLM_TPM_TOKEN_RESERVATION_ENABLED", raising=False)
+    api_key: Final = hash_token("sk-mcp-auto-execute-tool-failure")
+    local_cache: Final = DualCache()
+    handler: Final = _PROXY_MaxParallelRequestsHandler(
+        internal_usage_cache=InternalUsageCache(local_cache)
+    )
+    user_api_key_dict: Final = UserAPIKeyAuth(api_key=api_key, max_parallel_requests=1, tpm_limit=10_000)
+    parallel_key: Final = f"{{api_key:{api_key}}}:max_parallel_requests"
+    tokens_key: Final = handler.create_rate_limit_keys(key="api_key", value=api_key, rate_limit_type="tokens")
+    owner_call_id: Final = "mcp-tool-failure-owner"
+    owner_data: Final[dict[str, object]] = {
+        "model": "gpt-4o-mini",
+        "messages": [{"role": "user", "content": "weather in Paris"}],
+        "max_tokens": 50,
+        "litellm_call_id": owner_call_id,
+    }
+
+    async def in_flight() -> int:
+        return handler._gauge_in_flight_from_cache_value(
+            await local_cache.async_get_cache(key=parallel_key)
+        )
+
+    async def reserved_tokens() -> int:
+        return int(await local_cache.async_get_cache(key=tokens_key) or 0)
+
+    async def pre_call(data: dict[str, object], call_type: str) -> None:
+        await handler.async_pre_call_hook(
+            user_api_key_dict=user_api_key_dict,
+            cache=local_cache,
+            data=data,
+            call_type=call_type,
+        )
+
+    await pre_call(owner_data, CallTypes.acompletion.value)
+    await pre_call(
+        {"model": "MCP: get_weather", "messages": [{"role": "user", "content": "Tool: get_weather"}]},
+        CallTypes.call_mcp_tool.value,
+    )
+    reserved_before_tool_failure: Final = await reserved_tokens()
+
+    await handler.async_post_call_failure_hook(
+        request_data={"call_type": CallTypes.call_mcp_tool.value, "litellm_call_id": f"{owner_call_id}_call_1"},
+        original_exception=HTTPException(status_code=504, detail="MCP tool call timed out after 3.0s"),
+        user_api_key_dict=user_api_key_dict,
+    )
+
+    assert reserved_before_tool_failure > 0
+    assert await reserved_tokens() == reserved_before_tool_failure
+    with pytest.raises(HTTPException) as rejected:
+        await contextvars.Context().run(
+            asyncio.create_task, pre_call({"model": "gpt-4o-mini", "input": "next"}, CallTypes.aresponses.value)
+        )
+    assert rejected.value.status_code == 429
+
+    await handler.async_post_call_failure_hook(
+        request_data=owner_data,
+        original_exception=Exception("request failed"),
+        user_api_key_dict=user_api_key_dict,
+    )
+
+    assert await in_flight() == 0
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     ("stream", "in_flight_after_first_round_logs"),