diff --git a/litellm/proxy/common_request_processing.py b/litellm/proxy/common_request_processing.py
index 315fbcba310..24adc011828 100644
--- a/litellm/proxy/common_request_processing.py
+++ b/litellm/proxy/common_request_processing.py
@@ -6,7 +6,7 @@ import math
 import traceback
 from collections.abc import AsyncGenerator, Awaitable, Callable, Mapping, Sequence
 from datetime import datetime
-from functools import lru_cache
+from functools import lru_cache, partial
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Final, Literal, NamedTuple, Protocol, TypeAlias, TypeVar, overload
 
@@ -170,6 +170,8 @@ StreamChunkSerializer = Callable[[Any], str]
 # Type alias for streaming error serializer (ProxyException -> wire format)
 StreamErrorSerializer = Callable[[ProxyException], str]
 
+_disconnect_stream_assembly_limiter: Final = anyio.CapacityLimiter(1)
+
 if TYPE_CHECKING:
     from litellm.proxy.proxy_server import ProxyConfig as _ProxyConfig
 
@@ -364,11 +366,16 @@ async def _bill_partial_streamed_spend_on_disconnect(request_data: dict, respons
     )
     messages: Final[object] = getattr(response, "messages", None)
     try:
-        partial_response: Final = litellm.stream_chunk_builder(
+        build_partial_response: Final = partial(
+            litellm.stream_chunk_builder,
             chunks=chunks,
             messages=messages if isinstance(messages, list) else None,
             logging_obj=logging_obj,
         )
+        partial_response: Final = await anyio.to_thread.run_sync(
+            build_partial_response,
+            limiter=_disconnect_stream_assembly_limiter,
+        )
     except Exception as e:  # noqa: BLE001  # partial billing is best-effort; never break stream teardown
         verbose_proxy_logger.debug("Failed to assemble partial streamed response for disconnect billing: %s", e)
         return False
diff --git a/tests/test_litellm/proxy/test_common_request_processing.py b/tests/test_litellm/proxy/test_common_request_processing.py
index 64318778bc2..0d9758577f7 100644
--- a/tests/test_litellm/proxy/test_common_request_processing.py
+++ b/tests/test_litellm/proxy/test_common_request_processing.py
@@ -2,6 +2,8 @@ import asyncio
 import copy
 import datetime
 import json
+import threading
+import time
 from types import SimpleNamespace
 from typing import AsyncGenerator, Callable, Final, Optional
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -5649,6 +5651,84 @@ class TestStreamingClientDisconnectBilling:
         assert standard_logging_object["total_tokens"] > 0
         assert standard_logging_object["response_cost"] >= 0.002
 
+    @pytest.mark.asyncio
+    async def test_disconnect_partial_response_assembly_does_not_block_event_loop(self):
+        response = await self._start_partial_stream()
+        builder_started = threading.Event()
+        builder_finished = threading.Event()
+
+        def blocking_builder(*args, **kwargs):
+            builder_started.set()
+            time.sleep(0.2)
+            builder_finished.set()
+
+        async def event_loop_progressed_during_assembly():
+            while not builder_started.is_set():
+                await asyncio.sleep(0)
+            await asyncio.sleep(0.01)
+            return not builder_finished.is_set()
+
+        with patch(
+            "litellm.proxy.common_request_processing.litellm.stream_chunk_builder",
+            side_effect=blocking_builder,
+        ):
+            billing_task = asyncio.create_task(
+                _bill_partial_streamed_spend_on_disconnect(
+                    {"litellm_logging_obj": response.logging_obj}, response
+                )
+            )
+            event_loop_progressed = await event_loop_progressed_during_assembly()
+            billed = await billing_task
+
+        assert event_loop_progressed is True
+        assert billed is False
+
+    @pytest.mark.asyncio
+    async def test_disconnect_partial_response_assembly_limits_cpu_concurrency(self):
+        response = await self._start_partial_stream()
+        first_builder_started = threading.Event()
+        release_first_builder = threading.Event()
+        second_builder_started = threading.Event()
+        builder_claim_lock = threading.Lock()
+
+        def blocking_builder(*args, **kwargs):
+            with builder_claim_lock:
+                if first_builder_started.is_set():
+                    second_builder_started.set()
+                    return
+                first_builder_started.set()
+            release_first_builder.wait(timeout=5)
+
+        with patch(
+            "litellm.proxy.common_request_processing.litellm.stream_chunk_builder",
+            side_effect=blocking_builder,
+        ):
+            billing_tasks = (
+                asyncio.create_task(
+                    _bill_partial_streamed_spend_on_disconnect(
+                        {"litellm_logging_obj": response.logging_obj}, response
+                    )
+                ),
+                asyncio.create_task(
+                    _bill_partial_streamed_spend_on_disconnect(
+                        {"litellm_logging_obj": response.logging_obj}, response
+                    )
+                ),
+            )
+            try:
+                for _ in range(100):
+                    if first_builder_started.is_set():
+                        break
+                    await asyncio.sleep(0.01)
+                assert first_builder_started.is_set()
+                await asyncio.sleep(0.05)
+                assert second_builder_started.is_set() is False
+            finally:
+                release_first_builder.set()
+                await asyncio.gather(*billing_tasks)
+
+        assert second_builder_started.is_set() is True
+
     @pytest.mark.asyncio
     async def test_completed_stream_does_not_double_bill_on_late_disconnect(self):
         recorder = _RecordingSuccessLogger()