diff --git a/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py b/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
index 6d2ce73624f..ca35be52fad 100644
--- a/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
+++ b/litellm/proxy/pass_through_endpoints/pass_through_endpoints.py
@@ -557,6 +557,7 @@ class HttpPassThroughEndpointHelpers(BasePassthroughUtils):
         # real parent span.
         _metadata["user_api_key"] = user_api_key_dict.api_key
         _metadata["litellm_parent_otel_span"] = user_api_key_dict.parent_otel_span
+        _metadata["user_api_key_budget_reservation"] = user_api_key_dict.budget_reservation
         _metadata.update(
             LiteLLMProxyRequestSetup.get_sanitized_user_information_from_key(user_api_key_dict=user_api_key_dict)
         )
diff --git a/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py b/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
index 9bddeda0723..6681558f8da 100644
--- a/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
+++ b/tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py
@@ -4877,3 +4877,119 @@ async def test_unusable_upstream_cost_records_zero_not_the_flat_estimate():
     assert len(payloads) == 1
     assert payloads[0]["response_cost"] == 0.0
     assert payloads[0]["total_tokens"] == 1874
+
+
+def _passthrough_kwargs_for_reservation(
+    user_api_key_dict: UserAPIKeyAuth, parsed_body: Optional[dict] = None
+) -> dict:
+    mock_request = MagicMock(spec=Request)
+    mock_request.method = "POST"
+    mock_request.url = (
+        "http://0.0.0.0:4000/gemini/v1beta/models/gemini-2.5-flash:generateContent"
+    )
+    mock_request.headers = Headers({})
+
+    return HttpPassThroughEndpointHelpers._init_kwargs_for_pass_through_endpoint(
+        request=mock_request,
+        user_api_key_dict=user_api_key_dict,
+        passthrough_logging_payload=MagicMock(),
+        logging_obj=MagicMock(),
+        _parsed_body=parsed_body if parsed_body is not None else {},
+        litellm_call_id="lit-5425-call-id",
+    )
+
+
+async def _track_cost_for_passthrough_kwargs(kwargs: dict) -> AsyncMock:
+    from datetime import datetime
+
+    from litellm.proxy.hooks.proxy_track_cost_callback import _ProxyDBLogger
+
+    callback_kwargs = {
+        **kwargs,
+        "stream": False,
+        "standard_logging_object": {
+            "response_cost": 0.002,
+            "request_tags": None,
+        },
+    }
+
+    increment_spend_counters = AsyncMock()
+    with (
+        patch("litellm.proxy.proxy_server.proxy_logging_obj") as mock_proxy_logging,
+        patch(
+            "litellm.proxy.proxy_server.increment_spend_counters",
+            increment_spend_counters,
+        ),
+        patch("litellm.proxy.proxy_server.update_cache", new_callable=AsyncMock),
+    ):
+        mock_proxy_logging.db_spend_update_writer.update_database = AsyncMock()
+        mock_proxy_logging.slack_alerting_instance.customer_spend_alert = AsyncMock()
+
+        await _ProxyDBLogger()._PROXY_track_cost_callback(
+            kwargs=callback_kwargs,
+            completion_response=None,
+            start_time=datetime.now(),
+            end_time=datetime.now(),
+        )
+
+    return increment_spend_counters
+
+
+@pytest.mark.asyncio
+async def test_passthrough_success_reconciles_budget_reservation():
+    """
+    A successful pass-through request must hand its pre-call budget reservation
+    to the spend-counter update so the reserved amount is reconciled down to the
+    actual cost. Without it the reservation stays in the shared Redis counter and
+    the actual cost is added on top, so the counter drifts above real spend until
+    the key falsely trips BudgetExceededError.
+    """
+    budget_reservation = {
+        "reserved_cost": 0.5,
+        "entries": [{"counter_key": "spend:key:hashed-token", "reserved_cost": 0.5}],
+    }
+    user_api_key_dict = UserAPIKeyAuth(
+        api_key="hashed-token",
+        user_id="u1",
+        budget_reservation=budget_reservation,
+    )
+
+    reservation = user_api_key_dict.budget_reservation
+    kwargs = _passthrough_kwargs_for_reservation(user_api_key_dict)
+    assert (
+        kwargs["litellm_params"]["metadata"]["user_api_key_budget_reservation"]
+        is reservation
+    )
+
+    increment_spend_counters = await _track_cost_for_passthrough_kwargs(kwargs)
+
+    increment_spend_counters.assert_awaited_once()
+    assert increment_spend_counters.await_args.kwargs["budget_reservation"] is reservation
+    assert increment_spend_counters.await_args.kwargs["budget_reservation"] == budget_reservation
+
+
+@pytest.mark.asyncio
+async def test_passthrough_body_cannot_forge_budget_reservation():
+    """
+    The reservation is an internal counter handle: a client-supplied metadata
+    field naming arbitrary counter keys must never reach the spend-counter
+    update, or a caller could decrement another entity's Redis counter.
+    """
+    forged = {
+        "reserved_cost": 99.0,
+        "entries": [{"counter_key": "spend:team:victim", "reserved_cost": 99.0}],
+    }
+    user_api_key_dict = UserAPIKeyAuth(api_key="hashed-token", user_id="u1")
+
+    kwargs = _passthrough_kwargs_for_reservation(
+        user_api_key_dict,
+        parsed_body={"litellm_metadata": {"user_api_key_budget_reservation": forged}},
+    )
+    assert (
+        kwargs["litellm_params"]["metadata"]["user_api_key_budget_reservation"] is None
+    )
+
+    increment_spend_counters = await _track_cost_for_passthrough_kwargs(kwargs)
+
+    increment_spend_counters.assert_awaited_once()
+    assert increment_spend_counters.await_args.kwargs["budget_reservation"] is None