diff --git a/tests/integration/contracts.json b/tests/integration/contracts.json
index 20cb34a5031..2535547dfef 100644
--- a/tests/integration/contracts.json
+++ b/tests/integration/contracts.json
@@ -122,6 +122,9 @@
     "tests/integration/spend/test_cache_and_quota.py::test_different_system_messages_do_not_share_a_cached_response": [
       "quota_management.response_cache.system_messages_partition_cache_identity"
     ],
+    "tests/integration/spend/test_passthrough_budget_reservation.py::test_repeated_gemini_passthrough_calls_stay_served_while_key_spend_is_below_max_budget": [
+      "spend.budget_reservation.gemini_passthrough_success_releases_reservation_from_spend_counter"
+    ],
     "tests/integration/database/test_transaction_atomicity.py::test_access_group_second_key_constraint_failure_rolls_back_all_writes": [
       "other.database.access_group.failed_second_write_rolls_back_first"
     ],
diff --git a/tests/integration/spend/test_passthrough_budget_reservation.py b/tests/integration/spend/test_passthrough_budget_reservation.py
new file mode 100644
index 00000000000..15e868ec00c
--- /dev/null
+++ b/tests/integration/spend/test_passthrough_budget_reservation.py
@@ -0,0 +1,102 @@
+import uuid
+from hashlib import sha256
+from typing import Final
+
+import pytest
+from integration._support.client import Gateway, eventually, object_value, string_value
+from integration._support.database import read_rows
+from integration._support.upstream import delete_scenario, register_scenario
+from integration.cost_calculation.cost_tracking_case import JsonResponse
+from pydantic import JsonValue
+
+INPUT_COST_PER_TOKEN: Final = 0.000001
+OUTPUT_COST_PER_TOKEN: Final = 0.001
+PROMPT_TOKENS: Final = 10
+CANDIDATE_TOKENS: Final = 5
+COST_PER_CALL: Final = PROMPT_TOKENS * INPUT_COST_PER_TOKEN + CANDIDATE_TOKENS * OUTPUT_COST_PER_TOKEN
+MAX_BUDGET: Final = 0.02
+CALLS_WITHIN_BUDGET: Final = 4
+
+
+def _key_spend(digest: str) -> float:
+    rows: Final = read_rows('SELECT spend FROM "LiteLLM_VerificationToken" WHERE token=%s', (digest,))
+    assert len(rows) == 1, rows
+    return float(rows[0]["spend"])
+
+
+def _generate_content_request(model: str) -> dict[str, JsonValue]:
+    return {"contents": [{"role": "user", "parts": [{"text": f"budget {model}"}]}]}
+
+
+def _generate_content_response(model: str) -> JsonResponse:
+    return JsonResponse(
+        content_type="application/json",
+        body={
+            "candidates": [
+                {
+                    "content": {"parts": [{"text": f"scripted answer {model}"}], "role": "model"},
+                    "finishReason": "STOP",
+                    "index": 0,
+                }
+            ],
+            "usageMetadata": {
+                "promptTokenCount": PROMPT_TOKENS,
+                "candidatesTokenCount": CANDIDATE_TOKENS,
+                "totalTokenCount": PROMPT_TOKENS + CANDIDATE_TOKENS,
+            },
+            "modelVersion": model,
+        },
+    )
+
+
+def _served_call(gateway: Gateway, model: str, key: str, scenario_id: str, call: int) -> None:
+    digest: Final = sha256(key.encode()).hexdigest()
+    spend_before: Final = _key_spend(digest)
+    assert spend_before == pytest.approx((call - 1) * COST_PER_CALL) and spend_before < MAX_BUDGET
+    response: Final = gateway.request(
+        "POST",
+        f"/gemini/v1beta/models/{model}:generateContent",
+        _generate_content_request(model),
+        headers={"x-goog-api-key": key, "x-pass-x-scripted-scenario": scenario_id},
+    )
+    assert response.status_code == 200, f"call {call} with key spend {spend_before}: {response.text}"
+    assert response.json() == _generate_content_response(model).body, response.text
+    eventually(lambda: _key_spend(digest), lambda spend: spend >= call * COST_PER_CALL - 1e-9, seconds=70)
+
+
+@pytest.mark.covers("spend.budget_reservation.gemini_passthrough_success_releases_reservation_from_spend_counter")
+def test_repeated_gemini_passthrough_calls_stay_served_while_key_spend_is_below_max_budget(gateway: Gateway) -> None:
+    with gateway.scenario() as scenario:
+        gateway.post(
+            "/config/update",
+            {"environment_variables": {"GEMINI_API_BASE": gateway.upstream_url, "GEMINI_API_KEY": "scripted"}},
+        )
+        model: Final = f"gemini-passthrough-{uuid.uuid4().hex}"
+        created: Final = gateway.post(
+            "/model/new",
+            {
+                "model_name": model,
+                "litellm_params": {
+                    "model": "gemini/gemini-2.5-flash",
+                    "api_key": "scripted",
+                    "api_base": gateway.upstream_url,
+                    "input_cost_per_token": INPUT_COST_PER_TOKEN,
+                    "output_cost_per_token": OUTPUT_COST_PER_TOKEN,
+                },
+                "model_info": {"id": model, "max_output_tokens": 10},
+            },
+        )
+        scenario.cleanups.callback(scenario.delete_model, string_value(object_value(created["model_info"])["id"]))
+        handle: Final = register_scenario(f"sc-{model}", _generate_content_response(model))
+        scenario.cleanups.callback(delete_scenario, handle)
+        key: Final = scenario.key(models=[model], max_budget=MAX_BUDGET)
+        for call in range(1, CALLS_WITHIN_BUDGET + 1):
+            _served_call(gateway, model, key, handle.scenario_id, call)
+        assert _key_spend(sha256(key.encode()).hexdigest()) == pytest.approx(CALLS_WITHIN_BUDGET * COST_PER_CALL)
+        denied: Final = gateway.request(
+            "POST",
+            f"/gemini/v1beta/models/{model}:generateContent",
+            _generate_content_request(model),
+            headers={"x-goog-api-key": key, "x-pass-x-scripted-scenario": handle.scenario_id},
+        )
+        assert denied.status_code == 422 and denied.json()["error"]["type"] == "budget_exceeded", denied.text