fix(voyage): route rerank on the key it authenticates with

get_complete_url picked the rerank host from the environment while the auth
header carried the request key, so an explicit MongoDB-issued key was posted
to api.voyageai.com. validate_environment now hands the resolved key to the
config instance and get_complete_url reads it back, so the host and the
credential always come from one key. The config is built per request, so
nothing carries over between them.

litellm/llms/voyage/rerank/transformation.py

@@ -25,6 +25,16 @@ from ..embedding.transformation import VoyageError
 
 
 class VoyageRerankConfig(BaseRerankConfig):
+    """
+    ``validate_environment`` stores the credential it authenticates with so ``get_complete_url``
+    can select the host that issued it. ``ProviderConfigManager.get_provider_rerank_config``
+    builds this config per request, so that key never reaches another one.
+    """
+
+    def __init__(self) -> None:
+        super().__init__()
+        self._api_key: str | None = None
+
     def get_supported_cohere_rerank_params(self, model: str) -> list:
         return ["query", "documents", "top_n", "return_documents"]
 
@@ -60,7 +70,7 @@ class VoyageRerankConfig(BaseRerankConfig):
         optional_params: dict | None = None,
     ) -> str:
         if api_base is None:
-            return f"{get_default_base_url()}/rerank"
+            return f"{get_default_base_url(self._api_key)}/rerank"
         api_base = api_base.rstrip("/")
         if not api_base.endswith("/v1/rerank"):
             if api_base.endswith("/v1"):
@@ -144,6 +154,7 @@ class VoyageRerankConfig(BaseRerankConfig):
         resolved_api_key: Final = get_voyage_api_key(api_key)
         if resolved_api_key is None:
             raise ValueError("Voyage AI API key is required. Set via `api_key` parameter or `VOYAGE_API_KEY` env var.")
+        self._api_key = resolved_api_key
         return {
             "Authorization": f"Bearer {resolved_api_key}",
             "content-type": "application/json",

tests/test_litellm/llms/voyage/test_common_utils.py

@@ -72,10 +72,61 @@ def test_explicit_api_base_overrides_key_routing(config, endpoint):
     assert url == f"https://gateway.internal/v1/{endpoint}"
 
 
+@pytest.mark.parametrize("api_key, expected_host", [("al-key", MONGODB_API_BASE), ("pa-key", VOYAGE_API_BASE)])
+def test_rerank_routes_by_request_key_prefix(api_key, expected_host):
+    config = VoyageRerankConfig()
+    config.validate_environment({}, "rerank-2.5", api_key=api_key)
+
+    assert config.get_complete_url(None, "rerank-2.5") == f"{expected_host}/rerank"
+
+
 @pytest.mark.parametrize("api_key, expected_host", [("al-key", MONGODB_API_BASE), ("pa-key", VOYAGE_API_BASE)])
 def test_rerank_routes_by_env_key_prefix(monkeypatch, api_key, expected_host):
     monkeypatch.setenv("VOYAGE_API_KEY", api_key)
-    assert VoyageRerankConfig().get_complete_url(None, "rerank-2.5") == f"{expected_host}/rerank"
+    config = VoyageRerankConfig()
+    config.validate_environment({}, "rerank-2.5")
+
+    assert config.get_complete_url(None, "rerank-2.5") == f"{expected_host}/rerank"
+
+
+def test_rerank_request_key_beats_env_key_for_routing(monkeypatch):
+    """A MongoDB key on the request must not be posted to the Voyage host the env key names"""
+    monkeypatch.setenv("VOYAGE_API_KEY", "pa-from-env")
+    config = VoyageRerankConfig()
+
+    headers = config.validate_environment({}, "rerank-2.5", api_key="al-on-request")
+
+    assert headers["Authorization"] == "Bearer al-on-request"
+    assert config.get_complete_url(None, "rerank-2.5") == f"{MONGODB_API_BASE}/rerank"
+
+
+def test_rerank_config_is_built_per_request_so_keys_cannot_leak(monkeypatch):
+    """get_complete_url reads a key off the instance, so each request must get its own instance"""
+    import litellm
+    from litellm.types.utils import LlmProviders
+    from litellm.utils import ProviderConfigManager
+
+    monkeypatch.delenv("VOYAGE_API_KEY", raising=False)
+    first = ProviderConfigManager.get_provider_rerank_config(
+        model="rerank-2.5", provider=LlmProviders.VOYAGE, api_base=None, present_version_params=[]
+    )
+    second = ProviderConfigManager.get_provider_rerank_config(
+        model="rerank-2.5", provider=LlmProviders.VOYAGE, api_base=None, present_version_params=[]
+    )
+    assert isinstance(first, litellm.VoyageRerankConfig) and first is not second
+
+    first.validate_environment({}, "rerank-2.5", api_key="al-first-request")
+    second.validate_environment({}, "rerank-2.5", api_key="pa-second-request")
+
+    assert first.get_complete_url(None, "rerank-2.5") == f"{MONGODB_API_BASE}/rerank"
+    assert second.get_complete_url(None, "rerank-2.5") == f"{VOYAGE_API_BASE}/rerank"
+
+
+def test_rerank_falls_back_to_env_when_validate_environment_did_not_run(monkeypatch):
+    """A caller that skips validate_environment keeps the pre-existing env-only behaviour"""
+    monkeypatch.setenv("VOYAGE_API_KEY", "al-from-env")
+
+    assert VoyageRerankConfig().get_complete_url(None, "rerank-2.5") == f"{MONGODB_API_BASE}/rerank"
 
 
 @pytest.mark.parametrize(
@@ -95,11 +146,12 @@ def test_auth_header_uses_the_key_the_url_was_routed_on(monkeypatch, config):
 
 def test_rerank_auth_header_uses_the_key_the_url_was_routed_on(monkeypatch):
     monkeypatch.setenv("VOYAGE_AI_TOKEN", "al-from-env")
+    config = VoyageRerankConfig()
 
-    headers = VoyageRerankConfig().validate_environment({}, "rerank-2.5")
+    headers = config.validate_environment({}, "rerank-2.5")
 
     assert headers["Authorization"] == "Bearer al-from-env"
-    assert VoyageRerankConfig().get_complete_url(None, "rerank-2.5").startswith(MONGODB_API_BASE)
+    assert config.get_complete_url(None, "rerank-2.5").startswith(MONGODB_API_BASE)
 
 
 def test_get_voyage_api_key_prefers_env_vars_in_documented_order(monkeypatch):