fix(proxy): stop /{provider}/v1/files from capturing /anthropic passthrough

The native files and batches routers mount before the passthrough router, so
/anthropic/v1/files matched /{provider}/v1/files with provider="anthropic" and
422'd on the OpenAI-only `purpose` field instead of forwarding to Anthropic's
Files API. Give /anthropic its own router mounted ahead of batches and files,
mirroring the fix already applied to /openai_passthrough in #36092.

litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py

@@ -63,6 +63,7 @@ from .passthrough_endpoint_router import PassthroughEndpointRouter
 vertex_llm_base: Final = VertexBase()
 router: Final = APIRouter()
 openai_passthrough_router: Final = APIRouter()
+anthropic_passthrough_router: Final = APIRouter()
 default_vertex_config: Final = None
 
 passthrough_endpoint_router: Final = PassthroughEndpointRouter()
@@ -563,7 +564,7 @@ async def is_streaming_request_fn(request: Request) -> bool:
     return False
 
 
-@router.api_route(
+@anthropic_passthrough_router.api_route(
     "/anthropic/{endpoint:path}",
     methods=["GET", "POST", "PUT", "DELETE", "PATCH"],
     tags=["Anthropic Pass-through", "pass-through"],

litellm/proxy/proxy_server.py

@@ -560,6 +560,7 @@ from litellm.proxy.openai_files_endpoints.files_endpoints import (
     set_files_config,
 )
 from litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints import (
+    anthropic_passthrough_router,
     openai_passthrough_router,
     passthrough_endpoint_router,
     vertex_ai_live_websocket_passthrough,
@@ -17280,6 +17281,7 @@ app.include_router(image_router)
 app.include_router(fine_tuning_router)
 app.include_router(credential_router)
 app.include_router(openai_passthrough_router)
+app.include_router(anthropic_passthrough_router)
 app.include_router(batches_router)
 app.include_router(openai_files_router)
 app.include_router(llm_passthrough_router)

tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py

@@ -2990,6 +2990,26 @@ def test_native_provider_routes_are_unchanged(method, path, expected_name):
     assert _resolve_route_name(method, path) == expected_name
 
 
+@pytest.mark.parametrize(
+    "method, path",
+    [
+        ("POST", "/anthropic/v1/files"),
+        ("GET", "/anthropic/v1/files"),
+        ("GET", "/anthropic/v1/files/file-abc123"),
+        ("DELETE", "/anthropic/v1/files/file-abc123"),
+        ("POST", "/anthropic/v1/batches"),
+        ("POST", "/anthropic/v1/messages"),
+    ],
+)
+def test_anthropic_passthrough_prefix_wins_over_native_provider_routes(method, path):
+    """
+    Anthropic's Files API has no `purpose` field and no `files_settings` config,
+    so the native /{provider}/v1/files and /{provider}/v1/batches routes must never
+    capture /anthropic/... with provider="anthropic".
+    """
+    assert _resolve_route_name(method, path) == "anthropic_proxy_route"
+
+
 class TestCursorProxyRoute:
     """Tests for the Cursor Cloud Agents pass-through route."""