Forward backend capabilities in ProxyProvider (#3956)

Co-authored-by: Claude <noreply@anthropic.com>

tests/server/providers/proxy/test_proxy_server.py

@@ -915,3 +915,110 @@ class TestProxyProviderCache:
             result = await proxy.call_tool("greet", {"name": "Alice"})
             mock_list.assert_not_called()
         assert result.content[0].text == "Hello, Alice!"  # type: ignore[union-attr]  # ty:ignore[unresolved-attribute]
+
+
+def _make_tools_only_backend() -> FastMCP:
+    """Create a FastMCP backend that only supports tools (no resources/prompts)."""
+    backend = FastMCP("ToolsOnlyBackend")
+
+    @backend.tool
+    def my_tool() -> str:
+        return "result"
+
+    # Remove resource and prompt handlers to simulate a tools-only MCP server
+    backend._mcp_server.request_handlers.pop(mcp_types.ListResourcesRequest, None)
+    backend._mcp_server.request_handlers.pop(
+        mcp_types.ListResourceTemplatesRequest, None
+    )
+    backend._mcp_server.request_handlers.pop(mcp_types.ReadResourceRequest, None)
+    backend._mcp_server.request_handlers.pop(mcp_types.ListPromptsRequest, None)
+    backend._mcp_server.request_handlers.pop(mcp_types.GetPromptRequest, None)
+    return backend
+
+
+class TestProxyCapabilityForwarding:
+    """Proxy advertises the backend's actual capabilities (issue #3948)."""
+
+    async def test_proxy_forwards_tools_only_capabilities(self):
+        """When backend only supports tools, proxy initialize response should not
+        include resources or prompts capabilities."""
+        backend = _make_tools_only_backend()
+        proxy = create_proxy(FastMCPTransport(backend))
+
+        async with Client(proxy) as client:
+            init_result = client.initialize_result
+            assert init_result is not None
+            caps = init_result.capabilities
+            assert caps.tools is not None
+            assert caps.resources is None
+            assert caps.prompts is None
+
+    async def test_proxy_full_backend_advertises_all_capabilities(
+        self, fastmcp_server: FastMCP
+    ):
+        """When backend supports tools, resources and prompts, proxy should
+        advertise all three."""
+        proxy = create_proxy(FastMCPTransport(fastmcp_server))
+
+        async with Client(proxy) as client:
+            init_result = client.initialize_result
+            assert init_result is not None
+            caps = init_result.capabilities
+            assert caps.tools is not None
+            assert caps.resources is not None
+            assert caps.prompts is not None
+
+    async def test_proxy_resources_list_returns_method_not_found(self):
+        """When backend does not support resources, resources/list on the proxy
+        must return a METHOD_NOT_FOUND error, not an empty list."""
+        backend = _make_tools_only_backend()
+        proxy = create_proxy(FastMCPTransport(backend))
+
+        async with Client(proxy) as client:
+            with pytest.raises(McpError) as exc_info:
+                await client.list_resources()
+            assert exc_info.value.error.code == mcp_types.METHOD_NOT_FOUND
+
+    async def test_proxy_prompts_list_returns_method_not_found(self):
+        """When backend does not support prompts, prompts/list on the proxy
+        must return a METHOD_NOT_FOUND error, not an empty list."""
+        backend = _make_tools_only_backend()
+        proxy = create_proxy(FastMCPTransport(backend))
+
+        async with Client(proxy) as client:
+            with pytest.raises(McpError) as exc_info:
+                await client.list_prompts()
+            assert exc_info.value.error.code == mcp_types.METHOD_NOT_FOUND
+
+    async def test_proxy_preserves_local_resources_even_without_backend_support(self):
+        """When proxy has local resources but backend doesn't, resources capability
+        should still be advertised and local resources should be accessible."""
+        backend = _make_tools_only_backend()
+        proxy = create_proxy(FastMCPTransport(backend))
+
+        @proxy.resource("local://data")
+        def local_data() -> str:
+            return "local"
+
+        async with Client(proxy) as client:
+            init_result = client.initialize_result
+            assert init_result is not None
+            caps = init_result.capabilities
+            assert caps.tools is not None
+            assert caps.resources is not None  # local resource present
+            assert caps.prompts is None  # still no prompts
+
+            resources = await client.list_resources()
+            assert any(str(r.uri) == "local://data" for r in resources)
+
+    async def test_proxy_backend_capabilities_preloaded_in_lifespan(self):
+        """ProxyProvider._backend_capabilities should be set after lifespan runs."""
+        backend = _make_tools_only_backend()
+        provider = ProxyProvider(lambda: ProxyClient(FastMCPTransport(backend)))
+
+        async with provider.lifespan():
+            caps = provider._backend_capabilities
+            assert caps is not None
+            assert caps.tools is not None
+            assert caps.resources is None
+            assert caps.prompts is None