Merge pull request #605 from davenpi/fix-unreachable-mounted-servers

Add graceful error handling for unreachable mounted servers

src/fastmcp/server/server.py

@@ -257,9 +257,15 @@ class FastMCP(Generic[LifespanResultT]):
         """Get all registered tools, indexed by registered key."""
         if (tools := self._cache.get("tools")) is self._cache.NOT_FOUND:
             tools: dict[str, Tool] = {}
-            for server in self._mounted_servers.values():
-                server_tools = await server.get_tools()
-                tools.update(server_tools)
+            for prefix, server in self._mounted_servers.items():
+                try:
+                    server_tools = await server.get_tools()
+                    tools.update(server_tools)
+                except Exception as e:
+                    logger.warning(
+                        f"Failed to get tools from mounted server '{prefix}': {e}"
+                    )
+                    continue
             tools.update(self._tool_manager.get_tools())
             self._cache.set("tools", tools)
         return tools
@@ -268,9 +274,15 @@ class FastMCP(Generic[LifespanResultT]):
         """Get all registered resources, indexed by registered key."""
         if (resources := self._cache.get("resources")) is self._cache.NOT_FOUND:
             resources: dict[str, Resource] = {}
-            for server in self._mounted_servers.values():
-                server_resources = await server.get_resources()
-                resources.update(server_resources)
+            for prefix, server in self._mounted_servers.items():
+                try:
+                    server_resources = await server.get_resources()
+                    resources.update(server_resources)
+                except Exception as e:
+                    logger.warning(
+                        f"Failed to get resources from mounted server '{prefix}': {e}"
+                    )
+                    continue
             resources.update(self._resource_manager.get_resources())
             self._cache.set("resources", resources)
         return resources
@@ -281,9 +293,16 @@ class FastMCP(Generic[LifespanResultT]):
             templates := self._cache.get("resource_templates")
         ) is self._cache.NOT_FOUND:
             templates: dict[str, ResourceTemplate] = {}
-            for server in self._mounted_servers.values():
-                server_templates = await server.get_resource_templates()
-                templates.update(server_templates)
+            for prefix, server in self._mounted_servers.items():
+                try:
+                    server_templates = await server.get_resource_templates()
+                    templates.update(server_templates)
+                except Exception as e:
+                    logger.warning(
+                        "Failed to get resource templates from mounted server "
+                        f"'{prefix}': {e}"
+                    )
+                    continue
             templates.update(self._resource_manager.get_templates())
             self._cache.set("resource_templates", templates)
         return templates
@@ -294,9 +313,15 @@ class FastMCP(Generic[LifespanResultT]):
         """
         if (prompts := self._cache.get("prompts")) is self._cache.NOT_FOUND:
             prompts: dict[str, Prompt] = {}
-            for server in self._mounted_servers.values():
-                server_prompts = await server.get_prompts()
-                prompts.update(server_prompts)
+            for prefix, server in self._mounted_servers.items():
+                try:
+                    server_prompts = await server.get_prompts()
+                    prompts.update(server_prompts)
+                except Exception as e:
+                    logger.warning(
+                        f"Failed to get prompts from mounted server '{prefix}': {e}"
+                    )
+                    continue
             prompts.update(self._prompt_manager.get_prompts())
             self._cache.set("prompts", prompts)
         return prompts

tests/server/test_mount.py

@@ -1,4 +1,5 @@
 import json
+import sys
 from contextlib import asynccontextmanager
 
 import pytest
@@ -7,7 +8,7 @@ from mcp.types import TextContent, TextResourceContents
 
 from fastmcp import FastMCP
 from fastmcp.client import Client
-from fastmcp.client.transports import FastMCPTransport
+from fastmcp.client.transports import FastMCPTransport, SSETransport
 from fastmcp.exceptions import NotFoundError
 from fastmcp.server.proxy import FastMCPProxy
 
@@ -182,6 +183,80 @@ class TestMultipleServerMount:
         # Second app's tool should be accessible
         assert "api_second_tool" in tools
 
+    @pytest.mark.skipif(
+        sys.platform == "win32", reason="Windows asyncio networking timeouts."
+    )
+    async def test_mount_with_unreachable_proxy_servers(self, caplog):
+        """Test graceful handling when multiple mounted servers fail to connect."""
+
+        main_app = FastMCP("MainApp")
+        working_app = FastMCP("WorkingApp")
+
+        @working_app.tool()
+        def working_tool() -> str:
+            return "Working tool"
+
+        @working_app.resource(uri="working://data")
+        def working_resource():
+            return "Working resource"
+
+        @working_app.prompt()
+        def working_prompt() -> str:
+            return "Working prompt"
+
+        # Mount the working server
+        main_app.mount("working", working_app)
+
+        # Use an unreachable port
+        unreachable_client = Client(
+            transport=SSETransport("http://127.0.0.1:99999/sse")
+        )
+
+        # Create a proxy server that will fail to connect
+        unreachable_proxy = FastMCP.as_proxy(unreachable_client)
+
+        # Mount the unreachable proxy
+        main_app.mount("unreachable", unreachable_proxy)
+
+        # All object types should work from working server despite unreachable proxy
+        async with Client(main_app) as client:
+            # Test tools
+            tools = await client.list_tools()
+            tool_names = [tool.name for tool in tools]
+            assert "working_working_tool" in tool_names
+
+            # Test calling a tool
+            result = await client.call_tool("working_working_tool", {})
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "Working tool"
+
+            # Test resources
+            resources = await client.list_resources()
+            resource_uris = [str(resource.uri) for resource in resources]
+            assert "working://working/data" in resource_uris
+
+            # Test prompts
+            prompts = await client.list_prompts()
+            prompt_names = [prompt.name for prompt in prompts]
+            assert "working_working_prompt" in prompt_names
+
+        # Verify that warnings were logged for the unreachable server
+        warning_messages = [
+            record.message for record in caplog.records if record.levelname == "WARNING"
+        ]
+        assert any(
+            "Failed to get tools from mounted server 'unreachable'" in msg
+            for msg in warning_messages
+        )
+        assert any(
+            "Failed to get resources from mounted server 'unreachable'" in msg
+            for msg in warning_messages
+        )
+        assert any(
+            "Failed to get prompts from mounted server 'unreachable'" in msg
+            for msg in warning_messages
+        )
+
 
 class TestDynamicChanges:
     """Test that changes to mounted servers are reflected dynamically."""