Add test for no prefix

src/fastmcp/server/server.py

@@ -1094,11 +1094,13 @@ class FastMCP(Generic[LifespanResultT]):
 
 def _validate_resource_prefix(prefix: str) -> None:
     valid_resource = "resource://path/to/resource"
+    test_case = f"{prefix}{valid_resource}"
     try:
-        AnyUrl(f"{prefix}{valid_resource}")
+        AnyUrl(test_case)
     except pydantic.ValidationError as e:
         raise ValueError(
-            f"Resource prefix or separator would result in an invalid resource URI: {e}"
+            "Resource prefix or separator would result in an "
+            f"invalid resource URI (test case was {test_case!r}): {e}"
         )
 
 

tests/server/test_mount.py

@@ -106,6 +106,21 @@ class TestBasicMount:
         with pytest.raises(NotFoundError, match="Unknown tool: sub_sub_tool"):
             await main_app._mcp_call_tool("sub_sub_tool", {})
 
+    async def test_mount_with_no_prefix(self):
+        main_app = FastMCP("MainApp")
+        sub_app = FastMCP("SubApp")
+
+        @sub_app.tool()
+        def sub_tool() -> str:
+            return "This is from the sub app"
+
+        main_app.mount(
+            prefix="", server=sub_app, tool_separator="", resource_separator=""
+        )
+
+        tools = await main_app.get_tools()
+        assert "sub_tool" in tools
+
 
 class TestMultipleServerMount:
     """Test mounting multiple servers simultaneously."""