fix: Add checks and better error messages on server object for mcp run/dev

mcp run/dev command expects the server object in the python file to be a
global variable, and it must be of the FastMCP type and not the low level
Server type. The previous error messages may be confusing to users who
are not aware of these facts.

Author: john0312

Diff for: src/mcp/cli/cli.py

@@ -6,7 +6,10 @@
 import subprocess
 import sys
 from pathlib import Path
-from typing import Annotated
+from typing import Annotated, Any
+
+from mcp.server import FastMCP
+from mcp.server import Server as LowLevelServer
 
 try:
     import typer
@@ -141,17 +144,48 @@ def _import_server(file: Path, server_object: str | None = None):
     module = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(module)
 
+    def _check_server_object(server_object: Any, object_name: str):
+        """Helper function to check that the server object is supported
+
+        Args:
+            server_object: The server object to check.
+
+        Returns:
+            True if it's supported.
+        """
+        if not isinstance(server_object, FastMCP):
+            logger.error(
+                f"The server object {object_name} is of type "
+                f"{type(server_object)} (expecting <class 'FastMCP'>)."
+            )
+            if isinstance(server_object, LowLevelServer):
+                logger.warn(
+                    "Note that only FastMCP server is supported. Low level "
+                    "Server class is not yet supported."
+                )
+            return False
+        return True
+
     # If no object specified, try common server names
     if not server_object:
         # Look for the most common server object names
         for name in ["mcp", "server", "app"]:
             if hasattr(module, name):
+                if not _check_server_object(getattr(module, name), f"{file}:{name}"):
+                    logger.error(
+                        f"Ignoring object '{file}:{name}' as it's not a valid "
+                        "server object"
+                    )
+                    continue
                 return getattr(module, name)
 
         logger.error(
             f"No server object found in {file}. Please either:\n"
             "1. Use a standard variable name (mcp, server, or app)\n"
-            "2. Specify the object name with file:object syntax",
+            "2. Specify the object name with file:object syntax"
+            "3. If the server creates the FastMCP object within main() "
+            "   or another function, refactor the FastMCP object to be a "
+            "   global variable named mcp, server, or app.",
             extra={"file": str(file)},
         )
         sys.exit(1)
@@ -172,6 +206,9 @@ def _import_server(file: Path, server_object: str | None = None):
         # Just object name
         server = getattr(module, server_object, None)
 
+    if not _check_server_object(server, server_object):
+        server = None
+
     if server is None:
         logger.error(
             f"Server object '{server_object}' not found",