[v2.0.x] Point imports of mcp.server.fastmcp at the migration guide (#3393)

tests/server/test_fastmcp.py

@@ -0,0 +1,55 @@
+"""The removed v1 import path `mcp.server.fastmcp` fails with a pointer to the migration guide."""
+
+import importlib
+import sys
+
+import pytest
+from inline_snapshot import snapshot
+
+import mcp.server
+from mcp.server.mcpserver import MCPServer
+
+
+def test_importing_fastmcp_raises_module_not_found_that_points_at_the_migration_guide() -> None:
+    """SDK-defined: the v1 path fails with the same exception type and `.name` as a module
+    that genuinely does not exist, but the message names the replacement and the guide."""
+    with pytest.raises(ModuleNotFoundError) as exc_info:
+        importlib.import_module("mcp.server.fastmcp")
+
+    assert exc_info.value.name == "mcp.server.fastmcp"
+    assert str(exc_info.value) == snapshot(
+        "No module named 'mcp.server.fastmcp'. This is mcp 2.x, where FastMCP was renamed to MCPServer "
+        "(from mcp.server.mcpserver import MCPServer) and other APIs changed; see the migration guide at "
+        "https://py.sdk.modelcontextprotocol.io/v2/migration/#fastmcp-renamed-to-mcpserver "
+        "or pin 'mcp<2' to keep running v1 code."
+    )
+    # A module that raises while executing is never cached, so nothing is left behind.
+    assert "mcp.server.fastmcp" not in sys.modules
+    assert not hasattr(mcp.server, "fastmcp")
+
+
+def test_importing_a_fastmcp_submodule_raises_the_parent_pointer() -> None:
+    """SDK-defined: a deep v1 path executes `mcp.server.fastmcp` first, so it fails with that
+    module's message and `.name` rather than a bare error for the leaf."""
+    with pytest.raises(ModuleNotFoundError) as parent:
+        importlib.import_module("mcp.server.fastmcp")
+    with pytest.raises(ModuleNotFoundError) as exc_info:
+        importlib.import_module("mcp.server.fastmcp.utilities.types")
+
+    assert exc_info.value.name == "mcp.server.fastmcp"
+    assert str(exc_info.value) == str(parent.value)
+
+
+def test_v1_first_import_shim_falls_back_to_mcpserver() -> None:
+    """SDK-defined: projects that support both majors try the v1 import and fall back on
+    `ModuleNotFoundError` (the narrowest guard seen in the wild), which is why the pointer is
+    raised as exactly that type and not as a bare `ImportError` or after a warning."""
+    fell_back = False
+    try:
+        server_class: type = importlib.import_module("mcp.server.fastmcp").FastMCP
+    except ModuleNotFoundError:
+        fell_back = True
+        server_class = MCPServer
+
+    assert fell_back
+    assert server_class is MCPServer