Mark transforming single-server config wrapper legacy-only

fastmcp_slim/fastmcp/mcp_config.py

@@ -164,7 +164,16 @@ class _TransformingMCPServerMixin(BaseModel):
                 )
             ) from exc
 
-        return FastMCPTransport(mcp=self._to_server_and_underlying_transport()[0])
+        transport = FastMCPTransport(mcp=self._to_server_and_underlying_transport()[0])
+        # The wrapped proxy talks to its upstream over the legacy era (it pins
+        # the backend client to `mode="legacy"` to forward the initialize
+        # handshake and server-initiated features). Mark the wrapper legacy-only
+        # so a default `Client(config)` on `mode="auto"` negotiates legacy with
+        # it too, keeping both legs on the same era — otherwise a modern
+        # frontend would receive a forwarded server-initiated request that the
+        # modern era has no back-channel for.
+        transport.legacy_only = True
+        return transport
 
 
 class StdioMCPServer(BaseModel):

tests/test_mcp_config.py

@@ -140,6 +140,23 @@ class TestConfigTransportLegacyOnly:
         transport = MCPConfigTransport(config)
         assert transport.legacy_only is True
 
+    def test_transforming_single_server_wrapper_is_legacy_only(self):
+        """A single-server config that uses tool transforms or tag filters wraps
+        a legacy-pinned proxy; the wrapper transport must advertise legacy-only
+        so a default `mode="auto"` frontend negotiates the same era as the
+        backend rather than negotiating modern against a legacy upstream."""
+        config = {
+            "mcpServers": {
+                "a": {
+                    "url": "https://a.example.com/mcp",
+                    "include_tags": ["public"],
+                },
+            },
+        }
+        mcp_config = MCPConfig.from_dict(config)
+        transport = mcp_config.mcpServers["a"].to_transport()
+        assert transport.legacy_only is True
+
 
 def test_parse_single_stdio_config():
     config = {