feat(auth): enforce set-once semantics on ServerSession.tenant_id

Make the tenant_id setter raise ValueError if attempting to change
to a different value once already set. This prevents accidental tenant
reassignment which could be a security issue. Setting to the same
value is allowed (idempotent).

Author: andylim-duo

src/mcp/server/session.py

@@ -116,7 +116,18 @@ def tenant_id(self) -> str | None:
 
     @tenant_id.setter
     def tenant_id(self, value: str | None) -> None:
-        """Set the tenant_id for this session."""
+        """Set the tenant_id for this session (set-once).
+
+        Once a session is bound to a tenant, the tenant_id cannot be changed.
+        This prevents accidental tenant reassignment which could be a security issue.
+
+        Raises:
+            ValueError: If tenant_id is already set to a different value.
+        """
+        if self._tenant_id is not None and value != self._tenant_id:
+            raise ValueError(
+                f"Cannot change tenant_id from '{self._tenant_id}' to '{value}': session is already bound to a tenant"
+            )
         self._tenant_id = value
 
     @property

tests/server/test_multi_tenancy_session.py

@@ -117,7 +117,7 @@ def test_server_request_context_inherits_tenant_id_from_base():
 
 @pytest.mark.anyio
 async def test_server_session_tenant_id_property(init_options: InitializationOptions):
-    """Test ServerSession tenant_id property and setter."""
+    """Test ServerSession tenant_id property with set-once semantics."""
     server_to_client_send, server_to_client_recv = anyio.create_memory_object_stream[SessionMessage](1)
     client_to_server_send, client_to_server_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
 
@@ -134,13 +134,20 @@ async def test_server_session_tenant_id_property(init_options: InitializationOpt
             session.tenant_id = "tenant-123"
             assert session.tenant_id == "tenant-123"
 
-            # Can change tenant_id
-            session.tenant_id = "tenant-456"
-            assert session.tenant_id == "tenant-456"
+            # Setting to the same value is allowed
+            session.tenant_id = "tenant-123"
+            assert session.tenant_id == "tenant-123"
 
-            # Can reset to None
-            session.tenant_id = None
-            assert session.tenant_id is None
+            # Cannot change to a different value
+            with pytest.raises(ValueError, match="Cannot change tenant_id"):
+                session.tenant_id = "tenant-456"
+
+            # Cannot reset to None once set
+            with pytest.raises(ValueError, match="Cannot change tenant_id"):
+                session.tenant_id = None
+
+            # Original value is preserved
+            assert session.tenant_id == "tenant-123"
 
 
 def test_get_tenant_id_from_auth_context():
@@ -317,12 +324,13 @@ async def test_server_session_isolation_between_instances(init_options: Initiali
             assert session1.tenant_id == "tenant-alpha"
             assert session2.tenant_id == "tenant-beta"
 
-            # Modify one session's tenant_id
-            session1.tenant_id = "tenant-gamma"
+            # Attempting to change one session's tenant_id raises
+            with pytest.raises(ValueError, match="Cannot change tenant_id"):
+                session1.tenant_id = "tenant-gamma"
 
-            # Verify the other session is unaffected
-            assert session1.tenant_id == "tenant-gamma"
-            assert session2.tenant_id == "tenant-beta"  # Still beta, not gamma
+            # Both sessions retain their original values
+            assert session1.tenant_id == "tenant-alpha"
+            assert session2.tenant_id == "tenant-beta"
 
 
 @pytest.mark.anyio