refactor: remove unused Error state, defer to Phase 2 of #1691

Github-Issue: #1691

Author: dgenio

src/mcp/server/session.py

@@ -61,7 +61,10 @@ class InitializationState(Enum):
     State transitions:
         NotInitialized -> Initializing -> Initialized -> Closing -> Closed
         Stateless -> Closing -> Closed
-        Any state -> Error (on unrecoverable failure)
+
+    .. note::
+        An ``Error`` state for unrecoverable failures is planned for Phase 2
+        of issue #1691.
     """
 
     NotInitialized = 1
@@ -70,40 +73,31 @@ class InitializationState(Enum):
     Stateless = 4
     Closing = 5
     Closed = 6
-    Error = 7
 
 
 # Valid state transitions: maps each state to the set of states it can transition to.
+# TODO(#1691 Phase 2): add Error state with transitions for unrecoverable failures.
 _VALID_TRANSITIONS: dict[InitializationState, set[InitializationState]] = {
     InitializationState.NotInitialized: {
         InitializationState.Initializing,
         InitializationState.Initialized,  # client may send notification without prior request
         InitializationState.Closing,
-        InitializationState.Error,
     },
     InitializationState.Initializing: {
         InitializationState.Initialized,
         InitializationState.Closing,
-        InitializationState.Error,
     },
     InitializationState.Initialized: {
         InitializationState.Initializing,  # re-initialization
         InitializationState.Closing,
-        InitializationState.Error,
     },
     InitializationState.Stateless: {
         InitializationState.Closing,
-        InitializationState.Error,
     },
     InitializationState.Closing: {
         InitializationState.Closed,
-        InitializationState.Error,
     },
     InitializationState.Closed: set(),
-    InitializationState.Error: {
-        InitializationState.Closing,
-        InitializationState.Closed,
-    },
 }
 
 

tests/server/test_session_lifecycle.py

@@ -71,7 +71,7 @@ class TestInitializationStateEnum:
     """Verify the expanded InitializationState enum values."""
 
     def test_all_states_present(self) -> None:
-        expected = {"NotInitialized", "Initializing", "Initialized", "Stateless", "Closing", "Closed", "Error"}
+        expected = {"NotInitialized", "Initializing", "Initialized", "Stateless", "Closing", "Closed"}
         actual = {s.name for s in InitializationState}
         assert actual == expected
 
@@ -95,11 +95,6 @@ def test_all_states_have_entry(self) -> None:
     def test_closed_is_terminal(self) -> None:
         assert _VALID_TRANSITIONS[InitializationState.Closed] == set()
 
-    def test_error_allows_recovery(self) -> None:
-        targets = _VALID_TRANSITIONS[InitializationState.Error]
-        assert InitializationState.Closing in targets
-        assert InitializationState.Closed in targets
-
 
 # ---------------------------------------------------------------------------
 # _transition_state tests
@@ -153,21 +148,6 @@ async def test_closed_to_anything_raises(self) -> None:
                 with pytest.raises(RuntimeError, match="Invalid session state transition"):
                     session._transition_state(state)
 
-    async def test_error_transition_from_any_active_state(self) -> None:
-        """Error can be reached from active states."""
-        for start_state in (
-            InitializationState.NotInitialized,
-            InitializationState.Initializing,
-            InitializationState.Initialized,
-            InitializationState.Stateless,
-            InitializationState.Closing,
-        ):
-            async with _session_context() as session:
-                # Force the session into the desired start state
-                session._initialization_state = start_state
-                session._transition_state(InitializationState.Error)
-                assert session.initialization_state == InitializationState.Error
-
 
 # ---------------------------------------------------------------------------
 # is_initialized property tests