diff --git a/pantograph/expr.py b/pantograph/expr.py
index 4ae0367..dee9711 100644
--- a/pantograph/expr.py
+++ b/pantograph/expr.py
@@ -60,6 +60,11 @@ class GoalState:
     state_id: int
     goals: list[Goal]
 
+    _sentinel: list[int]
+
+    def __del__(self):
+        self._sentinel.append(self.state_id)
+
     @property
     def is_solved(self) -> bool:
         """
@@ -68,10 +73,10 @@ class GoalState:
         return not self.goals
 
     @staticmethod
-    def parse(payload: dict) -> Self:
+    def parse(payload: dict, _sentinel: list[int]) -> Self:
         state_id = payload["nextStateId"]
         goals = [Goal.parse(g) for g in payload["goals"]]
-        return GoalState(state_id, goals)
+        return GoalState(state_id, goals, _sentinel)
 
 @dataclass(frozen=True)
 class TacticHave:
diff --git a/pantograph/server.py b/pantograph/server.py
index 52a7661..19f087f 100644
--- a/pantograph/server.py
+++ b/pantograph/server.py
@@ -36,6 +36,9 @@ class Server:
         self.proc = None
         self.restart()
 
+        # List of goal states that should be garbage collected
+        self.to_remove_goal_states = []
+
     def restart(self):
         if self.proc is not None:
             self.proc.close()
@@ -48,14 +51,26 @@ class Server:
         self.proc.setecho(False)
 
     def run(self, cmd, payload):
+        """
+        Runs a raw JSON command. Preferably use one of the commands below.
+        """
         s = json.dumps(payload)
         self.proc.sendline(f"{cmd} {s}")
         try:
             self.proc.expect("{.*}\r\n", timeout=self.timeout)
             output = self.proc.match.group()
             return json.loads(output)
-        except pexpect.exceptions.TIMEOUT:
-            raise pexpect.exceptions.TIMEOUT
+        except pexpect.exceptions.TIMEOUT as exc:
+            raise exc
+
+    def gc(self):
+        """
+        Garbage collect deleted goal states.
+
+        Must be called periodically.
+        """
+        self.run('goal.delete', {'stateIds': self.to_remove_goal_states})
+        self.to_remove_goal_states.clear()
 
     def reset(self):
         return self.run("reset", {})
@@ -64,7 +79,7 @@ class Server:
         result = self.run('goal.start', {"expr": str(expr)})
         if "error" in result:
             raise ServerError(result["desc"])
-        return GoalState(state_id = result["stateId"], goals = [Goal.sentence(expr)])
+        return GoalState(state_id=result["stateId"], goals=[Goal.sentence(expr)], _sentinel=self.to_remove_goal_states)
 
     def goal_tactic(self, state: GoalState, goal_id: int, tactic: Tactic) -> GoalState:
         args = {"stateId": state.state_id, "goalId": goal_id}
@@ -83,7 +98,7 @@ class Server:
             raise ServerError(result["tacticErrors"])
         if "parseError" in result:
             raise ServerError(result["parseError"])
-        return GoalState.parse(result)
+        return GoalState.parse(result, self.to_remove_goal_states)
 
     def goal_conv_begin(self, state: GoalState, goal_id: int) -> GoalState:
         result = self.run('goal.tactic', {"stateId": state.state_id, "goalId": goal_id, "conv": True})
@@ -93,7 +108,7 @@ class Server:
             raise ServerError(result["tacticErrors"])
         if "parseError" in result:
             raise ServerError(result["parseError"])
-        return GoalState.parse(result)
+        return GoalState.parse(result, self.to_remove_goal_states)
 
     def goal_conv_end(self, state: GoalState) -> GoalState:
         result = self.run('goal.tactic', {"stateId": state.state_id, "goalId": 0, "conv": False})
@@ -103,7 +118,7 @@ class Server:
             raise ServerError(result["tacticErrors"])
         if "parseError" in result:
             raise ServerError(result["parseError"])
-        return GoalState.parse(result)
+        return GoalState.parse(result, self.to_remove_goal_states)
 
 
 def get_version():
@@ -122,6 +137,7 @@ class TestServer(unittest.TestCase):
     def test_goal_start(self):
         server = Server()
         state0 = server.goal_start("forall (p q: Prop), Or p q -> Or q p")
+        self.assertEqual(len(server.to_remove_goal_states), 0)
         self.assertEqual(state0.state_id, 0)
         state1 = server.goal_tactic(state0, goal_id=0, tactic="intro a")
         self.assertEqual(state1.state_id, 1)
@@ -132,6 +148,17 @@ class TestServer(unittest.TestCase):
         )])
         self.assertEqual(str(state1.goals[0]),"a : Prop\n⊢ ∀ (q : Prop), a ∨ q → q ∨ a")
 
+        del state0
+        self.assertEqual(len(server.to_remove_goal_states), 1)
+        server.gc()
+        self.assertEqual(len(server.to_remove_goal_states), 0)
+
+        state0b = server.goal_start("forall (p: Prop), p -> p")
+        del state0b
+        self.assertEqual(len(server.to_remove_goal_states), 1)
+        server.gc()
+        self.assertEqual(len(server.to_remove_goal_states), 0)
+
     def test_conv_calc(self):
         server = Server()
         state0 = server.goal_start("∀ (a b: Nat), (b = 2) -> 1 + a + 1 = a + b")