diff --git a/pydantic_ai_slim/pydantic_ai/agent.py b/pydantic_ai_slim/pydantic_ai/agent.py
index aa01963a8..7fb1e084d 100644
--- a/pydantic_ai_slim/pydantic_ai/agent.py
+++ b/pydantic_ai_slim/pydantic_ai/agent.py
@@ -1124,7 +1124,7 @@ def _set_result_tool_return(self, return_content: str) -> list[_messages.ModelMe
         raise LookupError(f'No tool call found with tool name {self.result.tool_name!r}.')
 
     @property
-    def is_ended(self):
+    def is_ended(self) -> bool:
         return self.graph_run.is_ended
 
     @property
diff --git a/pydantic_graph/pydantic_graph/graph.py b/pydantic_graph/pydantic_graph/graph.py
index 7724bec2c..710cfedee 100644
--- a/pydantic_graph/pydantic_graph/graph.py
+++ b/pydantic_graph/pydantic_graph/graph.py
@@ -536,7 +536,7 @@ def __init__(
         self._result: End[RunEndT] | None = None
 
     @property
-    def is_ended(self):
+    def is_ended(self) -> bool:
         return self._result is not None
 
     @property
@@ -557,6 +557,11 @@ async def next(
         self: GraphRun[StateT, DepsT, T], node: BaseNode[StateT, DepsT, T]
     ) -> BaseNode[StateT, DepsT, T] | End[T]:
         """Note: this method behaves very similarly to an async generator's `asend` method."""
+        if not self._started:
+            raise exceptions.GraphRuntimeError(
+                'You must enter the GraphRun as a contextmanager before you can call `next` on it.'
+            )
+
         history = self.history
         state = self.state
         deps = self.deps