Merge pull request #2552 from langchain-ai/vb/remove-assertion

langgraph: relax graph validation to handle nodes without return typehints

libs/langgraph/langgraph/graph/graph.py

@@ -374,6 +374,11 @@ class Graph:
             if source not in self.nodes and source != START:
                 raise ValueError(f"Found edge starting at unknown node '{source}'")
 
+        if START not in all_sources:
+            raise ValueError(
+                "Graph must have an entrypoint: add at least one edge from START to another node"
+            )
+
         # assemble targets
         all_targets = {end for _, end in self._all_edges}
         for start, branches in self.branches.items():
@@ -395,10 +400,6 @@ class Graph:
         for name, spec in self.nodes.items():
             if spec.ends:
                 all_targets.update(spec.ends)
-        # validate targets
-        for node in self.nodes:
-            if node not in all_targets:
-                raise ValueError(f"Node `{node}` is not reachable")
         for target in all_targets:
             if target not in self.nodes and target != END:
                 raise ValueError(f"Found edge ending at unknown node `{target}`")

libs/langgraph/tests/test_pregel.py

@@ -160,7 +160,7 @@ def test_graph_validation() -> None:
     workflow = Graph()
     workflow.add_node("agent", logic)
     workflow.set_finish_point("agent")
-    with pytest.raises(ValueError, match="not reachable"):
+    with pytest.raises(ValueError, match="must have an entrypoint"):
         workflow.compile()
 
     workflow = Graph()
@@ -207,18 +207,6 @@ def test_graph_validation() -> None:
     with pytest.raises(ValueError, match="unknown"):  # extra is not defined
         workflow.compile()
 
-    workflow = Graph()
-    workflow.add_node("agent", logic)
-    workflow.add_node("tools", logic)
-    workflow.add_node("extra", logic)
-    workflow.set_entry_point("agent")
-    workflow.add_conditional_edges("agent", logic, {"continue": "tools", "exit": END})
-    workflow.add_edge("tools", "agent")
-    with pytest.raises(
-        ValueError, match="Node `extra` is not reachable"
-    ):  # extra is not reachable
-        workflow.compile()
-
     workflow = Graph()
     workflow.add_node("agent", logic)
     workflow.add_node("tools", logic)
@@ -276,6 +264,25 @@ def test_graph_validation() -> None:
         graph.invoke({"hello": "there"})
 
 
+def test_graph_validation_with_command() -> None:
+    class State(TypedDict):
+        foo: str
+        bar: str
+
+    def node_a(state: State):
+        return GraphCommand(goto="b", update={"foo": "bar"})
+
+    def node_b(state: State):
+        return GraphCommand(goto=END, update={"bar": "baz"})
+
+    builder = StateGraph(State)
+    builder.add_node("a", node_a)
+    builder.add_node("b", node_b)
+    builder.add_edge(START, "a")
+    graph = builder.compile()
+    assert graph.invoke({"foo": ""}) == {"foo": "bar", "bar": "baz"}
+
+
 def test_checkpoint_errors() -> None:
     class FaultyGetCheckpointer(MemorySaver):
         def get_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]: