diff --git a/pyagentspec/src/pyagentspec/adapters/langgraph/_node_execution.py b/pyagentspec/src/pyagentspec/adapters/langgraph/_node_execution.py
index 4bcb1260..b2aa13b8 100644
--- a/pyagentspec/src/pyagentspec/adapters/langgraph/_node_execution.py
+++ b/pyagentspec/src/pyagentspec/adapters/langgraph/_node_execution.py
@@ -1067,9 +1067,17 @@ def is_single_string_output(expected_outputs: List[AgentSpecProperty]) -> bool:
     adapter takes it directly from the agent's final message rather than forcing
     structured generation. Mirrors ``LlmNodeExecutor``'s single-string handling
     and lets a string output work on models without structured-output support.
+
+    An enum is excluded: its schema names the only values the output may take, so
+    free text cannot satisfy it. Treating it as free text left the allowed set
+    unenforced — the agent replied in prose and a branch keyed on the value
+    matched nothing.
     """
     outputs = expected_outputs or []
-    return len(outputs) == 1 and outputs[0].type == "string"
+    if len(outputs) != 1 or outputs[0].type != "string":
+        return False
+    schema = outputs[0].json_schema or {}
+    return not schema.get("enum")
 
 
 def extract_outputs_from_invoke_result(
diff --git a/pyagentspec/tests/adapters/langgraph/flows/test_agentnode.py b/pyagentspec/tests/adapters/langgraph/flows/test_agentnode.py
index 8930aede..afd53dce 100644
--- a/pyagentspec/tests/adapters/langgraph/flows/test_agentnode.py
+++ b/pyagentspec/tests/adapters/langgraph/flows/test_agentnode.py
@@ -107,6 +107,26 @@ def test_is_single_string_output() -> None:
     assert is_single_string_output([StringProperty(title="a"), StringProperty(title="b")]) is False
 
 
+def test_a_single_enum_output_is_not_free_text() -> None:
+    """An enum names the only values its output may take, so it must go through
+    structured generation rather than being filled from the final message.
+
+    Treated as free text, the allowed set was never enforced: the agent replied in
+    prose and a BranchingNode keyed on the value matched none of its paths.
+    """
+    from pyagentspec.adapters.langgraph._node_execution import is_single_string_output
+    from pyagentspec.property import Property, StringProperty
+
+    enum_output = Property(
+        title="sentiment",
+        json_schema={"type": "string", "enum": ["positive", "negative", "neutral"]},
+    )
+
+    assert is_single_string_output([enum_output]) is False
+    # A plain string output keeps the free-text shortcut.
+    assert is_single_string_output([StringProperty(title="answer")]) is True
+
+
 def test_single_string_output_taken_from_final_message_without_structured_generation() -> None:
     """An AgentNode whose agent declares a single string output should resolve
     that output from the agent's final message — no structured generation, so