diff --git a/elm/tree.py b/elm/tree.py
index d2ea49d..ecb28ab 100644
--- a/elm/tree.py
+++ b/elm/tree.py
@@ -50,11 +50,16 @@ def __init__(self, graph):
             Directed acyclic graph where nodes are LLM prompts and edges are
             logical transitions based on the response. Must have high-level
             graph attribute "api" which is an ApiBase instance. Nodes should
-            have attribute "prompt" which can have {format} named arguments
-            that will be filled from the high-level graph attributes. Edges can
-            have attribute "condition" that is a callable to be executed on the
-            LLM response text. An edge from a node without a condition acts as
-            an "else" statement if no other edge conditions are satisfied. A
+            have attribute "prompt" which is a string that can have {format}
+            named arguments that will be filled from the high-level graph
+            attributes. Nodes can also have "callback" attributes that are
+            callables that act on the LLM response in an arbitrary way. The
+            function signature for a callback must be
+            ``callback(llm_response, decision_tree, node_name)``.
+            Edges can have attribute "condition" that is a callable to be
+            executed on the LLM response text that determines the edge
+            transition. An edge from a node without a condition acts as an
+            "else" statement if no other edge conditions are satisfied. A
             single edge from node to node does not need a condition.
         """
         self._g = graph
@@ -62,6 +67,19 @@ def __init__(self, graph):
         assert isinstance(self.graph, nx.DiGraph)
         assert 'api' in self.graph.graph
 
+    def __getitem__(self, key):
+        """Retrieve a node by name (str) or edge by (node0, node1) tuple"""
+        out = None
+        if key in self.graph.nodes:
+            out = self.graph.nodes[key]
+        elif key in self.graph.edges:
+            out = self.graph.edges[key]
+        else:
+            msg = (f'Could not find "{key}" in graph')
+            logger.error(msg)
+            raise KeyError(msg)
+        return out
+
     @property
     def api(self):
         """Get the ApiBase object.
@@ -112,13 +130,13 @@ def graph(self):
         """
         return self._g
 
-    def call_node(self, node0):
+    def call_node(self, node_name):
         """Call the LLM with the prompt from the input node and search the
         successor edges for a valid transition condition
 
         Parameters
         ----------
-        node0 : str
+        node_name : str
             Name of node being executed.
 
         Returns
@@ -126,22 +144,30 @@ def call_node(self, node0):
         out : str
             Next node or LLM response if at a leaf node.
         """
-        prompt = self._prepare_graph_call(node0)
+
+        node = self[node_name]
+        prompt = self._prepare_graph_call(node_name)
         out = self.api.chat(prompt)
-        return self._parse_graph_output(node0, out)
+        node['response'] = out
+
+        if 'callback' in node:
+            callback = node['callback']
+            callback(out, self, node_name)
+
+        return self._parse_graph_output(node_name, out)
 
-    def _prepare_graph_call(self, node0):
+    def _prepare_graph_call(self, node_name):
         """Prepare a graph call for given node."""
-        prompt = self.graph.nodes[node0]['prompt']
+        prompt = self[node_name]['prompt']
         txt_fmt = {k: v for k, v in self.graph.graph.items() if k != 'api'}
         prompt = prompt.format(**txt_fmt)
-        self._history.append(node0)
+        self._history.append(node_name)
         return prompt
 
     def _parse_graph_output(self, node0, out):
         """Parse graph output for given node and LLM call output. """
         successors = list(self.graph.successors(node0))
-        edges = [self.graph.edges[(node0, node1)] for node1 in successors]
+        edges = [self[(node0, node1)] for node1 in successors]
         conditions = [edge.get('condition', None) for edge in edges]
 
         if len(successors) == 0:
diff --git a/tests/test_tree.py b/tests/test_tree.py
index 87c7d56..c6e679a 100644
--- a/tests/test_tree.py
+++ b/tests/test_tree.py
@@ -39,12 +39,20 @@ def test_chunk_and_embed(mocker):
     graph = nx.DiGraph(text='hello', name='Grant',
                        api=ApiBase(model='gpt-35-turbo'))
 
+    response_dict = {}
+
+    # pylint: disable=unused-argument
+    def callback(response, graph, node_name):
+        response_dict.update({node_name: response})
+
     graph.add_node('init', prompt='Say {text} to {name}')
     graph.add_edge('init', 'next', condition=lambda x: 'Grant' in x)
-    graph.add_node('next', prompt='How are you?')
+    graph.add_node('next', prompt='How are you?', callback=callback)
 
     tree = DecisionTree(graph)
     tree.run()
 
     assert 'init' in tree.history
     assert 'next' in tree.history
+    assert isinstance(tree['next']['response'], str)
+    assert isinstance(response_dict['next'], str)