--- a/taskcluster/taskgraph/actions/util.py
+++ b/taskcluster/taskgraph/actions/util.py
@@ -12,17 +12,17 @@ import logging
 import os
 import re
 
 from six import text_type
 
 from requests.exceptions import HTTPError
 
 from taskgraph import create
-from taskgraph.decision import read_artifact, write_artifact
+from taskgraph.decision import read_artifact, write_artifact, rename_artifact
 from taskgraph.taskgraph import TaskGraph
 from taskgraph.optimize import optimize_task_graph
 from taskgraph.util.taskcluster import (
     get_session,
     get_artifact,
     list_tasks,
     parse_time,
     CONCURRENCY,
@@ -164,26 +164,53 @@ def create_tasks(graph_config, to_run, f
         optimized_task_graph,
         label_to_taskid,
         params,
         decision_task_id,
     )
     return label_to_taskid
 
 
+def _update_reducer(accumulator, new_value):
+    "similar to set or dict `update` method, but returning the modified object"
+    accumulator.update(new_value)
+    return accumulator
+
 def combine_task_graph_files(suffixes):
     """Combine task-graph-{suffix}.json files into a single task-graph.json file.
 
     Since Chain of Trust verification requires a task-graph.json file that
     contains all children tasks, we can combine the various task-graph-0.json
-    type files into a master task-graph.json file at the end."""
-    all = {}
-    for suffix in suffixes:
-        all.update(read_artifact('task-graph-{}.json'.format(suffix)))
-    write_artifact('task-graph.json', all)
+    type files into a master task-graph.json file at the end.
+
+    Actions also look for various artifacts, so we combine those in a similar
+    fashion.
+
+    In the case where there is only one suffix, we simply rename it to avoid the
+    additional cost of uploading two copies of the same data.
+    """
+
+    if len(suffixes) == 1:
+        for filename in ['task-graph', 'label-to-taskid', 'to-run']:
+            rename_artifact(
+                "{}-{}.json".format(filename, suffixes[0]),
+                "{}.json".format(filename))
+        return
+
+    def combine(file_contents, base):
+        return reduce(_update_reducer, file_contents, base)
+
+    files = [read_artifact("task-graph-{}.json".format(suffix)) for suffix in suffixes]
+    write_artifact("task-graph.json", combine(files, dict()))
+
+    files = [read_artifact("label-to-taskid-{}.json".format(suffix)) for suffix in suffixes]
+    write_artifact("label-to-taskid.json", combine(files, dict()))
+
+    files = [read_artifact("to-run-{}.json".format(suffix)) for suffix in suffixes]
+    write_artifact("to-run.json", list(combine(files, set())))
 
 
 def relativize_datestamps(task_def):
     """
     Given a task definition as received from the queue, convert all datestamps
     to {relative_datestamp: ..} format, with the task creation time as "now".
     The result is useful for handing to ``create_task``.
     """

--- a/taskcluster/taskgraph/decision.py
+++ b/taskcluster/taskgraph/decision.py
@@ -400,8 +400,12 @@ def read_artifact(filename):
         with open(path, 'r') as f:
             return json.load(f)
     elif filename.endswith('.gz'):
         import gzip
         with gzip.open(path, 'rb') as f:
             return json.load(f)
     else:
         raise TypeError("Don't know how to read {}".format(filename))
+
+
+def rename_artifact(src, dest):
+    os.rename(os.path.join(ARTIFACTS_DIR, src), os.path.join(ARTIFACTS_DIR, dest))

--- a/taskcluster/taskgraph/test/test_actions_util.py
+++ b/taskcluster/taskgraph/test/test_actions_util.py
@@ -1,18 +1,22 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import absolute_import, print_function, unicode_literals
 
 import unittest
-from mozunit import main
+import json
+from mock import patch
+from mozunit import main, MockedOpen
+from taskgraph.decision import read_artifact
 from taskgraph.actions.util import (
-    relativize_datestamps
+    relativize_datestamps,
+    combine_task_graph_files,
 )
 
 TASK_DEF = {
     'created': '2017-10-10T18:33:03.460Z',
     # note that this is not an even number of seconds off!
     'deadline': '2017-10-11T18:33:03.461Z',
     'dependencies': [],
     'expires': '2018-10-10T18:33:04.461Z',
@@ -37,10 +41,49 @@ class TestRelativize(unittest.TestCase):
         pprint.pprint(rel)
         assert rel['created'] == {'relative-datestamp': '0 seconds'}
         assert rel['deadline'] == {'relative-datestamp': '86400 seconds'}
         assert rel['expires'] == {'relative-datestamp': '31536001 seconds'}
         assert rel['payload']['artifacts']['public']['expires'] == \
             {'relative-datestamp': '31536000 seconds'}
 
 
+class TestCombineTaskGraphFiles(unittest.TestCase):
+
+    def test_no_suffixes(self):
+        with MockedOpen({}):
+            combine_task_graph_files([])
+            self.assertRaises(Exception, open('artifacts/to-run.json'))
+
+    @patch('taskgraph.actions.util.rename_artifact')
+    def test_one_suffix(self, rename_artifact):
+        combine_task_graph_files(['0'])
+        rename_artifact.assert_any_call('task-graph-0.json', 'task-graph.json')
+        rename_artifact.assert_any_call('label-to-taskid-0.json', 'label-to-taskid.json')
+        rename_artifact.assert_any_call('to-run-0.json', 'to-run.json')
+
+    def test_several_suffixes(self):
+        files = {
+            'artifacts/task-graph-0.json': json.dumps({'taska': {}}),
+            'artifacts/label-to-taskid-0.json': json.dumps({'taska': 'TASKA'}),
+            'artifacts/to-run-0.json': json.dumps(['taska']),
+            'artifacts/task-graph-1.json': json.dumps({'taskb': {}}),
+            'artifacts/label-to-taskid-1.json': json.dumps({'taskb': 'TASKB'}),
+            'artifacts/to-run-1.json': json.dumps(['taskb']),
+        }
+        with MockedOpen(files):
+            combine_task_graph_files(['0', '1'])
+            self.assertEqual(read_artifact('task-graph.json'), {
+                'taska': {},
+                'taskb': {},
+            })
+            self.assertEqual(read_artifact('label-to-taskid.json'), {
+                'taska': 'TASKA',
+                'taskb': 'TASKB',
+            })
+            self.assertEqual(sorted(read_artifact('to-run.json')), [
+                'taska',
+                'taskb',
+            ])
+
+
 if __name__ == '__main__':
     main()