--- a/.taskcluster.yml
+++ b/.taskcluster.yml
@@ -70,16 +70,17 @@ tasks:
         env:
           # checkout-gecko uses these to check out the source; the inputs
           # to `mach taskgraph decision` are all on the command line.
           GECKO_BASE_REPOSITORY: 'https://hg.mozilla.org/mozilla-unified'
           GECKO_HEAD_REPOSITORY: '{{{url}}}'
           GECKO_HEAD_REF: '{{revision}}'
           GECKO_HEAD_REV: '{{revision}}'
           HG_STORE_PATH: /home/worker/checkouts/hg-store
+          TASKCLUSTER_PROXY: 1
 
         cache:
           level-{{level}}-checkouts: /home/worker/checkouts
 
         features:
           taskclusterProxy: true
           chainOfTrust: true
 

--- a/taskcluster/taskgraph/optimize.py
+++ b/taskcluster/taskgraph/optimize.py
@@ -188,17 +188,17 @@ def optimization(name):
     return wrap
 
 
 @optimization('index-search')
 def opt_index_search(task, params, index_path):
     try:
         task_id = find_task_id(
             index_path,
-            use_proxy=bool(os.environ.get('TASK_ID')))
+            use_proxy=bool(os.environ.get('TASKCLUSTER_PROXY')))
 
         return True, task_id
     except requests.exceptions.HTTPError:
         pass
 
     return False, None
 
 

--- a/taskcluster/taskgraph/transforms/task.py
+++ b/taskcluster/taskgraph/transforms/task.py
@@ -538,16 +538,19 @@ def build_docker_worker_payload(config, 
         task_def['scopes'].append(
             'assume:project:taskcluster:level-{level}-sccache-buckets'.format(
                 level=config.params['level'])
         )
         worker['env']['USE_SCCACHE'] = '1'
     else:
         worker['env']['SCCACHE_DISABLE'] = '1'
 
+    if features.get('taskclusterProxy'):
+        worker['env']['TASKCLUSTER_PROXY'] = '1'
+
     capabilities = {}
 
     for lo in 'audio', 'video':
         if worker.get('loopback-' + lo):
             capitalized = 'loopback' + lo.capitalize()
             devices = capabilities.setdefault('devices', {})
             devices[capitalized] = True
             task_def['scopes'].append('docker-worker:capability:device:' + capitalized)