--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -1877,16 +1877,21 @@ class PackageFrontend(MachCommandBase):
                                         record.digest)
                 records[record.filename] = DownloadRecord(
                     url, record.filename, record.size, record.digest,
                     record.algorithm, unpack=record.unpack,
                     version=record.version, visibility=record.visibility,
                     setup=record.setup)
 
         if from_build:
+            if 'TASK_ID' in os.environ:
+                self.log(logging.ERROR, 'artifact', {},
+                         'Do not use --from-build in automation; all dependencies '
+                         'should be determined in the decision task.')
+                return 1
             from taskgraph.optimize import IndexSearch
             params = {
                 'message': '',
                 'project': '',
                 'level': os.environ.get('MOZ_SCM_LEVEL', '3'),
                 'base_repository': '',
                 'head_repository': '',
                 'head_rev': '',