--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -1877,16 +1877,20 @@ 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.')
             toolchains = load_toolchain_definitions()
             for b in from_build:
                 user_value = b
 
                 if b.startswith('toolchain-'):
                     b = b.replace('toolchain-', '')
 
                 if b not in toolchains: