diff --git a/src/cmd-buildprep b/src/cmd-buildprep
index c8b1c48f68..9f4335bf53 100755
--- a/src/cmd-buildprep
+++ b/src/cmd-buildprep
@@ -16,7 +16,7 @@ from tenacity import retry, retry_if_exception_type
 
 sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
 
-from cosalib.builds import Builds
+from cosalib.builds import Builds, BUILDFILES
 from cosalib.cmdlib import load_json, rm_allow_noent, retry_stop, retry_s3_exception, retry_callback  # noqa: E402
 
 retry_requests_exception = (retry_if_exception_type(requests.Timeout) |
@@ -39,7 +39,22 @@ def main():
 
     builds = None
     if fetcher.exists('builds.json'):
-        fetcher.fetch_json('builds.json')
+        if not args.refresh:
+            if os.path.isfile(BUILDFILES['sourcedata']):
+                # If we have local builds, don't overwrite that by default.
+                if subprocess.call(['cmp', BUILDFILES['sourcedata'], BUILDFILES['list']],
+                                   stdout=subprocess.DEVNULL,
+                                   stderr=subprocess.DEVNULL) != 0:
+                    raise SystemExit(f"{BUILDFILES['list']} modified locally, run with --refresh")
+            fetcher.fetch_json('builds.json')
+        else:
+            fetcher.fetch('builds.json', dest=BUILDFILES['sourcedata'])
+            print(f"Updated {BUILDFILES['sourcedata']}")
+            return
+        # Record the origin and original state
+        with open(BUILDFILES['sourceurl'], 'w') as f:
+            f.write(args.url + '\n')
+        subprocess.check_call(['cp', '-pf', '--reflink=auto', BUILDFILES['list'], BUILDFILES['sourcedata']])
         builds = Builds()
 
     if not builds or builds.is_empty():
@@ -87,6 +102,8 @@ def parse_args():
                         help="URL from which to fetch metadata")
     parser.add_argument("--ostree", action='store_true',
                         help="Also download full OSTree commit")
+    parser.add_argument("--refresh", action='store_true',
+                        help="Assuming local changes, only update {BUILDFILES['sourcedata']}")
     return parser.parse_args()
 
 
diff --git a/src/cmd-buildupload b/src/cmd-buildupload
index 34a6c242ff..9a31302499 100755
--- a/src/cmd-buildupload
+++ b/src/cmd-buildupload
@@ -8,6 +8,7 @@ import json
 import os
 import sys
 import tempfile
+import subprocess
 import boto3
 from botocore.exceptions import ClientError
 from tenacity import retry
@@ -20,7 +21,7 @@ CACHE_MAX_AGE_ARTIFACT = 60 * 60 * 24 * 365
 
 # set metadata caching to 5m
 CACHE_MAX_AGE_METADATA = 60 * 5
-from cosalib.builds import Builds
+from cosalib.builds import Builds, BUILDFILES
 from cosalib.cmdlib import load_json, retry_stop, retry_s3_exception, retry_callback  # noqa: E402
 
 
@@ -56,6 +57,9 @@ def parse_args():
 def cmd_upload_s3(args):
     bucket, prefix = args.url.split('/', 1)
     builds = Builds()
+    # This can't be an error for backcompat reasons, but let's print something
+    if not os.path.isfile(BUILDFILES['sourceurl']):
+        print(f"NOTICE: No {BUILDFILES['sourceurl']} file; uploading without buildprep?")
     if args.build == 'latest':
         args.build = builds.get_latest()
     print(f"Targeting build: {args.build}")
@@ -72,8 +76,12 @@ def cmd_upload_s3(args):
         s3_copy(f'builds/{args.build}/{f}', bucket, f'{prefix}/{args.build}/{f}',
                 CACHE_MAX_AGE_METADATA, args.acl)
     if not args.skip_builds_json:
-        s3_copy('builds/builds.json', bucket, f'{prefix}/builds.json',
+        s3_copy(BUILDFILES['list'], bucket, f'{prefix}/builds.json',
                 CACHE_MAX_AGE_METADATA, args.acl, extra_args={}, dry_run=args.dry_run)
+        # And now update our cached copy to note we've successfully sync'd.
+        with open(BUILDFILES['sourceurl'], 'w') as f:
+            f.write(f"s3://{bucket}/{prefix}\n")
+        subprocess.check_call(['cp', '-pf', '--reflink=auto', BUILDFILES['list'], BUILDFILES['sourcedata']])
 
 
 def s3_upload_build(args, builddir, bucket, prefix):
diff --git a/src/cosalib/builds.py b/src/cosalib/builds.py
index ca98b296ff..e6c1be7c69 100644
--- a/src/cosalib/builds.py
+++ b/src/cosalib/builds.py
@@ -16,11 +16,20 @@
     load_json,
     write_json)
 
+BUILDFILES = {
+    # The list of builds.
+    'list': 'builds/builds.json',
+    # This copy of builds.json tracks what we last downloaded from the source
+    'sourcedata': 'tmp/builds-source.json',
+    # This tracks the URL passed to buildprep
+    'sourceurl': 'tmp/builds-source.txt',
+}
+
 
 class Builds:  # pragma: nocover
     def __init__(self, workdir=None):
         self._workdir = workdir
-        self._fn = self._path("builds/builds.json")
+        self._fn = self._path(BUILDFILES['list'])
         if not os.path.isdir(self._path("builds")):
             raise Exception("No builds/ dir found!")
         elif os.path.isfile(self._fn):