diff --git a/scripts/profile_sync.py b/scripts/profile_sync.py
index 818e7bb54..a61f5b05f 100644
--- a/scripts/profile_sync.py
+++ b/scripts/profile_sync.py
@@ -4158,9 +4158,10 @@ def build_parser() -> argparse.ArgumentParser:
         help="the profiles the branch adds against --base, as CI checks them",
     )
     parser.add_argument(
-        "--base", default="origin/main",
+        "--base",
         help="revision --changed and --added compare with, through its merge "
-        "base with HEAD (default: %(default)s)",
+        "base with HEAD (default: the project's main line, upstream/main or "
+        "origin/main when that remote names the project, fetched otherwise)",
     )
     parser.add_argument(
         "--emulators-dir", default="emulators",
@@ -4327,14 +4328,33 @@ def branch_profiles(emulators_dir: str, base: str, added_only: bool) -> list[str
     })
 
 
+def project_base() -> str:
+    """The project's main line, the base --changed compares with by default.
+
+    In a fork's clone origin/main is the fork: once the branch is pushed it
+    is HEAD, and --changed --strict found nothing and passed while the PR's
+    CI then failed. validate_pr.py resolves the same line the same way.
+    """
+    from validate_pr import _git_lines as git_lines, _project_main_line
+
+    line = _project_main_line()
+    if line:
+        return line
+    for remote in ("upstream", "origin"):
+        if git_lines("rev-parse", "--verify", "--quiet", f"{remote}/main"):
+            return f"{remote}/main"
+    return "HEAD"
+
+
 def select_profiles(profiles: dict, args: argparse.Namespace) -> dict:
     """Profiles matching the selection flags."""
     if args.changed or args.added:
-        names = branch_profiles(args.emulators_dir, args.base, args.added)
+        base = args.base or project_base()
+        names = branch_profiles(args.emulators_dir, base, args.added)
         selected = {name: profiles[name] for name in names if name in profiles}
         if not selected:
             verb = "adds" if args.added else "adds or edits"
-            print(f"the branch {verb} no profile against {args.base}", file=sys.stderr)
+            print(f"the branch {verb} no profile against {base}", file=sys.stderr)
         return selected
     if args.emulator:
         names = [args.emulator] if isinstance(args.emulator, str) else args.emulator
diff --git a/tests/test_profile_sync.py b/tests/test_profile_sync.py
index 843d85fe4..8bb0e78db 100644
--- a/tests/test_profile_sync.py
+++ b/tests/test_profile_sync.py
@@ -5896,6 +5896,24 @@ class TestBranchProfiles(unittest.TestCase):
             ["draft", "edited", "new"],
         )
 
+    def test_a_fork_clone_compares_with_the_project(self):
+        """origin is the fork, pushed: origin/main is HEAD, and the default
+        base found nothing to check while the PR's CI failed."""
+        base = subprocess.run(
+            ["git", "rev-parse", "main"], cwd=self.root, capture_output=True,
+            text=True, check=True,
+        ).stdout.strip()
+        self._git("remote", "add", "origin", "https://github.com/someone/retrobios")
+        self._git("remote", "add", "upstream", "https://github.com/Abdess/retrobios")
+        self._git("update-ref", "refs/remotes/origin/main", "HEAD")
+        self._git("update-ref", "refs/remotes/upstream/main", base)
+        args = profile_sync.build_parser().parse_args(["--changed"])
+        profiles = {"kept": {}, "edited": {}, "new": {}, "draft": {}}
+        self.assertEqual(
+            list(profile_sync.select_profiles(profiles, args)),
+            ["draft", "edited", "new"],
+        )
+
 
 class TestBranchCheckIsWired(unittest.TestCase):
     """CONTRIBUTING and CI name the branch's profiles through the flag."""
diff --git a/wiki/tools.md b/wiki/tools.md
index 1aa6aef87..214920b78 100644
--- a/wiki/tools.md
+++ b/wiki/tools.md
@@ -447,8 +447,10 @@ to show one cited file and `--context N` lines around each hunk. `--strict`
 exits 1 when a profile has refs to review, and CI runs it on every profile a change
 adds. `--offline` answers from `--cache-dir` alone. `--changed` selects the
 profiles the branch adds or edits, uncommitted ones included, and `--added`
-the ones it adds, both against the merge base of `--base` (`origin/main` by
-default) and HEAD.
+the ones it adds, both against the merge base of `--base` and HEAD. By
+default `--base` is the project's main line: `upstream/main` or `origin/main`
+when that remote is the project, fetched from the project in a fork's clone,
+where `origin/main` is the fork.
 
 ```bash
 python scripts/profile_sync.py --all --system sega-saturn --changed-only