diff --git a/scripts/registryctl-build-output.py b/scripts/registryctl-build-output.py
index 1393ced..b150b68 100755
--- a/scripts/registryctl-build-output.py
+++ b/scripts/registryctl-build-output.py
@@ -56,12 +56,13 @@ def parse_build_output(
     if not isinstance(output_value, str) or not output_value:
         raise BuildReportError("the JSON report has no output root")
     output = Path(output_value)
-    if not output.is_absolute():
-        raise BuildReportError("the JSON report output root is not absolute")
-
     try:
         project_root = project_directory.resolve(strict=True)
-        output_root = output.resolve(strict=True)
+        output_root = (
+            output.resolve(strict=True)
+            if output.is_absolute()
+            else (project_root / output).resolve(strict=True)
+        )
         output_root.relative_to(project_root)
     except (FileNotFoundError, RuntimeError, ValueError) as error:
         raise BuildReportError(
diff --git a/scripts/test_registryctl_build_output.py b/scripts/test_registryctl_build_output.py
index 54a1834..fb14e67 100644
--- a/scripts/test_registryctl_build_output.py
+++ b/scripts/test_registryctl_build_output.py
@@ -59,6 +59,16 @@ def test_accepts_the_versioned_project_owned_build_root(self) -> None:
             self.output.resolve(),
         )
 
+    def test_accepts_the_portable_project_relative_build_root(self) -> None:
+        self.assertEqual(
+            MODULE.parse_build_output(
+                self.report(output=".registry-stack/build/local"),
+                project_directory=self.project,
+                environment="local",
+            ),
+            self.output.resolve(),
+        )
+
     def test_rejects_the_wrong_environment_without_echoing_report_values(self) -> None:
         with self.assertRaisesRegex(
             MODULE.BuildReportError,
@@ -84,6 +94,19 @@ def test_rejects_an_output_root_outside_the_project(self) -> None:
                 environment="local",
             )
 
+    def test_rejects_a_relative_output_root_that_escapes_the_project(self) -> None:
+        outside = Path(self.temporary.name) / "outside"
+        outside.mkdir()
+        with self.assertRaisesRegex(
+            MODULE.BuildReportError,
+            "not a real project-owned directory",
+        ):
+            MODULE.parse_build_output(
+                self.report(output="../outside"),
+                project_directory=self.project,
+                environment="local",
+            )
+
     def test_rejects_an_incomplete_product_closure(self) -> None:
         (self.output / "private" / "notary" / "config" / "notary.yaml").unlink()
         with self.assertRaisesRegex(