diff --git a/scripts/check_deliberate_break.py b/scripts/check_deliberate_break.py
index 6c1e1bca1..6d51f9224 100644
--- a/scripts/check_deliberate_break.py
+++ b/scripts/check_deliberate_break.py
@@ -68,6 +68,13 @@ def _json_result(verdict: str, **fields: object) -> dict[str, object]:
     return {"verdict": verdict, **fields}
 
 
+def _subprocess_output_text(value: str | bytes | None) -> str | None:
+    """Normalize captured subprocess output for JSON result payloads."""
+    if isinstance(value, bytes):
+        return value.decode(errors="replace")
+    return value
+
+
 def _write_github_output(**fields: str) -> None:
     output_path = os.environ.get("GITHUB_OUTPUT")
     if not output_path:
@@ -823,8 +830,8 @@ def verify_spec(
             reason="archive-command-failed",
             command=list(exc.cmd) if isinstance(exc.cmd, (tuple, list)) else str(exc.cmd),
             returncode=exc.returncode,
-            stdout=exc.stdout,
-            stderr=exc.stderr,
+            stdout=_subprocess_output_text(exc.stdout),
+            stderr=_subprocess_output_text(exc.stderr),
         )
     except OSError as exc:
         return _json_result(
diff --git a/templates/consumer-repo/scripts/check_deliberate_break.py b/templates/consumer-repo/scripts/check_deliberate_break.py
index 6c1e1bca1..6d51f9224 100644
--- a/templates/consumer-repo/scripts/check_deliberate_break.py
+++ b/templates/consumer-repo/scripts/check_deliberate_break.py
@@ -68,6 +68,13 @@ def _json_result(verdict: str, **fields: object) -> dict[str, object]:
     return {"verdict": verdict, **fields}
 
 
+def _subprocess_output_text(value: str | bytes | None) -> str | None:
+    """Normalize captured subprocess output for JSON result payloads."""
+    if isinstance(value, bytes):
+        return value.decode(errors="replace")
+    return value
+
+
 def _write_github_output(**fields: str) -> None:
     output_path = os.environ.get("GITHUB_OUTPUT")
     if not output_path:
@@ -823,8 +830,8 @@ def verify_spec(
             reason="archive-command-failed",
             command=list(exc.cmd) if isinstance(exc.cmd, (tuple, list)) else str(exc.cmd),
             returncode=exc.returncode,
-            stdout=exc.stdout,
-            stderr=exc.stderr,
+            stdout=_subprocess_output_text(exc.stdout),
+            stderr=_subprocess_output_text(exc.stderr),
         )
     except OSError as exc:
         return _json_result(
diff --git a/tests/scripts/test_check_deliberate_break.py b/tests/scripts/test_check_deliberate_break.py
index 13fb35e92..829c92632 100644
--- a/tests/scripts/test_check_deliberate_break.py
+++ b/tests/scripts/test_check_deliberate_break.py
@@ -1,3 +1,4 @@
+import json
 import os
 import subprocess
 import sys
@@ -1286,8 +1287,8 @@ def test_base_archive_command_failure_is_not_dependency_failure(tmp_path, monkey
     error = subprocess.CalledProcessError(
         17,
         ["git", "archive", base],
-        output="archive output",
-        stderr="bad ref",
+        output=b"archive output",
+        stderr=b"bad ref",
     )
     monkeypatch.setattr(
         deliberate_break,
@@ -1305,6 +1306,7 @@ def test_base_archive_command_failure_is_not_dependency_failure(tmp_path, monkey
         "stdout": "archive output",
         "stderr": "bad ref",
     }
+    json.dumps(result)
 
 
 def test_base_setup_failure_is_not_dependency_failure(tmp_path, monkeypatch) -> None: