diff --git a/cloudsmith_cli/cli/commands/push.py b/cloudsmith_cli/cli/commands/push.py
index 1dbe1b30..96059807 100644
--- a/cloudsmith_cli/cli/commands/push.py
+++ b/cloudsmith_cli/cli/commands/push.py
@@ -2,6 +2,7 @@
 
 # pylint: disable=too-many-lines
 
+import json
 import math
 import os
 import shlex
@@ -694,7 +695,7 @@ def display_status(current):
             # When using stderr for logs, avoid an interactive progress bar and just poll for status.
             while True:
                 res = get_package_status(owner, repo, slug)
-                ok, failed, _, _, _, _ = res
+                ok, failed, _, status_str, stage_str, reason = res
                 if ok or failed:
                     break
 
@@ -811,6 +812,34 @@ def display_status(current):
             attempts=attempts,
         )
     else:
+        if use_stderr:
+            # In JSON output mode the human-readable text above went to
+            # stderr, so stdout still needs a machine-readable envelope.
+            # Mirror the shape produced by ``handle_api_exceptions`` so a
+            # sync failure is parseable the same way as an API failure.
+            error_data = {
+                "detail": reason or "Package failed to synchronise.",
+                "help": {
+                    "context": context_msg,
+                    "hint": None,
+                },
+                "meta": {
+                    "status": status_str,
+                    "stage": stage_str or "Unknown",
+                    "seconds": seconds,
+                },
+            }
+
+            metadata_context = getattr(opts, "push_metadata_info", None)
+            if metadata_context is not None:
+                error_data["metadata_attachment"] = metadata_context
+
+            click.echo(
+                json.dumps(
+                    error_data, indent=4 if opts.output == "pretty_json" else None
+                )
+            )
+
         ctx.exit(1)
 
 
diff --git a/cloudsmith_cli/cli/tests/test_push.py b/cloudsmith_cli/cli/tests/test_push.py
index 0c6670df..1b6c3fe6 100644
--- a/cloudsmith_cli/cli/tests/test_push.py
+++ b/cloudsmith_cli/cli/tests/test_push.py
@@ -16,6 +16,7 @@
     resolve_push_metadata_options,
     upload_files_and_create_package,
     validate_metadata_payload,
+    wait_for_package_sync,
 )
 from ..metadata_common import ResolvedMetadata
 
@@ -1466,3 +1467,54 @@ def test_options_metadata_failure_mode_none_is_noop():
     opts = Options()
     opts.metadata_failure_mode = None
     assert opts.metadata_failure_mode is None
+
+
+def test_wait_for_package_sync_json_mode_prints_status_reason_on_failure(capsys):
+    """Failed sync under JSON output must print the server's status_reason.
+
+    The JSON poll path used to discard status_reason, then crash with
+    UnboundLocalError at ``if reason:`` instead of showing the message.
+    """
+    ctx = click.Context(click.Command("push"))
+    opts = SimpleNamespace(output="json")
+    status_reason = (
+        "A package with name 'eng-13978-repro' already exists. "
+        "This package should be deleted."
+    )
+    failed_status = (
+        False,
+        True,
+        100,
+        "Failed",
+        "Parsing Package Metadata",
+        status_reason,
+    )
+
+    with patch(
+        "cloudsmith_cli.cli.commands.push.get_package_status",
+        return_value=failed_status,
+    ):
+        with pytest.raises(click.exceptions.Exit) as exc_info:
+            wait_for_package_sync(
+                ctx=ctx,
+                opts=opts,
+                owner="bart-demo-org-terraform",
+                repo="eng-13978-cli-repro",
+                slug="eng-13978-repro-100-alpha4tgz",
+                wait_interval=1.0,
+                skip_errors=False,
+                attempts=1,
+            )
+
+    assert exc_info.value.exit_code == 1
+    captured = capsys.readouterr()
+    assert "Package failed to synchronise" in captured.err
+    assert "Parsing Package Metadata" in captured.err
+    assert "Reason given:" in captured.err
+    assert status_reason in captured.err
+
+    # JSON mode must still leave a machine-readable envelope on stdout.
+    envelope = json.loads(captured.out)
+    assert envelope["detail"] == status_reason
+    assert envelope["meta"]["stage"] == "Parsing Package Metadata"
+    assert envelope["meta"]["status"] == "Failed"