diff --git a/redmine_reporter/cli.py b/redmine_reporter/cli.py
index a018e7d..e3ee63b 100644
--- a/redmine_reporter/cli.py
+++ b/redmine_reporter/cli.py
@@ -17,6 +17,26 @@ from .mailer import send_report
 from .report_builder import build_grouped_report, calculate_summary
 from .yaml_config import ensure_config_dir, resolve_output_path, save_period_to_config
 
+# Маскируем API-ключ Redmine в тексте ошибок (#55):
+#  - query-параметр key=<значение> в URL (?key=... / &key=...);
+#  - заголовок X-Redmine-API-Key: <значение>.
+_SANITIZE_PATTERNS = [
+    (re.compile(r"([?&]key=)[^\s&\"')]+", re.IGNORECASE), r"\1***"),
+    (
+        re.compile(
+            r"(X-Redmine-API-Key[\"']?\s*[:=]\s*[\"']?)[^\s\"',}]+", re.IGNORECASE
+        ),
+        r"\1***",
+    ),
+]
+
+
+def _sanitize_error_text(text: str) -> str:
+    """Маскирует API-ключ Redmine в тексте ошибки перед выводом в stderr (#55)."""
+    for pattern, replacement in _SANITIZE_PATTERNS:
+        text = pattern.sub(replacement, text)
+    return text
+
 
 def parse_date_range(date_arg: str) -> tuple[str, str]:
     if "--" not in date_arg:
@@ -226,7 +246,7 @@ def _save_and_maybe_send(
             )
             print(f"📧 Report sent to {', '.join(email_config.to)}")
         except RedmineAPIError as e:
-            print(f"❌ {e.message}", file=sys.stderr)
+            print(f"❌ {_sanitize_error_text(e.message)}", file=sys.stderr)
             return 1
 
     return 0
@@ -392,12 +412,14 @@ def main(argv: Optional[List[str]] = None) -> int:
             dedup_before=_compute_dedup_cutoff(),
         )
     except RedmineAPIError as e:
-        print(f"❌ {e.message}", file=sys.stderr)
-        if args.debug and e.original is not None:
+        print(f"❌ {_sanitize_error_text(e.message)}", file=sys.stderr)
+        if (args.verbose or args.debug) and e.original is not None:
             logging.exception("Original Redmine API error")
         return 1
     except Exception as e:
-        print(f"❌ Unexpected error: {e}", file=sys.stderr)
+        print(f"❌ Unexpected error: {_sanitize_error_text(str(e))}", file=sys.stderr)
+        if args.verbose or args.debug:
+            logging.exception("Unexpected error")
         return 1
 
     if issue_hours is None:
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 95c6610..e2fdb2d 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1452,3 +1452,119 @@ def test_cli_dynamic_datetime_period_without_date(mock_fetch, tmp_path):
     # Следующий период после 2026-06-01T00:00:00--2026-06-30T23:59:59
     assert from_date == "2026-07-01T00:00:00"
     assert to_date == "2026-07-30T23:59:59"
+
+
+# ---------------------------------------------------------------------------
+# #55: sanitize Redmine exception output in CLI
+# ---------------------------------------------------------------------------
+
+
+class TestSanitizeErrorOutput:
+    """CLI не должен выводить API-ключ из текста исключений в stderr (#55)."""
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_api_key_in_url_is_masked(self, mock_fetch, capsys):
+        """key=SECRET в URL внутри сообщения исключения маскируется."""
+        from redmine_reporter.client import RedmineAPIError
+
+        mock_fetch.side_effect = RedmineAPIError(
+            "Network error while calling Redmine: "
+            "HTTPSConnectionPool(host='red.example.com', port=443): "
+            "Failed https://red.example.com/issues.json?key=SECRET123&limit=100"
+        )
+        code = main(["--date", "2026-01-01--2026-01-31"])
+        captured = capsys.readouterr()
+        assert code == 1
+        assert "SECRET123" not in captured.err
+        assert "key=***" in captured.err
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_api_key_header_is_masked(self, mock_fetch, capsys):
+        """Заголовок X-Redmine-API-Key в тексте исключения маскируется."""
+        from redmine_reporter.client import RedmineAPIError
+
+        mock_fetch.side_effect = RedmineAPIError(
+            "Request failed: headers {'X-Redmine-API-Key': 'SECRET123'}"
+        )
+        code = main(["--date", "2026-01-01--2026-01-31"])
+        captured = capsys.readouterr()
+        assert code == 1
+        assert "SECRET123" not in captured.err
+        assert "X-Redmine-API-Key" in captured.err
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_unexpected_exception_is_masked(self, mock_fetch, capsys):
+        """Общее исключение с key=SECRET в тексте тоже санитизируется."""
+        mock_fetch.side_effect = RuntimeError(
+            "boom while requesting https://red.example.com/t.json?key=SECRET123"
+        )
+        code = main(["--date", "2026-01-01--2026-01-31"])
+        captured = capsys.readouterr()
+        assert code == 1
+        assert "SECRET123" not in captured.err
+        assert "Unexpected error" in captured.err
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_error_message_readability_preserved(self, mock_fetch, capsys):
+        """Санитизация не ломает обычные сообщения об ошибках."""
+        from redmine_reporter.client import RedmineAPIError
+
+        mock_fetch.side_effect = RedmineAPIError("Authentication failed: bad key")
+        code = main(["--date", "2026-01-01--2026-01-31"])
+        captured = capsys.readouterr()
+        assert code == 1
+        assert "Authentication failed: bad key" in captured.err
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_debug_shows_full_traceback(self, mock_fetch, caplog):
+        """--debug выводит полный стектрейс исходного исключения."""
+        import logging as _logging
+
+        from redmine_reporter.client import RedmineAPIError
+
+        original = ValueError("raw details https://red.example.com?key=SECRET123")
+        error = RedmineAPIError("safe message", original=original)
+        error.__cause__ = original
+        mock_fetch.side_effect = error
+        with caplog.at_level(_logging.ERROR):
+            code = main(["--date", "2026-01-01--2026-01-31", "--debug"])
+        assert code == 1
+        assert "raw details" in caplog.text
+        assert "Traceback" in caplog.text
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_verbose_shows_full_traceback(self, mock_fetch, caplog):
+        """--verbose тоже даёт доступ к деталям/стектрейсу исходной ошибки."""
+        import logging as _logging
+
+        from redmine_reporter.client import RedmineAPIError
+
+        original = ValueError("raw details https://red.example.com?key=SECRET123")
+        error = RedmineAPIError("safe message", original=original)
+        error.__cause__ = original
+        mock_fetch.side_effect = error
+        with caplog.at_level(_logging.INFO):
+            code = main(["--date", "2026-01-01--2026-01-31", "--verbose"])
+        assert code == 1
+        assert "raw details" in caplog.text
+
+    @mock.patch.dict(os.environ, VALID_ENV, clear=True)
+    @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+    def test_no_flags_hides_original_details(self, mock_fetch, caplog, capsys):
+        """Без флагов детали исходного исключения не печатаются."""
+        from redmine_reporter.client import RedmineAPIError
+
+        original = ValueError("raw details https://red.example.com?key=SECRET123")
+        mock_fetch.side_effect = RedmineAPIError("safe message", original=original)
+        code = main(["--date", "2026-01-01--2026-01-31"])
+        captured = capsys.readouterr()
+        assert code == 1
+        assert "raw details" not in captured.err
+        assert "raw details" not in caplog.text
+        assert "safe message" in captured.err