diff --git a/artifact.metrics_analysis/src/artifact_metrics_analysis/cli.py b/artifact.metrics_analysis/src/artifact_metrics_analysis/cli.py
new file mode 100644
index 0000000..3fee350
--- /dev/null
+++ b/artifact.metrics_analysis/src/artifact_metrics_analysis/cli.py
@@ -0,0 +1,75 @@
+import argparse
+import json
+from pathlib import Path
+import logging
+from typing import Any
+from artifact_metrics_analysis.core import analyze_metrics
+
+
+def main() -> None:
+    """Command line interface for comparing Evidence Card metrics."""
+    parser = argparse.ArgumentParser(
+        description=(
+            "Analyze and compare metrics between Evidence Card #40 and #42, "
+            "producing a JSON summary of retry_tail_p99, band_width, and delta_band_width."
+        )
+    )
+    parser.add_argument(
+        "--card40", required=True, type=str,
+        help="Path to JSON metrics file for Evidence Card #40"
+    )
+    parser.add_argument(
+        "--card42", required=True, type=str,
+        help="Path to JSON metrics file for Evidence Card #42"
+    )
+    parser.add_argument(
+        "--output", required=False, type=str,
+        help="Path for the output JSON file (metrics comparison results)"
+    )
+    args = parser.parse_args()
+
+    logging.basicConfig(
+        level=logging.INFO,
+        format="[%(levelname)s] %(message)s"
+    )
+
+    try:
+        path_40 = Path(args.card40)
+        path_42 = Path(args.card42)
+
+        if not path_40.exists() or not path_42.exists():
+            raise FileNotFoundError("One or both input JSON files do not exist.")
+
+        with path_40.open('r', encoding='utf-8') as f40:
+            card_data_40: dict[str, Any] = json.load(f40)
+        with path_42.open('r', encoding='utf-8') as f42:
+            card_data_42: dict[str, Any] = json.load(f42)
+
+        # Input validation
+        assert isinstance(card_data_40, dict), "card_data_40 must be a dict"
+        assert isinstance(card_data_42, dict), "card_data_42 must be a dict"
+
+        result = analyze_metrics(card_data_40, card_data_42)
+        result_dict = result.to_dict() if hasattr(result, 'to_dict') else dict(result)
+
+        if args.output:
+            out_path = Path(args.output)
+            out_path.parent.mkdir(parents=True, exist_ok=True)
+            with out_path.open('w', encoding='utf-8') as outf:
+                json.dump(result_dict, outf, indent=2)
+            logging.info(f"Metrics comparison results written to {out_path}")
+        else:
+            print(json.dumps(result_dict, indent=2))
+
+    except AssertionError as ae:
+        logging.error(f"Input validation failed: {ae}")
+    except FileNotFoundError as fe:
+        logging.error(f"File not found: {fe}")
+    except json.JSONDecodeError as je:
+        logging.error(f"Invalid JSON input: {je}")
+    except Exception as exc:
+        logging.error(f"Unexpected error: {exc}")
+
+
+if __name__ == "__main__":
+    main()
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