fix: handle invalid receipt files in CLI

CHANGELOG.md

@@ -13,6 +13,11 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixed
+
+- `verify` and `inspect` now report unreadable or invalid receipt files as
+  one-line CLI errors instead of raising tracebacks.
+
 ## [0.1.0] - 2026-07-27
 
 ### Added

src/answerproof/cli.py

@@ -23,9 +23,17 @@
 from .verifier import verify_receipt
 
 
+class ReceiptLoadError(Exception):
+    """Raised when a receipt file cannot be read or validated."""
+
+
 def _load_receipt(path: str) -> Receipt:
-    text = Path(path).read_text(encoding="utf-8")
-    return Receipt.from_json(text)
+    try:
+        text = Path(path).read_text(encoding="utf-8")
+        return Receipt.from_json(text)
+    except (OSError, UnicodeError, ValueError) as exc:
+        detail = str(exc).splitlines()[0]
+        raise ReceiptLoadError(f"cannot load receipt {path!r}: {detail}") from exc
 
 
 def cmd_keygen(args: argparse.Namespace) -> int:
@@ -130,7 +138,11 @@ def build_parser() -> argparse.ArgumentParser:
 def main(argv: list[str] | None = None) -> int:
     parser = build_parser()
     args = parser.parse_args(argv)
-    return args.func(args)
+    try:
+        return args.func(args)
+    except ReceiptLoadError as exc:
+        print(f"error: {exc}", file=sys.stderr)
+        return 2
 
 
 if __name__ == "__main__":

tests/test_cli.py

@@ -62,6 +62,35 @@ def test_inspect(tmp_path, capsys, receipt):
     assert "merkle_root" in out
 
 
+@pytest.mark.parametrize("command", ["verify", "inspect"])
+def test_receipt_command_reports_missing_file(command, tmp_path, capsys):
+    missing = tmp_path / "missing.json"
+
+    rc = main([command, str(missing)])
+
+    captured = capsys.readouterr()
+    assert rc == 2
+    assert captured.out == ""
+    assert captured.err.startswith(f"error: cannot load receipt '{missing}':")
+    assert "Traceback" not in captured.err
+    assert len(captured.err.splitlines()) == 1
+
+
+@pytest.mark.parametrize("command", ["verify", "inspect"])
+def test_receipt_command_reports_invalid_json(command, tmp_path, capsys):
+    invalid = tmp_path / "invalid.json"
+    invalid.write_text("not json", encoding="utf-8")
+
+    rc = main([command, str(invalid)])
+
+    captured = capsys.readouterr()
+    assert rc == 2
+    assert captured.out == ""
+    assert captured.err.startswith(f"error: cannot load receipt '{invalid}':")
+    assert "Traceback" not in captured.err
+    assert len(captured.err.splitlines()) == 1
+
+
 def test_missing_command_errors():
     with pytest.raises(SystemExit):
         main([])