fix(cli): a bad input path is a message, not a traceback

postflight/__main__.py

@@ -42,7 +42,18 @@ def _load(args: argparse.Namespace) -> list[Turn]:
     if args.otel:
         from .adapters.otel import turns_from_jsonl
 
-        return turns_from_jsonl(args.otel)
+        # A typo'd path and a file that is not JSONL are both ordinary mistakes, and a
+        # traceback answers neither of them. SystemExit prints the message and sets a
+        # non-zero status without pretending the tool crashed.
+        try:
+            return turns_from_jsonl(args.otel)
+        except OSError as exc:
+            raise SystemExit(f"cannot read {args.otel}: {exc.strerror}") from exc
+        except json.JSONDecodeError as exc:
+            raise SystemExit(
+                f"{args.otel} is not newline-delimited JSON: {exc} "
+                "(expected one exported span object per line)"
+            ) from exc
 
     from .adapters.langfuse import LangfuseAdapter, LangfuseClient
 

tests/test_cli.py

@@ -59,3 +59,17 @@ def test_langfuse_without_credentials_fails_loudly(monkeypatch):
 def test_a_source_is_required():
     with pytest.raises(SystemExit):
         main([])
+
+
+def test_a_missing_file_is_a_message_not_a_traceback():
+    """A typo'd path is an ordinary mistake. Answering it with a stack trace tells the
+    user the tool crashed, which is both unhelpful and untrue."""
+    with pytest.raises(SystemExit, match="cannot read"):
+        main(["--otel", "/nonexistent/spans.jsonl"])
+
+
+def test_a_file_that_is_not_jsonl_says_so(tmp_path):
+    bad = tmp_path / "notes.txt"
+    bad.write_text("these are my notes, not spans\n")
+    with pytest.raises(SystemExit, match="not newline-delimited JSON"):
+        main(["--otel", str(bad)])