diff --git a/README.md b/README.md
index 91baa27..54fa158 100644
--- a/README.md
+++ b/README.md
@@ -104,7 +104,7 @@ From a clone, for development:
 
 ```bash
 pip install -e ".[dev]"
-pytest -q          # 17 tests
+pytest -q          # 22 tests
 ```
 
 No session of your own to look at yet? A synthetic one ships with the repo:
@@ -132,4 +132,4 @@ as such rather than vanish from the report.
 
 ## Status
 
-Working, 17 tests, validated against a real 34MB session with 152 subagent runs.
+Working, 22 tests, validated against a real 34MB session with 152 subagent runs.
diff --git a/agentrace/cli.py b/agentrace/cli.py
index 7890bcb..da59a72 100644
--- a/agentrace/cli.py
+++ b/agentrace/cli.py
@@ -14,6 +14,7 @@
 from pathlib import Path
 
 from rich.console import Console
+from rich.markup import escape
 from rich.table import Table
 
 from .checks import analyse
@@ -31,7 +32,11 @@ def _load(args) -> list[AgentRun]:
         sessions = parse_all(Path(args.dir) if args.dir else None)
     runs = [r for s in sessions for r in s.runs]
     if not runs:
-        console.print("[yellow]No subagent runs found.[/] Looked in ~/.claude/projects unless --dir was given.")
+        source = args.file or args.dir or "~/.claude/projects"
+        console.print(
+            f"[yellow]No subagent runs found.[/] Looked in {escape(source)}.",
+            soft_wrap=True,
+        )
     return runs
 
 
@@ -155,10 +160,17 @@ def cmd_stats(args) -> int:
     return 0
 
 
+def _existing_file(value: str) -> str:
+    if not Path(value).is_file():
+        raise argparse.ArgumentTypeError(f"not a file: {value}")
+    return value
+
+
 def main(argv: list[str] | None = None) -> int:
     p = argparse.ArgumentParser(prog="agentrace", description=__doc__.split("\n")[0])
-    p.add_argument("--dir", help="transcript root (default ~/.claude/projects)")
-    p.add_argument("--file", help="a single .jsonl transcript")
+    source = p.add_mutually_exclusive_group()
+    source.add_argument("--dir", help="transcript root (default ~/.claude/projects)")
+    source.add_argument("--file", type=_existing_file, help="a single .jsonl transcript")
     sub = p.add_subparsers(dest="cmd", required=True)
 
     sub.add_parser("list", help="list subagent runs").set_defaults(func=cmd_list)
diff --git a/tests/test_cli.py b/tests/test_cli.py
new file mode 100644
index 0000000..ecaf0fc
--- /dev/null
+++ b/tests/test_cli.py
@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+import subprocess
+import sys
+
+
+def run_cli(*args: str) -> subprocess.CompletedProcess[str]:
+    return subprocess.run(
+        [sys.executable, "-m", "agentrace.cli", *args],
+        capture_output=True,
+        check=False,
+        text=True,
+    )
+
+
+def test_file_and_dir_are_mutually_exclusive(tmp_path):
+    transcript = tmp_path / "session.jsonl"
+    transcript.write_text("")
+
+    result = run_cli("--dir", str(tmp_path), "--file", str(transcript), "list")
+
+    assert result.returncode == 2
+    assert "argument --file: not allowed with argument --dir" in result.stderr
+    assert result.stdout == ""
+
+
+def test_missing_file_is_a_clean_parser_error(tmp_path):
+    missing = tmp_path / "missing.jsonl"
+
+    result = run_cli("--file", str(missing), "list")
+
+    assert result.returncode == 2
+    assert f"not a file: {missing}" in result.stderr
+    assert "Traceback" not in result.stderr
+    assert result.stdout == ""
+
+
+def test_empty_result_names_selected_file(tmp_path):
+    transcript = tmp_path / "empty.jsonl"
+    transcript.write_text("")
+
+    result = run_cli("--file", str(transcript), "list")
+
+    assert result.returncode == 0
+    assert "No subagent runs found." in result.stdout
+    assert f"Looked in {transcript}." in result.stdout
+
+
+def test_empty_result_names_selected_directory(tmp_path):
+    result = run_cli("--dir", str(tmp_path), "list")
+
+    assert result.returncode == 0
+    assert "No subagent runs found." in result.stdout
+    assert f"Looked in {tmp_path}." in result.stdout