diff --git a/README.md b/README.md
index 183e8f1..ec6f3c0 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,10 @@ agentrace stats --json          # aggregate, machine-readable
 
 Point it somewhere else with `--dir` or at one file with `--file`.
 
+`show --max N` limits each prompt and result to at most `N` characters (default
+4000). `N` must be a non-negative integer; zero hides the text, while negative
+values are usage errors (exit code 2).
+
 ## Install
 
 **PyPI:** https://pypi.org/project/agentrace-cli/
diff --git a/agentrace/cli.py b/agentrace/cli.py
index eedb373..476759c 100644
--- a/agentrace/cli.py
+++ b/agentrace/cli.py
@@ -176,6 +176,13 @@ def cmd_stats(args) -> int:
     return 0
 
 
+def _nonnegative_int(value: str) -> int:
+    number = int(value)
+    if number < 0:
+        raise argparse.ArgumentTypeError("must be a non-negative integer")
+    return number
+
+
 def _existing_file(value: str) -> str:
     if not Path(value).is_file():
         raise argparse.ArgumentTypeError(f"not a file: {value}")
@@ -199,7 +206,12 @@ def main(argv: list[str] | None = None) -> int:
 
     s = sub.add_parser("show", help="read one run in full")
     s.add_argument("id", help="tool_use_id or its last 8 chars")
-    s.add_argument("--max", type=int, default=4000, help="truncate long text")
+    s.add_argument(
+        "--max",
+        type=_nonnegative_int,
+        default=4000,
+        help="maximum characters per prompt/result (non-negative; default: 4000)",
+    )
     s.set_defaults(func=cmd_show)
 
     st = sub.add_parser("stats", help="aggregate stats")
diff --git a/tests/test_show_limit.py b/tests/test_show_limit.py
new file mode 100644
index 0000000..6f0898c
--- /dev/null
+++ b/tests/test_show_limit.py
@@ -0,0 +1,34 @@
+import pytest
+
+from agentrace import cli
+
+
+@pytest.mark.parametrize("value", ["-1", "-5"])
+def test_negative_limit_rejected_before_loading(value, monkeypatch, capsys):
+    def unexpected_load(args):
+        pytest.fail("invalid --max must be rejected before loading transcripts")
+
+    monkeypatch.setattr(cli, "_load", unexpected_load)
+    with pytest.raises(SystemExit) as exc:
+        cli.main(["show", "abc", "--max", value])
+    assert exc.value.code == 2
+    captured = capsys.readouterr()
+    assert "--max" in captured.err
+    assert "non-negative" in captured.err
+    assert captured.out == ""
+
+
+@pytest.mark.parametrize(
+    "options, expected",
+    [([], 4000), (["--max", "0"], 0), (["--max", "1"], 1), (["--max", "100"], 100)],
+)
+def test_nonnegative_limit_and_default(options, expected, monkeypatch):
+    seen = []
+
+    def show(args):
+        seen.append(args.max)
+        return 0
+
+    monkeypatch.setattr(cli, "cmd_show", show)
+    assert cli.main(["show", "abc", *options]) == 0
+    assert seen == [expected]