diff --git a/README.md b/README.md
index ee3313f..cfcf006 100644
--- a/README.md
+++ b/README.md
@@ -92,7 +92,7 @@ usage: pf1e-sim [-h] --map MAP [--side NAME [FILE ...]] [--runs RUNS]
 | `--side NAME [FILE ...]` | Camp `NAME` avec un ou plusieurs JSON de monstres ; répétable pour chaque camp | — |
 | `--runs RUNS` | Nombre de combats simulés | `1000` |
 | `--seed SEED` | Graine du générateur aléatoire | `1` |
-| `--round-cap ROUND_CAP` | Plafond de rounds par combat (au-delà, combat nul) | `100` |
+| `--round-cap ROUND_CAP` | Plafond de rounds par combat, entier ≥ 1 (au-delà, combat nul) | `100` |
 | `--transcript` | Imprimer le déroulé du premier combat **en tableau** avant le rapport (voir ci-dessous) | désactivé |
 | `--simple-monster-death` | Mort simplifiée des monstres : `hp ≤ 0` au lieu de `hp ≤ -CON` (les PJ ne sont jamais affectés) | désactivé |
 
diff --git a/src/pf1e_simulator/cli.py b/src/pf1e_simulator/cli.py
index c22039b..e9d29fc 100644
--- a/src/pf1e_simulator/cli.py
+++ b/src/pf1e_simulator/cli.py
@@ -105,6 +105,9 @@ def render_transcript_table(transcript: Sequence[str]) -> str:
 
 
 def _build_spec(args: argparse.Namespace) -> EncounterSpec:
+    if args.round_cap < 1:
+        msg = f"--round-cap must be >= 1, got {args.round_cap}"
+        raise CliError(msg)
     map_spec = load_map(args.map)
     provided = [tuple(raw) for raw in args.side]
     provided_names = {raw[0] for raw in provided}
@@ -189,7 +192,7 @@ def main(argv: list[str] | None = None) -> int:
         "--round-cap",
         type=int,
         default=100,
-        help="max rounds per battle (default: 100)",
+        help="max rounds per battle, >= 1 (default: 100)",
     )
     parser.add_argument(
         "--transcript",
diff --git a/tests/test_cli.py b/tests/test_cli.py
index ac4eaac..843e005 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -84,6 +84,48 @@ def test_cli_rejects_duplicate_side_names(capsys: pytest.CaptureFixture[str]) ->
     assert "players" in err
 
 
+def test_cli_rejects_negative_round_cap(capsys: pytest.CaptureFixture[str]) -> None:
+    rc = main(
+        [
+            "--map",
+            str(MAP),
+            "--side",
+            "players",
+            str(GOBLIN),
+            "--side",
+            "monsters",
+            str(ORC),
+            "--runs",
+            "10",
+            "--round-cap",
+            "-3",
+        ]
+    )
+    assert rc == 2
+    assert "--round-cap must be >= 1" in capsys.readouterr().err
+
+
+def test_cli_rejects_zero_round_cap(capsys: pytest.CaptureFixture[str]) -> None:
+    rc = main(
+        [
+            "--map",
+            str(MAP),
+            "--side",
+            "players",
+            str(GOBLIN),
+            "--side",
+            "monsters",
+            str(ORC),
+            "--runs",
+            "10",
+            "--round-cap",
+            "0",
+        ]
+    )
+    assert rc == 2
+    assert "--round-cap must be >= 1" in capsys.readouterr().err
+
+
 def test_cli_transcript_prints_first_battle_log(capsys: pytest.CaptureFixture[str]) -> None:
     rc = main(
         [