diff --git a/src/multi_swarm/protocol/validator.py b/src/multi_swarm/protocol/validator.py
index 6802d13..0e7ade7 100644
--- a/src/multi_swarm/protocol/validator.py
+++ b/src/multi_swarm/protocol/validator.py
@@ -6,6 +6,17 @@ from .parser import Node, Strategy
 KNOWN_INDICATORS: frozenset[str] = frozenset({"sma", "rsi", "atr", "macd", "realized_vol"})
 KNOWN_FEATURES: frozenset[str] = frozenset({"open", "high", "low", "close", "volume"})
 
+# Numero di parametri numerici accettati dopo il nome dell'indicatore.
+# La tupla (min, max) include solo i numeri (gli argomenti di tipo Node sono
+# proibiti dal compiler - gli indicatori non sono annidabili in Phase 1).
+INDICATOR_ARITY: dict[str, tuple[int, int]] = {
+    "sma": (1, 1),           # length
+    "rsi": (1, 1),           # length
+    "atr": (1, 1),           # length
+    "realized_vol": (1, 1),  # window
+    "macd": (0, 3),          # fast, slow, signal (tutti opzionali con default)
+}
+
 
 class ValidationError(Exception):
     """Raised when an AST violates Phase 1 protocol semantics."""
@@ -55,12 +66,25 @@ def _validate_node(node: Node, _expect_bool: bool) -> None:
         return
 
     if node.kind == "indicator":
-        if len(node.args) < 2:
-            raise ValidationError("'indicator' needs >=2 args (name, length)")
+        if len(node.args) < 1:
+            raise ValidationError("'indicator' needs >=1 args (name [, params...])")
         name_node = node.args[0]
         ind_name = name_node.kind if isinstance(name_node, Node) else str(name_node)
         if ind_name not in KNOWN_INDICATORS:
             raise ValidationError(f"unknown indicator: {ind_name}")
+        # Gli indicatori non accettano Node come params (no-nesting in Phase 1).
+        for a in node.args[1:]:
+            if isinstance(a, Node):
+                raise ValidationError(
+                    f"indicator '{ind_name}' does not accept nested expressions; "
+                    f"only numeric literals (got node {a.kind})"
+                )
+        n_params = len(node.args) - 1
+        min_p, max_p = INDICATOR_ARITY[ind_name]
+        if not (min_p <= n_params <= max_p):
+            raise ValidationError(
+                f"indicator '{ind_name}' arity {n_params} out of [{min_p},{max_p}]"
+            )
         return
 
     if node.kind == "feature":