diff --git a/src/multi_swarm/protocol/validator.py b/src/multi_swarm/protocol/validator.py
new file mode 100644
index 0000000..6802d13
--- /dev/null
+++ b/src/multi_swarm/protocol/validator.py
@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+from .grammar import COMPARATOR_VERBS, LOGICAL_VERBS
+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"})
+
+
+class ValidationError(Exception):
+    """Raised when an AST violates Phase 1 protocol semantics."""
+
+
+def validate_strategy(strategy: Strategy) -> None:
+    """Check semantic constraints on a parsed Strategy AST.
+
+    The parser already enforces verb-set membership; this pass adds:
+      * arity checks for logical/comparator/data verbs,
+      * known-indicator / known-feature whitelists.
+    """
+    for rule in strategy.rules:
+        _validate_node(rule.condition, _expect_bool=True)
+
+
+def _validate_node(node: Node, _expect_bool: bool) -> None:
+    if node.kind in LOGICAL_VERBS:
+        if node.kind == "not":
+            if len(node.args) != 1:
+                raise ValidationError(f"'not' needs 1 arg, got {len(node.args)}")
+            arg = node.args[0]
+            if isinstance(arg, Node):
+                _validate_node(arg, _expect_bool=True)
+        else:
+            if len(node.args) < 2:
+                raise ValidationError(f"'{node.kind}' needs >=2 args")
+            for a in node.args:
+                if isinstance(a, Node):
+                    _validate_node(a, _expect_bool=True)
+        return
+
+    if node.kind in COMPARATOR_VERBS:
+        if len(node.args) != 2:
+            raise ValidationError(f"'{node.kind}' needs 2 args, got {len(node.args)}")
+        for a in node.args:
+            if isinstance(a, Node):
+                _validate_node(a, _expect_bool=False)
+        return
+
+    if node.kind in {"crossover", "crossunder"}:
+        if len(node.args) != 2:
+            raise ValidationError(f"'{node.kind}' needs 2 args")
+        for a in node.args:
+            if isinstance(a, Node):
+                _validate_node(a, _expect_bool=False)
+        return
+
+    if node.kind == "indicator":
+        if len(node.args) < 2:
+            raise ValidationError("'indicator' needs >=2 args (name, length)")
+        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}")
+        return
+
+    if node.kind == "feature":
+        if len(node.args) != 1:
+            raise ValidationError("'feature' needs 1 arg")
+        feat_node = node.args[0]
+        feat_name = feat_node.kind if isinstance(feat_node, Node) else str(feat_node)
+        if feat_name not in KNOWN_FEATURES:
+            raise ValidationError(f"unknown feature: {feat_name}")
+        return
+
+    raise ValidationError(f"unexpected node kind in expression: {node.kind}")
diff --git a/tests/unit/test_protocol_validator.py b/tests/unit/test_protocol_validator.py
new file mode 100644
index 0000000..2aa6b4f
--- /dev/null
+++ b/tests/unit/test_protocol_validator.py
@@ -0,0 +1,38 @@
+import pytest
+
+from multi_swarm.protocol.parser import parse_strategy
+from multi_swarm.protocol.validator import ValidationError, validate_strategy
+
+
+def test_valid_strategy_passes() -> None:
+    src = "(strategy (when (gt (indicator rsi 14) 70.0) (entry-short)))"
+    ast = parse_strategy(src)
+    validate_strategy(ast)  # no exception
+
+
+def test_indicator_unknown_name_fails() -> None:
+    src = "(strategy (when (gt (indicator wibble 14) 70.0) (entry-short)))"
+    ast = parse_strategy(src)
+    with pytest.raises(ValidationError, match="unknown indicator"):
+        validate_strategy(ast)
+
+
+def test_indicator_wrong_arity_fails() -> None:
+    src = "(strategy (when (gt (indicator rsi) 70.0) (entry-short)))"
+    ast = parse_strategy(src)
+    with pytest.raises(ValidationError):
+        validate_strategy(ast)
+
+
+def test_comparator_wrong_arity_fails() -> None:
+    src = "(strategy (when (gt 1.0) (entry-long)))"
+    ast = parse_strategy(src)
+    with pytest.raises(ValidationError):
+        validate_strategy(ast)
+
+
+def test_feature_unknown_column_fails() -> None:
+    src = "(strategy (when (gt (feature wibble) 100.0) (entry-long)))"
+    ast = parse_strategy(src)
+    with pytest.raises(ValidationError, match="unknown feature"):
+        validate_strategy(ast)