diff --git a/src/harel/cel.py b/src/harel/cel.py
index 09871c4..a2d485d 100644
--- a/src/harel/cel.py
+++ b/src/harel/cel.py
@@ -41,9 +41,33 @@ def _to_cel(value: Any) -> Any:
     return value
 
 
+def _from_cel(value: Any) -> Any:
+    """Normalize a celpy result to a canonical native/JSON Python value (SPEC §5.1).
+
+    No guard-language wrapper type (``celpy.celtypes.*``) may cross the engine boundary,
+    so every CEL result is coerced to its native equivalent here — the single choke point
+    for esv assignments, published payloads, and spawn args.
+    """
+    if isinstance(value, celtypes.BoolType):  # subclasses int — check before IntType
+        return bool(value)
+    if isinstance(value, (celtypes.IntType, celtypes.UintType)):
+        return int(value)
+    if isinstance(value, celtypes.DoubleType):
+        return float(value)
+    if isinstance(value, celtypes.StringType):
+        return str(value)
+    if isinstance(value, celtypes.BytesType):
+        return bytes(value)
+    if isinstance(value, dict):  # MapType (and native dict): normalize keys + values
+        return {_from_cel(k): _from_cel(v) for k, v in value.items()}
+    if isinstance(value, list):  # ListType (and native list)
+        return [_from_cel(v) for v in value]
+    return value
+
+
 def evaluate(expr: str, bindings: dict[str, Any]) -> Any:
-    """Evaluate a CEL expression against ``bindings`` (esvs + event/id/parent)."""
+    """Evaluate a CEL expression, returning a canonical native/JSON value (§5.1)."""
     try:
-        return _program(expr).evaluate(_to_cel(bindings))
+        return _from_cel(_program(expr).evaluate(_to_cel(bindings)))
     except celpy.CELEvalError as exc:  # type: ignore[attr-defined]
         raise CelError(str(exc)) from exc
diff --git a/tests/test_native_values.py b/tests/test_native_values.py
new file mode 100644
index 0000000..8c16e82
--- /dev/null
+++ b/tests/test_native_values.py
@@ -0,0 +1,99 @@
+"""Values crossing the boundary are canonical native/JSON types (SPEC §5.1).
+
+CEL-produced values (assign RHS, published payloads, …) must not surface celpy wrapper
+types (``IntType``/``DoubleType``/``BoolType``/``StringType``/``MapType``/``ListType``)
+through the public API, snapshots, or `--json`.
+"""
+
+from __future__ import annotations
+
+import json
+
+from harel import Host, load_definitions
+
+TYPES = """\
+id: types
+events:
+  go: {}
+top:
+  esvs:
+    i: { type: int, init: 0 }
+    f: { type: float, init: 0.0 }
+    b: { type: bool, init: false }
+    s: { type: string, init: "" }
+    m: { type: map, init: {} }
+    l: { type: list, init: [] }
+  initial: { transition_to: a }
+  states:
+    a:
+      on_events:
+        go:
+          action:
+            - { assign: { i: "1 + 2" } }
+            - { assign: { f: "1.5 + 0.5" } }
+            - { assign: { b: "1 < 2" } }
+            - { assign: { s: "'a' + 'b'" } }
+            - { assign: { m: "{'k': 1 + 1}" } }
+            - { assign: { l: "[1, 2, 3]" } }
+          transition_to: done_
+    done_: {}
+"""
+
+
+def _run() -> object:
+    host = Host()
+    host.register_all(load_definitions(TYPES))
+    inst = host.create_root(host.machines["types"], "r")
+    host.run_to_quiescence()
+    host.deliver("r", "go")
+    host.run_to_quiescence()
+    return inst
+
+
+def test_cel_assignments_are_native_python() -> None:
+    esvs = _run().resolved_esvs()
+    assert type(esvs["i"]) is int
+    assert type(esvs["f"]) is float
+    assert type(esvs["b"]) is bool
+    assert type(esvs["s"]) is str
+    assert type(esvs["m"]) is dict
+    assert type(esvs["l"]) is list
+    # nested values too (no wrappers inside containers)
+    assert type(esvs["m"]["k"]) is int and esvs["m"]["k"] == 2
+    assert [type(x) for x in esvs["l"]] == [int, int, int]
+    assert esvs["i"] == 3 and esvs["f"] == 2.0 and esvs["b"] is True and esvs["s"] == "ab"
+
+
+def test_snapshot_contains_only_native_json_values() -> None:
+    snap = _run().to_snapshot()
+    json.dumps(snap)  # must be plain-serializable
+
+    def _walk(v: object) -> None:
+        # celpy wrappers subclass their natives, so json.dumps alone wouldn't catch them.
+        assert "celpy" not in type(v).__module__, f"celpy type in snapshot: {type(v)}"
+        if isinstance(v, dict):
+            for k, val in v.items():
+                _walk(k)
+                _walk(val)
+        elif isinstance(v, list):
+            for x in v:
+                _walk(x)
+
+    _walk(snap)
+
+
+def test_no_celpy_type_leaks_anywhere_in_esvs() -> None:
+    esvs = _run().resolved_esvs()
+
+    def _assert_native(v: object) -> None:
+        assert type(v).__module__ == "builtins", f"non-native value leaked: {type(v)}"
+        if isinstance(v, dict):
+            for k, val in v.items():
+                _assert_native(k)
+                _assert_native(val)
+        elif isinstance(v, list):
+            for x in v:
+                _assert_native(x)
+
+    for val in esvs.values():
+        _assert_native(val)