diff --git a/src/openjd/model/v2023_09/_model.py b/src/openjd/model/v2023_09/_model.py
index 56caa30c..afe189be 100644
--- a/src/openjd/model/v2023_09/_model.py
+++ b/src/openjd/model/v2023_09/_model.py
@@ -921,6 +921,11 @@ class ScriptInterpreter(str, Enum):
 LET_MAX_BINDINGS = 50
 _LET_NAME_RE = re.compile(r"^[a-z_][A-Za-z0-9_]*$")
 
+# §3.6.1: maximum length of a `let` binding's `<UserIdentifier>`. Flat, so not
+# the §7.1 cap NameIdentifierLengthMixin applies: that one is 64 without
+# FEATURE_BUNDLE_1, and a 512-character name must be accepted with EXPR alone.
+LET_MAX_IDENTIFIER_LEN = 512
+
 
 def parse_let_bindings(value: Any) -> list[tuple[str, str]]:
     """Parse a ``let`` field value (list of ``"name = expression"`` strings)
@@ -941,6 +946,14 @@ def parse_let_bindings(value: Any) -> list[tuple[str, str]]:
         expr = expr.strip()
         if not _LET_NAME_RE.match(name):
             raise ValueError(f"A 'let' binding name must be a valid identifier: {name!r}")
+        # Truncated rather than omitted: the caller is a field_validator on the
+        # whole list, so the error path is `let` with no index to identify which
+        # binding is over.
+        if len(name) > LET_MAX_IDENTIFIER_LEN:
+            raise ValueError(
+                f"A 'let' binding name must be at most {LET_MAX_IDENTIFIER_LEN} "
+                f"characters long: {name[:32]!r}... ({len(name)} characters)"
+            )
         if not expr:
             raise ValueError(f"A 'let' binding must define an expression: {binding!r}")
         result.append((name, expr))
diff --git a/test/openjd/model_v0/v2023_09/test_let_bindings.py b/test/openjd/model_v0/v2023_09/test_let_bindings.py
index ac0ddc6a..64171b48 100644
--- a/test/openjd/model_v0/v2023_09/test_let_bindings.py
+++ b/test/openjd/model_v0/v2023_09/test_let_bindings.py
@@ -41,6 +41,29 @@ def test_script_let(self):
             _job([{"name": "S", "script": {"let": ["a = 2"], **_onrun("{{a}}")}}]),
         )
 
+    # §3.6.1 boundary: 512 characters is the maximum and must be accepted, with
+    # EXPR alone, since the cap does not depend on FEATURE_BUNDLE_1.
+    def test_name_512_chars(self):
+        name = "a" * 512
+        # Referenced, not just declared: a cap further down the path would
+        # otherwise be invisible here.
+        _decode(_job([{"name": "S", "let": [f"{name} = 1"], "script": _onrun(f"{{{{{name}}}}}")}]))
+
+    def test_name_512_chars_with_fb1(self):
+        name = "a" * 512
+        _decode(
+            _job(
+                [{"name": "S", "let": [f"{name} = 1"], "script": _onrun(f"{{{{{name}}}}}")}],
+                extensions=("EXPR", "FEATURE_BUNDLE_1"),
+            )
+        )
+
+    def test_name_512_chars_script(self):
+        name = "a" * 512
+        _decode(
+            _job([{"name": "S", "script": {"let": [f"{name} = 1"], **_onrun(f"{{{{{name}}}}}")}}])
+        )
+
     def test_chained_and_functions(self):
         _decode(
             _job(
@@ -98,6 +121,40 @@ def test_self_reference(self):
         with pytest.raises(DecodeValidationError, match="cannot reference itself"):
             _decode(_job([{"name": "S", "let": ["x = x + 1"], "script": _onrun("hi")}]))
 
+    # §3.6.1 caps a `<UserIdentifier>` at 512 characters. `_job` declares EXPR
+    # alone, so these pin the cap independently of FEATURE_BUNDLE_1.
+    def test_name_513_chars(self):
+        name = "a" * 513
+        with pytest.raises(
+            DecodeValidationError,
+            match=r"at most 512 characters long: 'a{32}'\.\.\. \(513 characters\)",
+        ):
+            _decode(_job([{"name": "S", "let": [f"{name} = 1"], "script": _onrun("hi")}]))
+
+    def test_name_513_chars_names_the_offending_binding(self):
+        # The validator is a field_validator on the whole list, so the error path
+        # is `let` with no index; the message has to identify the binding itself.
+        name = "b" * 513
+        with pytest.raises(DecodeValidationError, match=r"'b{32}'\.\.\. \(513 characters\)"):
+            _decode(_job([{"name": "S", "let": ["ok = 1", f"{name} = 2"], "script": _onrun("hi")}]))
+
+    def test_name_513_chars_with_fb1(self):
+        name = "a" * 513
+        with pytest.raises(DecodeValidationError, match="at most 512 characters"):
+            _decode(
+                _job(
+                    [{"name": "S", "let": [f"{name} = 1"], "script": _onrun("hi")}],
+                    extensions=("EXPR", "FEATURE_BUNDLE_1"),
+                )
+            )
+
+    def test_name_513_chars_script(self):
+        name = "a" * 513
+        with pytest.raises(DecodeValidationError, match="at most 512 characters"):
+            _decode(
+                _job([{"name": "S", "script": {"let": [f"{name} = 1"], **_onrun("hi")}}]),
+            )
+
     def test_comprehension_shadows_let(self):
         with pytest.raises(DecodeValidationError, match="shadows"):
             _decode(