diff --git a/crates/expr/src/host.rs b/crates/expr/src/host.rs
index bce803c56..505eb7146 100644
--- a/crates/expr/src/host.rs
+++ b/crates/expr/src/host.rs
@@ -138,9 +138,10 @@ pub fn footage_data_json(f: &effectcraft_project::Footage) -> Option<String> {
                 Some((h, b)) => (h.clone(), b.to_vec()),
                 None => (vec![], vec![]),
             };
-            // Every row object repeats every header name: a long header over many rows is far
-            // more than the text itself.
-            let keys: usize = header.iter().map(|k| k.len() + 4).sum();
+            // Every row object repeats every distinct header name (a repeated name keeps its last
+            // column), as serialized: a long header over many rows is far more than the text itself.
+            let distinct: std::collections::HashSet<&String> = header.iter().collect();
+            let keys: usize = distinct.iter().map(|k| serde_json::to_string(k).map_or(k.len() * 6 + 2, |j| j.len()) + 2).sum();
             if keys.saturating_mul(body.len()) > MAX_DATA_JSON_BYTES {
                 return None;
             }
diff --git a/crates/expr/src/prelude.js b/crates/expr/src/prelude.js
index c545482f3..d1d9bb4b9 100644
--- a/crates/expr/src/prelude.js
+++ b/crates/expr/src/prelude.js
@@ -1205,10 +1205,13 @@ function nearestKey(t) { return thisProperty.nearestKey(t); }
 (function () {
   var nest = __nest, stack = __stack, apply = Reflect.apply, construct = Reflect.construct, str = String;
   var define = Object.defineProperty, describe = Object.getOwnPropertyDescriptor, proto = Object.getPrototypeOf;
+  // The original `join`, kept before the guard below replaces it. Descriptors below have a null
+  // prototype: one that inherits `get`/`set` from a changed `Object.prototype` would be refused.
+  var join = Array.prototype.join;
   // Replace `obj[name]` keeping its attributes.
   function replace(obj, name, f) {
     var d = describe(obj, name);
-    define(obj, name, { value: f, writable: d.writable, enumerable: d.enumerable, configurable: true });
+    define(obj, name, { __proto__: null, value: f, writable: d.writable, enumerable: d.enumerable, configurable: true });
   }
   function guard(obj, name) {
     var f = obj[name];
@@ -1232,19 +1235,20 @@ function nearestKey(t) { return thisProperty.nearestKey(t); }
     // in helpers (`push`, `slice`, `join`, `String`) can be replaced by the expression, and
     // must not make the check see other text than the constructor gets.
     var w = function () {
-      var n = arguments.length, args = [], params = '';
+      var n = arguments.length, args = [], heads = [];
       for (var i = 0; i < n; i++) {
         var s = str(arguments[i]);
         if (typeof s !== 'string') s = '';
-        define(args, i, { value: s, writable: true, enumerable: true, configurable: true });
-        if (i < n - 1) params = i ? params + ',' + s : s;
+        define(args, i, { __proto__: null, value: s, writable: true, enumerable: true, configurable: true });
+        if (i < n - 1) define(heads, i, { __proto__: null, value: s, writable: true, enumerable: true, configurable: true });
       }
       stack();
-      nest(params);
+      // Joined in one pass: adding each piece to a growing string copies it every time.
+      nest(apply(join, heads, [',']));
       nest(n ? args[n - 1] : '');
       return construct(F, args);
     };
-    define(w, 'name', { value: F.name, configurable: true });
+    define(w, 'name', { __proto__: null, value: F.name, configurable: true });
     w.prototype = F.prototype;
     replace(F.prototype, 'constructor', w);
     return w;
diff --git a/crates/expr/src/tests.rs b/crates/expr/src/tests.rs
index 80a81f7fc..823d8ecbd 100644
--- a/crates/expr/src/tests.rs
+++ b/crates/expr/src/tests.rs
@@ -2114,3 +2114,22 @@ fn delimited_data_row_objects_are_bounded() {
     };
     assert!(crate::host::footage_data_json(&f).unwrap().contains("\"a\":1"));
 }
+
+/// The row-object estimate counts serialized (escaped) header bytes and each distinct name once.
+#[test]
+fn delimited_data_estimate_counts_escaped_and_distinct_headers() {
+    let data = |text: String| effectcraft_project::Footage {
+        path: "/d.csv".into(),
+        kind: effectcraft_project::FootageKind::Data,
+        data: Some(text),
+        ..Default::default()
+    };
+    // A control character serializes as six bytes: 200 KiB of them over 100 rows is ~120 MiB.
+    let mut text = format!("{}\n", "\u{1}".repeat(200 << 10));
+    text.push_str(&"1\n".repeat(100));
+    assert!(crate::host::footage_data_json(&data(text)).is_none());
+    // A name repeated across many columns is one key per row object.
+    let mut text = format!("{}\n", vec!["x"; 100].join(","));
+    text.push_str(&"1\n".repeat(140_000));
+    assert!(crate::host::footage_data_json(&data(text)).is_some());
+}
diff --git a/crates/expr/src/tests/deep_nesting.rs b/crates/expr/src/tests/deep_nesting.rs
index 292d23edf..0d379e5fb 100644
--- a/crates/expr/src/tests/deep_nesting.rs
+++ b/crates/expr/src/tests/deep_nesting.rs
@@ -69,6 +69,12 @@ fn eval_and_function_still_work() {
     assert_eq!(f.num("Object.getPrototypeOf(function* () { yield 7; }).constructor('yield 7')().next().value", 0.0), 7.0);
 }
 
+#[test]
+fn function_constructor_ignores_changes_to_object_prototype() {
+    let f = fx();
+    assert_eq!(f.num("Object.prototype.get = 1; Object.prototype.set = 1; Function('a', 'b', 'return a + b')(2, 3)", 0.0), 5.0);
+}
+
 #[test]
 fn deep_array_result_is_an_error_or_text_not_a_crash() {
     // The result itself: detaching it used to `toString` it past depth 4.