diff --git a/crates/engine/src/cmd/insert.rs b/crates/engine/src/cmd/insert.rs
index d2f0e3511..9c383fad6 100644
--- a/crates/engine/src/cmd/insert.rs
+++ b/crates/engine/src/cmd/insert.rs
@@ -777,16 +777,27 @@ fn insert_shape(s: &mut Session, p: &Json) -> Result<Json> {
     })
 }
 
+/// A text box holds at most as many characters as a cell.
+const SHAPE_TEXT_MAX_CHARS: usize = 32_767;
+
 fn shape_set_text(s: &mut Session, p: &Json) -> Result<Json> {
     let id =
         p.get("id").and_then(Json::as_u64).and_then(|id| u32::try_from(id).ok()).ok_or_else(|| bad("shape.setText", "`id` must be a text box id"))?;
     let text = str_param(p, "text").ok_or_else(|| bad("shape.setText", "`text` must be a string"))?;
+    if text.chars().nth(SHAPE_TEXT_MAX_CHARS).is_some() {
+        return Err(bad("shape.setText", "`text` is longer than 32,767 characters"));
+    }
     let d = s.doc()?;
     let sheet = d.wb.active_sheet;
-    let shape = d.wb.active().and_then(|sh| sh.shapes.iter().find(|shape| shape.id == id)).ok_or_else(|| bad("shape.setText", "no such text box"))?;
+    let sh = d.wb.active().ok_or_else(|| bad("shape.setText", "no such text box"))?;
+    let shape = sh.shapes.iter().find(|shape| shape.id == id).ok_or_else(|| bad("shape.setText", "no such text box"))?;
     if shape.kind != ShapeKind::TextBox {
         return Err(bad("shape.setText", "the shape is not a text box"));
     }
+    // Sheet protection locks drawing objects (there is no "edit objects" allowance yet).
+    if sh.is_protected() {
+        return Err(EngineError::Other(PROTECTED.into()));
+    }
     if shape.text == text {
         return ok();
     }
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 5ecaea7b6..b6111a2ff 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -168,6 +168,36 @@ fn shape_text_rejects_invalid_targets_and_params() {
     assert!(s.doc().unwrap().wb.active().unwrap().shapes.is_empty());
 }
 
+#[test]
+fn shape_text_respects_sheet_protection() {
+    let mut s = s();
+    let id = s.execute("insert.textBox", json!({"text": "Original"})).unwrap()["shape"].clone();
+    s.execute("review.protectSheet", json!({})).unwrap();
+    let before = s.doc().unwrap().wb.clone();
+    let undo_len = s.doc().unwrap().undo.len();
+    assert!(s.execute("shape.setText", json!({"id": id, "text": "Changed"})).is_err());
+    assert!(std::sync::Arc::ptr_eq(&s.doc().unwrap().wb, &before));
+    assert_eq!(s.doc().unwrap().undo.len(), undo_len);
+    s.execute("review.unprotectSheet", json!({})).unwrap();
+    s.execute("shape.setText", json!({"id": id, "text": "Changed"})).unwrap();
+    assert_eq!(s.doc().unwrap().wb.active().unwrap().shapes[0].text, "Changed");
+}
+
+#[test]
+fn shape_text_is_capped_like_a_cell() {
+    let mut s = s();
+    let id = s.execute("insert.textBox", json!({"text": "Original"})).unwrap()["shape"].clone();
+    // The cap counts characters, not bytes.
+    let longest = "é".repeat(32_767);
+    s.execute("shape.setText", json!({"id": id, "text": longest})).unwrap();
+    assert_eq!(s.doc().unwrap().wb.active().unwrap().shapes[0].text.chars().count(), 32_767);
+    let undo_len = s.doc().unwrap().undo.len();
+    let too_long = "x".repeat(32_768);
+    assert!(matches!(s.execute("shape.setText", json!({"id": id, "text": too_long})), Err(crate::EngineError::BadParams { .. })));
+    assert_eq!(s.doc().unwrap().wb.active().unwrap().shapes[0].text.chars().count(), 32_767);
+    assert_eq!(s.doc().unwrap().undo.len(), undo_len);
+}
+
 #[test]
 fn shape_text_edit_survives_xlsx_roundtrip() {
     let mut s = s();
diff --git a/crates/ui-egui/src/text_box.rs b/crates/ui-egui/src/text_box.rs
index f9d8dec70..1e83f62ec 100644
--- a/crates/ui-egui/src/text_box.rs
+++ b/crates/ui-egui/src/text_box.rs
@@ -31,9 +31,15 @@ impl SheetApp {
             return false;
         }
         let Some(d) = self.session.active() else { return false };
-        let Some(shape) = d.wb.active().and_then(|sh| sh.shapes.iter().find(|s| s.id == id && s.kind == ShapeKind::TextBox)) else {
+        let Some(sh) = d.wb.active() else { return false };
+        let Some(shape) = sh.shapes.iter().find(|s| s.id == id && s.kind == ShapeKind::TextBox) else {
             return false;
         };
+        // shape.setText refuses edits on a protected sheet; don't open an editor that can't commit.
+        if sh.is_protected() {
+            self.message = Some(("Text Box".into(), "The cell or chart you're trying to change is on a protected sheet.".into()));
+            return false;
+        }
         self.text_box_editor = Some(EditState {
             id,
             text: shape.text.clone(),
@@ -150,6 +156,8 @@ pub(crate) fn show(app: &mut SheetApp, ui: &mut egui::Ui, geo: &Geo, sh: &gridcr
                 .margin(egui::Margin::ZERO)
                 .desired_width(rect.width())
                 .desired_rows(1)
+                // shape.setText's limit (a cell's).
+                .char_limit(32_767)
                 .min_size(rect.size())
                 .lock_focus(true),
         );
diff --git a/crates/ui-egui/tests/text_box_editing.rs b/crates/ui-egui/tests/text_box_editing.rs
index 64e4ccbae..1d3b053ec 100644
--- a/crates/ui-egui/tests/text_box_editing.rs
+++ b/crates/ui-egui/tests/text_box_editing.rs
@@ -228,3 +228,14 @@ fn saving_and_switching_context_commit_to_the_original_text_box() {
     assert_eq!(shape_text(&docs[1].wb, 0, other_doc_id), "Other workbook");
     assert_eq!(docs[0].wb.sheet(0).unwrap().value(at("B2")), Value::text("cell stays"));
 }
+
+#[test]
+fn protected_sheet_refuses_to_open_the_text_box_editor() {
+    let (mut h, id) = fixture();
+    h.state_mut().session.execute("review.protectSheet", json!({})).unwrap();
+    edit_box(&mut h, id);
+    assert!(h.state().text_box_editor.is_none(), "a protected sheet's text box must not open an editor");
+    assert!(h.state().message.as_ref().is_some_and(|(_, m)| m.contains("protected sheet")));
+    assert_eq!(shape(&h, id).text, ORIGINAL);
+    assert_cell_unchanged(&h);
+}