diff --git a/crates/ui-egui/src/dialogs/composite_fonts.rs b/crates/ui-egui/src/dialogs/composite_fonts.rs
index 31472503b2..6e3b6f84e1 100644
--- a/crates/ui-egui/src/dialogs/composite_fonts.rs
+++ b/crates/ui-egui/src/dialogs/composite_fonts.rs
@@ -42,12 +42,21 @@ const CHARS_PER_ROW: usize = 20;
 /// and a few custom sets.
 const TABLE_HEIGHT: f32 = 340.0;
 
-pub(super) const SPEC: DialogSpec = DialogSpec { heading: |_| tl!("Composite Fonts").into(), body, confirm, min_width: 760.0, ..DialogSpec::FORM };
+/// `char` is held only while a character is picked, so `ui.dialog.set` takes it from `inputs`.
+pub(super) const SPEC: DialogSpec = DialogSpec {
+    heading: |_| tl!("Composite Fonts").into(),
+    body,
+    confirm,
+    min_width: 760.0,
+    inputs: |_| &["custom", "entry", "char"],
+    ..DialogSpec::FORM
+};
 
-/// Open Composite Fonts on the active document's composite fonts, the first one picked.
+/// Open Composite Fonts on the active document's composite fonts, the first one picked, with the
+/// custom character sets hidden and nothing typed.
 pub fn open(app: &mut VectorcraftApp) -> Result<Value, String> {
     let fonts = app.session.active().ok_or("no document open")?.doc.composite_fonts.to_vec();
-    let mut d = Dialog::new(KIND, json!({ "fonts": fonts, "selected": 0, "entryMode": "direct" }));
+    let mut d = Dialog::new(KIND, json!({ "fonts": fonts, "selected": 0, "custom": false, "entryMode": "direct", "entry": "" }));
     show_selected(&mut d);
     app.ui.dialog = Some(d);
     Ok(Value::Null)
diff --git a/crates/ui-egui/src/dialogs/composite_fonts/tests.rs b/crates/ui-egui/src/dialogs/composite_fonts/tests.rs
index b4d89c64ea..e50ce1b566 100644
--- a/crates/ui-egui/src/dialogs/composite_fonts/tests.rs
+++ b/crates/ui-egui/src/dialogs/composite_fonts/tests.rs
@@ -244,6 +244,27 @@ fn custom_character_sets_are_edited_in_the_dialog() {
     assert_eq!((f.custom.len(), f.custom[0].name.as_str(), f.custom[0].characters.as_str()), (1, "Letters", "Aあ"));
 }
 
+/// `ui.dialog.set` takes `custom`, `entry` and `char` as soon as the dialog opens, before Custom is
+/// clicked or anything is typed, and Add then adds the characters it set.
+#[test]
+fn custom_set_fields_are_set_right_after_open() {
+    let mut app = app();
+    let ctx = egui::Context::default();
+    let font =
+        json!({"name": "UI Composite Open", "parts": {"kanji": {"font": "Inter"}}, "custom": [{"name": "Marks", "characters": "", "font": "Inter"}]});
+    app.run(COMMAND, json!({ "fonts": [font] })).unwrap();
+    open(&mut app).unwrap();
+    for (field, value) in [("custom", json!(true)), ("entry", json!("※〒")), ("char", json!(0))] {
+        let (req, _) = crate::control::ControlRequest::new("ui.dialog.set", json!({"field": field, "value": value}));
+        let crate::control::Outcome::Done(r) = crate::control::handle(&mut app, &ctx, &req) else { panic!("not done") };
+        assert!(r.get("error").is_none(), "{field}: {r}");
+        assert_eq!(dialog(&mut app).fields.get(field), Some(&value), "{field}");
+    }
+    frame(&mut app);
+    press(&mut app, Action::AddChars);
+    assert_eq!(sets(&mut app)[0].characters, "※〒");
+}
+
 /// Two sets of the same name keep the dialog open and change nothing.
 #[test]
 fn custom_sets_sharing_a_name_keep_the_dialog_open() {
diff --git a/docs/mcp.md b/docs/mcp.md
index 1549d77362..8a0651f067 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -9333,7 +9333,8 @@ picked. Custom shows the font's custom character sets: Character Set picks one,
 Delete Set takes it out, and Name renames it. Character › Direct Entry adds the characters typed, and Character ›
 Unicode adds the characters of hexadecimal codes (`3042`, `U+3042`, several separated by spaces or commas); Add skips
 characters the set already lists. Delete takes the character picked out of the set. OK runs `type.compositeFonts.set`
-with every font as one undo step; a refused font keeps the dialog open.
+with every font as one undo step; a refused font keeps the dialog open. `ui.dialog.set` takes `custom`, `entry` and
+`char` (the index of the character picked in the set) as soon as the dialog opens.
 
 ```json
 {"name":"run_command","arguments":{"command":"type.compositeFonts.set","params":{"fonts":[{"name":"My Composite","parts":{"kanji":{"font":"Source Han Sans"},"roman":{"font":"Source Sans 3","size":95,"baseline":-2},"numerals":{"font":"Source Sans 3"}}}]}}}