diff --git a/crates/effects/src/catalog.rs b/crates/effects/src/catalog.rs
index e000b930f..d10d931d6 100644
--- a/crates/effects/src/catalog.rs
+++ b/crates/effects/src/catalog.rs
@@ -42,6 +42,20 @@ pub fn is_length(id: &str, key: &str, relative: bool) -> bool {
     catalog_index().get(id).is_some_and(|info| info.lengths.keys(relative).any(|k| k == key))
 }
 
+/// The range in points that distance parameter `key` of effect `id` is kept within, as its
+/// parameter documentation gives it (`{maxRadius: pt 4..127 (8), …}`); `None` when it gives none.
+pub fn length_range(id: &str, key: &str) -> Option<std::ops::RangeInclusive<f64>> {
+    let doc = catalog_index().get(id)?.params;
+    let starts = |at: usize| doc.get(..at).and_then(|s| s.chars().last()).is_some_and(|c| c == '{' || c == ' ');
+    let rest = doc.match_indices(key).map(|(at, _)| at).filter(|at| starts(*at)).find_map(|at| {
+        let rest = doc.get(at + key.len()..)?;
+        rest.strip_prefix(": pt ").or_else(|| rest.strip_prefix("?: pt "))
+    })?;
+    let (lo, hi) = rest.split_whitespace().next()?.split_once("..")?;
+    let (lo, hi) = (lo.parse::<f64>().ok()?, hi.trim_end_matches(',').parse::<f64>().ok()?);
+    (lo.is_finite() && hi.is_finite() && lo <= hi).then_some(lo..=hi)
+}
+
 /// The distance parameters of effect `id` (none for unknown effects).
 fn lengths_of(id: &str) -> Lengths {
     let always = |always| Lengths { always, absolute: &[] };
diff --git a/crates/effects/src/lib.rs b/crates/effects/src/lib.rs
index b6db2f4d9..a732e2dff 100644
--- a/crates/effects/src/lib.rs
+++ b/crates/effects/src/lib.rs
@@ -94,7 +94,7 @@ mod catalog;
 mod geometry;
 mod params;
 
-pub use catalog::{EffectInfo, Lengths, WARP_STYLES, effect_catalog, effect_info, is_length, plugin_effects};
+pub use catalog::{EffectInfo, Lengths, WARP_STYLES, effect_catalog, effect_info, is_length, length_range, plugin_effects};
 pub(crate) use geometry::{Budget, apply_one};
 pub use geometry::{GeomContext, apply_geometry, apply_geometry_bez, apply_geometry_with};
 #[cfg(test)]
diff --git a/crates/effects/src/tests.rs b/crates/effects/src/tests.rs
index 0241fa8b9..198da329e 100644
--- a/crates/effects/src/tests.rs
+++ b/crates/effects/src/tests.rs
@@ -1207,3 +1207,17 @@ fn a_repeat_of_a_symbol_whose_art_is_a_repeat_shares_the_instance_budget() {
     let small = Node::new(NodeId(0), NodeKind::Repeat(RepeatSpec::mirror(vec![instance("Small")], 90.0, 5.0)));
     assert_eq!(expand_live(Some(&d), &small).len(), 2);
 }
+
+#[test]
+fn a_distance_parameters_range_comes_from_its_documentation() {
+    assert_eq!(length_range("pixelate.colorHalftone", "maxRadius"), Some(4.0..=127.0));
+    // The effect applies the range's floor to a radius below it.
+    assert_eq!(
+        crate::pixel::parse("pixelate.colorHalftone", &json!({"maxRadius": 1})),
+        crate::pixel::parse("pixelate.colorHalftone", &json!({"maxRadius": 4}))
+    );
+    // A distance documented without a range, and unknown effects and parameters, have none.
+    assert_eq!(length_range("path.offsetPath", "offset"), None);
+    assert_eq!(length_range("pixelate.colorHalftone", "channel1"), None);
+    assert_eq!(length_range("no.such", "maxRadius"), None);
+}
diff --git a/crates/ui-egui/src/dialogs/form.rs b/crates/ui-egui/src/dialogs/form.rs
index b82fe54f8..941687ae4 100644
--- a/crates/ui-egui/src/dialogs/form.rs
+++ b/crates/ui-egui/src/dialogs/form.rs
@@ -44,8 +44,22 @@ pub(super) fn length_field(ui: &mut egui::Ui, d: &mut Dialog, key: &str, label:
 /// "5 mm", wins), kept in points (a string with a unit, "12 pt", also reads). Returns true when it
 /// changed.
 pub(super) fn length(ui: &mut egui::Ui, d: &mut Dialog, key: &str, unit: Unit, width: f32) -> bool {
+    length_within(ui, d, key, unit, width, None)
+}
+
+/// [`length`] whose value is kept within `range` (points): a value typed outside it becomes the
+/// nearer end, so the field shows the distance the command applies (#4637).
+pub(super) fn length_within(
+    ui: &mut egui::Ui,
+    d: &mut Dialog,
+    key: &str,
+    unit: Unit,
+    width: f32,
+    range: Option<std::ops::RangeInclusive<f64>>,
+) -> bool {
     let value = d.fields.contains_key(key).then(|| d.f64(key, 0.0));
     let Some(v) = crate::widgets::num_field(ui, ("dlg-len", key), value, unit, width) else { return false };
+    let v = range.map_or(v, |r| v.max(*r.start()).min(*r.end()));
     d.fields.insert(key.into(), json!(v));
     true
 }
@@ -365,7 +379,9 @@ pub(super) fn param_fields(
                     return;
                 }
                 if is_length(&k) {
-                    changed |= length(ui, d, &k, unit, 140.0);
+                    // An effect's documented range keeps the field to the distance it applies.
+                    let range = vectorcraft_effects::length_range(&d.str("__effect"), &k);
+                    changed |= length_within(ui, d, &k, unit, 140.0, range);
                     return;
                 }
                 if let Some(options) = choices(&k) {
diff --git a/crates/ui-egui/src/dialogs/tests.rs b/crates/ui-egui/src/dialogs/tests.rs
index 66f37e10f..ae9fb664f 100644
--- a/crates/ui-egui/src/dialogs/tests.rs
+++ b/crates/ui-egui/src/dialogs/tests.rs
@@ -2486,3 +2486,30 @@ fn artboard_options_selected_fit_spans_other_boards_and_off_board_art() {
     app.run("edit.redo", json!({})).unwrap();
     assert_eq!(app.session.doc().unwrap().doc, after);
 }
+
+/// An effect dialog keeps a distance typed outside the effect's range to the range, so the field
+/// shows what the effect applies: Color Halftone's Max Radius below 4 pt is 4 pt (#4637).
+#[test]
+fn an_effect_dialog_keeps_a_typed_distance_within_the_effects_range() {
+    let enter = || egui::Event::Key { key: egui::Key::Enter, physical_key: None, pressed: true, repeat: false, modifiers: Default::default() };
+    for (typed, applied) in [("1 pt", 4.0), ("0.5 px", 4.0), ("200 pt", 127.0), ("10 pt", 10.0)] {
+        let mut app = app();
+        let id = app.run("shape.rectangle", json!({"x": 10, "y": 10, "width": 50, "height": 50})).unwrap()["id"].as_u64().unwrap();
+        let ctx = egui::Context::default();
+        theme::install_fonts(&ctx);
+        theme::apply(&ctx, Default::default());
+        let frame = |app: &mut VectorcraftApp, events: Vec<egui::Event>| {
+            let mut out = ctx.run_ui(egui::RawInput { events, ..Default::default() }, |ui| show(app, ui.ctx()));
+            out.textures_delta.clear();
+        };
+        app.run("select.all", json!({})).unwrap();
+        app.run("effect.dialog", json!({"effect": "pixelate.colorHalftone"})).unwrap();
+        // Max Radius, the first field, is focused: type over it and press Enter.
+        frame(&mut app, vec![]);
+        frame(&mut app, vec![egui::Event::Text(typed.into())]);
+        frame(&mut app, vec![enter()]);
+        assert!(app.ui.dialog.is_none(), "{typed}");
+        let node = app.session.doc().unwrap().doc.node(vectorcraft_doc::NodeId(id)).unwrap().clone();
+        assert_eq!(node.appearance.effects.last().map(|e| e.params["maxRadius"].clone()), Some(json!(applied)), "{typed}");
+    }
+}