diff --git a/crates/engine/src/cmd/addons/typetools.rs b/crates/engine/src/cmd/addons/typetools.rs
index 29ca6f6bf..320dce6b9 100644
--- a/crates/engine/src/cmd/addons/typetools.rs
+++ b/crates/engine/src/cmd/addons/typetools.rs
@@ -89,12 +89,6 @@ fn text_tweaker(s: &mut Session, p: &Value) -> Result<Value> {
     let mut rng = Rng::new(whole_or(p, "seed", 0, C)?);
     let skip = switch_or(p, "skipSpaces", true, C)?;
     let setup = s.doc()?.doc.setup.clone();
-    // A range given as anything but whole bytes is refused, not read as all of the text.
-    for k in ["start", "end"] {
-        if p.get(k).is_some_and(|v| !v.is_null() && v.as_u64().is_none()) {
-            return Err(bad(C, format!("`{k}` must be a whole number of bytes")));
-        }
-    }
     // At a caret, all of the object's text.
     let ranges = type_ranges(s, p, C, |text, _| Some((0, text.len())))?;
     // Each object's changed characters: (byte, its change), worked out before the edit.
diff --git a/crates/engine/src/cmd/addons/typetools/tests.rs b/crates/engine/src/cmd/addons/typetools/tests.rs
index fec04f9c6..6c56f6769 100644
--- a/crates/engine/src/cmd/addons/typetools/tests.rs
+++ b/crates/engine/src/cmd/addons/typetools/tests.rs
@@ -196,7 +196,7 @@ fn bad_params_are_refused_without_a_change() {
         json!({"ids": [999_999]}),
         json!({"id": id.0, "start": -1}),
         json!({"id": id.0, "start": 1, "end": 1.5}),
-        json!({"end": "2"}),
+        json!({"id": id.0, "end": "2"}),
     ] {
         assert!(s.execute(C, &p).is_err(), "{p}");
         assert_eq!(s.doc().unwrap().doc, before, "{p}");
diff --git a/crates/engine/src/cmd/textedit.rs b/crates/engine/src/cmd/textedit.rs
index f0a331b0a..03943029e 100644
--- a/crates/engine/src/cmd/textedit.rs
+++ b/crates/engine/src/cmd/textedit.rs
@@ -7,6 +7,7 @@ use vectorcraft_doc::{Appearance, CharStyle, Node, NodeId, NodeKind, TextKind, T
 use vectorcraft_geom::{Affine, Point, Vec2};
 use vectorcraft_text::edit;
 
+use super::menucmds::whole_or;
 use super::stroke::StrokeChange;
 use super::typecmd::refresh_bounds;
 use super::{
@@ -233,12 +234,21 @@ fn byte_param(p: &Value, k: &str) -> Option<usize> {
     p.get(k).and_then(Value::as_u64).map(index)
 }
 
+/// The byte offset that `p` gives for `k`, or `default` when `p` leaves it out or gives null. Any
+/// other value that is not a whole number of at least 0, such as `"2"`, `-1` or `1.5`, is an error.
+fn offset_or(p: &Value, k: &str, default: usize, cmd: &str) -> Result<usize> {
+    match p.get(k) {
+        None | Some(Value::Null) => Ok(default),
+        Some(_) => Ok(index(whole_or(p, k, 0, cmd)?)),
+    }
+}
+
 /// The bytes `start..end` of `p` in text of `len` bytes, clamped and ordered (without them, all of
-/// it). A `start` or `end` that is not a number or null, such as `"2"`, is an error.
+/// it). A `start` or `end` that is not a whole number of at least 0 or null, such as `"2"`, `-1`
+/// or `1.5`, is an error.
 fn range_of(p: &Value, len: usize, cmd: &str) -> Result<(usize, usize)> {
-    check_numbers(p, &["start", "end"], cmd)?;
-    let a = byte_param(p, "start").unwrap_or(0).min(len);
-    let b = byte_param(p, "end").unwrap_or(len).min(len);
+    let a = offset_or(p, "start", 0, cmd)?.min(len);
+    let b = offset_or(p, "end", len, cmd)?.min(len);
     Ok((a.min(b), a.max(b)))
 }
 
@@ -310,10 +320,9 @@ fn clear_param(s: &Session, p: &Value, id: NodeId, c: &str) -> Result<Vec<NodeId
 fn start_list(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "text.startList";
     let id = id_param(p, "id").ok_or_else(|| bad(C, "missing `id`"))?;
-    check_numbers(p, &["at"], C)?;
     let t = text_ref(s, id)?;
     let len = edit::runs_len(&t.runs);
-    let at = byte_param(p, "at").unwrap_or(len).min(len);
+    let at = offset_or(p, "at", len, C)?.min(len);
     let Some((start, list)) = t.typed_list_at(at) else {
         return Ok(json!({"id": id.0, "started": false, "caret": at}));
     };
diff --git a/crates/engine/src/tests/textedit.rs b/crates/engine/src/tests/textedit.rs
index 9e346bf65..fa73e9d50 100644
--- a/crates/engine/src/tests/textedit.rs
+++ b/crates/engine/src/tests/textedit.rs
@@ -1200,6 +1200,33 @@ fn start_list_command_removes_the_typed_characters_and_starts_the_list() {
     assert!(s.execute("text.startList", &json!({"at": 3})).is_err(), "id required");
 }
 
+/// A byte offset given as a negative or fractional number is an error, not the end of the text
+/// (`at`) or its start or end (`start`, `end`), and the text is left as it was. Null keeps the default.
+#[test]
+fn byte_offsets_that_are_not_whole_numbers_are_refused() {
+    let mut s = session();
+    let id = text(&mut s, "x\n# ");
+    for at in [json!(-1), json!(1.5)] {
+        let e = s.execute("text.startList", &json!({"id": id.0, "at": at})).unwrap_err().to_string();
+        assert!(e.contains("`at`"), "{e}");
+    }
+    assert_eq!(obj(&s, id).plain_text(), "x\n# ", "no list was started");
+    assert_eq!(s.execute("text.startList", &json!({"id": id.0, "at": null})).unwrap()["started"], json!(true));
+    let id = text(&mut s, "Hello");
+    for (cmd, p, key) in [
+        ("text.getRange", json!({"id": id.0, "start": -1, "end": 2}), "start"),
+        ("text.getRange", json!({"id": id.0, "start": 1, "end": 2.5}), "end"),
+        ("text.editRange", json!({"id": id.0, "start": 1.5, "end": 2, "insert": "!"}), "start"),
+        ("text.editRange", json!({"id": id.0, "start": 1, "end": -2, "insert": "!"}), "end"),
+        ("text.setRangeStyle", json!({"id": id.0, "start": 0.5, "size": 30}), "start"),
+    ] {
+        let e = s.execute(cmd, &p).unwrap_err().to_string();
+        assert!(e.contains(&format!("`{key}`")), "{cmd} {p}: {e}");
+    }
+    assert_eq!(obj(&s, id).plain_text(), "Hello");
+    assert_eq!(s.execute("text.getRange", &json!({"id": id.0, "start": 1.0, "end": null})).unwrap()["text"], json!("ello"));
+}
+
 /// #1881: a Cmd/Ctrl drag while typing (the Selection tool lent to the Type tool) moves the type
 /// and keeps what was typed, and typing goes on after it.
 #[test]
diff --git a/docs/mcp.md b/docs/mcp.md
index 398e0fdbd..7c0499a06 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -1207,6 +1207,11 @@ and `amount` in `color.harmony`; `angle` in `paint.editGradient` and in the `gra
 `width` and `tolerance` in `livePaint.strokeEdge`. A null is read as a key left out. A gradient stop that names a
 swatch refuses a `tint` that is not a number with `` stop 0: `tint` must be a number ``.
 
+A byte offset must be a whole number of at least 0. `at` in `text.startList`, and `start` and `end` in
+`text.editRange`, `text.getRange`, `text.setRangeStyle` and the `type.step`, `type.size.*`, `type.resetTracking`,
+`type.resetHorizontalScale` and `addon.type.textTweaker` commands given an `id`, refuse a negative or fractional number with
+`` `at` must be a whole number ``, naming the key, and change nothing. `2.0` is read as `2`, and a null as the key left out.
+
 A step can use an earlier step's result, in `run` as in the `command_batch` tool. A parameter string that is exactly
 `$N` or `$N.path` takes that value from the result of step `N` (counting from 0, in the order they ran; `run --in`'s
 open isn't a step), and `$last…` from the step before. A path goes into objects by key and arrays by index: