diff --git a/crates/doc/src/lib.rs b/crates/doc/src/lib.rs
index f7415b5aae..c8d88f369c 100644
--- a/crates/doc/src/lib.rs
+++ b/crates/doc/src/lib.rs
@@ -1207,6 +1207,18 @@ impl Document {
         }
     }
 
+    /// Repair every paragraph's runs (see [`Paragraph::normalize`]); for models from untrusted
+    /// JSON, whose run lengths may overflow, fall short or split a character.
+    pub fn normalize_runs(&mut self) {
+        let mut stories: Vec<&mut Blocks> = vec![&mut self.body];
+        stories.extend(self.parts.values_mut().map(|p| &mut p.blocks));
+        for blocks in stories {
+            for b in blocks.iter_mut() {
+                edit::each_para_mut(Arc::make_mut(b), 0, &mut Paragraph::normalize);
+            }
+        }
+    }
+
     /// Number of paragraphs in the body (all depths).
     pub fn paragraph_count(&self) -> usize {
         self.para_paths(StoryRef::Body).len()
diff --git a/crates/doc/src/para.rs b/crates/doc/src/para.rs
index a6b08857b6..f1379bd354 100644
--- a/crates/doc/src/para.rs
+++ b/crates/doc/src/para.rs
@@ -1033,32 +1033,32 @@ impl Paragraph {
 
     /// Merge adjacent equal runs and drop empty ones.
     pub fn normalize(&mut self) {
+        // Runs must end on character boundaries and cover the text exactly (never expected, but
+        // files lie): clamp each end to the text, push it past a split character, pad the tail.
+        let n = self.text.len();
+        let mut end = 0usize;
         let mut out: Vec<Run> = Vec::with_capacity(self.runs.len());
         for r in self.runs.drain(..) {
-            if r.len == 0 {
+            let mut e = end.saturating_add(r.len).min(n);
+            while !self.text.is_char_boundary(e) {
+                e += 1;
+            }
+            if e == end {
                 continue;
             }
             match out.last_mut() {
-                Some(l) if l.props == r.props => l.len += r.len,
-                _ => out.push(r),
+                Some(l) if l.props == r.props => l.len += e - end,
+                _ => out.push(Run { len: e - end, props: r.props }),
+            }
+            end = e;
+        }
+        if end < n {
+            match out.last_mut() {
+                Some(l) => l.len += n - end,
+                None => out.push(Run { len: n, props: self.mark.clone() }),
             }
         }
         self.runs = out;
-        // Repair a run list that doesn't cover the text exactly (never expected, but files lie).
-        let total: usize = self.runs.iter().map(|r| r.len).sum();
-        if total != self.text.len() {
-            if total < self.text.len() {
-                let p = self.runs.last().map(|r| r.props.clone()).unwrap_or_else(|| self.mark.clone());
-                self.runs.push(Run { len: self.text.len() - total, props: p });
-            } else {
-                let mut keep = self.text.len();
-                for r in &mut self.runs {
-                    r.len = r.len.min(keep);
-                    keep -= r.len;
-                }
-                self.runs.retain(|r| r.len > 0);
-            }
-        }
     }
 
     /// Delete `a..b` (bytes). Objects inside are removed.
diff --git a/crates/engine/src/io.rs b/crates/engine/src/io.rs
index 7bd35f19c6..d96b4ebe42 100644
--- a/crates/engine/src/io.rs
+++ b/crates/engine/src/io.rs
@@ -188,7 +188,11 @@ fn read_bytes(name: &str, bytes: &[u8], password: Option<&str>, page: &wordcraft
         }
         // Plain text goes through the formats importer: BOMs, unmarked UTF-16, Windows-1252.
         "txt" | "text" => wordcraft_formats::txt::import_on(bytes, page),
-        "json" => serde_json::from_slice::<Document>(bytes).map_err(|e| format!("{name}: {e}"))?,
+        "json" => {
+            let mut d = serde_json::from_slice::<Document>(bytes).map_err(|e| format!("{name}: {e}"))?;
+            d.normalize_runs();
+            d
+        }
         other => match crate::io_ext::open(other, bytes, page) {
             Some(r) => r?,
             None => return open_by_content(name, other, bytes, password, page),
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index b69d4ff1ea..34d7bda848 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -5807,6 +5807,27 @@ fn insert_picture_takes_a_windows_metafile() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+#[test]
+fn json_open_repairs_hostile_run_lengths() {
+    use wordcraft_doc::{Block, Document};
+    // "héllo": é is two bytes (1..3); a run ending at 2 splits it, usize::MAX overflows a sum.
+    for lens in [vec![2, 4], vec![usize::MAX, usize::MAX], vec![1], vec![], vec![0, 99]] {
+        let mut d = Document::from_text("héllo");
+        if let Some(Block::Para(p)) = d.body.first_mut().map(std::sync::Arc::make_mut) {
+            p.runs = lens.iter().map(|&len| wordcraft_doc::para::Run { len, props: Default::default() }).collect();
+        }
+        let bytes = serde_json::to_vec(&d).unwrap();
+        let (doc, _) = crate::io::open_bytes_with("x.json", &bytes, None).unwrap();
+        let Some(Block::Para(p)) = doc.body.first().map(|b| &**b) else { panic!("no paragraph") };
+        let mut end = 0;
+        for (r, _) in p.run_ranges() {
+            assert!(p.text.is_char_boundary(r.end) && r.end > r.start, "{lens:?}");
+            end = r.end;
+        }
+        assert_eq!(end, p.text.len(), "{lens:?}");
+    }
+}
+
 #[test]
 fn outline_aliases_add_one_undo_step() {
     let mut s = s();