diff --git a/Sources/TextBoxInput.swift b/Sources/TextBoxInput.swift
index d2d6309817c..34e7e4359b2 100644
--- a/Sources/TextBoxInput.swift
+++ b/Sources/TextBoxInput.swift
@@ -3636,6 +3636,7 @@ final class TextBoxInputTextView: NSTextView {
     }
 
     override func insertText(_ insertString: Any, replacementRange: NSRange) {
+        let replacementRange = sanitizedTextStorageReplacementRange(replacementRange)
         queueAutomaticAttachmentFileCleanup(in: replacementRange)
         super.insertText(insertString, replacementRange: replacementRange)
         flushAutomaticAttachmentFileCleanup()
@@ -3643,7 +3644,12 @@ final class TextBoxInputTextView: NSTextView {
     }
 
     override func setMarkedText(_ string: Any, selectedRange: NSRange, replacementRange: NSRange) {
-        super.setMarkedText(string, selectedRange: selectedRange, replacementRange: replacementRange)
+        let markedTextLength = Self.textInputStringLength(string)
+        super.setMarkedText(
+            string,
+            selectedRange: Self.sanitizedMarkedTextSelectionRange(selectedRange, markedTextLength: markedTextLength),
+            replacementRange: sanitizedTextStorageReplacementRange(replacementRange)
+        )
         onMarkedTextStateChanged(hasMarkedText())
     }
 
@@ -3657,6 +3663,35 @@ final class TextBoxInputTextView: NSTextView {
         flushAutomaticAttachmentFileCleanup()
     }
 
+    private func sanitizedTextStorageReplacementRange(_ range: NSRange) -> NSRange {
+        guard range.location != NSNotFound else { return range }
+        return Self.sanitizedRange(range, upperBound: attributedString().length)
+    }
+
+    private static func sanitizedRange(_ range: NSRange, upperBound: Int) -> NSRange {
+        guard range.location != NSNotFound else { return range }
+        let upperBound = max(0, upperBound)
+        let location = min(max(0, range.location), upperBound)
+        let length = min(max(0, range.length), upperBound - location)
+        return NSRange(location: location, length: length)
+    }
+
+    private static func sanitizedMarkedTextSelectionRange(_ range: NSRange, markedTextLength: Int) -> NSRange {
+        let markedTextLength = max(0, markedTextLength)
+        guard range.location != NSNotFound else {
+            return NSRange(location: markedTextLength, length: 0)
+        }
+        return sanitizedRange(range, upperBound: markedTextLength)
+    }
+
+    private static func textInputStringLength(_ string: Any) -> Int {
+        if let attributed = string as? NSAttributedString {
+            return attributed.length
+        }
+        let plain = (string as? String) ?? String(describing: string)
+        return (plain as NSString).length
+    }
+
     override func copy(_ sender: Any?) {
         if copySelectedAttachments(to: .general) {
             return
diff --git a/cmuxTests/TextBoxMentionCompletionTests.swift b/cmuxTests/TextBoxMentionCompletionTests.swift
index 0e45e98c3f5..0355d78dd0e 100644
--- a/cmuxTests/TextBoxMentionCompletionTests.swift
+++ b/cmuxTests/TextBoxMentionCompletionTests.swift
@@ -165,6 +165,32 @@ struct TextBoxMentionCompletionTests {
         ))
     }
 
+    @Test
+    func testTextBoxInsertTextClampsStaleIMERangeAtUTF16End() {
+        let textView = TextBoxInputTextView(frame: NSRect(x: 0, y: 0, width: 320, height: 30))
+        textView.string = "日本語"
+        textView.setSelectedRange(NSRange(location: 3, length: 0))
+
+        textView.insertText("、", replacementRange: NSRange(location: 3, length: 1))
+
+        #expect(textView.string == "日本語、")
+        #expect(textView.selectedRange() == NSRange(location: 4, length: 0))
+    }
+
+    @Test
+    func testTextBoxSetMarkedTextNormalizesMissingSelectedRangeToMarkedTextEnd() {
+        let textView = TextBoxInputTextView(frame: NSRect(x: 0, y: 0, width: 320, height: 30))
+
+        textView.setMarkedText(
+            "日本語",
+            selectedRange: NSRange(location: NSNotFound, length: 0),
+            replacementRange: NSRange(location: NSNotFound, length: 0)
+        )
+
+        #expect(textView.string == "日本語")
+        #expect(textView.selectedRange() == NSRange(location: 3, length: 0))
+    }
+
     @Test
     func testTextBoxPublishesCommittedIMETextBeforeClearingMarkedState() {
         var text = ""