agent: Fix a panic when splitting streamed-in edits inside of a multibyte character (#57100)

This PR fixes a panic that could occur in the `edit_file` tool where
streaming in text could split in the middle of a multibyte character.

Closes FR-3 and [ZED-7ZX](https://zed-dev.sentry.io/issues/7480598098).

Release Notes:

- Fixed a panic that could occur when streaming in text with the
`edit_file` tool.

crates/agent/src/tools/edit_session/streaming_parser.rs

@@ -113,7 +113,7 @@ impl StreamingParser {
             {
                 if partial.new_text.is_some() && !state.buffer_new_text_until_old_text_done {
                     // new_text appeared after old_text, so old_text is done — emit everything.
-                    let start = state.old_text_emitted_len.min(old_text.len());
+                    let start = find_char_boundary(old_text, state.old_text_emitted_len);
                     let chunk = normalize_done_chunk(old_text[start..].to_string());
                     state.old_text_done = true;
                     state.old_text_emitted_len = old_text.len();
@@ -124,9 +124,10 @@ impl StreamingParser {
                     });
                 } else {
                     let safe_end = safe_emit_end_for_edit_text(old_text);
+                    let safe_start = find_char_boundary(old_text, state.old_text_emitted_len);
 
-                    if safe_end > state.old_text_emitted_len {
-                        let chunk = old_text[state.old_text_emitted_len..safe_end].to_string();
+                    if safe_end > safe_start {
+                        let chunk = old_text[safe_start..safe_end].to_string();
                         state.old_text_emitted_len = safe_end;
                         events.push(EditEvent::OldTextChunk {
                             edit_index: index,
@@ -143,9 +144,10 @@ impl StreamingParser {
                 && !state.new_text_done
             {
                 let safe_end = safe_emit_end_for_edit_text(new_text);
+                let safe_start = find_char_boundary(new_text, state.new_text_emitted_len);
 
-                if safe_end > state.new_text_emitted_len {
-                    let chunk = new_text[state.new_text_emitted_len..safe_end].to_string();
+                if safe_end > safe_start {
+                    let chunk = new_text[safe_start..safe_end].to_string();
                     state.new_text_emitted_len = safe_end;
                     events.push(EditEvent::NewTextChunk {
                         edit_index: index,
@@ -343,8 +345,10 @@ impl StreamingParser {
 /// held back because it may be an artifact of the partial JSON fixer closing
 /// an incomplete escape sequence (e.g. turning a half-received `\n` into `\\`).
 /// The next partial will reveal the correct character.
+///
+/// The returned position is always a valid UTF-8 character boundary.
 fn safe_emit_end(text: &str) -> usize {
-    if text.as_bytes().last() == Some(&b'\\') {
+    if text.ends_with('\\') {
         text.len() - 1
     } else {
         text.len()
@@ -353,13 +357,35 @@ fn safe_emit_end(text: &str) -> usize {
 
 fn safe_emit_end_for_edit_text(text: &str) -> usize {
     let safe_end = safe_emit_end(text);
-    if safe_end > 0 && text.as_bytes()[safe_end - 1] == b'\n' {
+    // Use string slicing to check the last character, ensuring we respect UTF-8 boundaries.
+    if safe_end > 0 && text[..safe_end].ends_with('\n') {
         safe_end - 1
     } else {
         safe_end
     }
 }
 
+/// Finds a valid UTF-8 character boundary at or before the target position.
+///
+/// When streaming partial JSON, the text structure can change between updates
+/// (e.g., an escape sequence being completed). This means a byte position that
+/// was valid in one partial may land inside a multi-byte character in the next.
+/// This function finds the nearest valid boundary at or before the target.
+fn find_char_boundary(text: &str, target: usize) -> usize {
+    if target >= text.len() {
+        return text.len();
+    }
+    if text.is_char_boundary(target) {
+        return target;
+    }
+    // Walk backwards to find a valid boundary.
+    let mut pos = target;
+    while pos > 0 && !text.is_char_boundary(pos) {
+        pos -= 1;
+    }
+    pos
+}
+
 fn normalize_done_chunk(mut chunk: String) -> String {
     if chunk.ends_with('\n') {
         chunk.pop();
@@ -1146,4 +1172,77 @@ mod tests {
             }]
         );
     }
+
+    #[test]
+    fn test_multibyte_char_with_trailing_backslash() {
+        // Reproduces a panic where the stored `old_text_emitted_len` from a previous
+        // partial lands inside a multi-byte UTF-8 character in the current partial.
+        //
+        // Scenario: The JSON fixer produces a literal backslash when the stream cuts
+        // mid-escape. If the *next* partial replaces that backslash with a multi-byte
+        // character (e.g., em-dash '—'), the stored byte position is no longer valid.
+        let mut parser = StreamingParser::default();
+
+        // First partial: text ends with backslash (held back by safe_emit_end).
+        // "abc" = 3 bytes, backslash held back, so emitted_len = 3.
+        let events = parser.push_edits(&[PartialEdit {
+            old_text: Some("abc\\".into()),
+            new_text: None,
+        }]);
+        assert_eq!(
+            events.as_slice(),
+            &[EditEvent::OldTextChunk {
+                edit_index: 0,
+                chunk: "abc".into(),
+                done: false,
+            }]
+        );
+
+        // Second partial: the backslash is replaced by em-dash '—' (3 bytes: E2 80 94).
+        // "ab—" = 2 + 3 = 5 bytes total, with em-dash at bytes 2..5.
+        // The stored emitted_len (3) is inside the em-dash!
+        // This should NOT panic.
+        let events = parser.push_edits(&[PartialEdit {
+            old_text: Some("ab—".into()),
+            new_text: None,
+        }]);
+        // The parser should handle this gracefully.
+        let _ = events;
+    }
+
+    #[test]
+    fn test_emitted_len_inside_multibyte_char_boundary() {
+        // More direct reproduction: emitted_len points inside a multi-byte character.
+        //
+        // This can happen when:
+        // 1. First partial has text where byte N is a valid boundary
+        // 2. Second partial has *different* text where byte N is inside a multi-byte char
+        let mut parser = StreamingParser::default();
+
+        // First partial: "ab" (2 bytes), backslash held back.
+        // After processing: emitted_len = 2
+        let events = parser.push_edits(&[PartialEdit {
+            old_text: Some("ab\\".into()),
+            new_text: None,
+        }]);
+        assert_eq!(
+            events.as_slice(),
+            &[EditEvent::OldTextChunk {
+                edit_index: 0,
+                chunk: "ab".into(),
+                done: false,
+            }]
+        );
+
+        // Second partial: "a—" where em-dash starts at byte 1 and spans bytes 1-3.
+        // Stored emitted_len = 2, but byte 2 is inside the em-dash!
+        // This should NOT panic.
+        let events = parser.push_edits(&[PartialEdit {
+            old_text: Some("a—".into()),
+            new_text: None,
+        }]);
+        // The parser should handle this gracefully.
+        // We don't care exactly what it emits, just that it doesn't panic.
+        let _ = events;
+    }
 }