fix(web): slice fetched HTML on UTF-8 char boundaries

Lowercasing non-ASCII text can change byte length, so offsets found in the
lowercased copy panicked when reused on the original (seen on 103 in
parse_ddg_results / strip_html). Use ASCII-only lowercasing, clamp slice
ends to char boundaries, and stop parsing on truncated result tags.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/goose/src/agents/platform_extensions/web.rs

@@ -60,32 +60,45 @@ impl WebClient {
         })
     }
 
+    fn floor_char_boundary(s: &str, index: usize) -> usize {
+        if index >= s.len() {
+            return s.len();
+        }
+        let mut i = index;
+        while !s.is_char_boundary(i) {
+            i -= 1;
+        }
+        i
+    }
+
     fn truncate(s: String) -> String {
         if s.len() > MAX_OUTPUT_CHARS {
-            format!("{}\n[output truncated]", &s[..MAX_OUTPUT_CHARS])
+            let end = Self::floor_char_boundary(&s, MAX_OUTPUT_CHARS);
+            format!("{}\n[output truncated]", &s[..end])
         } else {
             s
         }
     }
 
+    // Byte offsets found in the lowercased copy are reused to slice the original,
+    // so lowercasing must preserve byte length: ASCII-only.
+    fn remove_blocks(mut text: String, open: &str, close: &str) -> String {
+        loop {
+            let lower = text.to_ascii_lowercase();
+            let Some(start) = lower.find(open) else {
+                break;
+            };
+            let Some(end) = lower[start..].find(close) else {
+                break;
+            };
+            text.replace_range(start..start + end + close.len(), "");
+        }
+        text
+    }
+
     fn strip_html(html: &str) -> String {
-        let mut text = html.to_string();
-
-        while let Some(start) = text.to_lowercase().find("<script") {
-            if let Some(end) = text.to_lowercase()[start..].find("</script>") {
-                text = format!("{}{}", &text[..start], &text[start + end + 9..]);
-            } else {
-                break;
-            }
-        }
-
-        while let Some(start) = text.to_lowercase().find("<style") {
-            if let Some(end) = text.to_lowercase()[start..].find("</style>") {
-                text = format!("{}{}", &text[..start], &text[start + end + 8..]);
-            } else {
-                break;
-            }
-        }
+        let text = Self::remove_blocks(html.to_string(), "<script", "</script>");
+        let text = Self::remove_blocks(text, "<style", "</style>");
 
         let mut result = String::new();
         let mut in_tag = false;
@@ -183,7 +196,7 @@ impl WebClient {
 
     fn parse_ddg_results(html: &str, max: usize) -> String {
         let mut results = Vec::new();
-        let lower = html.to_lowercase();
+        let lower = html.to_ascii_lowercase();
 
         let mut pos = 0;
         while results.len() < max {
@@ -194,15 +207,15 @@ impl WebClient {
 
             let before = &html[..link_start + search_str.len()];
             let a_start = before.rfind('<').unwrap_or(0);
-            let a_block = &html[a_start
-                ..link_start
-                    + search_str.len()
-                    + 100.min(html.len() - link_start - search_str.len())];
+            let a_end = Self::floor_char_boundary(html, link_start + search_str.len() + 100);
+            let a_block = &html[a_start..a_end];
 
             let href = Self::extract_attr(a_block, "href").unwrap_or_default();
 
             let after_a_tag_end = link_start + search_str.len();
-            let close_offset = html[after_a_tag_end..].find('>').unwrap_or(0);
+            let Some(close_offset) = html[after_a_tag_end..].find('>') else {
+                break;
+            };
             let content_start = after_a_tag_end + close_offset + 1;
             let close_a = html[content_start..].find("</a>").unwrap_or(0);
             let title = Self::strip_html(&html[content_start..content_start + close_a]);
@@ -247,7 +260,7 @@ impl WebClient {
 
     fn extract_attr(tag: &str, attr: &str) -> Option<String> {
         let search = format!("{attr}=\"");
-        let lower = tag.to_lowercase();
+        let lower = tag.to_ascii_lowercase();
         let start = lower.find(&search)? + search.len();
         let end = tag[start..].find('"')?;
         Some(tag[start..start + end].to_string())
@@ -329,3 +342,63 @@ impl McpClientTrait for WebClient {
         Some(&self.info)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::WebClient;
+    use super::MAX_OUTPUT_CHARS;
+
+    #[test]
+    fn truncate_does_not_split_multibyte_chars() {
+        let s = format!("a{}", "中".repeat(MAX_OUTPUT_CHARS));
+        let out = WebClient::truncate(s);
+        assert!(out.ends_with("\n[output truncated]"));
+        let body = out.trim_end_matches("\n[output truncated]");
+        assert!(body.len() <= MAX_OUTPUT_CHARS);
+        assert!(body.chars().skip(1).all(|c| c == '中'));
+    }
+
+    #[test]
+    fn truncate_keeps_short_input() {
+        assert_eq!(WebClient::truncate("短文本".to_string()), "短文本");
+    }
+
+    #[test]
+    fn strip_html_handles_chars_that_change_length_when_lowercased() {
+        let html =
+            "<p>İstanbul Ⱥ 中文</p><SCRIPT>var x = 'İ';</SCRIPT><Style>.a{}</Style><p>尾部</p>";
+        let text = WebClient::strip_html(html);
+        assert_eq!(text, "İstanbul Ⱥ 中文尾部");
+    }
+
+    #[test]
+    fn strip_html_keeps_text_when_block_is_unclosed() {
+        assert_eq!(WebClient::strip_html("<p>前</p><script>x"), "前x");
+    }
+
+    #[test]
+    fn parse_ddg_results_handles_multibyte_near_link() {
+        let html = format!(
+            "<div>İİİ<a class=\"result__a\" href=\"https://example.com/中\">{}标题</a>\
+             <a class=\"result__snippet\">摘要 İ</a></div>",
+            "长".repeat(60)
+        );
+        let out = WebClient::parse_ddg_results(&html, 5);
+        assert!(out.starts_with("1. "));
+        assert!(out.contains("标题"));
+        assert!(out.contains("https://example.com/中"));
+        assert!(out.contains("摘要 İ"));
+    }
+
+    #[test]
+    fn parse_ddg_results_handles_truncated_link_tag() {
+        assert_eq!(
+            WebClient::parse_ddg_results("<a class=\"result__a\"", 5),
+            "No results found."
+        );
+        assert_eq!(
+            WebClient::parse_ddg_results("<a class=\"result__a\"中文", 5),
+            "No results found."
+        );
+    }
+}