fix(openab-agent): use XML format for skills prompt

openab-agent/src/skills.rs

@@ -99,20 +99,38 @@ fn parse_frontmatter(content: &str) -> Option<(String, String)> {
     Some((name, description))
 }
 
+/// Escape XML special characters in user-controlled strings to preserve prompt structure.
+fn xml_escape(s: &str) -> String {
+    s.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+}
+
 /// Format skills as a system prompt section listing available skills.
 pub fn format_skills_prompt(skills: &[Skill]) -> String {
     if skills.is_empty() {
         return String::new();
     }
-    let mut out = String::from("\n\n## Available Skills\n\nThe following skills are available. Use the `read` tool to load the full SKILL.md when you need a skill's instructions.\n\n");
-    for skill in skills {
+
+    // Sort skills by name to ensure deterministic output
+    let mut sorted_skills = skills.to_vec();
+    sorted_skills.sort_by(|a, b| a.name.cmp(&b.name));
+
+    let mut out = String::from(concat!(
+        "\n\n## Skills\n\n",
+        "Before replying, scan the skills below. If one matches or may be relevant to the user's task, ",
+        "use the `read` tool to load the full SKILL.md at the listed location and follow its instructions.\n\n",
+        "<available_skills>\n",
+    ));
+    for skill in sorted_skills {
         out.push_str(&format!(
-            "- **{}** ({}): {}\n",
-            skill.name,
-            skill.path.join("SKILL.md").display(),
-            skill.description
+            "  <skill>\n    <name>{}</name>\n    <description>{}</description>\n    <location>{}</location>\n  </skill>\n",
+            xml_escape(&skill.name),
+            xml_escape(&skill.description),
+            xml_escape(&skill.path.join("SKILL.md").to_string_lossy()),
         ));
     }
+    out.push_str("</available_skills>\n");
     out
 }
 
@@ -210,15 +228,51 @@ mod tests {
     }
 
     #[test]
-    fn format_skills_prompt_includes_path() {
+    fn format_skills_prompt_uses_xml_format() {
         let skills = vec![Skill {
             name: "test".to_string(),
             description: "A test skill".to_string(),
             path: PathBuf::from("/home/agent/.openab/skills/test"),
         }];
         let prompt = format_skills_prompt(&skills);
-        assert!(prompt.contains("test"));
-        assert!(prompt.contains("A test skill"));
-        assert!(prompt.contains("SKILL.md"));
+        assert!(prompt.contains("<available_skills>"));
+        assert!(prompt.contains("<name>test</name>"));
+        assert!(prompt.contains("<description>A test skill</description>"));
+        assert!(prompt.contains("<location>/home/agent/.openab/skills/test/SKILL.md</location>"));
+        assert!(prompt.contains("</available_skills>"));
+        assert!(prompt.contains("Before replying, scan the skills below"));
+    }
+
+    #[test]
+    fn format_skills_prompt_escapes_xml_chars() {
+        let skills = vec![Skill {
+            name: "a<b".to_string(),
+            description: "Use <tag> & \"quotes\" </description> injection test".to_string(),
+            path: PathBuf::from("/skills/test&cool"),
+        }];
+        let prompt = format_skills_prompt(&skills);
+        assert!(prompt.contains("<name>a&lt;b</name>"));
+        assert!(prompt.contains("<description>Use &lt;tag&gt; &amp; \"quotes\" &lt;/description&gt; injection test</description>"));
+        assert!(prompt.contains("<location>/skills/test&amp;cool/SKILL.md</location>"));
+    }
+
+    #[test]
+    fn format_skills_prompt_is_deterministic() {
+        let skills = vec![
+            Skill {
+                name: "zoo".to_string(),
+                description: "Zoo skill".to_string(),
+                path: PathBuf::from("/skills/zoo"),
+            },
+            Skill {
+                name: "apple".to_string(),
+                description: "Apple skill".to_string(),
+                path: PathBuf::from("/skills/apple"),
+            },
+        ];
+        let prompt = format_skills_prompt(&skills);
+        let apple_idx = prompt.find("<name>apple</name>").unwrap();
+        let zoo_idx = prompt.find("<name>zoo</name>").unwrap();
+        assert!(apple_idx < zoo_idx);
     }
 }