feat(core): improve activate_skill tool and use lowercase XML tags (#16009)

packages/core/src/tools/activate-skill.ts

@@ -18,6 +18,7 @@ import type {
 import { BaseDeclarativeTool, BaseToolInvocation, Kind } from './tools.js';
 import type { Config } from '../config/config.js';
 import { ACTIVATE_SKILL_TOOL_NAME } from './tool-names.js';
+import { ToolErrorType } from './tool-error.js';
 
 /**
  * Parameters for the ActivateSkill tool
@@ -51,7 +52,7 @@ class ActivateSkillToolInvocation extends BaseToolInvocation<
     if (skill) {
       return `"${skillName}": ${skill.description}`;
     }
-    return `"${skillName}" (⚠️ unknown skill)`;
+    return `"${skillName}" (?) unknown skill`;
   }
 
   private async getOrFetchFolderStructure(
@@ -107,9 +108,15 @@ ${folderStructure}`,
 
     if (!skill) {
       const skills = skillManager.getSkills();
+      const availableSkills = skills.map((s) => s.name).join(', ');
+      const errorMessage = `Skill "${skillName}" not found. Available skills are: ${availableSkills}`;
       return {
-        llmContent: `Error: Skill "${skillName}" not found. Available skills are: ${skills.map((s) => s.name).join(', ')}`,
-        returnDisplay: `Skill "${skillName}" not found.`,
+        llmContent: `Error: ${errorMessage}`,
+        returnDisplay: `Error: ${errorMessage}`,
+        error: {
+          message: errorMessage,
+          type: ToolErrorType.INVALID_TOOL_PARAMS,
+        },
       };
     }
 
@@ -126,15 +133,15 @@ ${folderStructure}`,
     );
 
     return {
-      llmContent: `<ACTIVATED_SKILL name="${skillName}">
-  <INSTRUCTIONS>
+      llmContent: `<activated_skill name="${skillName}">
+  <instructions>
     ${skill.body}
-  </INSTRUCTIONS>
+  </instructions>
 
-  <AVAILABLE_RESOURCES>
+  <available_resources>
     ${folderStructure}
-  </AVAILABLE_RESOURCES>
-</ACTIVATED_SKILL>`,
+  </available_resources>
+</activated_skill>`,
       returnDisplay: `Skill **${skillName}** activated. Resources loaded from \`${path.dirname(skill.location)}\`:\n\n${folderStructure}`,
     };
   }
@@ -169,10 +176,15 @@ export class ActivateSkillTool extends BaseDeclarativeTool<
       });
     }
 
+    const availableSkillsHint =
+      skillNames.length > 0
+        ? ` (Available: ${skillNames.map((n) => `'${n}'`).join(', ')})`
+        : '';
+
     super(
       ActivateSkillTool.Name,
       'Activate Skill',
-      "Activates a specialized agent skill by name. Returns the skill's instructions wrapped in `<ACTIVATED_SKILL>` tags. These provide specialized guidance for the current task. Use this when you identify a task that matches a skill's description.",
+      `Activates a specialized agent skill by name${availableSkillsHint}. Returns the skill's instructions wrapped in \`<activated_skill>\` tags. These provide specialized guidance for the current task. Use this when you identify a task that matches a skill's description. ONLY use names exactly as they appear in the \`<available_skills>\` section.`,
       Kind.Other,
       zodToJsonSchema(schema),
       messageBus,