Give skill instructions in system prompt

Author: jahooma

agents/base2/base2.ts

@@ -63,6 +63,7 @@ export function createBase2(
       'propose_write_file',
       !noAskUser && 'ask_user',
       'set_output',
+      'skill',
     ),
     spawnableAgents: buildArray(
       !isMax && 'file-picker',
@@ -232,6 +233,7 @@ ${isDefault || isMax
 
 ${PLACEHOLDER.FILE_TREE_PROMPT_SMALL}
 ${PLACEHOLDER.KNOWLEDGE_FILES_CONTENTS}
+${PLACEHOLDER.SKILLS_PROMPT}
 ${PLACEHOLDER.SYSTEM_INFO_PROMPT}
 
 # Initial Git Changes

agents/types/secret-agent-definition.ts

@@ -7,6 +7,7 @@ export type AllToolNames =
   | 'add_subgoal'
   | 'browser_logs'
   | 'create_plan'
+  | 'skill'
   | 'spawn_agent_inline'
   | 'update_subgoal'
 
@@ -31,6 +32,7 @@ const placeholderNames = [
   'KNOWLEDGE_FILES_CONTENTS',
   'PROJECT_ROOT',
   'REMAINING_STEPS',
+  'SKILLS_PROMPT',
   'SYSTEM_INFO_PROMPT',
   'TOOLS_PROMPT',
   'USER_CWD',

common/src/util/skills.ts

@@ -3,7 +3,7 @@ import type { SkillsMap } from '../types/skill'
 /**
  * Escapes special XML characters in a string.
  */
-function escapeXml(str: string): string {
+export function escapeXml(str: string): string {
   return str
     .replace(/&/g, '&')
     .replace(/</g, '&lt;')
@@ -30,3 +30,29 @@ export function formatAvailableSkillsXml(skills: SkillsMap): string {
 
   return `<available_skills>\n${skillsXml}\n</available_skills>`
 }
+
+/**
+ * Formats skills as a system prompt section for injection into agent prompts.
+ * Returns a markdown section with available skills and instructions on using the skill tool.
+ * Returns empty string if no skills are available.
+ */
+export function formatSkillsSystemPrompt(skills: SkillsMap | undefined): string {
+  if (!skills) {
+    return ''
+  }
+
+  const skillEntries = Object.values(skills)
+  if (skillEntries.length === 0) {
+    return ''
+  }
+
+  const skillsXml = formatAvailableSkillsXml(skills)
+
+  return `# Available Skills
+
+The following skills are available to help you complete tasks. Each skill provides specialized instructions and behaviors.
+
+${skillsXml}
+
+Use the \`skill\` tool to load a skill's full instructions when relevant to the current task. Skills are loaded on-demand - only load them when you need their specific guidance. Always load any relevant skills immediately: You should bias toward loading too many skills as early as possible.`
+}

packages/agent-runtime/src/templates/strings.ts

@@ -1,4 +1,5 @@
 import { KNOWLEDGE_FILE_NAMES_LOWERCASE } from '@codebuff/common/constants/knowledge'
+import { formatSkillsSystemPrompt } from '@codebuff/common/util/skills'
 import { escapeString } from '@codebuff/common/util/string'
 import { z } from 'zod/v4'
 
@@ -131,6 +132,7 @@ export async function formatPrompt(
           return `\`\`\`${path}\n${content.trim()}\n\`\`\``
         })
         .join('\n\n'),
+    [PLACEHOLDER.SKILLS_PROMPT]: () => formatSkillsSystemPrompt(fileContext.skills),
   }
 
   for (const varName of placeholderValues) {

packages/agent-runtime/src/templates/types.ts

@@ -21,6 +21,7 @@ const placeholderNames = [
   'KNOWLEDGE_FILES_CONTENTS',
   'PROJECT_ROOT',
   'REMAINING_STEPS',
+  'SKILLS_PROMPT',
   'SYSTEM_INFO_PROMPT',
   'USER_CWD',
   'USER_INPUT_PROMPT',