diff --git a/electron/main/ai/agent/prompt-builder.ts b/electron/main/ai/agent/prompt-builder.ts
index 8b33ede4..bf253480 100644
--- a/electron/main/ai/agent/prompt-builder.ts
+++ b/electron/main/ai/agent/prompt-builder.ts
@@ -1,107 +1,29 @@
 /**
- * Agent 系统提示词构建
+ * Agent system prompt builder — Electron adapter.
+ * Injects the Electron i18n `t` function into the shared builder.
  */
 
 import { t as i18nT } from '../../i18n'
-import type { OwnerInfo } from '../tools/types'
-import type { SkillContext } from './types'
-import type { ToolContext } from '../tools/types'
+import { buildSystemPrompt as buildSystemPromptCore } from '@openchatlab/node-runtime'
+import type { OwnerInfo, SkillContext, MentionedMember } from '@openchatlab/node-runtime'
 
-function agentT(key: string, locale: string, options?: Record<string, unknown>): string {
-  return i18nT(key, { lng: locale, ...options })
-}
+export type { OwnerInfo, SkillContext, MentionedMember }
 
-/**
- * 获取系统锁定部分的提示词（策略说明、时间处理等）
- *
- * 工具定义通过 Function Calling 的 tools 参数传递给 LLM，
- * 无需在 System Prompt 中重复描述。
- */
-function getLockedPromptSection(
-  chatType: 'group' | 'private',
-  ownerInfo?: OwnerInfo,
-  locale: string = 'zh-CN',
-  mentionedMembers?: ToolContext['mentionedMembers']
-): string {
-  const now = new Date()
-  const dateLocale = locale.startsWith('zh') ? 'zh-CN' : 'en-US'
-  const currentDate = now.toLocaleDateString(dateLocale, {
-    year: 'numeric',
-    month: 'long',
-    day: 'numeric',
-    weekday: 'long',
-  })
-
-  const isPrivate = chatType === 'private'
-  const chatContext = agentT(`ai.agent.chatContext.${chatType}`, locale)
-
-  const ownerNote = ownerInfo
-    ? agentT('ai.agent.ownerNote', locale, {
-        displayName: ownerInfo.displayName,
-        platformId: ownerInfo.platformId,
-        chatContext,
-      })
-    : ''
-
-  const memberNote = isPrivate
-    ? agentT('ai.agent.memberNotePrivate', locale)
-    : agentT('ai.agent.memberNoteGroup', locale)
-
-  const mentionedMembersNote =
-    mentionedMembers && mentionedMembers.length > 0
-      ? `${agentT('ai.agent.mentionedMembersNote', locale)}\n${mentionedMembers
-          .map((member) => {
-            const aliasPart = member.aliases.length > 0 ? ` | aliases=${member.aliases.join(',')}` : ''
-            return `- member_id=${member.memberId} | mention=${member.mentionText} | display_name=${member.displayName} | platform_id=${member.platformId}${aliasPart}`
-          })
-          .join('\n')}\n`
-      : ''
-
-  const year = now.getFullYear()
-
-  return `${agentT('ai.agent.currentDateIs', locale)} ${currentDate}。
-${ownerNote}
-${mentionedMembersNote}
-${memberNote}
-${agentT('ai.agent.timeParamsIntro', locale)}
-${agentT('ai.agent.defaultYearNote', locale, { year })}
-
-${agentT('ai.agent.responseInstruction', locale)}`
-}
-
-function getFallbackRoleDefinition(chatType: 'group' | 'private', locale: string = 'zh-CN'): string {
-  return agentT(`ai.agent.fallbackRoleDefinition.${chatType}`, locale)
-}
-
-/**
- * 构建完整的系统提示词
- *
- * 系统提示词优先使用当前助手配置；若助手不存在，再退回内置默认角色定义。
- *
- * 最终格式：{助手系统提示词}\n\n{系统锁定段(日期/owner/时间参数/通用指引)}
- */
 export function buildSystemPrompt(
   chatType: 'group' | 'private' = 'group',
   assistantSystemPrompt?: string,
   ownerInfo?: OwnerInfo,
   locale: string = 'zh-CN',
   skillCtx?: SkillContext,
-  mentionedMembers?: ToolContext['mentionedMembers']
+  mentionedMembers?: MentionedMember[]
 ): string {
-  const systemPrompt = assistantSystemPrompt || getFallbackRoleDefinition(chatType, locale)
-  const lockedSection = getLockedPromptSection(chatType, ownerInfo, locale, mentionedMembers)
-
-  let skillSection = ''
-  if (skillCtx?.skillDef) {
-    skillSection =
-      `\n## ${agentT('ai.agent.currentTask', locale)}：${skillCtx.skillDef.name}\n` +
-      `${agentT('ai.agent.skillPriorityNote', locale)}\n` +
-      skillCtx.skillDef.prompt
-  } else if (skillCtx?.skillMenu) {
-    skillSection = `\n${skillCtx.skillMenu}`
-  }
-
-  return `${systemPrompt}${skillSection}
-
-${lockedSection}`
+  return buildSystemPromptCore({
+    t: i18nT,
+    chatType,
+    assistantSystemPrompt,
+    ownerInfo,
+    locale,
+    skillCtx,
+    mentionedMembers,
+  })
 }
diff --git a/packages/node-runtime/src/ai/agent/__tests__/prompt-builder.test.ts b/packages/node-runtime/src/ai/agent/__tests__/prompt-builder.test.ts
new file mode 100644
index 00000000..182ca105
--- /dev/null
+++ b/packages/node-runtime/src/ai/agent/__tests__/prompt-builder.test.ts
@@ -0,0 +1,84 @@
+/**
+ * Tests for shared prompt-builder.
+ *
+ * Run: npx tsx --test packages/node-runtime/src/ai/agent/__tests__/prompt-builder.test.ts
+ */
+
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+import { buildSystemPrompt } from '../prompt-builder'
+import type { TranslateFn } from '../prompt-builder'
+
+const mockT: TranslateFn = (key, options) => {
+  if (key.startsWith('ai.agent.fallbackRoleDefinition')) return `[fallback:${key}]`
+  if (key === 'ai.agent.currentDateIs') return 'Current date is'
+  if (key === 'ai.agent.ownerNote') return `Owner: ${(options as Record<string, unknown>)?.displayName}`
+  if (key === 'ai.agent.memberNoteGroup') return '[member-group]'
+  if (key === 'ai.agent.memberNotePrivate') return '[member-private]'
+  if (key === 'ai.agent.timeParamsIntro') return '[time-params]'
+  if (key === 'ai.agent.defaultYearNote') return `[year:${(options as Record<string, unknown>)?.year}]`
+  if (key === 'ai.agent.responseInstruction') return '[response-instruction]'
+  if (key === 'ai.agent.mentionedMembersNote') return 'Mentioned members:'
+  if (key === 'ai.agent.currentTask') return 'Current task'
+  if (key === 'ai.agent.skillPriorityNote') return '[skill-priority]'
+  return `[${key}]`
+}
+
+describe('buildSystemPrompt', () => {
+  it('uses fallback role when no assistant prompt', () => {
+    const result = buildSystemPrompt({ t: mockT, chatType: 'group' })
+    assert.ok(result.includes('[fallback:ai.agent.fallbackRoleDefinition.group]'))
+  })
+
+  it('uses assistant prompt when provided', () => {
+    const result = buildSystemPrompt({ t: mockT, assistantSystemPrompt: 'Custom assistant' })
+    assert.ok(result.includes('Custom assistant'))
+    assert.ok(!result.includes('[fallback'))
+  })
+
+  it('includes owner info when provided', () => {
+    const result = buildSystemPrompt({
+      t: mockT,
+      ownerInfo: { displayName: 'Alice', platformId: 'alice123' },
+    })
+    assert.ok(result.includes('Owner: Alice'))
+  })
+
+  it('includes member note for private chat', () => {
+    const result = buildSystemPrompt({ t: mockT, chatType: 'private' })
+    assert.ok(result.includes('[member-private]'))
+  })
+
+  it('includes mentioned members', () => {
+    const result = buildSystemPrompt({
+      t: mockT,
+      mentionedMembers: [{ memberId: 1, platformId: 'p1', displayName: 'Bob', aliases: ['B'], mentionText: '@Bob' }],
+    })
+    assert.ok(result.includes('member_id=1'))
+    assert.ok(result.includes('aliases=B'))
+  })
+
+  it('includes skill definition when active', () => {
+    const result = buildSystemPrompt({
+      t: mockT,
+      skillCtx: { skillDef: { name: 'Summarizer', prompt: 'Summarize the chat.' } },
+    })
+    assert.ok(result.includes('Current task'))
+    assert.ok(result.includes('Summarizer'))
+    assert.ok(result.includes('Summarize the chat.'))
+  })
+
+  it('includes skill menu when no active skill', () => {
+    const result = buildSystemPrompt({
+      t: mockT,
+      skillCtx: { skillMenu: '[SKILL MENU TEXT]' },
+    })
+    assert.ok(result.includes('[SKILL MENU TEXT]'))
+  })
+
+  it('includes date and response instruction', () => {
+    const result = buildSystemPrompt({ t: mockT })
+    assert.ok(result.includes('Current date is'))
+    assert.ok(result.includes('[response-instruction]'))
+  })
+})
diff --git a/packages/node-runtime/src/ai/agent/prompt-builder.ts b/packages/node-runtime/src/ai/agent/prompt-builder.ts
new file mode 100644
index 00000000..419342a2
--- /dev/null
+++ b/packages/node-runtime/src/ai/agent/prompt-builder.ts
@@ -0,0 +1,126 @@
+/**
+ * Agent system prompt builder — shared implementation.
+ *
+ * The i18n translation function is injected via `t` parameter,
+ * making this module platform-agnostic.
+ */
+
+export interface OwnerInfo {
+  platformId: string
+  displayName: string
+}
+
+export interface MentionedMember {
+  memberId: number
+  platformId: string
+  displayName: string
+  aliases: string[]
+  mentionText: string
+}
+
+export interface SkillContext {
+  skillDef?: { name: string; prompt: string }
+  skillMenu?: string
+}
+
+export type TranslateFn = (key: string, options?: Record<string, unknown>) => string
+
+export interface BuildSystemPromptOptions {
+  t: TranslateFn
+  chatType?: 'group' | 'private'
+  assistantSystemPrompt?: string
+  ownerInfo?: OwnerInfo
+  locale?: string
+  skillCtx?: SkillContext
+  mentionedMembers?: MentionedMember[]
+}
+
+function agentT(t: TranslateFn, key: string, locale: string, options?: Record<string, unknown>): string {
+  return t(key, { lng: locale, ...options })
+}
+
+function getLockedPromptSection(
+  t: TranslateFn,
+  chatType: 'group' | 'private',
+  ownerInfo: OwnerInfo | undefined,
+  locale: string,
+  mentionedMembers: MentionedMember[] | undefined
+): string {
+  const now = new Date()
+  const dateLocale = locale.startsWith('zh') ? 'zh-CN' : 'en-US'
+  const currentDate = now.toLocaleDateString(dateLocale, {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+    weekday: 'long',
+  })
+
+  const isPrivate = chatType === 'private'
+  const chatContext = agentT(t, `ai.agent.chatContext.${chatType}`, locale)
+
+  const ownerNote = ownerInfo
+    ? agentT(t, 'ai.agent.ownerNote', locale, {
+        displayName: ownerInfo.displayName,
+        platformId: ownerInfo.platformId,
+        chatContext,
+      })
+    : ''
+
+  const memberNote = isPrivate
+    ? agentT(t, 'ai.agent.memberNotePrivate', locale)
+    : agentT(t, 'ai.agent.memberNoteGroup', locale)
+
+  const mentionedMembersNote =
+    mentionedMembers && mentionedMembers.length > 0
+      ? `${agentT(t, 'ai.agent.mentionedMembersNote', locale)}\n${mentionedMembers
+          .map((member) => {
+            const aliasPart = member.aliases.length > 0 ? ` | aliases=${member.aliases.join(',')}` : ''
+            return `- member_id=${member.memberId} | mention=${member.mentionText} | display_name=${member.displayName} | platform_id=${member.platformId}${aliasPart}`
+          })
+          .join('\n')}\n`
+      : ''
+
+  const year = now.getFullYear()
+
+  return `${agentT(t, 'ai.agent.currentDateIs', locale)} ${currentDate}。
+${ownerNote}
+${mentionedMembersNote}
+${memberNote}
+${agentT(t, 'ai.agent.timeParamsIntro', locale)}
+${agentT(t, 'ai.agent.defaultYearNote', locale, { year })}
+
+${agentT(t, 'ai.agent.responseInstruction', locale)}`
+}
+
+function getFallbackRoleDefinition(t: TranslateFn, chatType: 'group' | 'private', locale: string): string {
+  return agentT(t, `ai.agent.fallbackRoleDefinition.${chatType}`, locale)
+}
+
+export function buildSystemPrompt(options: BuildSystemPromptOptions): string {
+  const {
+    t,
+    chatType = 'group',
+    assistantSystemPrompt,
+    ownerInfo,
+    locale = 'zh-CN',
+    skillCtx,
+    mentionedMembers,
+  } = options
+
+  const systemPrompt = assistantSystemPrompt || getFallbackRoleDefinition(t, chatType, locale)
+  const lockedSection = getLockedPromptSection(t, chatType, ownerInfo, locale, mentionedMembers)
+
+  let skillSection = ''
+  if (skillCtx?.skillDef) {
+    skillSection =
+      `\n## ${agentT(t, 'ai.agent.currentTask', locale)}：${skillCtx.skillDef.name}\n` +
+      `${agentT(t, 'ai.agent.skillPriorityNote', locale)}\n` +
+      skillCtx.skillDef.prompt
+  } else if (skillCtx?.skillMenu) {
+    skillSection = `\n${skillCtx.skillMenu}`
+  }
+
+  return `${systemPrompt}${skillSection}
+
+${lockedSection}`
+}
diff --git a/packages/node-runtime/src/ai/index.ts b/packages/node-runtime/src/ai/index.ts
index fe849a6b..e0a28692 100644
--- a/packages/node-runtime/src/ai/index.ts
+++ b/packages/node-runtime/src/ai/index.ts
@@ -62,6 +62,16 @@ export type { PreprocessingPipelineOptions, PreprocessingPipelineResult } from '
 export type { AgentCoreOptions, AgentCoreEvent, AgentCoreResult, AgentTokenUsage, SimpleHistoryMessage } from './agent'
 export { runAgentCore } from './agent'
 
+// Agent Prompt Builder
+export { buildSystemPrompt } from './agent/prompt-builder'
+export type {
+  BuildSystemPromptOptions,
+  OwnerInfo,
+  MentionedMember,
+  SkillContext,
+  TranslateFn,
+} from './agent/prompt-builder'
+
 // LLM Model Builder
 export { buildPiModel, normalizeAnthropicBaseUrl, normalizeOpenAICompatibleBaseUrl } from './llm-builder'
 export type { PiModelConfig, BuildPiModelOptions } from './llm-builder'
diff --git a/packages/node-runtime/src/index.ts b/packages/node-runtime/src/index.ts
index 07d61d34..051203b8 100644
--- a/packages/node-runtime/src/index.ts
+++ b/packages/node-runtime/src/index.ts
@@ -88,6 +88,10 @@ export type {
 export type { AgentCoreOptions, AgentCoreEvent, AgentCoreResult, AgentTokenUsage, SimpleHistoryMessage } from './ai'
 export { runAgentCore } from './ai'
 
+// Agent Prompt Builder
+export { buildSystemPrompt } from './ai'
+export type { BuildSystemPromptOptions, OwnerInfo, MentionedMember, SkillContext, TranslateFn } from './ai'
+
 // LLM Model Builder
 export { buildPiModel, normalizeAnthropicBaseUrl, normalizeOpenAICompatibleBaseUrl } from './ai'
 export type { PiModelConfig, BuildPiModelOptions } from './ai'