diff --git a/src/core/assembler.ts b/src/core/assembler.ts
new file mode 100644
index 0000000..717c726
--- /dev/null
+++ b/src/core/assembler.ts
@@ -0,0 +1,102 @@
+import { existsSync } from "node:fs";
+import { readFile } from "node:fs/promises";
+import { join } from "node:path";
+import type { RuneConfig } from "../types.ts";
+import { getChangeDir } from "./config.ts";
+import { parseTasks } from "./task-parser.ts";
+
+export function assembleDiscussPrompt(config: RuneConfig): string {
+  const discuss = config.stages.discuss;
+  if (!discuss) throw new Error("discuss 阶段未配置");
+  return discuss.prompt;
+}
+
+export async function assemblePlanPrompt(
+  config: RuneConfig,
+  projectRoot: string,
+  changeName: string,
+): Promise<string> {
+  const plan = config.stages.plan;
+  if (!plan) throw new Error("plan 阶段未配置");
+
+  const changeDir = getChangeDir(projectRoot, changeName);
+  const parts: string[] = [];
+
+  parts.push(`# 规划阶段：${changeName}\n`);
+  parts.push("请为当前变更生成以下文档：\n");
+
+  for (const doc of plan.documents) {
+    parts.push(`## 文档：${doc.name}.md`);
+    parts.push(doc.prompt);
+
+    const docPath = join(changeDir, `${doc.name}.md`);
+    if (existsSync(docPath)) {
+      const existing = await readFile(docPath, "utf-8");
+      parts.push(`\n### 已有内容（请在此基础上修订）：\n${existing}`);
+    }
+
+    if (doc.template) {
+      const rendered = doc.template.replace(/\{\{change-name\}\}/g, changeName);
+      parts.push(`\n### 格式模板：\n${rendered}`);
+    }
+
+    parts.push("");
+  }
+
+  parts.push(`请将文档写入目录：${changeDir}`);
+  return parts.join("\n");
+}
+
+export async function assembleBuildPrompt(
+  config: RuneConfig,
+  projectRoot: string,
+  changeName: string,
+): Promise<string> {
+  const build = config.stages.build;
+  if (!build) throw new Error("build 阶段未配置");
+
+  const changeDir = getChangeDir(projectRoot, changeName);
+  const taskPath = join(changeDir, "task.md");
+
+  let taskContent: string;
+  try {
+    taskContent = await readFile(taskPath, "utf-8");
+  } catch {
+    throw new Error(`task.md not found in ${changeDir}`);
+  }
+
+  const tasks = parseTasks(taskContent);
+  const pendingTasks = tasks.filter((t) => !t.checked);
+
+  if (pendingTasks.length === 0) {
+    return `所有任务已完成。变更 "${changeName}" 可以归档。`;
+  }
+
+  const parts: string[] = [];
+  parts.push(`# 构建阶段：${changeName}\n`);
+  parts.push(build.prompt);
+  parts.push(`\n## 任务列表\n`);
+  parts.push(taskContent);
+  parts.push(`\n## 待执行任务（共 ${pendingTasks.length} 项）`);
+  for (const task of pendingTasks) {
+    parts.push(`- [ ] ${task.text}`);
+  }
+  parts.push(
+    `\n请从第一个待执行任务开始。完成后更新 ${taskPath} 中的 checkbox。`,
+  );
+
+  return parts.join("\n");
+}
+
+export function assembleArchivePrompt(
+  config: RuneConfig,
+  changeName: string,
+): string {
+  const archive = config.stages.archive;
+  if (!archive) throw new Error("archive 阶段未配置");
+
+  const parts: string[] = [];
+  parts.push(`# 归档阶段：${changeName}\n`);
+  parts.push(archive.prompt);
+  return parts.join("\n");
+}
diff --git a/tests/core/assembler.test.ts b/tests/core/assembler.test.ts
new file mode 100644
index 0000000..5ea71e0
--- /dev/null
+++ b/tests/core/assembler.test.ts
@@ -0,0 +1,144 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+import { mkdir, writeFile, rm } from "node:fs/promises";
+import { join } from "node:path";
+import {
+  assembleDiscussPrompt,
+  assemblePlanPrompt,
+  assembleBuildPrompt,
+  assembleArchivePrompt,
+} from "../../src/core/assembler.ts";
+import type { RuneConfig } from "../../src/types.ts";
+import { defaultConfig } from "../../src/defaults/config.ts";
+
+const TMP_DIR = join(import.meta.dir, "__tmp_assembler_test__");
+
+beforeEach(async () => {
+  await mkdir(TMP_DIR, { recursive: true });
+});
+
+afterEach(async () => {
+  await rm(TMP_DIR, { recursive: true, force: true });
+});
+
+describe("assembleDiscussPrompt", () => {
+  it("返回默认 discuss 提示词", () => {
+    const prompt = assembleDiscussPrompt(defaultConfig);
+    expect(prompt).toBeTruthy();
+    expect(prompt).toContain("软件架构师");
+  });
+
+  it("返回自定义 discuss 提示词", () => {
+    const config: RuneConfig = {
+      stages: { discuss: { prompt: "自定义讨论" } },
+    };
+    const prompt = assembleDiscussPrompt(config);
+    expect(prompt).toBe("自定义讨论");
+  });
+});
+
+describe("assemblePlanPrompt", () => {
+  it("包含变更名称和文档指引", async () => {
+    const prompt = await assemblePlanPrompt(
+      defaultConfig,
+      TMP_DIR,
+      "user-auth",
+    );
+    expect(prompt).toContain("user-auth");
+    expect(prompt).toContain("design");
+    expect(prompt).toContain("task");
+    expect(prompt).toContain("格式模板");
+  });
+
+  it("包含已有文档内容（重复 plan 场景）", async () => {
+    const changeDir = join(TMP_DIR, ".rune", "changes", "user-auth");
+    await mkdir(changeDir, { recursive: true });
+    await writeFile(join(changeDir, "design.md"), "# 已有设计");
+    const prompt = await assemblePlanPrompt(
+      defaultConfig,
+      TMP_DIR,
+      "user-auth",
+    );
+    expect(prompt).toContain("已有设计");
+    expect(prompt).toContain("在此基础上修订");
+  });
+
+  it("替换模板中的 {{change-name}}", async () => {
+    const prompt = await assemblePlanPrompt(
+      defaultConfig,
+      TMP_DIR,
+      "user-auth",
+    );
+    expect(prompt).toContain("user-auth 设计文档");
+    expect(prompt).toContain("user-auth 任务列表");
+    expect(prompt).not.toContain("{{change-name}}");
+  });
+
+  it("使用自定义 plan 配置", async () => {
+    const config: RuneConfig = {
+      stages: {
+        plan: {
+          documents: [
+            {
+              name: "spec",
+              prompt: "生成规格",
+              template: "# {{change-name}} 规格",
+            },
+          ],
+        },
+      },
+    };
+    const prompt = await assemblePlanPrompt(config, TMP_DIR, "my-feature");
+    expect(prompt).toContain("spec");
+    expect(prompt).toContain("my-feature 规格");
+    expect(prompt).not.toContain("design");
+  });
+});
+
+describe("assembleBuildPrompt", () => {
+  it("包含待执行任务列表", async () => {
+    const changeDir = join(TMP_DIR, ".rune", "changes", "user-auth");
+    await mkdir(changeDir, { recursive: true });
+    await writeFile(
+      join(changeDir, "task.md"),
+      `- [x] 任务一\n- [ ] 任务二\n- [ ] 任务三`,
+    );
+    const prompt = await assembleBuildPrompt(
+      defaultConfig,
+      TMP_DIR,
+      "user-auth",
+    );
+    expect(prompt).toContain("任务二");
+    expect(prompt).toContain("待执行任务");
+    expect(prompt).toContain("共 2 项");
+  });
+
+  it("所有任务完成时提示可归档", async () => {
+    const changeDir = join(TMP_DIR, ".rune", "changes", "user-auth");
+    await mkdir(changeDir, { recursive: true });
+    await writeFile(
+      join(changeDir, "task.md"),
+      `- [x] 任务一\n- [x] 任务二`,
+    );
+    const prompt = await assembleBuildPrompt(
+      defaultConfig,
+      TMP_DIR,
+      "user-auth",
+    );
+    expect(prompt).toContain("已完成");
+    expect(prompt).toContain("归档");
+  });
+
+  it("task.md 不存在时抛出错误", async () => {
+    await expect(
+      assembleBuildPrompt(defaultConfig, TMP_DIR, "nonexistent"),
+    ).rejects.toThrow("task.md not found");
+  });
+});
+
+describe("assembleArchivePrompt", () => {
+  it("返回归档提示词", () => {
+    const prompt = assembleArchivePrompt(defaultConfig, "user-auth");
+    expect(prompt).toContain("user-auth");
+    expect(prompt).toContain("归档");
+  });
+});