diff --git a/mcp/src/lib/library-map.ts b/mcp/src/lib/library-map.ts
new file mode 100644
index 0000000..113192c
--- /dev/null
+++ b/mcp/src/lib/library-map.ts
@@ -0,0 +1,129 @@
+/**
+ * 藏書地圖（library-map）共用邏輯 — SDD M4（system-dev/docs/3-specs/library-map/design.md §4）
+ *
+ * 兩個 consumer 共用本檔：
+ *   1. kbdb_get_map MCP tool（tools/kbdb_map.ts）— 薄殼呼 GET /map／/map/:library
+ *   2. MCP server instructions 注入（mcp-handler.ts）— 連線時把全館地圖渲染成緊湊文字嵌入
+ *
+ * 走既有 KBDB service binding（kbdbFetch），不新增 binding、不碰 D1（薄殼鐵律）。
+ *
+ * 鐵律（design §4）：注入失敗絕不能讓 MCP 連線失敗——buildLibraryMapInstructions 任何錯誤
+ * （超時/HTTP 錯/空庫/JSON 壞）一律回 null，caller 靜默略過，instructions 沒地圖照常可用。
+ *
+ * 快取抉擇（design §4 給了「快取＋TTL 或每次現拉」兩個選項，這裡選 isolate 內 TTL 快取）：
+ * 本 MCP 用 stateless StreamableHTTP（sessionIdGenerator: undefined）——每個 HTTP request 都
+ * 重建 McpServer，也就是說「每次現拉」實際上是每個 tool call 都多打一次 /map，不是每條連線一次。
+ * 地圖只在 ingest recompute 後才變，所以用 module 層（isolate 內）TTL 快取：成功 5 分鐘、
+ * 失敗 1 分鐘（避免 kbdb 掛掉時每個 request 都白等 timeout）。isolate 回收即自然失效，無需失效協議。
+ */
+
+import type { Env } from "../types.js";
+import { kbdbFetch } from "./kbdb-client.js";
+
+/** 全館視圖一行（kbdb GET /map 的 libraries[] 元素；top_entities 已是 top-3 名字）。 */
+export interface LibraryMapRow {
+  library: string;
+  narrative: string | null;
+  top_entities: unknown; // 正常是 string[]；防禦：舊部署可能回 JSON 字串形
+  triplet_count: number | string;
+  updated_at?: number;
+}
+
+/**
+ * slot 值容錯 parse：kbdb 正常已回 parsed 陣列，但 slot 底層存的是 JSON 字串
+ * （live 曾觀測到字串形直出）——遇字串就 JSON.parse，parse 失敗/非陣列一律回空陣列（不 crash）。
+ */
+export function parseSlotArray<T>(raw: unknown): T[] {
+  if (Array.isArray(raw)) return raw as T[];
+  if (typeof raw === "string") {
+    try {
+      const v = JSON.parse(raw);
+      return Array.isArray(v) ? (v as T[]) : [];
+    } catch {
+      return [];
+    }
+  }
+  return [];
+}
+
+/** top_entities 統一成名字清單（元素可能是 "name" 字串或 {name, degree} 物件）。 */
+export function entityNames(raw: unknown, limit: number): string[] {
+  return parseSlotArray<unknown>(raw)
+    .map((e) => {
+      if (typeof e === "string") return e;
+      if (e && typeof e === "object" && typeof (e as { name?: unknown }).name === "string") {
+        return (e as { name: string }).name;
+      }
+      return null;
+    })
+    .filter((n): n is string => !!n)
+    .slice(0, limit);
+}
+
+/** 全館地圖渲染上限：庫行數與 narrative 截斷長度（instructions 總量控制在數百 token，R3）。 */
+const MAX_LIBRARY_LINES = 30;
+const MAX_NARRATIVE_CHARS = 60;
+
+/** 把 GET /map 的 libraries[] 渲染成緊湊文字（每庫一行，design §4 指定格式）。空清單回 null。 */
+export function renderLibraryMapLines(libraries: LibraryMapRow[]): string | null {
+  const rows = libraries.filter((l) => l && typeof l.library === "string" && l.library);
+  if (rows.length === 0) return null;
+  const lines = rows.slice(0, MAX_LIBRARY_LINES).map((l) => {
+    const narrative = (l.narrative ?? "").trim() || "（narrative 待補）";
+    const clipped =
+      narrative.length > MAX_NARRATIVE_CHARS ? `${narrative.slice(0, MAX_NARRATIVE_CHARS)}…` : narrative;
+    const core = entityNames(l.top_entities, 3);
+    const count = Number(l.triplet_count ?? 0) || 0;
+    return `- ${l.library}：${clipped}｜核心：${core.length ? core.join("、") : "（尚無）"}｜${count} triplets`;
+  });
+  const omitted = rows.length > MAX_LIBRARY_LINES ? `\n（其餘 ${rows.length - MAX_LIBRARY_LINES} 庫略，kbdb_get_map 可看全部）` : "";
+  return lines.join("\n") + omitted;
+}
+
+/** 拉 /map 的逾時上限（ms）：instructions 是加分不是依賴，不值得讓連線多等。 */
+const MAP_FETCH_TIMEOUT_MS = 1500;
+/** 快取 TTL：成功 5 分鐘（地圖只在 ingest recompute 後變）、失敗/空 1 分鐘（別每 request 白等）。 */
+const CACHE_TTL_OK_MS = 5 * 60 * 1000;
+const CACHE_TTL_FAIL_MS = 60 * 1000;
+
+let instructionsCache: { text: string | null; expiresAt: number } | null = null;
+
+/** 測試用：清掉 isolate 內快取（prod 不呼叫）。 */
+export function __resetLibraryMapInstructionsCacheForTests(): void {
+  instructionsCache = null;
+}
+
+/**
+ * 組 MCP server instructions 的藏書地圖段（design §4 / §6「session 啟動 → instructions 已含
+ * 全館地圖（push 零查詢）」）。任何失敗（超時/HTTP 錯/空庫/壞 JSON）→ null（caller 靜默略過）。
+ */
+export async function buildLibraryMapInstructions(env: Env): Promise<string | null> {
+  const now = Date.now();
+  if (instructionsCache && instructionsCache.expiresAt > now) return instructionsCache.text;
+
+  let text: string | null = null;
+  try {
+    const res = await Promise.race([
+      kbdbFetch(env, "/map"),
+      new Promise<never>((_, reject) =>
+        setTimeout(() => reject(new Error("library map fetch timeout")), MAP_FETCH_TIMEOUT_MS),
+      ),
+    ]);
+    if (res.ok) {
+      const data = (await res.json()) as { libraries?: LibraryMapRow[] };
+      const body = renderLibraryMapLines(Array.isArray(data.libraries) ? data.libraries : []);
+      if (body) {
+        text =
+          "【藏書地圖】KBDB 全館現況（每庫一行）。查資料前先看這裡定位該進哪個庫；" +
+          "需要某庫細節呼叫 kbdb_get_map(library)，不確定該查什麼時先呼叫 kbdb_get_map。\n" +
+          body;
+      }
+    }
+  } catch {
+    // 鐵律：地圖是加分不是依賴——任何錯誤都不往外拋，instructions 沒地圖照常可用。
+    text = null;
+  }
+
+  instructionsCache = { text, expiresAt: now + (text ? CACHE_TTL_OK_MS : CACHE_TTL_FAIL_MS) };
+  return text;
+}
diff --git a/mcp/src/mcp-handler.ts b/mcp/src/mcp-handler.ts
index d062552..ccd92d8 100644
--- a/mcp/src/mcp-handler.ts
+++ b/mcp/src/mcp-handler.ts
@@ -1,6 +1,7 @@
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js";
 import { registerAllTools } from "./tools/registry.js";
+import { buildLibraryMapInstructions } from "./lib/library-map.js";
 import { Env } from "./types.js";
 
 export async function handleMcpRequest(
@@ -9,8 +10,16 @@ export async function handleMcpRequest(
   orgNamespace: string,
   partnerToken: string,
 ): Promise<Response> {
+  // library-map SDD M4（design §4/§6）：連線時把全館藏書地圖嵌進 server instructions，
+  // session 一開就知道館裡有哪些庫（push 零查詢）。builder 內建 timeout＋isolate TTL 快取
+  //（選型理由見 lib/library-map.ts 檔頭）；任何失敗回 null → 靜默略過，絕不擋 MCP 連線（鐵律）。
+  const mapInstructions = await buildLibraryMapInstructions(env);
+
   const transport = new WebStandardStreamableHTTPServerTransport({ sessionIdGenerator: undefined });
-  const server = new McpServer({ name: "arcrun-mcp-server", version: "1.0.0" });
+  const server = new McpServer(
+    { name: "arcrun-mcp-server", version: "1.0.0" },
+    mapInstructions ? { instructions: mapInstructions } : undefined,
+  );
 
   registerAllTools(server, env, orgNamespace, partnerToken);
   await server.connect(transport);
diff --git a/mcp/src/tools/kbdb_map.ts b/mcp/src/tools/kbdb_map.ts
new file mode 100644
index 0000000..80397f0
--- /dev/null
+++ b/mcp/src/tools/kbdb_map.ts
@@ -0,0 +1,140 @@
+/**
+ * 藏書地圖 MCP 薄殼（library-map SDD M4；源頭 Arcrun#39）
+ *
+ * rule 07 §5（薄殼鐵律）：地圖的聚合 SQL 全住 kbdb base（actions/library-map.ts，D6 裁定），
+ * MCP 只做介面轉換——經既有 KBDB service binding（kbdbFetch）打 GET /map／/map/:library，
+ * 不碰 D1、不新增 binding。與 #68 kbdb_graph_neighbors 同族（D17 KBDB MCP 面，kbdb_* 前綴）。
+ *
+ * 端點契約（kbdb/src/routes/map.ts，M2 已 merge）：
+ *   GET /map           → { success, libraries:[{library, narrative, top_entities(名字 top3),
+ *                          triplet_count, updated_at}], count }
+ *   GET /map/:library  → { success, map:{record_id, library, narrative, content, top_entities,
+ *                          relation_profile, bridges, triplet_count, commit_hash, status, updated_at} }
+ *   404 → { success:false, error:'not found' }（該庫從未 recompute）
+ *
+ * slot 值防禦：top_entities/relation_profile/bridges 底層存 JSON 字串，正常 base 已 parse；
+ * 但仍容錯「字串形直出」（live 曾觀測）——字串就 parse、失敗當空陣列，絕不 crash（鐵律）。
+ */
+
+import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import { z } from "zod";
+import type { Env } from "../types.js";
+import { kbdbFetch } from "../lib/kbdb-client.js";
+import { errorResponse, successResponse } from "../lib/cypher-client.js";
+import { entityNames, parseSlotArray, type LibraryMapRow } from "../lib/library-map.js";
+
+/** 空庫/404 時的 backfill 指引（誠實回報＋給下一步，鐵律：不假綠）。 */
+const RECOMPUTE_HINTS = [
+  "地圖由 ingest 尾端自動重算（M3）；尚未接鏈的庫要手動 backfill：對 kbdb 呼 POST /map/recompute?library=<庫名>（可帶 body {narrative, source_prefix}）",
+  "backfill 過渡期（triplet 還沒有 library slot 值）用 source_prefix 以 source_uri 前綴歸庫，如 {\"source_prefix\":\"gitea:Leo/kb@\"}",
+];
+
+/** 註冊全部藏書地圖工具（library-map M4）。 */
+export function registerAllKbdbMapTools(server: McpServer, env: Env) {
+  registerGetMap(server, env);
+}
+
+/** 單庫詳圖回傳形狀（GET /map/:library 的 map，slot 陣列已 parse 成物件）。 */
+interface LibraryMapDetail {
+  record_id?: string;
+  library?: string;
+  narrative?: string | null;
+  content?: string | null;
+  top_entities?: unknown;
+  relation_profile?: unknown;
+  bridges?: unknown;
+  triplet_count?: number | string;
+  commit_hash?: string | null;
+  status?: string;
+  updated_at?: number;
+}
+
+/**
+ * kbdb_get_map — 藏書地圖。無參數＝全館（每庫一行）；帶 library＝該庫詳圖。
+ * design §6 retrieval 流程的第一站：地圖 → get_map(library) 細節 → graph/search 進庫。
+ */
+export function registerGetMap(server: McpServer, env: Env) {
+  server.tool(
+    "kbdb_get_map",
+    "藏書地圖：KBDB 全館導覽。不帶參數＝全館地圖（每庫一行：庫名＋narrative＋核心 top 3 entities＋" +
+      "triplet 數），帶 library 參數＝該庫詳圖（完整 top_entities/relation_profile/跨庫 bridges）。" +
+      "不確定該查什麼時，先呼叫此工具——先看地圖定位該進哪個庫，再用 kbdb_search（關鍵字/語義）或 " +
+      "kbdb_graph_neighbors（關係遍歷）進庫查細節。",
+    {
+      library: z.string().min(1).optional().describe(
+        "庫名（如 'kb'／'notes'）。帶了回該庫詳圖；不帶回全館地圖（先看全館再挑庫）",
+      ),
+      owner_id: z.string().optional().describe("限定某資料歸屬範圍（選填，與其他 kbdb_* 工具同義）"),
+    },
+    async ({ library, owner_id }) => {
+      try {
+        const qs = owner_id ? `?owner_id=${encodeURIComponent(owner_id)}` : "";
+
+        if (!library) {
+          // 全館地圖：每庫一行（library＋narrative＋top 3 entities＋triplet_count）。
+          const res = await kbdbFetch(env, `/map${qs}`);
+          if (!res.ok) {
+            return errorResponse(
+              "map_fetch_failed",
+              `取全館地圖失敗 HTTP ${res.status}`,
+              ["稍後重試", ...RECOMPUTE_HINTS],
+              await res.text().catch(() => ""),
+            );
+          }
+          const data = (await res.json()) as { libraries?: LibraryMapRow[]; count?: number };
+          const libraries = (Array.isArray(data.libraries) ? data.libraries : []).map((l) => ({
+            ...l,
+            // 防禦：top_entities 若是 JSON 字串形就 parse 成名字清單（失敗當空，誠實不 crash）。
+            top_entities: entityNames(l.top_entities, 3),
+            triplet_count: Number(l.triplet_count ?? 0) || 0,
+          }));
+          if (libraries.length === 0) {
+            // 空庫誠實回報：不是錯誤（端點正常、就是還沒有地圖），給 backfill 指引。
+            return successResponse({ libraries: [], count: 0 }, [
+              "全館地圖是空的：還沒有任何庫跑過 recompute",
+              ...RECOMPUTE_HINTS,
+            ]);
+          }
+          return successResponse({ libraries, count: libraries.length }, [
+            "要看某庫細節：kbdb_get_map(library='庫名')",
+            "進庫查內容：kbdb_search（關鍵字/語義）；查關係：kbdb_graph_neighbors",
+          ]);
+        }
+
+        // 單庫詳圖：完整 slots（slot 陣列 parse 成物件再回）。
+        const res = await kbdbFetch(env, `/map/${encodeURIComponent(library)}${qs}`);
+        if (res.status === 404) {
+          return errorResponse(
+            "map_not_found",
+            `庫「${library}」還沒有地圖（從未 recompute，或庫名打錯）`,
+            ["kbdb_get_map 不帶參數看全館有哪些庫（確認庫名）", ...RECOMPUTE_HINTS],
+          );
+        }
+        if (!res.ok) {
+          return errorResponse(
+            "map_fetch_failed",
+            `取庫「${library}」詳圖失敗 HTTP ${res.status}`,
+            ["稍後重試", ...RECOMPUTE_HINTS],
+            await res.text().catch(() => ""),
+          );
+        }
+        const data = (await res.json()) as { map?: LibraryMapDetail };
+        const raw = data.map ?? {};
+        const map = {
+          ...raw,
+          // slot 字串容錯 parse（任務規格：JSON 字串形要 parse 成物件再回，失敗當空陣列）。
+          top_entities: parseSlotArray<{ name: string; degree: number }>(raw.top_entities),
+          relation_profile: parseSlotArray<{ predicate: string; count: number }>(raw.relation_profile),
+          bridges: parseSlotArray<{ entity: string; libraries: string[] }>(raw.bridges),
+          triplet_count: Number(raw.triplet_count ?? 0) || 0,
+        };
+        return successResponse({ map }, [
+          "bridges＝此庫 entity 同時出現在哪些其他庫（M3 backfill 前會偏稀疏，是誠實現況不是 bug）",
+          "沿核心 entity 挖關係：kbdb_graph_neighbors(subject=entity 名)",
+        ]);
+      } catch (e) {
+        return errorResponse("internal_error", e instanceof Error ? e.message : String(e), ["稍後重試"]);
+      }
+    },
+  );
+}
diff --git a/mcp/src/tools/registry.ts b/mcp/src/tools/registry.ts
index 285ee5a..974c996 100644
--- a/mcp/src/tools/registry.ts
+++ b/mcp/src/tools/registry.ts
@@ -19,6 +19,7 @@ import { registerAllSkillExampleTools } from "./arcrun_skills_examples.js";
 import { registerAllRecipeTools } from "./arcrun_recipe.js";
 import { registerAllKbdbDataTools } from "./kbdb_data.js";
 import { registerAllKbdbGraphTools } from "./kbdb_graph.js";
+import { registerAllKbdbMapTools } from "./kbdb_map.js";
 import { registerWhoami } from "./arcrun_whoami.js";
 
 export function registerAllTools(server: McpServer, env: Env, orgNamespace: string, partnerToken: string) {
@@ -53,6 +54,9 @@ export function registerAllTools(server: McpServer, env: Env, orgNamespace: stri
   // issue #68: KBDB graph 查詢薄殼（kbdb_graph_neighbors，調 /q/:ns/graph_neighbors 同步查詢端點）
   // 補齊 D17「KBDB MCP＝RAG 套餐」第三模式：關鍵字/語義之外的圖（關係遍歷）
   registerAllKbdbGraphTools(server, env, orgNamespace);
+  // library-map SDD M4（Arcrun#39）: 藏書地圖薄殼（kbdb_get_map，調 kbdb GET /map／/map/:library）
+  // retrieval 第一站：先看地圖定位庫，再 search/graph 進庫（design §6）
+  registerAllKbdbMapTools(server, env);
   // §7.8 P1 D2: whoami（與 CLI acr whoami 對齊，AI 不繞 CLI 自己 curl 猜帳號）
   registerWhoami(server, env, orgNamespace);
 }
diff --git a/mcp/tests/unit/tools/kbdb-map.test.ts b/mcp/tests/unit/tools/kbdb-map.test.ts
new file mode 100644
index 0000000..3a20612
--- /dev/null
+++ b/mcp/tests/unit/tools/kbdb-map.test.ts
@@ -0,0 +1,297 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import type { Env } from "../../../src/types.js";
+import { registerGetMap } from "../../../src/tools/kbdb_map.js";
+import {
+  buildLibraryMapInstructions,
+  renderLibraryMapLines,
+  __resetLibraryMapInstructionsCacheForTests,
+} from "../../../src/lib/library-map.js";
+
+// ── 假 McpServer：只攔 tool 註冊，抓出 handler 直接呼叫（比照 kbdb-graph.test.ts）──────
+type ToolHandler = (args: Record<string, unknown>) => Promise<{
+  content: { type: string; text: string }[];
+  isError?: boolean;
+}>;
+
+function makeServer() {
+  const tools = new Map<string, { description: string; handler: ToolHandler }>();
+  const server = {
+    tool(name: string, description: string, _schema: unknown, handler: ToolHandler) {
+      tools.set(name, { description, handler });
+    },
+  };
+  return { server: server as unknown as McpServer, tools };
+}
+
+// ── 假 KBDB service binding：記錄請求、回預設 response ─────────────────────────────
+function makeEnv(respond: (url: URL, init?: RequestInit) => Response) {
+  const calls: { url: URL; init?: RequestInit }[] = [];
+  const env = {
+    KBDB: {
+      fetch: async (input: string, init?: RequestInit) => {
+        const url = new URL(input);
+        calls.push({ url, init });
+        return respond(url, init);
+      },
+    },
+    KBDB_INTERNAL_TOKEN: "test-token",
+  } as unknown as Env;
+  return { env, calls };
+}
+
+function parseResult(r: { content: { text: string }[] }) {
+  return JSON.parse(r.content[0].text) as Record<string, unknown>;
+}
+
+const KB_ROW = {
+  library: "kb",
+  narrative: "leo 的知識庫主庫",
+  top_entities: ["00-INDEX", "kb/00-INDEX", "Gitea"],
+  triplet_count: 111,
+  updated_at: 1784451880,
+};
+
+describe("kbdb_get_map: registration", () => {
+  it("registers under kbdb_* prefix (D17) with the 'call this first' hint in description", () => {
+    const { server, tools } = makeServer();
+    const { env } = makeEnv(() => new Response("{}"));
+    registerGetMap(server, env);
+    expect(tools.has("kbdb_get_map")).toBe(true);
+    // 任務規格：description 必含「不確定該查什麼時，先呼叫此工具」
+    expect(tools.get("kbdb_get_map")!.description).toContain("不確定該查什麼時，先呼叫此工具");
+  });
+});
+
+describe("kbdb_get_map: 全館地圖（無參數）", () => {
+  it("hits GET /map via KBDB binding and returns per-library rows", async () => {
+    const { server, tools } = makeServer();
+    const { env, calls } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [KB_ROW], count: 1 })),
+    );
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({});
+
+    expect(calls).toHaveLength(1);
+    expect(calls[0].url.pathname).toBe("/map");
+    // kbdbFetch 慣例：KBDB_INTERNAL_TOKEN 注入 Authorization
+    expect(new Headers(calls[0].init?.headers).get("Authorization")).toBe("Bearer test-token");
+
+    const body = parseResult(res);
+    expect(body.ok).toBe(true);
+    const data = body.data as { libraries: { library: string; top_entities: string[] }[]; count: number };
+    expect(data.count).toBe(1);
+    expect(data.libraries[0].library).toBe("kb");
+    expect(data.libraries[0].top_entities).toEqual(["00-INDEX", "kb/00-INDEX", "Gitea"]);
+  });
+
+  it("owner_id is forwarded as query param", async () => {
+    const { server, tools } = makeServer();
+    const { env, calls } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [], count: 0 })),
+    );
+    registerGetMap(server, env);
+    await tools.get("kbdb_get_map")!.handler({ owner_id: "leo" });
+    expect(calls[0].url.searchParams.get("owner_id")).toBe("leo");
+  });
+
+  it("top_entities in JSON-string form is parsed defensively (live-observed slot shape)", async () => {
+    const { server, tools } = makeServer();
+    const row = {
+      ...KB_ROW,
+      top_entities: '[{"name":"00-INDEX","degree":29},{"name":"Gitea","degree":7}]',
+      triplet_count: "111", // slot 底層存字串 → 要轉數字
+    };
+    const { env } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [row], count: 1 })),
+    );
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({});
+    const data = parseResult(res).data as {
+      libraries: { top_entities: string[]; triplet_count: number }[];
+    };
+    expect(data.libraries[0].top_entities).toEqual(["00-INDEX", "Gitea"]);
+    expect(data.libraries[0].triplet_count).toBe(111);
+  });
+
+  it("空庫誠實回報＋提示跑 POST /map/recompute backfill（鐵律：不假綠不 crash）", async () => {
+    const { server, tools } = makeServer();
+    const { env } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [], count: 0 })),
+    );
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({});
+    const body = parseResult(res);
+    expect(body.ok).toBe(true);
+    expect((body.data as { count: number }).count).toBe(0);
+    expect(JSON.stringify(body.hints)).toContain("POST /map/recompute");
+  });
+
+  it("HTTP error → map_fetch_failed with recompute hint, not a crash", async () => {
+    const { server, tools } = makeServer();
+    const { env } = makeEnv(() => new Response("boom", { status: 500 }));
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({});
+    expect(res.isError).toBe(true);
+    const body = parseResult(res);
+    expect(body.error_code).toBe("map_fetch_failed");
+    expect(JSON.stringify(body.next_actions)).toContain("POST /map/recompute");
+  });
+});
+
+describe("kbdb_get_map: 單庫詳圖（library 參數）", () => {
+  const DETAIL = {
+    record_id: "e_123",
+    library: "kb",
+    narrative: "leo 的知識庫主庫",
+    content: "kb：leo 的知識庫主庫。核心：00-INDEX",
+    top_entities: [{ name: "00-INDEX", degree: 29 }],
+    relation_profile: [{ predicate: "連結至", count: 48 }],
+    bridges: [{ entity: "Gitea", libraries: ["notes"] }],
+    triplet_count: 111,
+    commit_hash: null,
+    status: "active",
+    updated_at: 1784451880,
+  };
+
+  it("hits GET /map/:library and returns full detail", async () => {
+    const { server, tools } = makeServer();
+    const { env, calls } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, map: DETAIL })),
+    );
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({ library: "kb" });
+    expect(calls[0].url.pathname).toBe("/map/kb");
+    const map = (parseResult(res).data as { map: typeof DETAIL }).map;
+    expect(map.record_id).toBe("e_123");
+    expect(map.relation_profile).toEqual([{ predicate: "連結至", count: 48 }]);
+    expect(map.bridges).toEqual([{ entity: "Gitea", libraries: ["notes"] }]);
+  });
+
+  it("JSON-string slot values are parsed into objects（任務規格）; broken JSON → 空陣列", async () => {
+    const { server, tools } = makeServer();
+    const raw = {
+      ...DETAIL,
+      top_entities: '[{"name":"00-INDEX","degree":29}]',
+      relation_profile: '[{"predicate":"連結至","count":48}]',
+      bridges: "not-json{{", // parse 失敗 → 當空陣列，不 crash
+      triplet_count: "111",
+    };
+    const { env } = makeEnv(() => new Response(JSON.stringify({ success: true, map: raw })));
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({ library: "kb" });
+    expect(res.isError).toBeUndefined();
+    const map = (parseResult(res).data as { map: Record<string, unknown> }).map;
+    expect(map.top_entities).toEqual([{ name: "00-INDEX", degree: 29 }]);
+    expect(map.relation_profile).toEqual([{ predicate: "連結至", count: 48 }]);
+    expect(map.bridges).toEqual([]);
+    expect(map.triplet_count).toBe(111);
+  });
+
+  it("404 → map_not_found with recompute backfill hint（誠實回報）", async () => {
+    const { server, tools } = makeServer();
+    const { env } = makeEnv(
+      () => new Response(JSON.stringify({ success: false, error: "not found" }), { status: 404 }),
+    );
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({ library: "ghost" });
+    expect(res.isError).toBe(true);
+    const body = parseResult(res);
+    expect(body.error_code).toBe("map_not_found");
+    expect(JSON.stringify(body.next_actions)).toContain("POST /map/recompute");
+  });
+
+  it("binding throws → internal_error, not an unhandled crash", async () => {
+    const { server, tools } = makeServer();
+    const env = {
+      KBDB: {
+        fetch: async () => {
+          throw new Error("binding down");
+        },
+      },
+    } as unknown as Env;
+    registerGetMap(server, env);
+    const res = await tools.get("kbdb_get_map")!.handler({ library: "kb" });
+    expect(res.isError).toBe(true);
+    expect(parseResult(res).error_code).toBe("internal_error");
+  });
+});
+
+// ── instructions 注入（design §4：拉不到＝靜默略過，絕不擋連線）────────────────────────
+describe("buildLibraryMapInstructions", () => {
+  beforeEach(() => __resetLibraryMapInstructionsCacheForTests());
+
+  it("renders one compact line per library in the指定格式", async () => {
+    const { env } = makeEnv(
+      () =>
+        new Response(
+          JSON.stringify({
+            success: true,
+            libraries: [
+              KB_ROW,
+              { library: "notes", narrative: "手機隨手筆記", top_entities: ["Gitea"], triplet_count: 108 },
+            ],
+            count: 2,
+          }),
+        ),
+    );
+    const text = await buildLibraryMapInstructions(env);
+    expect(text).not.toBeNull();
+    // design §4 格式：{library}：{narrative}｜核心：{top3}｜{triplet_count} triplets
+    expect(text!).toContain("kb：leo 的知識庫主庫｜核心：00-INDEX、kb/00-INDEX、Gitea｜111 triplets");
+    expect(text!).toContain("notes：手機隨手筆記｜核心：Gitea｜108 triplets");
+    // 導引句：把地圖跟 get_map 工具接起來（design §6 retrieval 流程）
+    expect(text!).toContain("kbdb_get_map");
+  });
+
+  it("HTTP error → null（靜默略過，不 throw 不擋連線）", async () => {
+    const { env } = makeEnv(() => new Response("boom", { status: 500 }));
+    await expect(buildLibraryMapInstructions(env)).resolves.toBeNull();
+  });
+
+  it("binding throws → null（靜默略過）", async () => {
+    const env = {
+      KBDB: {
+        fetch: async () => {
+          throw new Error("kbdb down");
+        },
+      },
+    } as unknown as Env;
+    await expect(buildLibraryMapInstructions(env)).resolves.toBeNull();
+  });
+
+  it("empty libraries → null（沒地圖就不注入，不塞空段落）", async () => {
+    const { env } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [], count: 0 })),
+    );
+    await expect(buildLibraryMapInstructions(env)).resolves.toBeNull();
+  });
+
+  it("caches within TTL：same isolate 第二次不再打 /map", async () => {
+    const { env, calls } = makeEnv(
+      () => new Response(JSON.stringify({ success: true, libraries: [KB_ROW], count: 1 })),
+    );
+    const first = await buildLibraryMapInstructions(env);
+    const second = await buildLibraryMapInstructions(env);
+    expect(second).toBe(first);
+    expect(calls).toHaveLength(1);
+  });
+});
+
+describe("renderLibraryMapLines", () => {
+  it("null narrative / string-form top_entities / string count 都容錯", () => {
+    const text = renderLibraryMapLines([
+      {
+        library: "kb",
+        narrative: null,
+        top_entities: '["00-INDEX"]',
+        triplet_count: "5",
+      },
+    ]);
+    expect(text).toBe("- kb：（narrative 待補）｜核心：00-INDEX｜5 triplets");
+  });
+
+  it("empty input → null", () => {
+    expect(renderLibraryMapLines([])).toBeNull();
+  });
+});
diff --git a/system-dev/docs/3-specs/library-map/tasks.md b/system-dev/docs/3-specs/library-map/tasks.md
index 75d1e14..ab2b612 100644
--- a/system-dev/docs/3-specs/library-map/tasks.md
+++ b/system-dev/docs/3-specs/library-map/tasks.md
@@ -7,7 +7,7 @@
 | M1 | `library_map` Template＋slots 定義（含 triplet 按庫過濾現況核實；不足則 Triplet template 加 optional library slot） | B | — | 🔨 PR 已開 | D6 零建表。核實：triplet 無 library slot → 已走預案（design §1 核實結果） |
 | M2 | kbdb base `POST /map/recompute?library=`＋`GET /map`／`GET /map/:library`（聚合 SQL 住基本盤；交易式 supersede） | B | M1 | 🔨 PR 已開 | PR＋測試（真 SQLite 驗聚合）；merge 後 gated 部署（leo 閘）＋逐庫 backfill recompute |
 | M3 | ingest 尾端接鏈：diff 涉及庫 → 逐庫呼 recompute（rag-ingest-cards v2＋個人庫 ingest 同款改版） | A | M2 | ⬜ | workflow 改版走 bundle 分發 |
-| M4 | MCP：instructions 注入全館地圖＋`get_map` 工具 | B | M2 | ⬜ | 與 #68 同族薄殼 |
+| M4 | MCP：instructions 注入全館地圖＋`get_map` 工具 | B | M2 | 🔨 PR 已開 | 與 #68 同族薄殼；`kbdb_get_map`＋connect 時注入（isolate TTL 快取，失敗靜默略過不擋連線）；merge 後 gated redeploy arcrun-mcp（leo 閘） |
 | M5 | GUI 首頁：全館地圖 render（console＋portal） | B | M2 | ⬜ | 取代空白搜尋框 |
 | M6 | D30 連動：map 層 embed＋semantic 庫路由第一跳 | B | M2 | ⬜ | #58/#59/#60 家族的第一片治本 |
 | M7 | dogfood：leo 庫（leo21c）首個實例 backfill＋驗收（requirements 驗收段全項） | A | M3-M5 | ⬜ | 過了才進 demo/客戶 |
diff --git a/system-dev/wiki/status.md b/system-dev/wiki/status.md
index 5ec3872..7352e6f 100644
--- a/system-dev/wiki/status.md
+++ b/system-dev/wiki/status.md
@@ -15,6 +15,17 @@ metadata:
 
 ## 📍 當前位置
 
+> **2026-07-19（#39 藏書地圖 M4，分支 `feat/mcp-library-map-inject`）**：**library-map SDD M4 PR 已開，
+> 等審＋gated 部署（merge 後需 leo 閘 redeploy arcrun-mcp）**。兩件（design §4）：
+> ① MCP tool `kbdb_get_map`（無參數＝全館每庫一行；帶 library＝詳圖，slot JSON 字串容錯 parse 成
+> 物件，parse 失敗當空陣列；404/空庫誠實回報＋POST /map/recompute backfill 指引；description 含
+> 「不確定該查什麼時，先呼叫此工具」）——走既有 KBDB service binding（kbdbFetch），與 #68 同族薄殼。
+> ② server instructions 注入：`buildLibraryMapInstructions` 連線時拉 GET /map 渲染成每庫一行
+> （`{library}：{narrative}｜核心：{top3}｜{n} triplets`）嵌 instructions；**快取選型＝isolate 內
+> TTL 快取（成功 5min／失敗 1min）**，因 stateless StreamableHTTP 每個 HTTP request 都重建
+> McpServer，「每次現拉」實際是每個 tool call 都多打一次；timeout 1.5s，任何失敗回 null 靜默略過
+> **絕不擋 MCP 連線**（鐵律）。測試：假 KBDB binding 17 新測（全 76/76 過）＋tsc 乾淨。
+>
 > **2026-07-19（#39 藏書地圖 M1+M2，分支 `feat/library-map-base`）**：**library-map SDD M1+M2 PR 已開，
 > 等審＋gated 部署（merge 後需 leo 閘 redeploy kbdb＋首次 backfill 逐庫呼 recompute）**。
 > M1：`library_map` template（migration 0003 seed＋runtime ensure，D6 零建表）；核實 triplet 按庫定位＝