diff --git a/ADOPT.md b/ADOPT.md
index cb173fa..d250f7f 100644
--- a/ADOPT.md
+++ b/ADOPT.md
@@ -49,7 +49,7 @@ Every migrated file gets a frontmatter `description` — one plain-language line
 1. Copy `code/tools/gen-book-index.mjs` and wire the script (see the `scripts` block in BoCode's `code/package.json`). If the project has no JavaScript tooling at all, a shell wrapper calling `node` is fine — the script is zero-dependency.
 2. Optional, recommended: copy `scripts/check-commit-message.mjs` and `.githooks/`, then run `git config core.hooksPath .githooks`.
 3. Optional: copy `.github/workflows/git-policy.yml`. **Check the branch names** — if the project's integration branch isn't `dev`, either ask the user to adopt the branch model or adapt the workflow's branch filters. Don't silently rewrite their branch model.
-4. Run the index generator: every book file must carry a `description`; zero warnings is the only passing state.
+4. Run the index generator: every book file must carry a `description` and every internal link must resolve; zero warnings is the only passing state.
 
 ## Step 5 — Install the skills
 
diff --git a/README.ja.md b/README.ja.md
index a656720..78ec622 100644
--- a/README.ja.md
+++ b/README.ja.md
@@ -135,8 +135,9 @@ diff を読まないと分からない summary は summary ではありません
 - すべての book ドキュメントは frontmatter の `description` で始まる——体裁ではなく内容を語る、人の言葉で一行
 - `code/tools/gen-book-index.mjs`（依存ゼロ、プレーン Node）が `book/README.md`——全ドキュメントのマスターインデックス——を再生成する
 - description の欠けたドキュメントはビルドを**失敗させる**（exit 1）。ゼロ警告だけが合格です
+- 内部リンクは解決必須——切れたリンクもビルドを失敗させます。book は vault を兼ねるため、切れたリンクはビルドバグです
 
-`book/README.md` は入口も兼ねます：エージェントはまずここで地図を手に入れる。フォルダはそのまま [Obsidian](https://obsidian.md) の vault として開けます。
+`book/README.md` は入口も兼ねます：エージェントはまずここで地図を手に入れる。フォルダはそのまま [Obsidian](https://obsidian.md) の vault として開けます——インデックス、タグ、バックリンク、グラフがすべて使えます（FAQ 参照）。
 
 ### bowrite（薄写）——人の言葉で書く
 
@@ -198,6 +199,9 @@ cp -r skills/bocode skills/boscope skills/book-writeback skills/bowrite <your-sk
 **どの AI ツールで動きますか？**
 `AGENTS.md` を指示ファイルとして読み、`SKILL.md` 形式をサポートするものなら何でも（ZCode、Claude Code、互換エージェント）。スキルがなくてもワークフローは劣化なしで機能します：guidelines が同じルールを文章で運びます。
 
+**book は Obsidian などの PKM ツールで読めますか？**
+はい、設定なしで。book は純粋な Markdown です——相対リンク、YAML frontmatter（description、tags）、独自形式は一切なし。`book/` を Obsidian の vault として開けば、インデックス、タグ、バックリンク、グラフがすべて使えます。ビルドはリンクも検証し、切れたリンクはビルドを失敗させるため、開くものはいつも辿れる網です。Obsidian 固定でもありません：Logseq、Foam、VS Code、Markdown を読めるものなら何でも。
+
 **インデックスを再生成しないとどうなりますか？**
 実行時には何も壊れません——しかしインデックスが古くなれば、システム全体が依存する発見可能性が崩れていきます。だからこそチェックは、description の欠落を派手に失敗させます。
 
diff --git a/README.md b/README.md
index 4c60449..9662840 100644
--- a/README.md
+++ b/README.md
@@ -137,8 +137,9 @@ Documentation you can't find doesn't exist. BoCode makes discoverability a build
 - Every book document starts with a frontmatter `description` — one plain-language line about the *content*, not the genre.
 - `code/tools/gen-book-index.mjs` (zero dependencies, plain Node) regenerates `book/README.md` — the master index of every document, grouped by directory.
 - A document missing its description makes the build **fail** (exit 1). Zero warnings is the only passing state.
+- Internal links must resolve — a broken link fails the build. The book doubles as an Obsidian vault; a broken link is a build bug.
 
-`book/README.md` doubles as the entry point: agents read it first to get the map; the folder also opens directly as an [Obsidian](https://obsidian.md) vault.
+`book/README.md` doubles as the entry point: agents read it first to get the map; the folder also opens directly as an [Obsidian](https://obsidian.md) vault — index, tags, backlinks, and graph all work (see FAQ).
 
 ### bowrite（薄写）— speak human
 
@@ -200,6 +201,9 @@ No. The book is Markdown; the two scripts are plain Node with zero dependencies.
 **Which AI tools does it work with?**
 Anything that reads `AGENTS.md` as its instruction file and supports the `SKILL.md` format (ZCode, Claude Code, and compatible agents). The workflow degrades gracefully without skills: the guidelines carry the same rules in prose.
 
+**Does the book work in Obsidian or other PKM tools?**
+Yes, with zero setup. The book is plain Markdown — relative links, YAML frontmatter (`description`, `tags`), no proprietary formats. Open `book/` as an Obsidian vault and the index, tag pane, backlinks, and graph all work. The build also validates links — a broken link fails the build — so what you open is always a navigable web. Not Obsidian-locked either: Logseq, Foam, VS Code, anything that reads Markdown works.
+
 **What if I don't regenerate the index?**
 Nothing breaks at runtime — but a stale index erodes the discoverability the whole system rests on, which is why the check makes missing descriptions fail loudly.
 
diff --git a/README.zh-CN.md b/README.zh-CN.md
index 8844b92..170616c 100644
--- a/README.zh-CN.md
+++ b/README.zh-CN.md
@@ -135,8 +135,9 @@ book 的文档按读者分流，而不是按文档类型（`book/notes/`）：
 - 每篇 book 文档开头有 frontmatter `description`——一行说人话的内容摘要，写内容，不写体裁
 - `code/tools/gen-book-index.mjs`（零依赖、纯 Node）重新生成 `book/README.md`——全库文档总索引
 - 缺 description 的文档让构建直接失败（退出码 1）。零警告是唯一通过状态
+- 内部链接必须走得通：断链同样让构建失败——book 是个 vault，断链就是构建 bug
 
-`book/README.md` 也是入口：agent 先读它拿地图；这个目录还能直接用 [Obsidian](https://obsidian.md) 打开当 vault。
+`book/README.md` 也是入口：agent 先读它拿地图；这个目录还能直接用 [Obsidian](https://obsidian.md) 等知识管理工具打开当 vault——索引、标签、反链、关系图全部可用（详见 FAQ）。
 
 ### 薄写（bowrite）：说人话
 
@@ -198,6 +199,9 @@ cp -r skills/bocode skills/boscope skills/book-writeback skills/bowrite <your-sk
 **支持哪些 AI 工具？**
 只要读 `AGENTS.md`、认 `SKILL.md` 格式就行（ZCode、Claude Code 和兼容工具）。没装 skill 也能跑：guidelines 用文字载着同样的规则。
 
+**book 能用 Obsidian 这类双链笔记工具读吗？**
+能，零设置。book 全是普通 Markdown：相对链接、YAML frontmatter（description、tags），没有任何专有格式。把 `book/` 当 vault 打开，索引、标签面板、反向链接、关系图全部可用。构建还会校验链接——断链直接构建失败，你打开的永远是一张走得通的网。也不锁 Obsidian：Logseq、Foam、VS Code，任何认 Markdown 的工具都行。
+
 **不刷索引会怎么样？**
 运行不会出问题——但索引一过期，文档就慢慢找不到了，而这套体系靠的正是"找得到"。所以检查让缺 description 的文档直接报错，而不是悄悄放过去。
 
diff --git a/book/guidelines/workflow.md b/book/guidelines/workflow.md
index d0e40d3..6f13cda 100644
--- a/book/guidelines/workflow.md
+++ b/book/guidelines/workflow.md
@@ -57,7 +57,7 @@ Phases 5 and 6 share one discipline: **record, never fix.** A problem found duri
 4. **Plans drive code** — `book/plans/` is the input to implementation. The agent reads the plan, builds in `code/`, writes back to `book/`.
 5. **Issues feed code** — `notes/issue/` records problems that become the direction of later iterations.
 6. **The book compounds** — every feature should leave the book more complete than it found it. The book is both the agent's context and the human's reference manual.
-7. **Refresh the index** — after any book change, run `cd code && npm run book:index` before wrapping up. New files must carry a frontmatter `description`; missing ones fail the build.
+7. **Refresh the index** — after any book change, run `cd code && npm run book:index` before wrapping up. New files must carry a frontmatter `description`, and internal links must resolve — either failing fails the build.
 
 ## Document routing
 
diff --git a/code/tools/gen-book-index.mjs b/code/tools/gen-book-index.mjs
index 2d1dd4b..7698e10 100755
--- a/code/tools/gen-book-index.mjs
+++ b/code/tools/gen-book-index.mjs
@@ -6,13 +6,15 @@
  * Usage:   cd code && node tools/gen-book-index.mjs
  *          (or: npm run book:index)
  *
- * Convention: every document starts with frontmatter whose `description`
- * is a one-line, plain-language summary of the content. Files missing it
- * are listed on stderr and the script exits 1 — zero warnings is the
- * only passing state. Zero dependencies; runs on plain Node (and bun).
+ * Conventions:
+ *   1. Every document starts with frontmatter whose `description` is a
+ *      one-line, plain-language summary. Missing → warning, exit 1.
+ *   2. Internal links must resolve. Broken relative links → warning, exit 1 —
+ *      the book doubles as an Obsidian vault, so a broken link is a build bug.
+ * Zero dependencies; runs on plain Node (and bun).
  */
-import { readdirSync, readFileSync, writeFileSync } from "node:fs";
-import { join, dirname, relative } from "node:path";
+import { readdirSync, readFileSync, writeFileSync, existsSync } from "node:fs";
+import { join, dirname, relative, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 
 const TOOLS_DIR = dirname(fileURLToPath(import.meta.url));
@@ -66,8 +68,7 @@ function walkMd(dir) {
   return out;
 }
 
-function parseDescription(abs) {
-  const text = readFileSync(abs, "utf8");
+function parseDescription(text) {
   if (!text.startsWith("---")) return null;
   const lines = text.split("\n");
   for (let i = 1; i < lines.length; i++) {
@@ -78,6 +79,17 @@ function parseDescription(abs) {
   return null;
 }
 
+function collectInternalLinks(text) {
+  const out = [];
+  const re = /(?:\]\(<([^>]+)>\)|\]\(([^)\s]+)\))/g;
+  let m;
+  while ((m = re.exec(text))) {
+    const target = (m[1] || m[2]).split("#")[0];
+    if (target && !/^[a-z]+:/.test(target)) out.push(target); // skip http:, mailto:, obsidian:…
+  }
+  return out;
+}
+
 function link(rel) {
   return /[\s()]/.test(rel) ? `<${rel}>` : rel;
 }
@@ -87,13 +99,19 @@ const files = walkMd(BOOK_ROOT)
   .sort();
 
 const warnings = [];
+const brokenLinks = [];
 const byDir = new Map();
 
 for (const abs of files) {
   const rel = relative(BOOK_ROOT, abs);
   const dir = dirname(rel) === "." ? "" : dirname(rel);
-  const desc = parseDescription(abs);
+  const text = readFileSync(abs, "utf8");
+  const desc = parseDescription(text);
   if (!desc) warnings.push(rel);
+  for (const target of collectInternalLinks(text)) {
+    const resolved = resolve(dirname(abs), target);
+    if (!existsSync(resolved)) brokenLinks.push(`${rel} → ${target}`);
+  }
   if (!byDir.has(dir)) byDir.set(dir, []);
   byDir.get(dir).push({ name: rel.split("/").pop(), rel, desc: desc ?? "(missing description)" });
 }
@@ -132,10 +150,16 @@ out += `<!-- ${files.length} documents -->\n`;
 
 writeFileSync(OUTPUT, out);
 
-if (warnings.length > 0) {
-  console.error(`Files missing frontmatter description (${warnings.length}):`);
-  for (const w of warnings) console.error(`  - ${w}`);
-  console.error(`Index written to ${OUTPUT} (${files.length} documents), but the files above lack a summary. Fix them and rerun.`);
+if (warnings.length > 0 || brokenLinks.length > 0) {
+  if (warnings.length > 0) {
+    console.error(`Files missing frontmatter description (${warnings.length}):`);
+    for (const w of warnings) console.error(`  - ${w}`);
+  }
+  if (brokenLinks.length > 0) {
+    console.error(`Broken internal links (${brokenLinks.length}):`);
+    for (const l of brokenLinks) console.error(`  - ${l}`);
+  }
+  console.error(`Index written to ${OUTPUT} (${files.length} documents), but the issues above fail the build. Fix them and rerun.`);
   process.exit(1);
 }
-console.log(`Index written to ${OUTPUT} — ${files.length} documents, zero warnings.`);
+console.log(`Index written to ${OUTPUT} — ${files.length} documents, zero warnings, all links resolve.`);