diff --git a/web/static/dev.html b/web/static/dev.html
index f52d706..ae1a453 100644
--- a/web/static/dev.html
+++ b/web/static/dev.html
@@ -222,6 +222,74 @@
   #new-task-modal .err { color: var(--accent); font-size: 12px; margin-top: 8px; min-height: 1em; }
   #new-task-modal .actions { margin-top: 14px; display: flex; gap: 8px; justify-content: flex-end; }
 
+  /* ───── file preview modal ───── */
+  #file-preview-modal {
+    position: fixed; inset: 0; background: rgba(0,0,0,0.5);
+    display: none; align-items: center; justify-content: center; z-index: 90;
+  }
+  #file-preview-modal.show { display: flex; }
+  #file-preview-modal .card {
+    background: var(--panel); border-radius: 6px;
+    width: 90vw; height: 90vh; max-width: 1200px;
+    display: flex; flex-direction: column;
+    box-shadow: 0 8px 24px rgba(0,0,0,.2);
+  }
+  #file-preview-modal .hdr {
+    display: flex; align-items: center; gap: 8px;
+    padding: 8px 12px; border-bottom: 1px solid var(--border);
+  }
+  #file-preview-modal .hdr .name {
+    flex: 1; font-weight: 500; overflow: hidden;
+    text-overflow: ellipsis; white-space: nowrap;
+  }
+  #file-preview-modal .body {
+    flex: 1; overflow: auto; padding: 12px; position: relative;
+  }
+  #file-preview-modal .body.center {
+    display: flex; align-items: center; justify-content: center;
+  }
+  #file-preview-modal .body .ph {
+    color: var(--muted); font-size: 13px; text-align: center;
+  }
+  #file-preview-modal .body img.preview-img {
+    max-width: 100%; max-height: 100%; object-fit: contain;
+    display: block; margin: 0 auto;
+  }
+  #file-preview-modal .body iframe.preview-frame {
+    width: 100%; height: 100%; border: 0;
+  }
+  #file-preview-modal .body pre.preview-text {
+    margin: 0; padding: 8px; background: var(--code-bg);
+    border-radius: 4px; white-space: pre-wrap; word-break: break-word;
+    font-family: ui-monospace, "SF Mono", Consolas, monospace;
+    font-size: 12px; line-height: 1.5;
+  }
+  #file-preview-modal .body .md-render {
+    max-width: 860px; margin: 0 auto; line-height: 1.7;
+  }
+  #file-preview-modal .body .md-render pre {
+    background: var(--code-bg); padding: 10px; border-radius: 4px; overflow: auto;
+  }
+  #file-preview-modal .body .md-render code { background: var(--code-bg); padding: 1px 4px; border-radius: 3px; }
+  #file-preview-modal .body .md-render pre code { background: transparent; padding: 0; }
+  #file-preview-modal .body .docx-host { background: #fff; }
+  #file-preview-modal .body .xlsx-tabs {
+    display: flex; gap: 4px; flex-wrap: wrap; margin-bottom: 8px;
+    border-bottom: 1px solid var(--border); padding-bottom: 6px;
+  }
+  #file-preview-modal .body .xlsx-tabs button.active {
+    background: var(--accent-soft); border-color: var(--accent); color: var(--accent);
+  }
+  #file-preview-modal .body .xlsx-sheet {
+    overflow: auto;
+  }
+  #file-preview-modal .body .xlsx-sheet table {
+    border-collapse: collapse; font-size: 12px;
+  }
+  #file-preview-modal .body .xlsx-sheet td, #file-preview-modal .body .xlsx-sheet th {
+    border: 1px solid var(--border); padding: 4px 8px; white-space: nowrap;
+  }
+
   .small { font-size: 12px; }
   .muted { color: var(--muted); }
 </style>
@@ -357,6 +425,19 @@
   </div>
 </div>
 
+<!-- ───── file preview modal ───── -->
+<div id="file-preview-modal">
+  <div class="card">
+    <div class="hdr">
+      <span class="name" id="fp-name"></span>
+      <span class="small muted" id="fp-meta"></span>
+      <button class="small" id="fp-download" title="下载原文件">下载</button>
+      <button class="small" id="fp-close" title="关闭 (Esc)">×</button>
+    </div>
+    <div class="body" id="fp-body"></div>
+  </div>
+</div>
+
 <script>
 const SENTINEL = "00000000-0000-0000-0000-000000000000";
 const LS_TOKEN = "zcbot.token";
@@ -1003,7 +1084,7 @@ function renderFiles(data) {
     el.onclick = () => {
       const rel = el.dataset.rel;
       if (el.dataset.isdir === "true") { state.filesPath = rel; loadFiles(); }
-      else { downloadFile(rel); }
+      else { openFilePreview(rel); }
     };
   });
   $("file-list").querySelectorAll(".del-file").forEach((btn) => {
@@ -1072,6 +1153,243 @@ function downloadFile(rel) {
   });
 }
 
+// ───── file preview ─────
+const PREVIEW_TEXT_MAX = 2 * 1024 * 1024;
+const PREVIEW_BIN_MAX = 50 * 1024 * 1024;
+
+const _scriptCache = new Map();
+function loadScript(src) {
+  if (_scriptCache.has(src)) return _scriptCache.get(src);
+  const p = new Promise((resolve, reject) => {
+    const s = document.createElement("script");
+    s.src = src;
+    s.onload = () => resolve();
+    s.onerror = () => { _scriptCache.delete(src); reject(new Error("load failed: " + src)); };
+    document.head.appendChild(s);
+  });
+  _scriptCache.set(src, p);
+  return p;
+}
+
+const _previewBlobUrls = new Set();
+function _trackBlobUrl(blob, mime) {
+  const b = mime ? new Blob([blob], { type: mime }) : blob;
+  const url = URL.createObjectURL(b);
+  _previewBlobUrls.add(url);
+  return url;
+}
+function _flushBlobUrls() {
+  for (const u of _previewBlobUrls) URL.revokeObjectURL(u);
+  _previewBlobUrls.clear();
+}
+
+const _EXT_GROUPS = {
+  image: new Set(["jpg","jpeg","png","gif","webp","bmp","svg","ico"]),
+  pdf:   new Set(["pdf"]),
+  md:    new Set(["md","markdown"]),
+  text:  new Set([
+    "txt","log","json","jsonl","yaml","yml","toml","ini","csv","tsv",
+    "py","js","mjs","ts","jsx","tsx","go","rs","java","c","cc","cpp","h","hpp",
+    "html","htm","xml","css","scss","sh","bash","zsh","sql","conf","env",
+  ]),
+  docx:  new Set(["docx"]),
+  xlsx:  new Set(["xlsx","xls"]),
+};
+function _categorize(rel) {
+  const m = /\.([a-z0-9]+)$/i.exec(rel);
+  const ext = m ? m[1].toLowerCase() : "";
+  for (const [cat, set] of Object.entries(_EXT_GROUPS)) if (set.has(ext)) return cat;
+  return "fallback";
+}
+
+let _fpCurrentRel = null;
+
+async function openFilePreview(rel) {
+  _fpCurrentRel = rel;
+  const name = rel.split("/").pop() || rel;
+  $("fp-name").textContent = name;
+  $("fp-meta").textContent = "";
+  const body = $("fp-body");
+  body.className = "body center";
+  body.innerHTML = `<div class="ph">加载中…</div>`;
+  $("file-preview-modal").classList.add("show");
+
+  const cat = _categorize(rel);
+  try {
+    const r = await fetch("/v1/files/download?path=" + encodeURIComponent(rel), {
+      headers: { "Authorization": "Bearer " + state.token },
+    });
+    if (!r.ok) throw new Error("HTTP " + r.status);
+    const blob = await r.blob();
+    $("fp-meta").textContent = humanSize(blob.size);
+
+    if (cat === "text" || cat === "md") {
+      if (blob.size > PREVIEW_TEXT_MAX) {
+        _showFallback(`文件过大 (${humanSize(blob.size)}),请下载查看`);
+        return;
+      }
+      const text = await blob.text();
+      if (cat === "md") _showMarkdown(text);
+      else _showText(text);
+      return;
+    }
+    if (blob.size > PREVIEW_BIN_MAX) {
+      _showFallback(`文件过大 (${humanSize(blob.size)}),请下载查看`);
+      return;
+    }
+    if (cat === "image") _showImage(blob);
+    else if (cat === "pdf") _showPdf(blob);
+    else if (cat === "docx") await _showDocx(blob);
+    else if (cat === "xlsx") await _showXlsx(blob);
+    else _showFallback("暂不支持在线预览此格式,请下载查看");
+  } catch (e) {
+    if (e.status === 401) { closeFilePreview(); logout(); return; }
+    _showFallback("加载失败:" + e.message);
+  }
+}
+
+function _showImage(blob) {
+  const url = _trackBlobUrl(blob);
+  const body = $("fp-body");
+  body.className = "body center";
+  body.innerHTML = "";
+  const img = document.createElement("img");
+  img.className = "preview-img";
+  img.src = url;
+  body.appendChild(img);
+}
+
+function _showPdf(blob) {
+  const url = _trackBlobUrl(blob, "application/pdf");
+  const body = $("fp-body");
+  body.className = "body";
+  body.innerHTML = `<iframe class="preview-frame" src="${url}"></iframe>`;
+}
+
+function _showText(text) {
+  const body = $("fp-body");
+  body.className = "body";
+  body.innerHTML = "";
+  const pre = document.createElement("pre");
+  pre.className = "preview-text";
+  pre.textContent = text;
+  body.appendChild(pre);
+}
+
+function _showMarkdown(text) {
+  const body = $("fp-body");
+  body.className = "body";
+  body.innerHTML = `<div class="md-render">${renderMd(text)}</div>`;
+  highlightIn(body);
+}
+
+async function _showDocx(blob) {
+  const body = $("fp-body");
+  body.className = "body center";
+  body.innerHTML = `<div class="ph">解析 docx 中…</div>`;
+  try {
+    await loadScript("/static/vendor/jszip.min.js");
+    await loadScript("/static/vendor/docx-preview.min.js");
+  } catch (e) {
+    _showFallback("docx 解析库加载失败:" + e.message);
+    return;
+  }
+  if (!window.docx || !window.docx.renderAsync) {
+    _showFallback("docx 解析库不可用");
+    return;
+  }
+  body.className = "body";
+  body.innerHTML = `<div class="docx-host"></div>`;
+  try {
+    await window.docx.renderAsync(blob, body.querySelector(".docx-host"), null, {
+      inWrapper: false,
+      ignoreLastRenderedPageBreak: true,
+    });
+  } catch (e) {
+    _showFallback("docx 渲染失败:" + e.message);
+  }
+}
+
+async function _showXlsx(blob) {
+  const body = $("fp-body");
+  body.className = "body center";
+  body.innerHTML = `<div class="ph">解析表格中…</div>`;
+  try {
+    await loadScript("/static/vendor/xlsx.full.min.js");
+  } catch (e) {
+    _showFallback("xlsx 解析库加载失败:" + e.message);
+    return;
+  }
+  if (!window.XLSX || !window.XLSX.read) {
+    _showFallback("xlsx 解析库不可用");
+    return;
+  }
+  let wb;
+  try {
+    const ab = await blob.arrayBuffer();
+    wb = window.XLSX.read(ab, { type: "array" });
+  } catch (e) {
+    _showFallback("xlsx 解析失败:" + e.message);
+    return;
+  }
+  const names = wb.SheetNames || [];
+  if (!names.length) { _showFallback("xlsx 内无 sheet"); return; }
+  body.className = "body";
+  const tabsHtml = names.map((n, i) =>
+    `<button class="small xlsx-tab${i===0?" active":""}" data-i="${i}">${escapeHtml(n)}</button>`
+  ).join("");
+  body.innerHTML = `<div class="xlsx-tabs">${tabsHtml}</div><div class="xlsx-sheet" id="fp-xlsx-sheet"></div>`;
+  const render = (i) => {
+    const ws = wb.Sheets[names[i]];
+    $("fp-xlsx-sheet").innerHTML = window.XLSX.utils.sheet_to_html(ws);
+  };
+  body.querySelectorAll(".xlsx-tab").forEach((btn) => {
+    btn.onclick = () => {
+      body.querySelectorAll(".xlsx-tab").forEach((b) => b.classList.remove("active"));
+      btn.classList.add("active");
+      render(parseInt(btn.dataset.i));
+    };
+  });
+  render(0);
+}
+
+function _showFallback(msg) {
+  const body = $("fp-body");
+  body.className = "body center";
+  body.innerHTML = "";
+  const ph = document.createElement("div");
+  ph.className = "ph";
+  ph.textContent = msg;
+  const br = document.createElement("br");
+  const dl = document.createElement("button");
+  dl.className = "primary";
+  dl.textContent = "下载原文件";
+  dl.style.marginTop = "12px";
+  dl.onclick = () => { if (_fpCurrentRel) downloadFile(_fpCurrentRel); };
+  ph.appendChild(document.createElement("br"));
+  ph.appendChild(br);
+  ph.appendChild(dl);
+  body.appendChild(ph);
+}
+
+function closeFilePreview() {
+  $("file-preview-modal").classList.remove("show");
+  $("fp-body").innerHTML = "";
+  _flushBlobUrls();
+  _fpCurrentRel = null;
+}
+
+$("fp-close").onclick = closeFilePreview;
+$("fp-download").onclick = () => { if (_fpCurrentRel) downloadFile(_fpCurrentRel); };
+$("file-preview-modal").addEventListener("click", (e) => {
+  if (e.target.id === "file-preview-modal") closeFilePreview();
+});
+document.addEventListener("keydown", (e) => {
+  if (e.key === "Escape" && $("file-preview-modal").classList.contains("show")) {
+    closeFilePreview();
+  }
+});
+
 async function uploadSelected() {
   const inp = $("upload-input");
   const files = Array.from(inp.files || []);