diff --git a/.cdp/verify-project-sync.cjs b/.cdp/verify-project-sync.cjs
new file mode 100644
index 0000000..01b3db3
--- /dev/null
+++ b/.cdp/verify-project-sync.cjs
@@ -0,0 +1,109 @@
+/* Same sync check as verify-sync.cjs, but on a Project board (verifies the
+   projectId branch of getBoardSnapshot + the provider's scoping). */
+const { spawn } = require("node:child_process");
+const { execFileSync } = require("node:child_process");
+const fs = require("node:fs");
+const http = require("node:http");
+
+const EDGE = "C:/Program Files (x86)/Microsoft/Edge/Application/msedge.exe";
+const PORT = 9342;
+const BASE = "http://localhost:3210";
+
+function httpJson(p) {
+  return new Promise((resolve, reject) => {
+    const req = http.request({ host: "127.0.0.1", port: PORT, path: p }, (res) => {
+      let d = ""; res.on("data", (c) => (d += c));
+      res.on("end", () => { try { resolve(JSON.parse(d)); } catch { resolve(d); } });
+    });
+    req.on("error", reject); req.end();
+  });
+}
+const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+function sql(statement) {
+  execFileSync("docker", ["exec", "organize-polltest-db", "psql", "-U", "organize", "-d", "organize", "-c", statement]);
+}
+
+(async () => {
+  const devUserId = execFileSync("docker", ["exec", "organize-polltest-db", "psql", "-U", "organize", "-d", "organize", "-tAc", "select id from \"User\" where email='dev@example.com'"]).toString().trim();
+
+  // Test project with one open to-do (clean slate regardless of prior runs).
+  sql(`delete from "Todo" where id='ptest-todo'; delete from "Group" where id='ptest-grp'; delete from "Category" where id='ptest-cat'; delete from "Project" where id='ptest-proj';`);
+  sql(`insert into "Project" (id, title, "ownerId", "createdAt", "updatedAt") values ('ptest-proj', 'Sync Test Project', '${devUserId}', now(), now());`);
+  sql(`insert into "Category" (id, name, "order", "projectId", "createdAt", "updatedAt") values ('ptest-cat', 'Lane A', 0, 'ptest-proj', now(), now());`);
+  sql(`insert into "Group" (id, title, color, "order", "noteContent", "categoryId", "createdAt", "updatedAt") values ('ptest-grp', 'P group', 'ocean', 0, '', 'ptest-cat', now(), now());`);
+  sql(`insert into "Todo" (id, title, "order", "groupId", "createdAt", "updatedAt") values ('ptest-todo', 'Project milk', 0, 'ptest-grp', now(), now());`);
+
+  const profile = fs.mkdtempSync("/tmp/edge-psync-");
+  const proc = spawn(EDGE, ["--headless=new", `--remote-debugging-port=${PORT}`, `--user-data-dir=${profile}`, "--no-first-run", "--no-default-browser-check", "--disable-gpu", "about:blank"], { stdio: "ignore" });
+  let targets = null;
+  for (let i = 0; i < 40; i++) { await sleep(500); try { targets = await httpJson("/json/list"); break; } catch {} }
+  if (!targets) { console.error("no targets"); process.exit(1); }
+  const page = targets.find((t) => t.type === "page");
+  const ws = new WebSocket(page.webSocketDebuggerUrl);
+  await new Promise((r) => (ws.onopen = r));
+  let id = 0;
+  const pending = new Map();
+  const send = (method, params = {}) => new Promise((resolve) => {
+    const myId = ++id; pending.set(myId, resolve);
+    ws.send(JSON.stringify({ id: myId, method, params }));
+  });
+  const errors = [];
+  ws.onmessage = (m) => {
+    const msg = JSON.parse(m.data);
+    if (msg.id) { const r = pending.get(msg.id); if (r) { pending.delete(msg.id); r(msg.result ?? msg.error); } return; }
+    if (msg.method === "Runtime.consoleAPICalled" && msg.params.type === "error") {
+      const txt = msg.params.args.map((a) => a.value ?? a.description ?? a.type).join(" ");
+      if (!/hydrated|React DevTools|HMR|Download the React DevTools/.test(txt)) errors.push(txt.slice(0, 200));
+    }
+  };
+  const evalJs = (expression) =>
+    send("Runtime.evaluate", { expression, awaitPromise: true, returnByValue: true })
+      .then((r) => (r?.result ? r.result.value : r));
+  await send("Runtime.enable");
+
+  await send("Page.enable");
+  await send("Page.navigate", { url: `${BASE}/login` });
+  for (let i = 0; i < 60; i++) { await sleep(500); if (await evalJs("document.readyState") === "complete") break; }
+  const login = await evalJs(`(async () => {
+    const c = await (await fetch('/api/auth/csrf')).json();
+    const r = await fetch('/api/auth/callback/credentials', { method: 'POST', redirect: 'manual',
+      body: new URLSearchParams({ csrfToken: c.csrfToken, email: 'dev@example.com', password: 'password123' }) });
+    return r.status; })()`);
+  console.log("login status:", login);
+
+  await send("Page.navigate", { url: `${BASE}/projects/ptest-proj` });
+  let checkboxFound = false;
+  for (let i = 0; i < 60; i++) {
+    await sleep(500);
+    checkboxFound = await evalJs(`!!Array.from(document.querySelectorAll('[data-todo-checkbox]'))
+      .some((el) => (el.getAttribute('aria-label') || '').includes('Project milk'))`);
+    if (checkboxFound) break;
+  }
+  if (!checkboxFound) { console.error("FAIL: project board checkbox not found"); process.exit(1); }
+  console.log("Project milk before:", await evalJs(`Array.from(document.querySelectorAll('[data-todo-checkbox]'))
+    .find((el) => (el.getAttribute('aria-label') || '').includes('Project milk')).getAttribute('aria-checked')`));
+
+  const t0 = Date.now();
+  console.log("simulating phone: completing the project's to-do in the DB...");
+  sql(`update "Todo" set "completed"=true, "completedAt"=now() where id='ptest-todo';`);
+  let done = false;
+  for (let i = 0; i < 60; i++) {
+    await sleep(1000);
+    const state = await evalJs(`(() => {
+      const el = Array.from(document.querySelectorAll('[data-todo-checkbox]'))
+        .find((el) => (el.getAttribute('aria-label') || '').includes('Project milk'));
+      if (el) return el.getAttribute('aria-checked');
+      return document.querySelector('[aria-label="Expand P group"]') ? "collapsed-done" : "missing";
+    })()`);
+    if (state === "true" || state === "collapsed-done") { done = true; break; }
+  }
+  const elapsed = Math.round((Date.now() - t0) / 1000);
+  console.log(`project to-do after (${elapsed}s): ${done ? "synced" : "NOT synced"}`);
+
+  sql(`delete from "Todo" where id='ptest-todo'; delete from "Group" where id='ptest-grp'; delete from "Category" where id='ptest-cat'; delete from "Project" where id='ptest-proj';`);
+  console.log("console errors:", errors.length ? errors : "none");
+  const ok = done && errors.length === 0;
+  console.log(ok ? "ALL PASS" : "FAILURES PRESENT");
+  ws.close(); proc.kill();
+  process.exit(ok ? 0 : 1);
+})().catch((e) => { console.error(e); process.exit(1); });
diff --git a/.cdp/verify-sync.cjs b/.cdp/verify-sync.cjs
new file mode 100644
index 0000000..5cc63ab
--- /dev/null
+++ b/.cdp/verify-sync.cjs
@@ -0,0 +1,153 @@
+/* Verifies cross-device board sync: a "second device" (direct DB write)
+   changes a to-do, and the open desktop tab must pick it up via the 30s
+   poll -- and immediately when the tab regains visibility. */
+const { spawn } = require("node:child_process");
+const fs = require("node:fs");
+const http = require("node:http");
+
+const EDGE = "C:/Program Files (x86)/Microsoft/Edge/Application/msedge.exe";
+const PORT = 9341;
+const BASE = "http://localhost:3210";
+
+function httpJson(p) {
+  return new Promise((resolve, reject) => {
+    const req = http.request({ host: "127.0.0.1", port: PORT, path: p }, (res) => {
+      let d = ""; res.on("data", (c) => (d += c));
+      res.on("end", () => { try { resolve(JSON.parse(d)); } catch { resolve(d); } });
+    });
+    req.on("error", reject); req.end();
+  });
+}
+const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+
+function dbTodo(title, completed) {
+  // Simulates the phone: a change that lands in the DB, not this tab.
+  return new Promise((resolve) => {
+    const { execFile } = require("node:child_process");
+    const sql = `UPDATE "Todo" SET "completed"=${completed}, "completedAt"=${completed ? "now()" : "NULL"} WHERE title='${title}' AND "completed"=${!completed};`;
+    execFile("docker", ["exec", "organize-polltest-db", "psql", "-U", "organize", "-d", "organize", "-c", sql], (err, out, errOut) => {
+      resolve({ ok: !err, out, errOut });
+    });
+  });
+}
+
+(async () => {
+  const profile = fs.mkdtempSync("/tmp/edge-sync-");
+  const proc = spawn(EDGE, ["--headless=new", `--remote-debugging-port=${PORT}`, `--user-data-dir=${profile}`, "--no-first-run", "--no-default-browser-check", "--disable-gpu", "about:blank"], { stdio: "ignore" });
+  let targets = null;
+  for (let i = 0; i < 40; i++) { await sleep(500); try { targets = await httpJson("/json/list"); break; } catch {} }
+  if (!targets) { console.error("no targets"); process.exit(1); }
+  const page = targets.find((t) => t.type === "page");
+  const ws = new WebSocket(page.webSocketDebuggerUrl);
+  await new Promise((r) => (ws.onopen = r));
+  let id = 0;
+  const pending = new Map();
+  const send = (method, params = {}) => new Promise((resolve) => {
+    const myId = ++id; pending.set(myId, resolve);
+    ws.send(JSON.stringify({ id: myId, method, params }));
+  });
+  const consoleErrors = [];
+  ws.onmessage = (m) => {
+    const msg = JSON.parse(m.data);
+    if (msg.id) { const r = pending.get(msg.id); if (r) { pending.delete(msg.id); r(msg.result ?? msg.error); } return; }
+    if (msg.method === "Runtime.consoleAPICalled" && msg.params.type === "error") {
+      const txt = msg.params.args.map((a) => a.value ?? a.description ?? a.type).join(" ");
+      if (!/hydrated|React DevTools|HMR|Download the React DevTools/.test(txt)) consoleErrors.push(txt.slice(0, 200));
+    }
+  };
+
+  const evalJs = (expression) =>
+    send("Runtime.evaluate", { expression, awaitPromise: true, returnByValue: true })
+      .then((r) => (r?.result ? r.result.value : r));
+  const goto = async (url) => {
+    await send("Page.enable");
+    await send("Page.navigate", { url });
+    // Wait for load to settle.
+    for (let i = 0; i < 60; i++) {
+      await sleep(500);
+      const s = await evalJs("document.readyState");
+      if (s === "complete") return;
+    }
+    throw new Error("page did not load: " + url);
+  };
+  const checkboxState = (title) =>
+    evalJs(`(() => {
+      const el = Array.from(document.querySelectorAll('[data-todo-checkbox]'))
+        .find((el) => (el.getAttribute('aria-label') || '').includes(${JSON.stringify(title)}));
+      return el ? el.getAttribute('aria-checked') : "missing";
+    })()`);
+
+  await send("Runtime.enable");
+
+  // --- Login ---
+  await goto(`${BASE}/login`);
+  const login = await evalJs(`(async () => {
+    const c = await (await fetch('/api/auth/csrf')).json();
+    const r = await fetch('/api/auth/callback/credentials', {
+      method: 'POST', redirect: 'manual',
+      body: new URLSearchParams({ csrfToken: c.csrfToken, email: 'dev@example.com', password: 'password123' }),
+    });
+    return r.status;
+  })()`);
+  console.log("login status:", login);
+
+  // --- Reset test data before the board loads (idempotent) ---
+  await dbTodo("Milk", false);
+  await dbTodo("Eggs", false);
+
+  // --- Open the Home board ---
+  await goto(`${BASE}/`);
+  for (let i = 0; i < 60; i++) {
+    await sleep(500);
+    if (await evalJs(`!!document.querySelector('[data-todo-checkbox]')`)) break;
+  }
+  console.log("Milk before:", await checkboxState('Milk'));
+  if ((await checkboxState("Milk")) === "missing") { console.error("FAIL: Milk checkbox not found"); process.exit(1); }
+
+  // --- Test 1: 30s poll picks up a remote change ---
+  const t0 = Date.now();
+  console.log("simulating phone: checking off 'Milk' in the DB...");
+  await dbTodo("Milk", true);
+  let saw = "missing";
+  for (let i = 0; i < 60; i++) {
+    await sleep(1000);
+    saw = await checkboxState("Milk");
+    if (saw === "true") break;
+  }
+  const elapsed = Math.round((Date.now() - t0) / 1000);
+  console.log(`Milk after (${elapsed}s):`, saw);
+  const test1 = saw === "true" && elapsed <= 45;
+  console.log(test1 ? "PASS: 30s poll applied the remote change" : "FAIL: poll did not apply the remote change");
+
+  // --- Test 2: hidden tab skips, visibilitychange catches up ---
+  await evalJs(`Object.defineProperty(document, 'visibilityState', { configurable: true, get: () => 'hidden' })`);
+  // Let a poll tick fire while "hidden" (interval is 30s; wait 32s).
+  await sleep(32000);
+  console.log("simulating phone: checking off 'Eggs' in the DB (tab hidden)...");
+  await dbTodo("Eggs", true);
+  await sleep(2000);
+  const whileHidden = await checkboxState("Eggs");
+  console.log("Eggs while still hidden:", whileHidden);
+  await evalJs(`Object.defineProperty(document, 'visibilityState', { configurable: true, get: () => 'visible' }); document.dispatchEvent(new Event('visibilitychange'))`);
+  // When the sync lands, 'Groceries' becomes fully completed and the card
+  // collapses (its checkboxes disappear from the DOM, replaced by an
+  // "Expand Groceries" disclosure) -- that is exactly how a fully-checked
+  // group renders, so assert the collapsed card rather than the checkbox.
+  let settled = false;
+  for (let i = 0; i < 15; i++) {
+    await sleep(1000);
+    const state = await checkboxState("Eggs");
+    const collapsed = await evalJs(`!!document.querySelector('[aria-label="Expand Groceries"]')`);
+    if (state === "missing" && collapsed) { settled = true; break; }
+    if (state === "true") { settled = true; break; }
+  }
+  const test2 = settled && whileHidden === "false";
+  console.log(`Eggs group after tab shown: ${settled ? "synced (card collapsed as fully-complete)" : "NOT synced"}`);
+  console.log(test2 ? "PASS: visibilitychange triggered immediate catch-up" : "FAIL: visibility catch-up");
+
+  console.log("console errors:", consoleErrors.length ? consoleErrors : "none");
+  const ok = test1 && test2 && consoleErrors.length === 0;
+  console.log(ok ? "ALL PASS" : "FAILURES PRESENT");
+  ws.close(); proc.kill();
+  process.exit(ok ? 0 : 1);
+})().catch((e) => { console.error(e); process.exit(1); });
diff --git a/components/board/board-context.tsx b/components/board/board-context.tsx
index 14d2d96..1751764 100644
--- a/components/board/board-context.tsx
+++ b/components/board/board-context.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { createContext, useContext, useState, useCallback } from "react";
+import { createContext, useContext, useState, useCallback, useEffect, useRef } from "react";
 import { toast } from "sonner";
 
 import type { CategoryDTO, GroupDTO, TodoDTO } from "@/types/board";
@@ -25,6 +25,13 @@ import {
   deleteTodo as deleteTodoAction,
 } from "@/lib/actions/todos";
 import { updateGroupNote } from "@/lib/actions/notes";
+import { getBoardSnapshot } from "@/lib/actions/board";
+
+// How often a visible tab checks for changes made on another device (see
+// the sync effect in BoardProvider). Deliberately not real time: a phone
+// tap lands here within a tick at most -- and immediately if the tab only
+// just came to the foreground.
+const POLL_INTERVAL_MS = 30_000;
 
 interface BoardContextValue {
   categories: CategoryDTO[];
@@ -75,6 +82,10 @@ interface BoardContextValue {
     completed: boolean
   ) => Promise<void>;
   removeTodo: (todoId: string, groupId: string, categoryId: string) => Promise<void>;
+  // True while a dnd-kit drag is live -- a state replacement mid-drag would
+  // swap the SortableContext items out from under it, so remote sync
+  // defers until the drag ends.
+  suspendRemoteSync: (suspended: boolean) => void;
 }
 
 const BoardContext = createContext<BoardContextValue | null>(null);
@@ -96,6 +107,94 @@ export function BoardProvider({
 }) {
   const [categories, setCategories] = useState(initialCategories);
 
+  // -- Cross-device sync ---------------------------------------------------
+  // The board is seeded once from the server and otherwise only changed by
+  // this tab's own optimistic mutations, so edits made on a second device
+  // (checking off a to-do on a phone, renaming a lane, ...) never reach
+  // this tab on their own. The poll below closes that gap: fetch a fresh
+  // snapshot and, if it differs from local state and no local write is
+  // pending or just landed, apply it. The refs are the guards that keep a
+  // poll from ever clobbering local work.
+  // Always mirrors `categories` for the diff below (updated by the effect
+  // under it rather than every setCategories call site).
+  const categoriesRef = useRef(categories);
+  // Local mutations in flight (optimistic state ahead of the server).
+  const inflightRef = useRef(0);
+  // True while a drag is live (set from Board's drag start/end).
+  const remoteSyncSuspendedRef = useRef(false);
+  // A poll is already running.
+  const pollingRef = useRef(false);
+  // When the most recent local mutation settled (success or rollback).
+  const lastLocalWriteAtRef = useRef(0);
+
+  useEffect(() => {
+    categoriesRef.current = categories;
+  }, [categories]);
+
+  // Marks a server action as "local work in progress" for the poll's
+  // duration, and stamps when it settled. Every mutation below goes
+  // through this so the sync effect can tell "state we are about to lose"
+  // from "state we will happily overwrite with the server's copy".
+  const track = useCallback(
+    <T,>(promise: Promise<T>): Promise<T> => {
+      inflightRef.current += 1;
+      return promise.finally(() => {
+        inflightRef.current -= 1;
+        lastLocalWriteAtRef.current = Date.now();
+      });
+    },
+    []
+  );
+
+  const suspendRemoteSync = useCallback((suspended: boolean) => {
+    remoteSyncSuspendedRef.current = suspended;
+  }, []);
+
+  useEffect(() => {
+    let disposed = false;
+
+    async function refreshBoard() {
+      if (disposed || pollingRef.current) return;
+      if (remoteSyncSuspendedRef.current || inflightRef.current > 0) return;
+      // No point fetching while the tab is hidden; the visibilitychange
+      // handler below catches up the moment it is shown again.
+      if (document.visibilityState !== "visible") return;
+      pollingRef.current = true;
+      try {
+        const board = await getBoardSnapshot(projectId ?? null);
+        if (disposed) return;
+        // Re-check the guards at apply time, not just at start: a mutation
+        // (or drag) that began while the fetch was in flight makes this
+        // snapshot stale relative to local state. A write that settled in
+        // the last couple of seconds may post-date the snapshot's read on
+        // the server too, so defer it -- the next tick picks everything up.
+        if (remoteSyncSuspendedRef.current || inflightRef.current > 0) return;
+        if (Date.now() - lastLocalWriteAtRef.current < 2000) return;
+        // The common case is "nothing changed" -- skip the setState so the
+        // whole board doesn't re-render every tick.
+        if (JSON.stringify(board) === JSON.stringify(categoriesRef.current)) return;
+        setCategories(board);
+      } catch {
+        // Offline or a server hiccup -- the next tick retries. (If the
+        // session itself is gone, the page's own navigation handles it.)
+      } finally {
+        pollingRef.current = false;
+      }
+    }
+
+    function handleVisibilityChange() {
+      if (document.visibilityState === "visible") void refreshBoard();
+    }
+
+    document.addEventListener("visibilitychange", handleVisibilityChange);
+    const timer = window.setInterval(() => void refreshBoard(), POLL_INTERVAL_MS);
+    return () => {
+      disposed = true;
+      document.removeEventListener("visibilitychange", handleVisibilityChange);
+      window.clearInterval(timer);
+    };
+  }, [projectId]);
+
   const updateCategory = useCallback(
     (categoryId: string, updater: (c: CategoryDTO) => CategoryDTO) => {
       setCategories((prev) => prev.map((c) => (c.id === categoryId ? updater(c) : c)));
@@ -116,13 +215,13 @@ export function BoardProvider({
   const addCategory = useCallback(
     async (name: string) => {
       try {
-        const category = await createCategory(name, projectId);
+        const category = await track(createCategory(name, projectId));
         setCategories((prev) => [...prev, category]);
       } catch {
         toast.error("Couldn't create category. Try again.");
       }
     },
-    [projectId]
+    [projectId, track]
   );
 
   const removeCategory = useCallback(async (categoryId: string) => {
@@ -132,7 +231,7 @@ export function BoardProvider({
       return prev.filter((c) => c.id !== categoryId);
     });
     try {
-      const result = await deleteCategoryAction(categoryId);
+      const result = await track(deleteCategoryAction(categoryId));
       if (result?.error) {
         setCategories(prevState);
         toast.error(result.error);
@@ -141,7 +240,7 @@ export function BoardProvider({
       setCategories(prevState);
       toast.error("Couldn't delete category. Try again.");
     }
-  }, []);
+  }, [track]);
 
   const reorderLanes = useCallback(
     async (orderedIds: string[]) => {
@@ -152,36 +251,36 @@ export function BoardProvider({
         return orderedIds.map((id, order) => ({ ...byId.get(id)!, order }));
       });
       try {
-        await reorderCategories(orderedIds, projectId);
+        await track(reorderCategories(orderedIds, projectId));
       } catch {
         setCategories(prevState);
         toast.error("Couldn't reorder categories. Try again.");
       }
     },
-    [projectId]
+    [projectId, track]
   );
 
   const addGroup = useCallback(async (categoryId: string, title: string, color: string) => {
     try {
-      const group = await createGroup(categoryId, title, color);
+      const group = await track(createGroup(categoryId, title, color));
       updateCategory(categoryId, (c) => ({ ...c, groups: [...c.groups, group] }));
       return group;
     } catch {
       toast.error("Couldn't create group. Try again.");
       return undefined;
     }
-  }, [updateCategory]);
+  }, [updateCategory, track]);
 
   const editGroup = useCallback(
     async (groupId: string, categoryId: string, data: { title?: string; color?: string }) => {
       try {
-        await updateGroupAction(groupId, data);
+        await track(updateGroupAction(groupId, data));
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, ...data }));
       } catch {
         toast.error("Couldn't update group. Try again.");
       }
     },
-    [updateGroupInState]
+    [updateGroupInState, track]
   );
 
   const removeGroup = useCallback(async (groupId: string, categoryId: string) => {
@@ -193,12 +292,12 @@ export function BoardProvider({
       );
     });
     try {
-      await deleteGroupAction(groupId);
+      await track(deleteGroupAction(groupId));
     } catch {
       setCategories(prevState);
       toast.error("Couldn't delete group. Try again.");
     }
-  }, []);
+  }, [track]);
 
   const archiveGroup = useCallback(async (groupId: string, categoryId: string) => {
     let prevState: CategoryDTO[] = [];
@@ -209,12 +308,12 @@ export function BoardProvider({
       );
     });
     try {
-      await archiveGroupAction(groupId);
+      await track(archiveGroupAction(groupId));
     } catch {
       setCategories(prevState);
       toast.error("Couldn't archive group. Try again.");
     }
-  }, []);
+  }, [track]);
 
   const reorderGroups = useCallback(async (categoryId: string, orderedIds: string[]) => {
     let prevState: CategoryDTO[] = [];
@@ -227,12 +326,12 @@ export function BoardProvider({
       });
     });
     try {
-      await reorderGroupsInCategory(categoryId, orderedIds);
+      await track(reorderGroupsInCategory(categoryId, orderedIds));
     } catch {
       setCategories(prevState);
       toast.error("Couldn't reorder groups. Try again.");
     }
-  }, []);
+  }, [track]);
 
   const moveGroup = useCallback(
     async (
@@ -273,19 +372,19 @@ export function BoardProvider({
         });
       });
       try {
-        await moveGroupToCategory(groupId, toCategoryId, orderedTarget, orderedSource);
+        await track(moveGroupToCategory(groupId, toCategoryId, orderedTarget, orderedSource));
       } catch {
         setCategories(prevState);
         toast.error("Couldn't move group. Try again.");
       }
     },
-    []
+    [track]
   );
 
   const saveNote = useCallback(
     async (groupId: string, categoryId: string, noteContent: string) => {
       try {
-        await updateGroupNote(groupId, noteContent);
+        await track(updateGroupNote(groupId, noteContent));
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, noteContent }));
         return true;
       } catch {
@@ -293,13 +392,13 @@ export function BoardProvider({
         return false;
       }
     },
-    [updateGroupInState]
+    [updateGroupInState, track]
   );
 
   const addTodo = useCallback(
     async (groupId: string, categoryId: string, title: string, details?: string) => {
       try {
-        const todo = await createTodo(groupId, title, details);
+        const todo = await track(createTodo(groupId, title, details));
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, todos: [...g.todos, todo] }));
         return true;
       } catch {
@@ -307,13 +406,13 @@ export function BoardProvider({
         return false;
       }
     },
-    [updateGroupInState]
+    [updateGroupInState, track]
   );
 
   const addTodos = useCallback(
     async (groupId: string, categoryId: string, todos: { title: string; details?: string }[]) => {
       try {
-        const created = await createTodos(groupId, todos);
+        const created = await track(createTodos(groupId, todos));
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, todos: [...g.todos, ...created] }));
         return true;
       } catch {
@@ -321,7 +420,7 @@ export function BoardProvider({
         return false;
       }
     },
-    [updateGroupInState]
+    [updateGroupInState, track]
   );
 
   const editTodo = useCallback(
@@ -332,7 +431,7 @@ export function BoardProvider({
       data: { title?: string; details?: string | null }
     ) => {
       try {
-        await updateTodoAction(todoId, data);
+        await track(updateTodoAction(todoId, data));
         // Approximates the server's own `updatedAt` (set by the same write,
         // a moment later) closely enough for display purposes, without
         // waiting on a round trip just to read it back.
@@ -347,7 +446,7 @@ export function BoardProvider({
         return false;
       }
     },
-    [updateGroupInState]
+    [updateGroupInState, track]
   );
 
   const toggleTodoDone = useCallback(
@@ -370,13 +469,13 @@ export function BoardProvider({
         }),
       }));
       try {
-        await toggleTodoAction(todoId, completed);
+        await track(toggleTodoAction(todoId, completed));
       } catch {
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, todos: prevTodos }));
         toast.error("Couldn't update to-do. Try again.");
       }
     },
-    [updateCategory, updateGroupInState]
+    [updateCategory, updateGroupInState, track]
   );
 
   const removeTodo = useCallback(
@@ -391,13 +490,13 @@ export function BoardProvider({
         }),
       }));
       try {
-        await deleteTodoAction(todoId);
+        await track(deleteTodoAction(todoId));
       } catch {
         updateGroupInState(categoryId, groupId, (g) => ({ ...g, todos: prevTodos }));
         toast.error("Couldn't delete to-do. Try again.");
       }
     },
-    [updateCategory, updateGroupInState]
+    [updateCategory, updateGroupInState, track]
   );
 
   return (
@@ -420,6 +519,7 @@ export function BoardProvider({
         editTodo,
         toggleTodoDone,
         removeTodo,
+        suspendRemoteSync,
       }}
     >
       {children}
diff --git a/components/board/kanban-board.tsx b/components/board/kanban-board.tsx
index 151c4a7..f7fbb45 100644
--- a/components/board/kanban-board.tsx
+++ b/components/board/kanban-board.tsx
@@ -38,7 +38,7 @@ function Board({
   projectId?: string;
   aiConfigured: boolean;
 }) {
-  const { categories, reorderLanes, reorderGroups, moveGroup } = useBoard();
+  const { categories, reorderLanes, reorderGroups, moveGroup, suspendRemoteSync } = useBoard();
   const [draggedGroup, setDraggedGroup] = useState<GroupDTO | null>(null);
   const [quickAddOpen, setQuickAddOpen] = useState(false);
   const pagerRef = useRef<HTMLDivElement>(null);
@@ -67,6 +67,11 @@ function Board({
   }
 
   function handleDragStart(event: DragStartEvent) {
+    // Hold cross-device sync off for the drag's duration: replacing the
+    // board state mid-drag would swap the SortableContext items out from
+    // under dnd-kit. Drag ends (including cancelled ones) always fire
+    // handleDragEnd, which resumes it.
+    suspendRemoteSync(true);
     const data = event.active.data.current;
     if (data?.type === "group") {
       const category = findCategory(data.categoryId as string);
@@ -76,6 +81,7 @@ function Board({
   }
 
   function handleDragEnd(event: DragEndEvent) {
+    suspendRemoteSync(false);
     const { active, over } = event;
     setDraggedGroup(null);
     if (!over) return;
diff --git a/lib/actions/board.ts b/lib/actions/board.ts
new file mode 100644
index 0000000..99d0c94
--- /dev/null
+++ b/lib/actions/board.ts
@@ -0,0 +1,27 @@
+"use server";
+
+import { prisma } from "@/lib/db";
+import { requireUserId } from "@/lib/auth-helpers";
+import { projectAccessFilter } from "@/lib/access";
+import { getBoard } from "@/lib/board";
+import type { CategoryDTO } from "@/types/board";
+
+/**
+ * A fresh read of one board for the client's cross-device sync (see the
+ * poll in board-context.tsx). Returns the exact same shape the page's
+ * server component renders, so BoardProvider can diff it against its own
+ * state and apply it only if something actually changed. `projectId` null
+ * is Home scope -- same convention as getScheduledBoard.
+ */
+export async function getBoardSnapshot(projectId: string | null): Promise<CategoryDTO[]> {
+  const userId = await requireUserId();
+  if (projectId) {
+    const project = await prisma.project.findFirst({
+      where: { id: projectId, ...projectAccessFilter(userId) },
+      select: { id: true },
+    });
+    if (!project) throw new Error("Project not found");
+    return getBoard({ projectId: project.id });
+  }
+  return getBoard({ userId, projectId: null });
+}