diff --git a/public/plugins/collab-buckets/backend.php b/public/plugins/collab-buckets/backend.php
index b7160d4..b3467b6 100644
--- a/public/plugins/collab-buckets/backend.php
+++ b/public/plugins/collab-buckets/backend.php
@@ -358,7 +358,7 @@ function collabBuildCardPayload(array $card): array {
  */
 function collabSyncCards(PDO $db): array {
     $relay = collabRelaySettings();
-    $stats = ['pushed' => 0, 'pulled' => 0, 'deleted_local' => 0, 'tombstones_sent' => 0, 'conflicts' => 0, 'moved' => 0];
+    $stats = ['pushed' => 0, 'pulled' => 0, 'deleted_local' => 0, 'tombstones_sent' => 0, 'conflicts' => 0, 'moved' => 0, 'blocked' => 0, 'blocked_by' => []];
     if (!$relay['url'] || !$relay['user'] || !$relay['key']) return $stats;
 
     // ── 1. Drain tombstones ────────────────────────────────
@@ -371,8 +371,15 @@ function collabSyncCards(PDO $db): array {
         if (($res['ok'] ?? false)) {
             $db->prepare('DELETE FROM collab_tombstones WHERE id = :id')->execute(['id' => $t['id']]);
             $stats['tombstones_sent']++;
+        } else if (($res['status'] ?? 0) === 423) {
+            // Lock holder is editing — don't delete under their feet.
+            // Tombstone stays queued; next poll retries after release.
+            $stats['blocked']++;
+            $by = $res['locked_by'] ?? '';
+            if ($by !== '' && !in_array($by, $stats['blocked_by'], true)) $stats['blocked_by'][] = $by;
         }
-        // If a tombstone push fails it stays queued and gets retried next poll.
+        // If a tombstone push fails for any other reason it stays queued
+        // and gets retried next poll.
     }
 
     // ── 1b. Detect moved cards ─────────────────────────────
@@ -457,7 +464,18 @@ function collabSyncCards(PDO $db): array {
             'card_uid'   => $relayUid,
             'payload'    => $payload,
         ]);
-        if (!($res['ok'] ?? false)) continue; // retry next poll
+        if (!($res['ok'] ?? false)) {
+            // 423 = relay refused because someone else holds an exclusive
+            // lock. Don't bump last_push_at — keep the card dirty so we
+            // retry on the next poll. Surface the holder name so the
+            // frontend can toast.
+            if (($res['status'] ?? 0) === 423) {
+                $stats['blocked']++;
+                $by = $res['locked_by'] ?? '';
+                if ($by !== '' && !in_array($by, $stats['blocked_by'], true)) $stats['blocked_by'][] = $by;
+            }
+            continue;
+        }
 
         $stateUpsert->execute([
             'cid'  => $card['id'],
@@ -798,6 +816,7 @@ PluginManager::instance()->onApiRoute(function (string $route, string $method) {
 
         collabJson([
             'ok'               => true,
+            'self'             => collabRelaySettings()['user'],
             'changed'          => $changed,
             'total'            => count($remote),
             'cards_changed'    => $cardsChanged,
@@ -867,7 +886,11 @@ PluginManager::instance()->onApiRoute(function (string $route, string $method) {
     if ($route === '/api/collab-locks' && $method === 'POST') {
         $input = json_decode(file_get_contents('php://input'), true) ?: [];
         $res = collabRelayCall('/api/collab-locks', 'POST', $input);
-        collabJson($res);
+        // 409 from the relay = another tenant already holds this lock.
+        // Surface the status so the frontend can take the editor over.
+        $code = ($res['ok'] ?? false) ? 200 : (int)($res['status'] ?? 500);
+        if ($code === 0) $code = 500;
+        collabJson($res, $code);
     }
     if (preg_match('#^/api/collab-locks/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$#', $route, $m) && $method === 'DELETE') {
         $res = collabRelayCall('/api/collab-locks/' . urlencode($m[1]) . '/' . urlencode($m[2]), 'DELETE');
diff --git a/public/plugins/collab-buckets/collab-buckets.css b/public/plugins/collab-buckets/collab-buckets.css
index 19d897e..a4c8908 100644
--- a/public/plugins/collab-buckets/collab-buckets.css
+++ b/public/plugins/collab-buckets/collab-buckets.css
@@ -235,27 +235,84 @@ body.anim-slow .bucket-tab.bucket-tab-collab.pulsing {
     font-style: italic;
 }
 
-/* ── Phase 4: edit-lock indicator on board cards ── */
+/* ── Phase 4: exclusive edit-lock indicator on board cards ──
+   Locked cards get a frosted overlay + centered lock icon. The card
+   content underneath stays visible enough to read but is clearly
+   "off-limits" — the cue is intentionally heavier than the old badge
+   so the user can't miss it. */
 .board-card.collab-locked {
-    box-shadow: inset 0 0 0 2px color-mix(in srgb, var(--t-accent2) 55%, transparent);
+    box-shadow: inset 0 0 0 2px color-mix(in srgb, var(--t-accent2) 70%, transparent);
+}
+.board-card.collab-locked > *:not(.collab-lock-badge) {
+    filter: blur(2px);
+    opacity: 0.55;
+    transition: filter 0.15s, opacity 0.15s;
 }
 .collab-lock-badge {
     position: absolute;
-    top: 6px;
-    right: 6px;
-    width: 20px;
-    height: 20px;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
     display: flex;
+    flex-direction: column;
     align-items: center;
     justify-content: center;
+    gap: 4px;
     color: var(--t-accent2);
-    background: color-mix(in srgb, var(--t-surface) 92%, transparent);
-    border: 1px solid color-mix(in srgb, var(--t-accent2) 60%, var(--t-border));
-    border-radius: 50%;
+    background: color-mix(in srgb, var(--t-surface) 60%, transparent);
+    backdrop-filter: blur(1px);
+    -webkit-backdrop-filter: blur(1px);
     z-index: 6;
     pointer-events: none;
-    font-size: 11px;
+    font-family: var(--t-font-pixel, 'Silkscreen', cursive);
+    font-size: 9px;
+    text-align: center;
+    padding: 4px 8px;
 }
+.collab-lock-badge i {
+    font-size: 22px;
+}
+.collab-lock-badge span {
+    color: var(--t-text);
+    max-width: 100%;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+/* ── Editor takeover (foreign lock arrived while editing) ── */
+.collab-readonly-modal #inp-title,
+.collab-readonly-modal #editor-body {
+    opacity: 0.7;
+    cursor: not-allowed;
+}
+.collab-readonly-modal #editor-body {
+    background: color-mix(in srgb, var(--t-danger) 6%, transparent) !important;
+}
+.collab-save-blocked {
+    opacity: 0.4 !important;
+    cursor: not-allowed !important;
+    pointer-events: auto;
+}
+.collab-takeover-banner {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    padding: 10px 14px;
+    background: color-mix(in srgb, var(--t-danger) 12%, transparent);
+    border-bottom: 1px solid var(--t-danger);
+    color: var(--t-bright);
+    font-family: var(--t-font-pixel, 'Silkscreen', cursive);
+    font-size: 10px;
+    line-height: 1.5;
+}
+.collab-takeover-banner i {
+    color: var(--t-danger);
+    font-size: 16px;
+    flex-shrink: 0;
+}
+.collab-takeover-banner b { color: var(--t-danger); }
 
 /* ── Conflict toast (LWW losers stashed into the Conflicts bucket) ── */
 .collab-conflict-toast {
@@ -302,3 +359,17 @@ body.anim-slow .bucket-tab.bucket-tab-collab.pulsing {
     align-items: center;
 }
 .collab-conflict-close:hover { color: var(--t-danger); }
+
+/* Lock toast: same shape as the conflict toast but accented with the
+   collab/lock color. Reused for both the editor-takeover notification
+   and the "your push was queued because someone else is editing" toast. */
+.collab-conflict-toast.collab-lock-toast {
+    border-color: var(--t-accent2);
+}
+.collab-conflict-toast.collab-lock-toast i.ph-lock-simple-fill {
+    color: var(--t-accent2);
+    flex-shrink: 0;
+}
+.collab-conflict-toast.collab-lock-toast b {
+    color: var(--t-accent2);
+}
diff --git a/public/plugins/collab-buckets/collab-buckets.js b/public/plugins/collab-buckets/collab-buckets.js
index b8ec215..685c57a 100644
--- a/public/plugins/collab-buckets/collab-buckets.js
+++ b/public/plugins/collab-buckets/collab-buckets.js
@@ -27,11 +27,12 @@ let collabByUid = new Map();
 let pollHandle = null;
 const POLL_INTERVAL_MS = 30000;
 
-// Phase 4: edit locks
+// Phase 4: edit locks (now exclusive — server-enforced)
 // card_uid -> { bucket_uid, holder_username, expires_at }
 let collabLocks = new Map();
-// Active heartbeat for the currently open editor (if editing a collab card)
-let activeLock = null; // { bucket_uid, card_uid, timer }
+// Tracks the editor session: { bucket_uid, card_uid, cardId, hbTimer,
+// inputCleanup, acquired, takeover }
+let editorSession = null;
 const LOCK_HEARTBEAT_MS = 30000;
 
 // ── Helpers ───────────────────────────────────────────────────────
@@ -61,6 +62,7 @@ async function pollOnce() {
     try {
         const res = await hyprcrd.api('api/collab-buckets/poll', { method: 'POST' });
         if (!res?.ok) return;
+        if (res.self) window._collabSelf = res.self;
         // Reload if:
         //   * a bucket changed (metadata / members)
         //   * the bucket set shrunk or grew
@@ -90,6 +92,25 @@ async function pollOnce() {
             collabLocks = next;
             window._collabLocks = collabLocks;
             decorateBoardLocks();
+            // If the editor is currently open on a card that just got
+            // locked by someone else, take over immediately. Covers the
+            // race where two users started typing nearly simultaneously
+            // and the relay arbitration only resolves on the next poll.
+            if (editorSession && !editorSession.takeover) {
+                const lock = collabLocks.get(editorSession.card_uid);
+                const self = window._collabSelf || null;
+                if (lock && lock.holder_username && lock.holder_username !== self) {
+                    enterEditorTakeover(lock.holder_username);
+                }
+            }
+        }
+        // The push phase couldn't deliver because someone else holds an
+        // exclusive lock on a dirty card. Surface so the user knows their
+        // edit isn't lost — it's just queued.
+        const blocked = res.card_stats?.blocked || 0;
+        if (blocked > 0) {
+            const by = Array.isArray(res.card_stats?.blocked_by) ? res.card_stats.blocked_by : [];
+            showBlockedToast(by);
         }
     } catch { /* silent */ }
 }
@@ -597,10 +618,13 @@ function decorateBoardLocks() {
             if (!badge) {
                 badge = document.createElement('div');
                 badge.className = 'collab-lock-badge';
-                badge.innerHTML = '<i class="ph ph-lock-simple text-xs"></i>';
                 el.appendChild(badge);
             }
-            badge.title = `Being edited by ${lock.holder_username}`;
+            badge.innerHTML = `
+                <i class="ph ph-lock-simple-fill"></i>
+                <span>${escapeHtml(lock.holder_username)}</span>
+            `;
+            badge.title = `Locked by ${lock.holder_username}`;
         } else if (badge) {
             badge.remove();
         }
@@ -630,13 +654,22 @@ function collabIdentityOf(card) {
     return { bucket_uid: bucket.collab_uid, card_uid: card.relay_uid };
 }
 
-async function acquireLock(bucket_uid, card_uid) {
+/** Acquire (or refresh) the exclusive lock. Returns:
+ *    { ok: true }                  on success
+ *    { ok: false, locked_by: '…' } when another tenant already holds it (409)
+ *    { ok: false }                 on transport / unknown error */
+async function tryAcquireLock(bucket_uid, card_uid) {
     try {
-        await hyprcrd.api('api/collab-locks', {
+        const res = await hyprcrd.api('api/collab-locks', {
             method: 'POST',
             body: { bucket_uid, card_uid },
         });
-    } catch { /* silent */ }
+        if (res?.ok) return { ok: true };
+        if (res?.locked_by) return { ok: false, locked_by: res.locked_by };
+        return { ok: false };
+    } catch {
+        return { ok: false };
+    }
 }
 
 async function releaseLock(bucket_uid, card_uid) {
@@ -648,41 +681,203 @@ async function releaseLock(bucket_uid, card_uid) {
     } catch { /* silent */ }
 }
 
-function startHeartbeat(bucket_uid, card_uid) {
-    stopHeartbeat();
-    activeLock = { bucket_uid, card_uid, timer: null };
-    acquireLock(bucket_uid, card_uid);
-    activeLock.timer = setInterval(() => acquireLock(bucket_uid, card_uid), LOCK_HEARTBEAT_MS);
+/** First-keystroke lock: sets up one-shot input listeners on the editor
+ *  inputs. The first input event tries to acquire the exclusive lock; on
+ *  success we start the heartbeat, on failure we slap the editor into
+ *  takeover mode. */
+function armFirstKeystrokeLock() {
+    if (!editorSession) return;
+    const inputs = [document.getElementById('inp-title'), document.getElementById('editor-body')].filter(Boolean);
+    let fired = false;
+    const onInput = async () => {
+        if (fired) return;
+        fired = true;
+        // Drop the listeners — heartbeat or takeover takes over from here
+        for (const el of inputs) el.removeEventListener('input', onInput);
+        if (!editorSession) return;
+        const { bucket_uid, card_uid } = editorSession;
+        const res = await tryAcquireLock(bucket_uid, card_uid);
+        if (!editorSession) return; // editor closed during the await
+        if (res.ok) {
+            editorSession.acquired = true;
+            editorSession.hbTimer = setInterval(() => tryAcquireLock(bucket_uid, card_uid), LOCK_HEARTBEAT_MS);
+        } else {
+            enterEditorTakeover(res.locked_by || 'someone');
+        }
+    };
+    for (const el of inputs) el.addEventListener('input', onInput);
+    editorSession.inputCleanup = () => {
+        for (const el of inputs) el.removeEventListener('input', onInput);
+    };
 }
 
-function stopHeartbeat() {
-    if (!activeLock) return;
-    const { bucket_uid, card_uid, timer } = activeLock;
-    if (timer) clearInterval(timer);
-    releaseLock(bucket_uid, card_uid);
-    activeLock = null;
+/** Switch the editor to read-only takeover mode. The user can still see /
+ *  copy their text but the inputs are disabled and save is blocked. */
+function enterEditorTakeover(holder) {
+    if (!editorSession || editorSession.takeover) return;
+    editorSession.takeover = true;
+    const modal = document.getElementById('modal');
+    const title = document.getElementById('inp-title');
+    const body  = document.getElementById('editor-body');
+    if (title) title.disabled = true;
+    if (body)  body.disabled  = true;
+    modal?.classList.add('collab-readonly-modal');
+
+    // Block save buttons by capturing clicks before the editor handler.
+    // stopImmediatePropagation also blocks any sibling listeners registered
+    // on the same element (the editor's own click handler) and skips both
+    // target + bubble phases for ancestors.
+    const saveIds = ['modal-save', 'modal-save-view', 'modal-save-close'];
+    const blockers = saveIds.map(id => {
+        const btn = document.getElementById(id);
+        if (!btn) return null;
+        const handler = (e) => {
+            e.stopImmediatePropagation();
+            e.preventDefault();
+        };
+        btn.addEventListener('click', handler, true); // capture phase
+        btn.setAttribute('disabled', 'disabled');
+        btn.classList.add('collab-save-blocked');
+        return { btn, handler };
+    }).filter(Boolean);
+    editorSession.saveBlockers = blockers;
+
+    // Also intercept Ctrl/Cmd+S — app.js binds this directly to saveCard/
+    // saveOnly without going through the buttons we just disabled.
+    const keyBlocker = (e) => {
+        if ((e.ctrlKey || e.metaKey) && (e.key === 's' || e.key === 'S')) {
+            e.preventDefault();
+            e.stopImmediatePropagation();
+        }
+    };
+    window.addEventListener('keydown', keyBlocker, true);
+    editorSession.keyBlocker = keyBlocker;
+
+    // Banner inside the modal explaining the situation
+    let banner = document.getElementById('collab-takeover-banner');
+    if (!banner) {
+        banner = document.createElement('div');
+        banner.id = 'collab-takeover-banner';
+        banner.className = 'collab-takeover-banner';
+        const inner = document.querySelector('#modal .modal-inner') || document.querySelector('#modal > div') || modal;
+        inner?.prepend(banner);
+    }
+    banner.innerHTML = `
+        <i class="ph ph-lock-simple-fill"></i>
+        <span><b>Locked by ${escapeHtml(holder)}.</b> This card was locked by another user while you were typing. Copy your text out — saving is disabled.</span>
+    `;
+
+    // Also show a one-off toast so the user can't miss it
+    showLockToast(holder);
+}
+
+function exitEditorTakeover() {
+    const modal = document.getElementById('modal');
+    const title = document.getElementById('inp-title');
+    const body  = document.getElementById('editor-body');
+    if (title) title.disabled = false;
+    if (body)  body.disabled  = false;
+    modal?.classList.remove('collab-readonly-modal');
+    document.getElementById('collab-takeover-banner')?.remove();
+    if (editorSession?.saveBlockers) {
+        for (const { btn, handler } of editorSession.saveBlockers) {
+            btn.removeEventListener('click', handler, true);
+            btn.removeAttribute('disabled');
+            btn.classList.remove('collab-save-blocked');
+        }
+        editorSession.saveBlockers = null;
+    }
+    if (editorSession?.keyBlocker) {
+        window.removeEventListener('keydown', editorSession.keyBlocker, true);
+        editorSession.keyBlocker = null;
+    }
+}
+
+function showLockToast(holder) {
+    const existing = document.getElementById('collab-lock-toast');
+    if (existing) existing.remove();
+    const toast = document.createElement('div');
+    toast.id = 'collab-lock-toast';
+    toast.className = 'collab-conflict-toast collab-lock-toast';
+    toast.innerHTML = `
+        <i class="ph ph-lock-simple-fill"></i>
+        <span><b>Locked by ${escapeHtml(holder)}.</b> Copy your text — saving is blocked.</span>
+        <button class="collab-conflict-close" aria-label="dismiss">✕</button>
+    `;
+    toast.querySelector('.collab-conflict-close').addEventListener('click', () => {
+        toast.classList.add('fade-out');
+        setTimeout(() => toast.remove(), 400);
+    });
+    document.body.appendChild(toast);
+    setTimeout(() => { toast.classList.add('fade-out'); setTimeout(() => toast.remove(), 400); }, 8000);
+}
+
+function showBlockedToast(by) {
+    const existing = document.getElementById('collab-blocked-toast');
+    if (existing) existing.remove();
+    const toast = document.createElement('div');
+    toast.id = 'collab-blocked-toast';
+    toast.className = 'collab-conflict-toast collab-lock-toast';
+    const who = by.length ? by.join(', ') : 'another user';
+    toast.innerHTML = `
+        <i class="ph ph-lock-simple-fill"></i>
+        <span>Your edit is queued — <b>${escapeHtml(who)}</b> is currently editing. It'll sync when they're done.</span>
+        <button class="collab-conflict-close" aria-label="dismiss">✕</button>
+    `;
+    toast.querySelector('.collab-conflict-close').addEventListener('click', () => {
+        toast.classList.add('fade-out');
+        setTimeout(() => toast.remove(), 400);
+    });
+    document.body.appendChild(toast);
+    setTimeout(() => { toast.classList.add('fade-out'); setTimeout(() => toast.remove(), 400); }, 6000);
+}
+
+/** Tear down everything we own for the active editor session. Called both
+ *  on editor-close and on plugin reload. */
+function endEditorSession() {
+    if (!editorSession) return;
+    const { hbTimer, inputCleanup, acquired, bucket_uid, card_uid } = editorSession;
+    if (hbTimer) clearInterval(hbTimer);
+    if (typeof inputCleanup === 'function') inputCleanup();
+    if (acquired) releaseLock(bucket_uid, card_uid);
+    exitEditorTakeover();
+    editorSession = null;
 }
 
 window.addEventListener('hyprcrd:editor-open', (e) => {
+    endEditorSession();
     const card = e.detail?.card;
     const id = collabIdentityOf(card);
-    if (id) startHeartbeat(id.bucket_uid, id.card_uid);
+    if (!id) return;
+    editorSession = {
+        bucket_uid: id.bucket_uid,
+        card_uid: id.card_uid,
+        cardId: card?.id,
+        hbTimer: null,
+        inputCleanup: null,
+        acquired: false,
+        takeover: false,
+        saveBlockers: null,
+    };
+    // If a foreign lock is already known from the last poll, take over
+    // immediately — no point letting the user type into a doomed editor.
+    const existing = collabLocks.get(id.card_uid);
+    const self = window._collabSelf || null;
+    if (existing && existing.holder_username && existing.holder_username !== self) {
+        enterEditorTakeover(existing.holder_username);
+        return;
+    }
+    armFirstKeystrokeLock();
 });
 window.addEventListener('hyprcrd:editor-close', () => {
-    stopHeartbeat();
+    endEditorSession();
 });
 // Graceful release on tab close
 window.addEventListener('beforeunload', () => {
-    if (activeLock) {
-        // Best-effort sync release; browsers may block regular fetch, so
-        // use sendBeacon when available.
+    if (editorSession?.acquired) {
         try {
-            if (navigator.sendBeacon) {
-                const url = 'api/collab-locks/' + encodeURIComponent(activeLock.bucket_uid) + '/' + encodeURIComponent(activeLock.card_uid);
-                // sendBeacon only supports POST; use fetch keepalive as the
-                // DELETE fallback. Both are best-effort at unload time.
-                fetch(url, { method: 'DELETE', keepalive: true }).catch(() => {});
-            }
+            const url = 'api/collab-locks/' + encodeURIComponent(editorSession.bucket_uid) + '/' + encodeURIComponent(editorSession.card_uid);
+            fetch(url, { method: 'DELETE', keepalive: true }).catch(() => {});
         } catch { /* ignore */ }
     }
 });
diff --git a/relay/inc/bootstrap.php b/relay/inc/bootstrap.php
index 576a89d..f29ffe4 100644
--- a/relay/inc/bootstrap.php
+++ b/relay/inc/bootstrap.php
@@ -227,6 +227,18 @@ $db->exec('CREATE INDEX IF NOT EXISTS idx_collab_cards_bucket_updated ON collab_
 // 60s TTL. Other clients poll the list and show a lock indicator on the
 // card. The lock is purely advisory — force-editing is still allowed and
 // LWW remains the authoritative conflict resolver.
+// Exclusive lock: PRIMARY KEY is (bucket_uid, card_uid). The first writer
+// to acquire holds it; everyone else gets a 409 Locked. The previous schema
+// had holder_id in the PK, which let multiple users "lock" the same card
+// simultaneously — locks were advisory only, and concurrent writers could
+// silently overwrite each other through LWW.
+//
+// Migration: if the existing table still has the 3-column PK, drop it.
+// Locks are ephemeral (60s TTL) so dropping discards nothing important.
+$lockTableInfo = $db->query("SELECT sql FROM sqlite_master WHERE type='table' AND name='collab_card_locks'")->fetchColumn();
+if ($lockTableInfo && stripos($lockTableInfo, 'PRIMARY KEY (bucket_uid, card_uid, holder_id)') !== false) {
+    $db->exec('DROP TABLE collab_card_locks');
+}
 $db->exec('CREATE TABLE IF NOT EXISTS collab_card_locks (
     bucket_uid TEXT NOT NULL,
     card_uid TEXT NOT NULL,
@@ -234,7 +246,7 @@ $db->exec('CREATE TABLE IF NOT EXISTS collab_card_locks (
     holder_username TEXT NOT NULL,
     acquired_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
     expires_at TEXT NOT NULL,
-    PRIMARY KEY (bucket_uid, card_uid, holder_id),
+    PRIMARY KEY (bucket_uid, card_uid),
     FOREIGN KEY (holder_id) REFERENCES tenants(id) ON DELETE CASCADE
 )');
 $db->exec('CREATE INDEX IF NOT EXISTS idx_collab_card_locks_expires ON collab_card_locks(expires_at)');
diff --git a/relay/inc/routes/collab-cards.php b/relay/inc/routes/collab-cards.php
index 334306a..08f3aa4 100644
--- a/relay/inc/routes/collab-cards.php
+++ b/relay/inc/routes/collab-cards.php
@@ -27,6 +27,21 @@ function collabCardsIsMember(PDO $db, string $bucketUid, int $tenantId): bool {
     return (bool)$stmt->fetchColumn();
 }
 
+/** If a foreign tenant currently holds a non-expired lock on this card,
+ *  return the lock row; otherwise null. Used to gate writes / deletes. */
+function collabCardsForeignLock(PDO $db, string $bucketUid, string $cardUid, int $tenantId): ?array {
+    $stmt = $db->prepare("
+        SELECT holder_id, holder_username, expires_at
+        FROM collab_card_locks
+        WHERE bucket_uid = :b AND card_uid = :c
+          AND holder_id != :t
+          AND expires_at > datetime('now')
+    ");
+    $stmt->execute(['b' => $bucketUid, 'c' => $cardUid, 't' => $tenantId]);
+    $row = $stmt->fetch();
+    return $row ?: null;
+}
+
 // ── POST: upsert a card ──────────────────────────────────────────
 
 if ($route === '/api/collab-cards' && $method === 'POST') {
@@ -42,6 +57,16 @@ if ($route === '/api/collab-cards' && $method === 'POST') {
     if (!collabCardsIsMember($db, $bucketUid, (int)$clientTenant['id'])) {
         jsonResponse(['error' => 'not a member of this bucket'], 403);
     }
+    // Exclusive lock enforcement: if another tenant holds the lock,
+    // refuse the write. Plugin retries on the next poll until the lock
+    // expires or is released. The local edit is preserved either way.
+    $foreign = collabCardsForeignLock($db, $bucketUid, $cardUid, (int)$clientTenant['id']);
+    if ($foreign) {
+        jsonResponse([
+            'error'     => 'card is being edited by ' . $foreign['holder_username'],
+            'locked_by' => $foreign['holder_username'],
+        ], 423);
+    }
     if (empty($clientTenant['secret_encrypted'])) {
         jsonResponse(['error' => 'sender has no client secret set — log into the relay UI and configure one in your account'], 400);
     }
@@ -148,6 +173,15 @@ if (preg_match('#^/api/collab-cards/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$#', $route
     if (!collabCardsIsMember($db, $bucketUid, (int)$clientTenant['id'])) {
         jsonResponse(['error' => 'not a member of this bucket'], 403);
     }
+    // Same lock enforcement on delete: if another user has the editor
+    // open on this card, deleting it under their feet would be rude.
+    $foreign = collabCardsForeignLock($db, $bucketUid, $cardUid, (int)$clientTenant['id']);
+    if ($foreign) {
+        jsonResponse([
+            'error'     => 'card is being edited by ' . $foreign['holder_username'],
+            'locked_by' => $foreign['holder_username'],
+        ], 423);
+    }
     // Mark as tombstone rather than actually deleting. Late pollers see it,
     // bootstrap.php cleanup prunes after 7 days.
     $db->prepare('
diff --git a/relay/inc/routes/collab-locks.php b/relay/inc/routes/collab-locks.php
index 2e2396c..953e8c6 100644
--- a/relay/inc/routes/collab-locks.php
+++ b/relay/inc/routes/collab-locks.php
@@ -30,18 +30,42 @@ if ($route === '/api/collab-locks' && $method === 'POST') {
     if (!collabLocksIsMember($db, $bucketUid, (int)$clientTenant['id'])) {
         jsonResponse(['error' => 'not a member of this bucket'], 403);
     }
+    // Drop expired before checking — bootstrap cleanup runs every request
+    // but this protects against a stale lock blocking acquisition in the
+    // narrow window between cleanup and this query.
+    $db->prepare("DELETE FROM collab_card_locks WHERE bucket_uid = :b AND card_uid = :c AND expires_at <= datetime('now')")
+        ->execute(['b' => $bucketUid, 'c' => $cardUid]);
+    // Existing live lock? If a different tenant holds it, refuse.
+    $cur = $db->prepare('SELECT holder_id, holder_username, expires_at FROM collab_card_locks WHERE bucket_uid = :b AND card_uid = :c');
+    $cur->execute(['b' => $bucketUid, 'c' => $cardUid]);
+    $existing = $cur->fetch();
+    if ($existing && (int)$existing['holder_id'] !== (int)$clientTenant['id']) {
+        jsonResponse([
+            'error'           => 'card is being edited by ' . $existing['holder_username'],
+            'locked_by'       => $existing['holder_username'],
+            'locked_by_id'    => (int)$existing['holder_id'],
+            'expires_at'      => $existing['expires_at'],
+        ], 409);
+    }
+    // Acquire / refresh — same holder bumps the expiry
     $db->prepare("
         INSERT INTO collab_card_locks (bucket_uid, card_uid, holder_id, holder_username, acquired_at, expires_at)
         VALUES (:b, :c, :hid, :hu, datetime('now'), datetime('now', '+60 seconds'))
-        ON CONFLICT(bucket_uid, card_uid, holder_id) DO UPDATE SET
-            expires_at = datetime('now', '+60 seconds')
+        ON CONFLICT(bucket_uid, card_uid) DO UPDATE SET
+            holder_id       = excluded.holder_id,
+            holder_username = excluded.holder_username,
+            expires_at      = excluded.expires_at
     ")->execute([
         'b'   => $bucketUid,
         'c'   => $cardUid,
         'hid' => $clientTenant['id'],
         'hu'  => $clientTenant['username'],
     ]);
-    jsonResponse(['ok' => true, 'expires_at' => (string)$db->query("SELECT datetime('now', '+60 seconds')")->fetchColumn()]);
+    jsonResponse([
+        'ok'         => true,
+        'holder_id'  => (int)$clientTenant['id'],
+        'expires_at' => (string)$db->query("SELECT datetime('now', '+60 seconds')")->fetchColumn(),
+    ]);
 }
 
 // ── DELETE: release own lock ─────────────────────────────────────