diff --git a/app/models/teller_enrollment.py b/app/models/teller_enrollment.py
index c5ad4fc..76f855c 100644
--- a/app/models/teller_enrollment.py
+++ b/app/models/teller_enrollment.py
@@ -52,3 +52,25 @@ class TellerAccount(db.Model):
 
     def __repr__(self):
         return f'<TellerAccount {self.account_name} → PFM #{self.pfm_account_id}>'
+
+
+class TellerSyncPreview(db.Model):
+    """
+    Temporary server-side storage for sync preview data.
+    Replaces cookie-based session storage which has a 4 KB limit —
+    large accounts (100+ transactions) exceeded that limit and caused
+    silent data loss on the confirm step.
+    One row per teller_account_id; overwritten on each new sync preview.
+    """
+    __tablename__ = 'teller_sync_previews'
+
+    id              = db.Column(db.Integer, primary_key=True)
+    teller_account_id = db.Column(
+        db.Integer, db.ForeignKey('teller_accounts.id'),
+        nullable=False, unique=True, index=True,
+    )
+    data_json       = db.Column(db.Text, nullable=False)
+    created_at      = db.Column(db.DateTime, default=datetime.utcnow)
+
+    def __repr__(self):
+        return f'<TellerSyncPreview account_id={self.teller_account_id}>'
diff --git a/app/routes/teller.py b/app/routes/teller.py
index 7ce1e8a..9004ace 100644
--- a/app/routes/teller.py
+++ b/app/routes/teller.py
@@ -7,7 +7,7 @@ from flask import (Blueprint, render_template, redirect, url_for, flash,
                    request, jsonify, current_app, session)
 from flask_login import login_required, current_user
 from app.extensions import db
-from app.models.teller_enrollment import TellerEnrollment, TellerAccount
+from app.models.teller_enrollment import TellerEnrollment, TellerAccount, TellerSyncPreview
 from app.models.account import Account
 from app.services.teller_service import (
     get_accounts, get_balance, sync_preview, import_transactions,
@@ -183,13 +183,14 @@ def sync_preview_view(teller_account_id):
         flash(f'Sync failed: {e}', 'danger')
         return redirect(url_for('teller.index'))
 
-    # Store preview in session for confirm step
-    session['teller_preview'] = [
+    # Store preview in DB (not session cookie — cookie limit is 4 KB;
+    # large accounts with 100+ transactions silently exceeded it).
+    preview_data = [
         {
             'teller_id':        p['teller_id'],
             'date':             p['date'].isoformat(),
             'transaction_type': p['transaction_type'],
-            'amount':           p['amount'],
+            'amount':           float(p['amount']),
             'description':      p['description'],
             'account_id':       p['account_id'],
             'category_id':      p['category_id'],
@@ -197,6 +198,19 @@ def sync_preview_view(teller_account_id):
         }
         for p in preview
     ]
+    # Upsert: one preview row per teller account (overwrite any stale preview)
+    existing_sp = TellerSyncPreview.query.filter_by(teller_account_id=teller_account_id).first()
+    if existing_sp:
+        existing_sp.data_json  = json.dumps(preview_data)
+        existing_sp.created_at = __import__('datetime').datetime.utcnow()
+        sp = existing_sp
+    else:
+        sp = TellerSyncPreview(teller_account_id=teller_account_id,
+                               data_json=json.dumps(preview_data))
+        db.session.add(sp)
+    db.session.commit()
+    # Only the tiny row ID goes into the session cookie
+    session['teller_preview_id'] = sp.id
     session['teller_account_id'] = teller_account_id
 
     from app.models.category import Category
@@ -213,13 +227,22 @@ def sync_preview_view(teller_account_id):
 @login_required
 def sync_confirm():
     """Import selected previewed transactions with optional type overrides."""
-    raw = session.pop('teller_preview', [])
-    ta_id = session.pop('teller_account_id', None)
+    preview_id = session.pop('teller_preview_id', None)
+    ta_id      = session.pop('teller_account_id', None)
 
-    if not raw or not ta_id:
+    if not preview_id or not ta_id:
         flash('No pending import. Please sync again.', 'warning')
         return redirect(url_for('teller.index'))
 
+    sp = db.session.get(TellerSyncPreview, preview_id)
+    if not sp:
+        flash('Preview expired or already imported. Please sync again.', 'warning')
+        return redirect(url_for('teller.index'))
+
+    raw = json.loads(sp.data_json)
+    db.session.delete(sp)
+    db.session.commit()
+
     ta = db.get_or_404(TellerAccount, ta_id)
 
     selected_ids = set(request.form.getlist('selected'))