diff --git a/src/app/api/merchants/route.ts b/src/app/api/merchants/route.ts
index e4d4a35..0655ab7 100644
--- a/src/app/api/merchants/route.ts
+++ b/src/app/api/merchants/route.ts
@@ -1,7 +1,10 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getMerchantSuggestions, getBankNames } from "@/lib/queries";
+import { getCurrentUser } from "@/lib/auth";
 
 export async function GET(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const search = req.nextUrl.searchParams.get("search");
   const type = req.nextUrl.searchParams.get("type");
 
diff --git a/src/app/api/participants/[id]/balance/route.ts b/src/app/api/participants/[id]/balance/route.ts
index 1950b17..eae3ae7 100644
--- a/src/app/api/participants/[id]/balance/route.ts
+++ b/src/app/api/participants/[id]/balance/route.ts
@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from "next/server";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
 
 interface BalanceRow {
   participant_id: number;
@@ -9,9 +10,11 @@ interface BalanceRow {
 }
 
 export async function GET(
-  _req: NextRequest,
+  req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
 
   const rows = await queryRaw<BalanceRow>(
diff --git a/src/app/api/rules/apply/route.ts b/src/app/api/rules/apply/route.ts
index baad0c1..a2dd7d5 100644
--- a/src/app/api/rules/apply/route.ts
+++ b/src/app/api/rules/apply/route.ts
@@ -151,11 +151,18 @@ export async function POST(req: NextRequest) {
 
       if (actions.apply_split?.length) {
         if (splitFrom && tx.transaction_date < splitFrom) continue;
-        await queryRaw(`DELETE FROM transaction_splits WHERE transaction_id = $1`, [tx.id]);
+        // Remove only participants no longer in the rule's split, and upsert the
+        // rest — a plain delete+reinsert would reset settled flags on every run.
+        await queryRaw(
+          `DELETE FROM transaction_splits WHERE transaction_id = $1 AND participant_id != ALL($2::int[])`,
+          [tx.id, actions.apply_split.map((s) => s.participant_id)]
+        );
         for (const s of actions.apply_split) {
           await queryRaw(
             `INSERT INTO transaction_splits (transaction_id, participant_id, share_percent)
-             VALUES ($1, $2, $3) ON CONFLICT DO NOTHING`,
+             VALUES ($1, $2, $3)
+             ON CONFLICT (transaction_id, participant_id)
+             DO UPDATE SET share_percent = EXCLUDED.share_percent`,
             [tx.id, s.participant_id, s.share_percent]
           );
         }
diff --git a/src/app/api/splits/settle/route.ts b/src/app/api/splits/settle/route.ts
index 1c3e706..bcfaa2b 100644
--- a/src/app/api/splits/settle/route.ts
+++ b/src/app/api/splits/settle/route.ts
@@ -1,29 +1,45 @@
 import { NextRequest, NextResponse } from "next/server";
-import { prisma } from "@/lib/db";
+import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
+
+// A split may be settled by the transaction's effective owner or by the
+// participant the split belongs to.
+const SCOPE = `
+  AND EXISTS (
+    SELECT 1 FROM transactions t
+    LEFT JOIN statements s ON s.id = t.statement_id
+    WHERE t.id = transaction_splits.transaction_id
+      AND (COALESCE(t.owner_id, s.owner_id) = $2 OR transaction_splits.participant_id = $2)
+  )`;
 
 export async function POST(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
+
   const body = await req.json();
   const { participant_id, split_ids } = body as {
     participant_id?: number;
     split_ids?: number[];
   };
 
-  const now = new Date();
-
   if (participant_id) {
-    const result = await prisma.transaction_splits.updateMany({
-      where: { participant_id, settled: false },
-      data: { settled: true, settled_at: now },
-    });
-    return NextResponse.json({ settled: result.count });
+    const rows = await queryRaw<{ id: number }>(
+      `UPDATE transaction_splits SET settled = true, settled_at = NOW()
+       WHERE participant_id = $1 AND settled = false ${SCOPE}
+       RETURNING id`,
+      [participant_id, user.id]
+    );
+    return NextResponse.json({ settled: rows.length });
   }
 
   if (split_ids?.length) {
-    const result = await prisma.transaction_splits.updateMany({
-      where: { id: { in: split_ids }, settled: false },
-      data: { settled: true, settled_at: now },
-    });
-    return NextResponse.json({ settled: result.count });
+    const rows = await queryRaw<{ id: number }>(
+      `UPDATE transaction_splits SET settled = true, settled_at = NOW()
+       WHERE id = ANY($1::int[]) AND settled = false ${SCOPE}
+       RETURNING id`,
+      [split_ids, user.id]
+    );
+    return NextResponse.json({ settled: rows.length });
   }
 
   return NextResponse.json({ error: "participant_id or split_ids required" }, { status: 400 });
diff --git a/src/app/api/statements/[id]/route.ts b/src/app/api/statements/[id]/route.ts
index 7134646..5530c9f 100644
--- a/src/app/api/statements/[id]/route.ts
+++ b/src/app/api/statements/[id]/route.ts
@@ -1,12 +1,17 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getStatementById } from "@/lib/queries";
+import { getCurrentUser } from "@/lib/auth";
 
 export async function GET(
-  _req: NextRequest,
+  req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
   const stmt = await getStatementById(Number(id));
-  if (!stmt) return NextResponse.json({ error: "Not found" }, { status: 404 });
+  if (!stmt || stmt.owner_id !== user.id) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   return NextResponse.json(stmt);
 }
diff --git a/src/app/api/tags/[id]/route.ts b/src/app/api/tags/[id]/route.ts
index 2cc9e78..aa674ff 100644
--- a/src/app/api/tags/[id]/route.ts
+++ b/src/app/api/tags/[id]/route.ts
@@ -1,7 +1,10 @@
 import { NextRequest, NextResponse } from "next/server";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
 
-export async function DELETE(_req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+export async function DELETE(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
   await queryRaw(`DELETE FROM tags WHERE id = $1`, [Number(id)]);
   return NextResponse.json({ ok: true });
diff --git a/src/app/api/tags/route.ts b/src/app/api/tags/route.ts
index d3d8b7a..dc32e23 100644
--- a/src/app/api/tags/route.ts
+++ b/src/app/api/tags/route.ts
@@ -1,13 +1,18 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getTags } from "@/lib/queries";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
 
-export async function GET() {
+export async function GET(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const tags = await getTags();
   return NextResponse.json(tags);
 }
 
 export async function POST(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { name, color } = await req.json();
   if (!name?.trim()) {
     return NextResponse.json({ error: "name required" }, { status: 400 });
diff --git a/src/app/api/transactions/[id]/route.ts b/src/app/api/transactions/[id]/route.ts
index a288cd2..29da865 100644
--- a/src/app/api/transactions/[id]/route.ts
+++ b/src/app/api/transactions/[id]/route.ts
@@ -1,15 +1,21 @@
 import { NextRequest, NextResponse } from "next/server";
-import { getTransactionById } from "@/lib/queries";
+import { getTransactionById, canAccessTransactions } from "@/lib/queries";
+import { getCurrentUser } from "@/lib/auth";
 import { prisma } from "@/lib/db";
 import { queryRaw } from "@/lib/db";
 
 const VALID_TYPES = ["debit", "credit", "payment", "refund", "fee", "interest", "transfer"];
 
 export async function GET(
-  _req: NextRequest,
+  req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
+  if (!(await canAccessTransactions(user.id, [Number(id)]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const txn = await getTransactionById(Number(id));
   if (!txn) return NextResponse.json({ error: "Not found" }, { status: 404 });
   return NextResponse.json(txn);
@@ -19,8 +25,13 @@ export async function PATCH(
   req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
   const transactionId = Number(id);
+  if (!(await canAccessTransactions(user.id, [transactionId]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const body = await req.json();
 
   const { category, merchant_normalized, notes, transaction_type, my_share_percent, description, amount, transaction_date, trip_id } = body as {
diff --git a/src/app/api/transactions/[id]/splits/route.ts b/src/app/api/transactions/[id]/splits/route.ts
index f6ccda4..1326952 100644
--- a/src/app/api/transactions/[id]/splits/route.ts
+++ b/src/app/api/transactions/[id]/splits/route.ts
@@ -1,6 +1,8 @@
 import { NextRequest, NextResponse } from "next/server";
 import { prisma } from "@/lib/db";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
+import { canAccessTransactions } from "@/lib/queries";
 
 interface SplitInput {
   participant_id: number;
@@ -19,10 +21,15 @@ interface SplitRow {
 }
 
 export async function GET(
-  _req: NextRequest,
+  req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
+  if (!(await canAccessTransactions(user.id, [Number(id)]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const splits = await queryRaw<SplitRow>(
     `SELECT ts.*, p.name
      FROM transaction_splits ts
@@ -38,8 +45,13 @@ export async function POST(
   req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
   const transactionId = Number(id);
+  if (!(await canAccessTransactions(user.id, [transactionId]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const { splits } = (await req.json()) as { splits: SplitInput[] };
 
   if (!splits || !Array.isArray(splits) || splits.length === 0) {
diff --git a/src/app/api/transactions/[id]/tags/route.ts b/src/app/api/transactions/[id]/tags/route.ts
index 88c70e3..351d451 100644
--- a/src/app/api/transactions/[id]/tags/route.ts
+++ b/src/app/api/transactions/[id]/tags/route.ts
@@ -1,8 +1,15 @@
 import { NextRequest, NextResponse } from "next/server";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
+import { canAccessTransactions } from "@/lib/queries";
 
 export async function POST(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
+  if (!(await canAccessTransactions(user.id, [Number(id)]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const { tag_id } = await req.json();
   if (!tag_id) return NextResponse.json({ error: "tag_id required" }, { status: 400 });
   await queryRaw(
@@ -13,7 +20,12 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
 }
 
 export async function DELETE(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
+  if (!(await canAccessTransactions(user.id, [Number(id)]))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
   const { tag_id } = await req.json();
   if (!tag_id) return NextResponse.json({ error: "tag_id required" }, { status: 400 });
   await queryRaw(
diff --git a/src/app/api/transactions/bulk/route.ts b/src/app/api/transactions/bulk/route.ts
index 8b15a63..fc44180 100644
--- a/src/app/api/transactions/bulk/route.ts
+++ b/src/app/api/transactions/bulk/route.ts
@@ -1,8 +1,11 @@
 import { NextRequest, NextResponse } from "next/server";
 import { prisma, queryRaw } from "@/lib/db";
-import { assignTransactionsToTrip } from "@/lib/queries";
+import { assignTransactionsToTrip, canAccessTransactions } from "@/lib/queries";
+import { getCurrentUser } from "@/lib/auth";
 
 export async function POST(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const body = await req.json();
   const { action, ids, category, merchant_normalized, splits, tag_id } = body as {
     action: string;
@@ -17,6 +20,10 @@ export async function POST(req: NextRequest) {
     return NextResponse.json({ error: "ids required" }, { status: 400 });
   }
 
+  if (!(await canAccessTransactions(user.id, ids.map(Number)))) {
+    return NextResponse.json({ error: "Not found" }, { status: 404 });
+  }
+
   if (action === "categorize" && category) {
     const ops = ids.map((id) =>
       prisma.transaction_overrides.upsert({
diff --git a/src/lib/queries.ts b/src/lib/queries.ts
index 6f2e452..097b82c 100644
--- a/src/lib/queries.ts
+++ b/src/lib/queries.ts
@@ -231,6 +231,22 @@ export async function getTransactions(ownerId: number, filters: TransactionFilte
   return { data, total, limit, offset };
 }
 
+// A user may act on a transaction they own (directly or via the parent
+// statement) or one they participate in via a split.
+export async function canAccessTransactions(ownerId: number, transactionIds: number[]): Promise<boolean> {
+  if (!transactionIds.length) return false;
+  const rows = await queryRaw<{ n: number }>(
+    `SELECT COUNT(*)::int AS n
+     FROM transactions t
+     LEFT JOIN statements s ON s.id = t.statement_id
+     WHERE t.id = ANY($2::int[])
+       AND (COALESCE(t.owner_id, s.owner_id) = $1
+            OR EXISTS (SELECT 1 FROM transaction_splits ts WHERE ts.transaction_id = t.id AND ts.participant_id = $1))`,
+    [ownerId, transactionIds]
+  );
+  return rows[0]?.n === transactionIds.length;
+}
+
 export async function getTransactionById(id: number) {
   const sql = `
     SELECT t.*,
@@ -654,7 +670,7 @@ export async function getTrips(ownerId: number): Promise<TripRow[]> {
     SELECT
       t.*,
       COALESCE(SUM(
-        CASE WHEN tx.transaction_type IN ('debit','fee','interest') THEN tx.amount ELSE 0 END
+        CASE WHEN tx.transaction_type IN ('debit','fee','interest') THEN COALESCE(tx.amount_aud, tx.amount) ELSE 0 END
       ), 0)::float AS total_spend,
       COUNT(o.transaction_id)::int AS transaction_count
     FROM trips t
@@ -671,7 +687,7 @@ export async function getTripById(id: number, ownerId: number): Promise<TripRow
     SELECT
       t.*,
       COALESCE(SUM(
-        CASE WHEN tx.transaction_type IN ('debit','fee','interest') THEN tx.amount ELSE 0 END
+        CASE WHEN tx.transaction_type IN ('debit','fee','interest') THEN COALESCE(tx.amount_aud, tx.amount) ELSE 0 END
       ), 0)::float AS total_spend,
       COUNT(o.transaction_id)::int AS transaction_count
     FROM trips t
@@ -691,7 +707,7 @@ export async function getTripAnalytics(tripId: number, ownerId: number): Promise
     queryRaw<{ category: string; amount: number; count: number }>(`
       SELECT
         COALESCE(o.category_override, tx.category, 'other') AS category,
-        SUM(tx.amount)::float AS amount,
+        SUM(COALESCE(tx.amount_aud, tx.amount))::float AS amount,
         COUNT(*)::int AS count
       FROM transaction_overrides o
       JOIN transactions tx ON tx.id = o.transaction_id
@@ -704,7 +720,7 @@ export async function getTripAnalytics(tripId: number, ownerId: number): Promise
     queryRaw<{ date: string; amount: number }>(`
       SELECT
         tx.transaction_date::text AS date,
-        SUM(tx.amount)::float AS amount
+        SUM(COALESCE(tx.amount_aud, tx.amount))::float AS amount
       FROM transaction_overrides o
       JOIN transactions tx ON tx.id = o.transaction_id
       WHERE o.trip_id = $1
@@ -716,7 +732,7 @@ export async function getTripAnalytics(tripId: number, ownerId: number): Promise
     queryRaw<{ merchant: string; amount: number; count: number }>(`
       SELECT
         COALESCE(o.merchant_normalized, tx.merchant_normalized, tx.merchant_name, tx.description) AS merchant,
-        SUM(tx.amount)::float AS amount,
+        SUM(COALESCE(tx.amount_aud, tx.amount))::float AS amount,
         COUNT(*)::int AS count
       FROM transaction_overrides o
       JOIN transactions tx ON tx.id = o.transaction_id
@@ -730,7 +746,7 @@ export async function getTripAnalytics(tripId: number, ownerId: number): Promise
     queryRaw<{ tag_id: number; name: string; color: string; amount: number; count: number }>(`
       SELECT
         tg.id AS tag_id, tg.name, tg.color,
-        SUM(tx.amount)::float AS amount,
+        SUM(COALESCE(tx.amount_aud, tx.amount))::float AS amount,
         COUNT(DISTINCT tx.id)::int AS count
       FROM transaction_overrides o
       JOIN transactions tx ON tx.id = o.transaction_id
@@ -746,9 +762,9 @@ export async function getTripAnalytics(tripId: number, ownerId: number): Promise
       SELECT
         p.id AS participant_id,
         p.name,
-        SUM(ts.share_percent / 100.0 * tx.amount)::float AS owed,
-        SUM(CASE WHEN ts.settled THEN ts.share_percent / 100.0 * tx.amount ELSE 0 END)::float AS settled,
-        SUM(CASE WHEN NOT ts.settled THEN ts.share_percent / 100.0 * tx.amount ELSE 0 END)::float AS unsettled
+        SUM(ts.share_percent / 100.0 * COALESCE(tx.amount_aud, tx.amount))::float AS owed,
+        SUM(CASE WHEN ts.settled THEN ts.share_percent / 100.0 * COALESCE(tx.amount_aud, tx.amount) ELSE 0 END)::float AS settled,
+        SUM(CASE WHEN NOT ts.settled THEN ts.share_percent / 100.0 * COALESCE(tx.amount_aud, tx.amount) ELSE 0 END)::float AS unsettled
       FROM transaction_overrides o
       JOIN transactions tx ON tx.id = o.transaction_id
       JOIN transaction_splits ts ON ts.transaction_id = tx.id