diff --git a/src/server/services/auth.service.ts b/src/server/services/auth.service.ts
index 5a800e6..b5c9ced 100644
--- a/src/server/services/auth.service.ts
+++ b/src/server/services/auth.service.ts
@@ -25,9 +25,9 @@ export async function getOrCreateUser(
 // ── API Key Management ───────────────────────────────────────────────
 
 export async function createApiKey(
-	db: Db = prodDb,
-	name: string,
+	db: Db,
 	userId: number,
+	name: string,
 ) {
 	const rawKey = randomBytes(32).toString("hex");
 	const keyHash = await Bun.password.hash(rawKey);
@@ -42,7 +42,7 @@ export async function createApiKey(
 }
 
 export async function verifyApiKey(
-	db: Db = prodDb,
+	db: Db,
 	rawKey: string,
 ): Promise<{ userId: number } | null> {
 	const prefix = rawKey.slice(0, 8);
@@ -59,7 +59,7 @@ export async function verifyApiKey(
 	return null;
 }
 
-export async function listApiKeys(db: Db = prodDb, userId: number) {
+export async function listApiKeys(db: Db, userId: number) {
 	return db
 		.select({
 			id: apiKeys.id,
@@ -71,11 +71,7 @@ export async function listApiKeys(db: Db = prodDb, userId: number) {
 		.where(eq(apiKeys.userId, userId));
 }
 
-export async function deleteApiKey(
-	db: Db = prodDb,
-	id: number,
-	userId: number,
-) {
+export async function deleteApiKey(db: Db, userId: number, id: number) {
 	await db
 		.delete(apiKeys)
 		.where(and(eq(apiKeys.id, id), eq(apiKeys.userId, userId)));
diff --git a/src/server/services/setup.service.ts b/src/server/services/setup.service.ts
index 49aef9d..da0456f 100644
--- a/src/server/services/setup.service.ts
+++ b/src/server/services/setup.service.ts
@@ -1,17 +1,24 @@
-import { eq, sql } from "drizzle-orm";
+import { and, eq, inArray, sql } from "drizzle-orm";
 import { db as prodDb } from "../../db/index.ts";
 import { categories, items, setupItems, setups } from "../../db/schema.ts";
 import type { CreateSetup, UpdateSetup } from "../../shared/types.ts";
 
 type Db = typeof prodDb;
 
-export async function createSetup(db: Db = prodDb, data: CreateSetup) {
-	const [row] = await db.insert(setups).values({ name: data.name }).returning();
+export async function createSetup(
+	db: Db,
+	userId: number,
+	data: CreateSetup,
+) {
+	const [row] = await db
+		.insert(setups)
+		.values({ name: data.name, userId })
+		.returning();
 
 	return row;
 }
 
-export async function getAllSetups(db: Db = prodDb) {
+export async function getAllSetups(db: Db, userId: number) {
 	return db
 		.select({
 			id: setups.id,
@@ -33,11 +40,19 @@ export async function getAllSetups(db: Db = prodDb) {
         WHERE setup_items.setup_id = setups.id
       ), 0)`.as("total_cost"),
 		})
-		.from(setups);
+		.from(setups)
+		.where(eq(setups.userId, userId));
 }
 
-export async function getSetupWithItems(db: Db = prodDb, setupId: number) {
-	const [setup] = await db.select().from(setups).where(eq(setups.id, setupId));
+export async function getSetupWithItems(
+	db: Db,
+	userId: number,
+	setupId: number,
+) {
+	const [setup] = await db
+		.select()
+		.from(setups)
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
 	if (!setup) return null;
 
 	const itemList = await db
@@ -66,42 +81,68 @@ export async function getSetupWithItems(db: Db = prodDb, setupId: number) {
 }
 
 export async function updateSetup(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	setupId: number,
 	data: UpdateSetup,
 ) {
 	const [existing] = await db
 		.select({ id: setups.id })
 		.from(setups)
-		.where(eq(setups.id, setupId));
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
 	if (!existing) return null;
 
 	const [row] = await db
 		.update(setups)
 		.set({ name: data.name, updatedAt: new Date() })
-		.where(eq(setups.id, setupId))
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)))
 		.returning();
 
 	return row;
 }
 
-export async function deleteSetup(db: Db = prodDb, setupId: number) {
+export async function deleteSetup(
+	db: Db,
+	userId: number,
+	setupId: number,
+) {
 	const [existing] = await db
 		.select({ id: setups.id })
 		.from(setups)
-		.where(eq(setups.id, setupId));
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
 	if (!existing) return false;
 
-	await db.delete(setups).where(eq(setups.id, setupId));
+	await db
+		.delete(setups)
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
 	return true;
 }
 
 export async function syncSetupItems(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	setupId: number,
 	itemIds: number[],
 ) {
 	return await db.transaction(async (tx) => {
+		// Verify the setup belongs to this user
+		const [setup] = await tx
+			.select({ id: setups.id })
+			.from(setups)
+			.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
+		if (!setup) return null;
+
+		// Verify all itemIds belong to this user
+		const validItems =
+			itemIds.length > 0
+				? await tx
+						.select({ id: items.id })
+						.from(items)
+						.where(and(eq(items.userId, userId), inArray(items.id, itemIds)))
+				: [];
+		const validItemIds = new Set(validItems.map((i) => i.id));
+		const filteredItemIds = itemIds.filter((id) => validItemIds.has(id));
+
 		// Save existing classifications before deleting
 		const existing = await tx
 			.select({
@@ -119,8 +160,8 @@ export async function syncSetupItems(
 		// Delete all existing items for this setup
 		await tx.delete(setupItems).where(eq(setupItems.setupId, setupId));
 
-		// Re-insert new items, preserving classifications for retained items
-		for (const itemId of itemIds) {
+		// Re-insert only user-owned items, preserving classifications
+		for (const itemId of filteredItemIds) {
 			await tx.insert(setupItems).values({
 				setupId,
 				itemId,
@@ -131,27 +172,43 @@ export async function syncSetupItems(
 }
 
 export async function updateItemClassification(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	setupId: number,
 	itemId: number,
 	classification: string,
 ) {
+	// Verify setup belongs to user
+	const [setup] = await db
+		.select({ id: setups.id })
+		.from(setups)
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
+	if (!setup) return null;
+
 	await db
 		.update(setupItems)
 		.set({ classification })
 		.where(
-			sql`${setupItems.setupId} = ${setupId} AND ${setupItems.itemId} = ${itemId}`,
+			and(eq(setupItems.setupId, setupId), eq(setupItems.itemId, itemId)),
 		);
 }
 
 export async function removeSetupItem(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	setupId: number,
 	itemId: number,
 ) {
+	// Verify setup belongs to user
+	const [setup] = await db
+		.select({ id: setups.id })
+		.from(setups)
+		.where(and(eq(setups.id, setupId), eq(setups.userId, userId)));
+	if (!setup) return null;
+
 	await db
 		.delete(setupItems)
 		.where(
-			sql`${setupItems.setupId} = ${setupId} AND ${setupItems.itemId} = ${itemId}`,
+			and(eq(setupItems.setupId, setupId), eq(setupItems.itemId, itemId)),
 		);
 }
diff --git a/src/server/services/thread.service.ts b/src/server/services/thread.service.ts
index e38e778..ad7383b 100644
--- a/src/server/services/thread.service.ts
+++ b/src/server/services/thread.service.ts
@@ -1,4 +1,4 @@
-import { asc, desc, eq, max, sql } from "drizzle-orm";
+import { and, asc, desc, eq, max, sql } from "drizzle-orm";
 import { db as prodDb } from "../../db/index.ts";
 import {
 	categories,
@@ -11,20 +11,34 @@ import type {
 	CreateThread,
 	ReorderCandidates,
 } from "../../shared/types.ts";
+import { getOrCreateUncategorized } from "./category.service.ts";
 
 type Db = typeof prodDb;
 
-export async function createThread(db: Db = prodDb, data: CreateThread) {
+export async function createThread(
+	db: Db,
+	userId: number,
+	data: CreateThread,
+) {
 	const [row] = await db
 		.insert(threads)
-		.values({ name: data.name, categoryId: data.categoryId })
+		.values({ name: data.name, categoryId: data.categoryId, userId })
 		.returning();
 
 	return row;
 }
 
-export async function getAllThreads(db: Db = prodDb, includeResolved = false) {
-	const query = db
+export async function getAllThreads(
+	db: Db,
+	userId: number,
+	includeResolved = false,
+) {
+	const baseCondition = eq(threads.userId, userId);
+	const whereCondition = includeResolved
+		? baseCondition
+		: and(baseCondition, eq(threads.status, "active"));
+
+	return db
 		.select({
 			id: threads.id,
 			name: threads.name,
@@ -50,22 +64,19 @@ export async function getAllThreads(db: Db = prodDb, includeResolved = false) {
 		})
 		.from(threads)
 		.innerJoin(categories, eq(threads.categoryId, categories.id))
+		.where(whereCondition)
 		.orderBy(desc(threads.createdAt));
-
-	if (!includeResolved) {
-		return query.where(eq(threads.status, "active"));
-	}
-	return query;
 }
 
 export async function getThreadWithCandidates(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	threadId: number,
 ) {
 	const [thread] = await db
 		.select()
 		.from(threads)
-		.where(eq(threads.id, threadId));
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 	if (!thread) return null;
 
 	const candidateList = await db
@@ -97,30 +108,31 @@ export async function getThreadWithCandidates(
 }
 
 export async function updateThread(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	threadId: number,
 	data: Partial<{ name: string; categoryId: number }>,
 ) {
 	const [existing] = await db
 		.select({ id: threads.id })
 		.from(threads)
-		.where(eq(threads.id, threadId));
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 	if (!existing) return null;
 
 	const [row] = await db
 		.update(threads)
 		.set({ ...data, updatedAt: new Date() })
-		.where(eq(threads.id, threadId))
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)))
 		.returning();
 
 	return row;
 }
 
-export async function deleteThread(db: Db = prodDb, threadId: number) {
+export async function deleteThread(db: Db, userId: number, threadId: number) {
 	const [thread] = await db
 		.select()
 		.from(threads)
-		.where(eq(threads.id, threadId));
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 	if (!thread) return null;
 
 	// Collect candidate image filenames for cleanup
@@ -131,7 +143,9 @@ export async function deleteThread(db: Db = prodDb, threadId: number) {
 			.where(eq(threadCandidates.threadId, threadId))
 	).filter((c) => c.imageFilename != null);
 
-	await db.delete(threads).where(eq(threads.id, threadId));
+	await db
+		.delete(threads)
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 
 	return {
 		...thread,
@@ -140,7 +154,8 @@ export async function deleteThread(db: Db = prodDb, threadId: number) {
 }
 
 export async function createCandidate(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	threadId: number,
 	data: Partial<CreateCandidate> & {
 		name: string;
@@ -149,6 +164,13 @@ export async function createCandidate(
 		imageSourceUrl?: string;
 	},
 ) {
+	// Verify the parent thread belongs to this user
+	const [thread] = await db
+		.select({ id: threads.id })
+		.from(threads)
+		.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
+	if (!thread) return null;
+
 	const [maxRow] = await db
 		.select({ maxOrder: max(threadCandidates.sortOrder) })
 		.from(threadCandidates)
@@ -179,7 +201,8 @@ export async function createCandidate(
 }
 
 export async function updateCandidate(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	candidateId: number,
 	data: Partial<{
 		name: string;
@@ -195,12 +218,22 @@ export async function updateCandidate(
 		cons: string;
 	}>,
 ) {
+	// Verify the candidate's parent thread belongs to this user
 	const [existing] = await db
-		.select({ id: threadCandidates.id })
+		.select({
+			id: threadCandidates.id,
+			threadId: threadCandidates.threadId,
+		})
 		.from(threadCandidates)
 		.where(eq(threadCandidates.id, candidateId));
 	if (!existing) return null;
 
+	const [thread] = await db
+		.select({ id: threads.id })
+		.from(threads)
+		.where(and(eq(threads.id, existing.threadId), eq(threads.userId, userId)));
+	if (!thread) return null;
+
 	const [row] = await db
 		.update(threadCandidates)
 		.set({ ...data, updatedAt: new Date() })
@@ -210,19 +243,33 @@ export async function updateCandidate(
 	return row;
 }
 
-export async function deleteCandidate(db: Db = prodDb, candidateId: number) {
+export async function deleteCandidate(
+	db: Db,
+	userId: number,
+	candidateId: number,
+) {
+	// Verify the candidate's parent thread belongs to this user
 	const [candidate] = await db
 		.select()
 		.from(threadCandidates)
 		.where(eq(threadCandidates.id, candidateId));
 	if (!candidate) return null;
 
+	const [thread] = await db
+		.select({ id: threads.id })
+		.from(threads)
+		.where(
+			and(eq(threads.id, candidate.threadId), eq(threads.userId, userId)),
+		);
+	if (!thread) return null;
+
 	await db.delete(threadCandidates).where(eq(threadCandidates.id, candidateId));
 	return candidate;
 }
 
 export async function reorderCandidates(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	threadId: number,
 	orderedIds: ReorderCandidates["orderedIds"],
 ): Promise<{ success: boolean; error?: string }> {
@@ -230,7 +277,7 @@ export async function reorderCandidates(
 		const [thread] = await tx
 			.select()
 			.from(threads)
-			.where(eq(threads.id, threadId));
+			.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 		if (!thread) {
 			return { success: false, error: "Thread not found" };
 		}
@@ -250,16 +297,17 @@ export async function reorderCandidates(
 }
 
 export async function resolveThread(
-	db: Db = prodDb,
+	db: Db,
+	userId: number,
 	threadId: number,
 	candidateId: number,
 ): Promise<{ success: boolean; item?: any; error?: string }> {
 	return await db.transaction(async (tx) => {
-		// 1. Check thread is active
+		// 1. Check thread is active and belongs to user
 		const [thread] = await tx
 			.select()
 			.from(threads)
-			.where(eq(threads.id, threadId));
+			.where(and(eq(threads.id, threadId), eq(threads.userId, userId)));
 		if (!thread || thread.status !== "active") {
 			return { success: false, error: "Thread not active" };
 		}
@@ -276,14 +324,21 @@ export async function resolveThread(
 			return { success: false, error: "Candidate not in thread" };
 		}
 
-		// 3. Verify categoryId still exists, fallback to Uncategorized (id=1)
+		// 3. Verify categoryId belongs to user, fallback to Uncategorized
 		const [category] = await tx
 			.select({ id: categories.id })
 			.from(categories)
-			.where(eq(categories.id, candidate.categoryId));
-		const safeCategoryId = category ? candidate.categoryId : 1;
+			.where(
+				and(
+					eq(categories.id, candidate.categoryId),
+					eq(categories.userId, userId),
+				),
+			);
+		const safeCategoryId = category
+			? candidate.categoryId
+			: await getOrCreateUncategorized(tx as unknown as Db, userId);
 
-		// 4. Create collection item from candidate data
+		// 4. Create collection item from candidate data — with userId
 		const [newItem] = await tx
 			.insert(items)
 			.values({
@@ -291,6 +346,7 @@ export async function resolveThread(
 				weightGrams: candidate.weightGrams,
 				priceCents: candidate.priceCents,
 				categoryId: safeCategoryId,
+				userId,
 				notes: candidate.notes,
 				productUrl: candidate.productUrl,
 				imageFilename: candidate.imageFilename,