diff --git a/apps/server/convex/chats.ts b/apps/server/convex/chats.ts
index 4ce9c08c..f85f6d81 100644
--- a/apps/server/convex/chats.ts
+++ b/apps/server/convex/chats.ts
@@ -11,6 +11,7 @@ const chatDoc = v.object({
 	createdAt: v.number(),
 	updatedAt: v.number(),
 	lastMessageAt: v.optional(v.number()),
+	deletedAt: v.optional(v.number()),
 });
 
 export const list = query({
@@ -19,11 +20,13 @@ export const list = query({
 	},
 	returns: v.array(chatDoc),
 	handler: async (ctx, args) => {
-		return await ctx.db
+		const chats = await ctx.db
 			.query("chats")
 			.withIndex("by_user", (q) => q.eq("userId", args.userId))
 			.order("desc")
 			.take(200);
+		// Filter out soft-deleted chats
+		return chats.filter((chat) => !chat.deletedAt);
 	},
 });
 
@@ -35,7 +38,7 @@ export const get = query({
 	returns: v.union(chatDoc, v.null()),
 	handler: async (ctx, args) => {
 		const chat = await ctx.db.get(args.chatId);
-		if (!chat || chat.userId !== args.userId) return null;
+		if (!chat || chat.userId !== args.userId || chat.deletedAt) return null;
 		return chat;
 	},
 });
@@ -67,15 +70,28 @@ export const remove = mutation({
 	returns: v.object({ ok: v.boolean() }),
 	handler: async (ctx, args) => {
 		const chat = await ctx.db.get(args.chatId);
-		if (!chat || chat.userId !== args.userId) {
+		if (!chat || chat.userId !== args.userId || chat.deletedAt) {
 			return { ok: false } as const;
 		}
+		const now = Date.now();
+		// Soft delete: mark chat as deleted instead of hard delete
+		await ctx.db.patch(args.chatId, {
+			deletedAt: now,
+		});
+		// Cascade soft delete to all messages in the chat (skip already deleted messages)
 		const messages = await ctx.db
 			.query("messages")
 			.withIndex("by_chat", (q) => q.eq("chatId", args.chatId))
 			.collect();
-		await Promise.all(messages.map((message) => ctx.db.delete(message._id)));
-		await ctx.db.delete(args.chatId);
+		await Promise.all(
+			messages
+				.filter((message) => !message.deletedAt)
+				.map((message) =>
+					ctx.db.patch(message._id, {
+						deletedAt: now,
+					}),
+				),
+		);
 		return { ok: true } as const;
 	},
 });
@@ -86,7 +102,7 @@ export async function assertOwnsChat(
 	userId: Id<"users">,
 ) {
 	const chat = await ctx.db.get(chatId);
-	if (!chat || chat.userId !== userId) {
+	if (!chat || chat.userId !== userId || chat.deletedAt) {
 		return null;
 	}
 	return chat;
diff --git a/apps/server/convex/messages.ts b/apps/server/convex/messages.ts
index 992d80f3..c2d2375e 100644
--- a/apps/server/convex/messages.ts
+++ b/apps/server/convex/messages.ts
@@ -13,6 +13,8 @@ const messageDoc = v.object({
 	content: v.string(),
 	createdAt: v.number(),
 	status: v.optional(v.string()),
+	userId: v.optional(v.id("users")),
+	deletedAt: v.optional(v.number()),
 });
 
 export const list = query({
@@ -24,11 +26,13 @@ export const list = query({
 	handler: async (ctx, args) => {
 		const chat = await assertOwnsChat(ctx, args.chatId, args.userId);
 		if (!chat) return [];
-		return await ctx.db
+		const messages = await ctx.db
 			.query("messages")
 			.withIndex("by_chat", (q) => q.eq("chatId", args.chatId))
 			.order("asc")
 			.collect();
+		// Filter out soft-deleted messages
+		return messages.filter((message) => !message.deletedAt);
 	},
 });
 
@@ -68,6 +72,7 @@ export const send = mutation({
 			createdAt: userCreatedAt,
 			clientMessageId: args.userMessage.clientMessageId,
 			status: "completed",
+			userId: args.userId,
 		});
 
 		let assistantMessageId: Id<"messages"> | null = null;
@@ -81,6 +86,7 @@ export const send = mutation({
 				createdAt: assistantCreatedAt,
 				clientMessageId: args.assistantMessage.clientMessageId,
 				status: "completed",
+				userId: args.userId,
 			});
 		}
 
@@ -126,6 +132,7 @@ export const streamUpsert = mutation({
 			status: args.status ?? "streaming",
 			clientMessageId: args.clientMessageId,
 			overrideId: args.messageId ?? undefined,
+			userId: args.userId,
 		});
 
 		if (args.status === "completed" && (args.role === "assistant" || args.role === "user")) {
@@ -140,6 +147,8 @@ export const streamUpsert = mutation({
 	},
 });
 
+const MAX_MESSAGE_CONTENT_LENGTH = 100 * 1024; // 100KB
+
 async function insertOrUpdateMessage(
 	ctx: MutationCtx,
 	args: {
@@ -150,8 +159,16 @@ async function insertOrUpdateMessage(
 		status: string;
 		clientMessageId?: string | null;
 		overrideId?: Id<"messages">;
+		userId?: Id<"users">;
 	},
 ) {
+	// Validate message content length (100KB max) - count actual bytes, not string length
+	const contentBytes = new TextEncoder().encode(args.content).length;
+	if (contentBytes > MAX_MESSAGE_CONTENT_LENGTH) {
+		throw new Error(
+			`Message content exceeds maximum length of ${MAX_MESSAGE_CONTENT_LENGTH} bytes`,
+		);
+	}
 	let targetId = args.overrideId;
 	if (!targetId && args.clientMessageId) {
 		const existing = await ctx.db
@@ -160,7 +177,8 @@ async function insertOrUpdateMessage(
 				q.eq("chatId", args.chatId).eq("clientMessageId", args.clientMessageId!),
 			)
 			.unique();
-		if (existing) {
+		// Only reuse the message if it hasn't been soft-deleted
+		if (existing && !existing.deletedAt) {
 			targetId = existing._id;
 		}
 	}
@@ -172,6 +190,7 @@ async function insertOrUpdateMessage(
 			content: args.content,
 			createdAt: args.createdAt,
 			status: args.status,
+			userId: args.userId ?? undefined,
 		});
 	} else {
 		await ctx.db.patch(targetId, {
@@ -180,6 +199,7 @@ async function insertOrUpdateMessage(
 			content: args.content,
 			createdAt: args.createdAt,
 			status: args.status,
+			userId: args.userId ?? undefined,
 		});
 	}
 	return targetId;
diff --git a/apps/server/convex/schema.ts b/apps/server/convex/schema.ts
index a067bc89..dbe8861c 100644
--- a/apps/server/convex/schema.ts
+++ b/apps/server/convex/schema.ts
@@ -16,15 +16,20 @@ export default defineSchema({
 		createdAt: v.number(),
 		updatedAt: v.number(),
 		lastMessageAt: v.optional(v.number()),
+		deletedAt: v.optional(v.number()),
 	}).index("by_user", ["userId", "updatedAt"]),
 	messages: defineTable({
 		chatId: v.id("chats"),
 		clientMessageId: v.optional(v.string()),
 		role: v.string(),
+		// Max length: 100KB (102400 bytes)
 		content: v.string(),
 		createdAt: v.number(),
 		status: v.optional(v.string()),
+		userId: v.optional(v.id("users")),
+		deletedAt: v.optional(v.number()),
 	})
 		.index("by_chat", ["chatId", "createdAt"])
-		.index("by_client_id", ["chatId", "clientMessageId"]),
+		.index("by_client_id", ["chatId", "clientMessageId"])
+		.index("by_user", ["userId"]),
 });