---
issue: 072
title: "Backend: chat messages stored as embedded array — unbounded document growth, 16MB ceiling"
severity: high
domain: Chat
labels: [performance, backend, data-model]
status: open
created: 2026-05-30
source: Full Codebase Audit 2026-05-30
---

# Backend: chat messages stored as embedded array — unbounded document growth, 16MB ceiling

**Severity:** high
**Domain:** Chat
**Labels:** performance, backend, data-model

## Description

`backend/src/models/Chat.ts:173` stores all messages as an embedded array in the Chat document. MongoDB's 16MB document size limit will be hit for active long-running chats. Reads also load the full message history into memory even when only the latest page is needed.

## Options

1. Migrate messages to a `Messages` collection keyed by `chatId` with pagination.
2. Cap embedded messages and archive older ones.
3. Keep embedded but project only needed messages (slice) on reads.

## Recommendation

Plan migration to a dedicated `Messages` collection. This is a large data-model migration that needs careful coordination.

## Affected Files

- `backend/src/models/Chat.ts:173`
- `backend/src/services/chat/ChatService.ts` — all read paths

## References

- [Full Codebase Audit 2026-05-30](../09%20-%20Audits/Full%20Codebase%20Audit%20-%202026-05-30.md) — DEC-42