---
issue: 045
title: "addParticipants frontend sends { participants: string[] } array but backend expects { userId: string } single user"
severity: major
domain: Chat
labels: [bug, frontend, major, chat]
status: open
created: 2026-05-29
source: Doc vs Code Audit 2026-05-29
---

# 🟠 addParticipants frontend sends { participants: string[] } array but backend expects { userId: string } single user

**Severity:** major
**Domain:** Chat
**Labels:** bug, frontend, major, chat

## Description

The frontend addParticipants action (chat.ts line 425) sends { participants: string[] } as the body. The API documents POST /api/chat/:id/participants with body { userId: string } — a single user. Backend expects a single userId, not an array. Bulk participant addition will be silently handled incorrectly.

## Current Behavior

addParticipants sends an array payload that the backend does not expect. Participant addition may fail or be ignored.

## Expected Behavior

Frontend should send { userId: string } and call the endpoint once per participant, or backend should be updated to accept an array.

## Affected Files

- `frontend/src/actions/chat.ts`

## References

- [Doc vs Code Audit Report](../09%20-%20Audits/Doc%20vs%20Code%20Audit%20Report%20-%202026-05-29.md)