---
title: Heartbeat
seo:
  title: Heartbeat | FIX Session
  description: "FIX session Heartbeat reference: field layout, repeating groups,
    supported values, and message-specific behavior."
sidebar:
  label: Heartbeat
---
`MsgType(35)=Heartbeat(0)`

Direction: **Both directions**.

Maintain a session or answer a test request.

## Field Layout

See [FIX Message Layouts](/reference/fix-message-layouts) for field ordering and presence rules.

| Field | Type | Presence | Behavior |
| --- | --- | --- | --- |
| `TestReqID(112)` | [`STRING`](/reference/fix-data-types#string) | Optional |  |