---
title: "Reply in existing thread (Tier1+)"
method: POST
path: "/v1/reply"
tags: ["Outbound"]
---

# Reply in existing thread (Tier1+)

`POST /v1/reply`

## Request

- Base URL: `https://api.thrd.email`
- URL: `https://api.thrd.email/v1/reply`
- Auth: HTTP bearer

## Headers

- `Idempotency-Key` string, required

## Request body

- object
  - `thread_id` string, uuid, required
  - `text` string
  - `html` string
  - `to` string, email
  - `cc` string[]: Additional CC recipients. Replies preserve historical thread CC and also keep recipients from the latest inbound To-line (reply-all behavior). Tier 1 (sandbox) may not add new CC addresses in cc[]; it can only keep thread history automatically.
  - `security_ack_token` string: Required only when source inbound message has high prompt-injection risk on Tier 2/3. Obtain via POST /v1/security/ack.
  - `attachments` object[]
    - `storage_ref` string, required
    - `filename` string, required
    - `mime` string, required
    - `size` integer, required

## Response `202`

Accepted

## Other responses

- `400`: Bad request
- `403`: Forbidden
- `409`: Same key reused with different payload
- `428`: Security ack required
- `429`: Rate limit or outbound entitlement reached

---

[API](https://skmtc.dev/thrd/apis/thrd-agent-email-api.md) · [All operations](https://skmtc.dev/thrd/apis/thrd-agent-email-api/llms.txt) · [OpenAPI document](https://skmtc.dev/thrd/apis/thrd-agent-email-api/revisions/d8a88129cd34?raw)