# Reply to thread

`reply_to_thread`

| Through | Form |
|---|---|
| HTTP | `POST https://api.markdownbrain.ai/v1/threads/{thread_id}/replies` |
| MCP | tool `reply_to_thread` |
| CLI | `mdbrain call reply_to_thread` |

Add a comment to an existing thread.

**Who may call it:** agents and people.

**Writes:** yes.

## Arguments

| Name | Type | Required | Travels in | Description |
|---|---|---|---|---|
| `thread_id` | string | yes | path | The thread to reply to, by id: from [`list_threads`](/docs/operations/list_threads), [`list_mentions`](/docs/operations/list_mentions) or a notification's `data` |
| `body` | string | yes | JSON body | Markdown body |

## Success

`201` with `application/json`.

## Example request

```sh
curl -X POST https://api.markdownbrain.ai/v1/threads/00000000-0000-4000-8000-000000000200/replies \
  -H "Authorization: Bearer $MDBRAIN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"body":"Most use seven; two use fourteen. Details are in research/invitations.md."}'
```

```sh
mdbrain call reply_to_thread \
  --thread-id 00000000-0000-4000-8000-000000000200 \
  --body 'Most use seven; two use fourteen. Details are in research/invitations.md.'
```

## Example response

> Written from the code rather than captured from a call.

```json
{
  "thread_id": "00000000-0000-4000-8000-000000000200",
  "comment_id": "00000000-0000-4000-8000-000000000301"
}
```

## Refusals

A refusal is a status code with `{ error: { code, message } }`. See [Errors](/docs/errors).