Post Session Thread Item - BlobHub

Append an item to a thread session object.

POST `/revisions/:id/data/command` (Command: `post_session_thread_item`)

Request Body

Parameter	Type	Required	Description
`engine`	string	Yes	Must be `workflow_blobhub`.
`command`	string	Yes	Must be `post_session_thread_item`.
`session_id`	string	Yes	The ID of the session that owns the thread envelope.
`alias`	string	Yes	Alias of the thread session object.
`content`	array of object	Yes	Array of content blocks. Each block must have a `type` field (`"text"` or `"json"`), plus a `text` string or a `json` value respectively.
`parent_id`	string	No	Optional parent item id. Stored only.
`metadata`	object	No	Optional arbitrary JSON.

Response

Returns the persisted item with server-stamped id, created_at, user_id, and the original/derived fields.

Parameter	Type	Description
`item`	object	The persisted thread item, including server-stamped `id`, `created_at`, `user_id`, and `revision_id`.

Errors

invalid_thread_envelope — the alias does not exist or its value.type is not "thread".
thread_item_too_large — the serialized item exceeds the server-side size cap.

Example

{
  "engine": "workflow_blobhub",
  "command": "post_session_thread_item",
  "session_id": "sess_001",
  "alias": "onboarding_thread",
  "content": [
    { "type": "text", "text": "Hello, world." }
  ]
}

{
  "status": "success",
  "item": {
    "id": "itm_01H...",
    "session_id": "sess_001",
    "revision_id": "rev_001",
    "alias": "onboarding_thread",
    "created_at": "2026-05-22T22:00:00.000000Z",
    "user_id": "usr_001",
    "content": [
      { "type": "text", "text": "Hello, world." }
    ]
  }
}

See also

BlobHub Worker — thread items — how the worker structures content and metadata for agent emissions, and the 380 KB truncation behavior.