Resume Chat Stream

{
  "summary": "Resume a chat response stream",
  "description": "Reconnects to an in-progress chat response — the resume counterpart to [`POST /api/chat`](/api-reference/chat/workflow), which never resumes. Returns the live Server-Sent Events stream when a response is still being generated, or `204 No Content` when there is nothing to resume. The chat must belong to the authenticated account.\n\n**Related endpoints**\n- [`POST /api/chat`](/api-reference/chat/workflow) — the interactive turn whose stream this resumes.\n- [`POST /api/chat/runs`](/api-reference/chat/runs) — start a **headless** run, then pass the returned `chatId` here to **watch its output live**.\n- [`GET /api/chat/runs/{runId}`](/api-reference/chat/runs-status) — check **whether** that run finished (status, not content).",
  "security": [
    {
      "apiKeyAuth": []
    },
    {
      "bearerAuth": []
    }
  ],
  "parameters": [
    {
      "name": "chatId",
      "in": "path",
      "required": true,
      "description": "UUID of the chat to resume.",
      "schema": {
        "type": "string",
        "format": "uuid"
      }
    },
    {
      "name": "account_id",
      "in": "query",
      "required": false,
      "description": "Act on a chat belonging to another account. Only honoured for a key that is authorised for that account — an organization key may pass a member account's id; a personal key passing anyone else's is rejected with `403 Access denied to specified account_id`. Omit it to act as the authenticated account.",
      "schema": {
        "type": "string",
        "format": "uuid"
      }
    },
    {
      "name": "startIndex",
      "in": "query",
      "required": false,
      "description": "Zero-based index of the first stream chunk to return, so a client that was already receiving this response resumes where it left off instead of replaying the whole turn.\n\nPass the number of chunks already received. Omit it to receive the response from the beginning — the right choice for a fresh reader such as a page load, or a client watching a headless [`POST /api/chat/runs`](/api-reference/chat/runs) run for the first time.\n\nA long turn's stream can end before the run does; reconnecting with `startIndex` is how a client keeps rendering without duplicating or skipping chunks.",
      "schema": {
        "type": "integer",
        "minimum": 0
      }
    }
  ],
  "responses": {
    "200": {
      "description": "Server-Sent Events stream of UI message parts for the in-progress response, compatible with the Vercel AI SDK. The `x-workflow-run-id` response header carries the run ID for this response, and `x-workflow-stream-tail-index` carries the index of the last chunk known when this read was opened — a base for computing absolute positions, not a record of where the response ended.",
      "headers": {
        "x-workflow-run-id": {
          "description": "Durable workflow run ID of the resumed run.",
          "schema": {
            "type": "string"
          }
        },
        "x-workflow-stream-tail-index": {
          "description": "Zero-based index of the last chunk known to the stream **at the moment this read was opened**. Because headers are sent before the body, a read that stays open past that point will deliver chunks beyond it — so this is a base for computing absolute positions, not a record of where the response ended. A client resuming precisely should count the chunks it receives on top of this value. Omitted if the runtime cannot report it.",
          "schema": {
            "type": "integer"
          }
        }
      },
      "content": {
        "text/event-stream": {
          "schema": {
            "type": "string",
            "description": "Server-Sent Events stream containing UI message parts"
          }
        }
      }
    },
    "204": {
      "description": "No active stream to resume for this chat."
    },
    "400": {
      "description": "`startIndex` was provided but is not a non-negative integer.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/ChatStreamErrorResponse"
          }
        }
      }
    },
    "401": {
      "description": "Missing or invalid credentials.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/ChatStreamErrorResponse"
          }
        }
      }
    },
    "403": {
      "description": "The authenticated account does not own the referenced chat.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/ChatStreamErrorResponse"
          }
        }
      }
    },
    "404": {
      "description": "Chat not found.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/ChatStreamErrorResponse"
          }
        }
      }
    }
  }
}