Featherless AI Chat Completions Streaming (HTTP + SSE)

asyncapi: '2.6.0'
id: 'urn:ai:featherless:v1:chat-completions:sse'
info:
  title: Featherless AI Chat Completions Streaming (HTTP + SSE)
  version: '1.0.0'
  description: |
    AsyncAPI 2.6 description of Featherless AI's **chat completion streaming**
    surface.

    Featherless AI's core inference API is OpenAI-compatible and does not
    publish a WebSocket API for chat. The asynchronous / event-style transport
    documented at https://featherless.ai/docs/completions is **HTTP
    Server-Sent Events (SSE)** delivered over the same REST endpoint
    (`POST /chat/completions`) when the request body sets `stream: true`. SSE
    is a one-way, server-to-client HTTP streaming channel; it is **not**
    WebSocket.

    This AsyncAPI document models only the streamed events emitted by the SSE
    response. The request body (model, messages, sampling params) is modeled
    in the companion OpenAPI document at `openapi/featherless-openapi.yml`.

    Featherless separately offers a Realtime API (beta) following the OpenAI
    Realtime audio protocol; that beta audio surface is not modeled here.
  contact:
    name: API Evangelist
    email: kin@apievangelist.com
    url: https://apievangelist.com
  license:
    name: API documentation - Featherless AI Terms of Service
    url: https://featherless.ai/terms
  x-transport-notes:
    transport: HTTP Server-Sent Events (SSE)
    protocol: https
    direction: server-to-client (one-way)
    mediaType: text/event-stream
    triggeredBy: 'POST https://api.featherless.ai/v1/chat/completions with request body { "stream": true }'
    terminator: 'data: [DONE]'
    notWebSocket: true
    source: https://featherless.ai/docs/completions
defaultContentType: text/event-stream
servers:
  featherless:
    url: api.featherless.ai/v1
    protocol: https
    description: |
      Featherless AI's OpenAI-compatible REST base. Chat completion streaming
      is delivered as HTTP Server-Sent Events over this base when
      `stream: true` is set on the JSON request body. AsyncAPI 2.6 does not
      define a dedicated SSE protocol identifier; `https` is used here and the
      SSE transport is documented in `info.x-transport-notes` and on each
      channel.
    security:
      - bearerAuth: []
channels:
  /chat/completions:
    description: |
      Chat completion SSE stream. The client opens this channel by issuing
      `POST /chat/completions` with `Content-Type: application/json` and a
      JSON body containing `stream: true`. The server responds with
      `Content-Type: text/event-stream` and emits a sequence of `data:` lines,
      each carrying one JSON-serialized `chat.completion.chunk` object,
      followed by a final `data: [DONE]` line.
    bindings:
      http:
        type: request
        method: POST
        bindingVersion: '0.3.0'
      x-sse:
        mediaType: text/event-stream
        eventField: 'data'
        terminator: '[DONE]'
    subscribe:
      operationId: streamChatCompletionChunks
      summary: Subscribe to streamed chat completion chunks (SSE).
      description: |
        After `POST /chat/completions` is issued with `stream: true`, the
        server emits an ordered sequence of SSE `data:` events. Each `data:`
        line either carries a JSON-serialized `ChatCompletionChunk` or the
        literal sentinel `[DONE]` marking end of stream.
      bindings:
        http:
          type: response
          bindingVersion: '0.3.0'
      message:
        oneOf:
          - $ref: '#/components/messages/ChatCompletionChunk'
          - $ref: '#/components/messages/StreamDone'
components:
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      bearerFormat: 'Featherless API key'
      description: |
        Set the `Authorization: Bearer <FEATHERLESS_API_KEY>` header on the
        `POST /chat/completions` request that opens the SSE stream.
  messages:
    ChatCompletionChunk:
      name: ChatCompletionChunk
      title: Streamed chat completion chunk
      summary: |
        A single SSE `data:` event carrying one JSON `chat.completion.chunk`
        object. Many of these are emitted per request, in order.
      contentType: application/json
      description: |
        Sent as `data: {json}\n\n` on the SSE stream. The JSON object's
        `object` field is always the literal string `chat.completion.chunk`.
      payload:
        $ref: '#/components/schemas/ChatCompletionChunk'
      examples:
        - name: openingChunk
          summary: First chunk - establishes role
          payload:
            id: chatcmpl-abc123
            object: chat.completion.chunk
            created: 1750464000
            model: Qwen/Qwen2.5-7B-Instruct
            choices:
              - index: 0
                delta:
                  role: assistant
                  content: ''
                finish_reason: null
        - name: contentChunk
          summary: Token delta
          payload:
            id: chatcmpl-abc123
            object: chat.completion.chunk
            created: 1750464000
            model: Qwen/Qwen2.5-7B-Instruct
            choices:
              - index: 0
                delta:
                  content: 'Hello'
                finish_reason: null
        - name: finalChunk
          summary: Final chunk - finish_reason set
          payload:
            id: chatcmpl-abc123
            object: chat.completion.chunk
            created: 1750464000
            model: Qwen/Qwen2.5-7B-Instruct
            choices:
              - index: 0
                delta: {}
                finish_reason: stop
    StreamDone:
      name: StreamDone
      title: Stream terminator
      summary: |
        The literal SSE event `data: [DONE]` that marks end of stream. Not
        JSON; the payload is the string `[DONE]`.
      contentType: text/plain
      description: |
        Clients must stop reading the stream when this sentinel is observed.
      payload:
        $ref: '#/components/schemas/StreamDoneSentinel'
      examples:
        - name: done
          summary: End-of-stream sentinel
          payload: '[DONE]'
  schemas:
    StreamDoneSentinel:
      type: string
      enum:
        - '[DONE]'
      description: |
        End-of-stream sentinel. The full SSE line is `data: [DONE]`.
    ChatCompletionChunk:
      type: object
      description: A streamed chunk of a chat completion response.
      required:
        - choices
        - created
        - id
        - model
        - object
      properties:
        id:
          type: string
          description: A unique identifier for the chat completion. Each chunk has the same ID.
        choices:
          type: array
          description: A list of chat completion choices.
          items:
            $ref: '#/components/schemas/ChatCompletionChunkChoice'
        created:
          type: integer
          description: Unix timestamp (seconds) of when the completion was created.
        model:
          type: string
          description: The model used to generate the completion.
        object:
          type: string
          enum:
            - chat.completion.chunk
          description: The object type, always `chat.completion.chunk`.
        usage:
          $ref: '#/components/schemas/CompletionUsage'
    ChatCompletionChunkChoice:
      type: object
      required:
        - delta
        - finish_reason
        - index
      properties:
        index:
          type: integer
        delta:
          $ref: '#/components/schemas/ChatCompletionStreamResponseDelta'
        finish_reason:
          type: string
          nullable: true
          enum:
            - stop
            - length
            - tool_calls
          description: |
            Reason the model stopped generating. Null on all chunks except
            the final content chunk.
    ChatCompletionStreamResponseDelta:
      type: object
      description: A chat completion delta generated by streamed model responses.
      properties:
        role:
          type: string
          enum:
            - system
            - user
            - assistant
            - tool
        content:
          type: string
          nullable: true
          description: The contents of the chunk message (token slice).
        tool_calls:
          type: array
          description: Streaming tool-call fragments.
          items:
            type: object
    CompletionUsage:
      type: object
      nullable: true
      description: Usage statistics; present on the final chunk when requested.
      properties:
        prompt_tokens:
          type: integer
        completion_tokens:
          type: integer
        total_tokens:
          type: integer