Skip to main content
Braintrust supports executing prompts, functions, and evaluations through the API and within the UI through the playground. Like popular LLM services, Braintrust supports streaming results using Server-Sent Events. The Braintrust SDK and UI automatically parse the SSE stream, and we have adapters for common libraries like the Vercel AI SDK, so you can quickly integrate with the rich and growing ecosystem of LLM tools. However, the SSE format itself is also purposefully simple, so if you need to parse it yourself, you can! To see more about how to use streaming data, see the prompts documentation.

​
Why does this exist

Streaming is a very powerful way to consume LLM outputs, but the predominant “chat” data structure produced by modern LLMs is more complex than most applications need. In fact, the most common use cases are to (a) convert the text of the first message into a string or (b) parse the arguments of the first tool call into a JSON object. The Braintrust SSE format is really optimized to make these use cases easy to parse, while also supporting more advanced scenarios like parallel tools calls.

​
Formal spec

SSE events consist of three fields: id (optional), event (optional), and data. The Braintrust SSE format always sets event and data, and never sets id. The SSE events in Braintrust follow this structure: 
<BraintrustSSEEvent> ::= <TextDeltaEvent> | <JSONDeltaEvent> | <DoneEvent>

<TextDeltaEvent> ::=
    event: "text_delta"
    data: <JSON-encoded-string>

<JSONDeltaEvent> ::=
    event: "json_delta"
    data: <JSON-snippet>

<ErrorEvent> ::=
    event: "error"
    data: <JSON-encoded-string>

<ProgressEvent> ::=
    event: "progress"
    data: <JSON-encoded-object>

<DoneEvent> ::=
    event: "done"
    data: ""

​
Text

A text_delta is a snippet of text, which is JSON-encoded. For example, you might receive: 
event: text_delta
data: "this is a line\nbreak"

event: text_delta
data: "with some \"nested quotes\"."

event: done
data:
As you process a text_delta, you can JSON-decode the string and display it directly.

​
JSON

A json_delta is a snippet of JSON-encoded data, which cannot necessarily be parsed on its own. For example: 
event: json_delta
data: {"name": "Cecil",

event: json_delta
data: "age": 30}

event: done
data:
As you process a json_delta events, concatenate the strings together and then parse them as JSON at the end of the stream.

​
Error

An error event is a JSON-encoded string that contains the error message. For example: 
event: error
data: "Something went wrong."

event: done
data:

​
Progress

A progress event is a JSON-encoded object that contains intermediate events produced by functions while they are executing. Each json object contains the following fields: 
{
    "id": "A span id for this event",
    "object_type": "prompt" | "tool" | "scorer" | "task",
    "format": "llm" | "code" | "global",
    "output_type": "completion" | "score" | "any",
    "name": "The name of the function or prompt",
    "event": "text_delta" | "json_delta" | "error" | "start" | "done",
    "data": "The delta or error message"
}
The event field is the type of event produced by the intermediate function call, and the data field is the same as the data field in the text_delta and json_delta events.

​
Start

A start event is a progress event with event: "start" and an empty string for data. Start is not guaranteed to be sent and is for display purposes only.

​
Done

A done event is a progress event with event: "done" and an empty string for data. Once a done event is received, you can safely assume that the function has completed and will send no more events.