# ServerSentEvents
[](https://github.com/benjreinhart/server_sent_events/actions/workflows/ci.yml)
[](https://github.com/benjreinhart/server_sent_events/blob/main/LICENSE.md)
[](https://hex.pm/packages/server_sent_events)
[](https://hexdocs.pm/server_sent_events/readme.html)
Lightweight, ultra-fast Server Sent Event parser for Elixir.
This module parses according to the official [Server Sent Events specification](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream) with a comprehensive [test suite](https://github.com/benjreinhart/server_sent_events/blob/main/test/server_sent_events/parser_test.exs). See [Behavior Boundary](#behavior-boundary) for more info.
## Installation
The package can be installed by adding `server_sent_events` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:server_sent_events, "~> 1.0.0"}
]
end
```
## Usage
Decode an enumerable of binary chunks with `ServerSentEvents.decode_stream/1`:
```elixir
events =
[
"event: message\n",
"data: {\"complete\":true}\n\n"
]
|> ServerSentEvents.decode_stream()
|> Enum.to_list()
IO.inspect(events)
# [%{event: "message", data: "{\"complete\":true}"}]
```
The decoder keeps state across arbitrary chunk boundaries:
```elixir
events =
[
"event: mes",
"sage\ndata: {\"complete\":",
"true}\n\n"
]
|> ServerSentEvents.decode_stream()
|> Enum.to_list()
IO.inspect(events)
# [%{event: "message", data: "{\"complete\":true}"}]
```
Events are maps that always include `:data`, and may also include `:id`, `:event`, or `:retry`.
The `:id`, `:event`, and `:data` values are binaries. The `:retry` value is a non-negative
integer when present.
### Real world example using Req
Req can expose the response body as an enumerable with `into: :self`. That body can be passed through `ServerSentEvents.decode_stream/1`:
From there, callers typically filter event types and JSON-decode the `data` field.
```elixir
%Req.Response{status: 200, body: response_body} =
Req.post!("https://api.anthropic.com/v1/messages",
json: request,
into: :self,
headers: %{
"x-api-key" => api_key(),
"anthropic-version" => "2023-06-01",
"anthropic-beta" => "adaptive-thinking-2026-01-28,effort-2025-11-24,max-effort-2026-01-24"
}
)
response_body
|> ServerSentEvents.decode_stream()
|> Stream.map(fn %{data: data} -> JSON.decode!(data) end)
|> Enum.each(&IO.inspect/1)
# %{
# "content_block" => %{"type" => "thinking", "signature" => "", "thinking" => ""},
# "index" => 0,
# "type" => "content_block_start"
# }
# %{
# "delta" => %{"type" => "thinking_delta", "thinking" => "Now"},
# "index" => 0,
# "type" => "content_block_delta"
# }
# %{
# "delta" => %{"type" => "thinking_delta", "thinking" => " I have a good understanding of the project. Let "},
# "index" => 0,
# "type" => "content_block_delta"
# }
#
# # etc...
#
# %{"index" => 11, "type" => "content_block_stop"}
# %{
# "delta" => %{"stop_details" => nil, "stop_reason" => "tool_use", "stop_sequence" => nil},
# "type" => "message_delta",
# "usage" => %{
# "cache_creation_input_tokens" => 2810,
# "cache_read_input_tokens" => 14451,
# "input_tokens" => 6,
# "output_tokens" => 10528
# }
# }
```
## Behavior Boundary
This library decodes the event stream syntax and applies the field-level parsing rules from the
specification. In particular, `id` fields containing NULL are ignored, and `retry` fields are
emitted as integers only when they contain ASCII digits. Events that do not contain a `data`
field are suppressed.
It intentionally leaves EventSource state and connection behavior to the caller, including:
- Tracking, resetting, or applying `lastEventId`.
- Applying retry delays.
- Supplying a default event type such as `"message"`.
- Opening HTTP connections, reconnecting, or interpreting response headers.
This decoder also assumes the input stream is UTF-8. It does not validate UTF-8, reject malformed input, or perform replacement-character decoding.
## Benchmarking
Run the local benchmark with:
```sh
mix bench
```
The benchmark exercises both large complete payloads and large payloads that end with an incomplete trailing event, and reports execution time and memory usage.
