Use when an Axum handler must push a one-way server-to-client stream over plain HTTP: live notifications, log tailing, progress updates, an LLM token feed, or any "the browser only ever receives" channel consumed by an EventSource. Prevents the compile error from returning a bare Stream of Event instead of the required Result<Event, E> item type, prevents the dropped-connection bug from omitting KeepAlive on a quiet long-lived stream, prevents the runtime panic from calling a single-use Event builder method twice, and prevents reaching for WebSockets when the client never sends anything back. Covers axum::response::sse::Sse, Event, KeepAlive, the TryStream<Ok = Event> bound on Sse::new, the .map(Ok) infallible lift, the Event builder methods (.data, .json_data, .event, .id, .retry, .comment) with their documented panics, the json and tokio feature flags, and the SSE versus WebSocket decision. Keywords: axum SSE, server-sent events, axum::response::sse, Sse::new, Event, KeepAlive, EventSource, text/event-strea
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Use when an Axum handler must push a one-way server-to-client stream over plain HTTP: live notifications, log tailing, progress updates, an LLM token feed, or any "the browser only ever receives" channel consumed by an EventSource. Prevents the compile error from returning a bare Stream of Event instead of the required Result<Event, E> item type, prevents the dropped-connection bug from omitting KeepAlive on a quiet long-lived stream, prevents the runtime panic from calling a single-use Event builder method twice, and prevents reaching for WebSockets when the client never sends anything back. Covers axum::response::sse::Sse, Event, KeepAlive, the TryStream<Ok = Event> bound on Sse::new, the .map(Ok) infallible lift, the Event builder methods (.data, .json_data, .event, .id, .retry, .comment) with their documented panics, the json and tokio feature flags, and the SSE versus WebSocket decision. Keywords: axum SSE, server-sent events, axum::response::sse, Sse::new, Event, KeepAlive, EventSource, text/event-stream, TryStream, map(Ok), Result Event Infallible, BoxError, keep_alive, throttle, broadcast stream, json_data, the trait bound Stream is not satisfied, expected Result found Event, my SSE connection keeps dropping, events stop after a minute, connection closed by proxy, panicked at data already set, how do I stream to the browser, how do I push live updates, what is SSE, SSE vs websocket, one-way push, live feed not updating.
license
MIT
compatibility
Designed for Claude Code. Requires Axum 0.7,0.8.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
axum-impl-sse
Overview
Server-Sent Events (SSE) is a one-way, server-to-client push channel over a
single long-lived HTTP response with body type text/event-stream. The browser
consumes it with new EventSource(url) and reconnects automatically when the
connection drops.
Axum exposes SSE through the axum::response::sse module. The module has
exactly three public types:
Type
Role
Sse<S>
The response. Wraps a Stream and implements IntoResponse.
Event
One SSE record, built fluently from Event::default().
KeepAlive
Idle-gap filler that holds quiet connections open.
The axum::response::sse module is API-stable between Axum 0.7 and 0.8. None of
the 0.7-to-0.8 breaking changes (path syntax, WebSocket Message types, removed
axum::Server) touch this module, so every snippet in this skill compiles
unchanged on both versions. There are no version-divergent code paths here.
Quick Reference
Need
Do this
Return SSE from a handler
Return Sse<impl Stream<Item = Result<Event, E>>>
Build the response
Sse::new(stream)
Lift an infallible Event stream
.map(Ok) so items become Result<Event, Infallible>
fn keep_alive(self, keep_alive: KeepAlive) -> Sse<KeepAliveStream<S>> (feature tokio, on by default)
Full API signatures and feature-flag detail: references/methods.md.
Decision Trees
SSE or WebSocket
Does the client ever send messages on this connection?
├── NO (client only receives: notifications, feeds, logs, progress, token stream)
│ -> Use SSE. This skill. axum::response::sse.
│ Plain HTTP, passes proxies, free browser auto-reconnect.
└── YES (chat, collaborative editing, multiplayer, interactive control)
-> Use WebSockets. See axum-impl-websockets.
Bidirectional, carries binary frames.
ALWAYS choose SSE when the data flow is push-only. NEVER reach for WebSockets
for a receive-only client: SSE is simpler, has less overhead, needs no protocol
upgrade, and gives free reconnection with Last-Event-ID resumption.
What item type does my stream yield
Can the stream itself fail at runtime?
├── NO (a timer, a counter, repeat_with, a fixed iterator)
│ -> Stream<Item = Event>, then .map(Ok)
│ -> Result<Event, Infallible>
│ Return: Sse<impl Stream<Item = Result<Event, Infallible>>>
└── YES (a database cursor, a broadcast::Receiver, a channel)
-> map each item into Result<Event, YourError>
YourError MUST satisfy Into<BoxError>
Return: Sse<impl Stream<Item = Result<Event, axum::BoxError>>>
Sse::new requires S: TryStream<Ok = Event>, which is exactly
Stream<Item = Result<Event, E>>. A bare Stream<Item = Event> does NOT
satisfy this bound. This is the single most common SSE compile error.
Do I need KeepAlive
Will this stream ever stay idle longer than a few seconds?
├── YES (almost always: event feeds, notifications, log tails)
│ -> ALWAYS attach .keep_alive(KeepAlive::new())
└── NO (a short fixed burst that completes in under a second)
-> KeepAlive is optional, but attaching it is still safe
When unsure, attach KeepAlive. The default 15-second comment interval is a
safe baseline and costs nothing on an active stream.
Patterns
Pattern: minimal infallible SSE handler
The canonical shape. Verified against the official tokio-rs/axum SSE example.
use axum::response::sse::{Event, KeepAlive, Sse};
use futures_util::stream::{self, Stream};
use std::{convert::Infallible, time::Duration};
use tokio_stream::StreamExt as _;
asyncfnsse_handler() -> Sse<implStream<Item = Result<Event, Infallible>>> {
letstream = stream::repeat_with(|| Event::default().data("hi!"))
.map(Ok) // Event -> Ok(Event); error type = Infallible
.throttle(Duration::from_secs(1)); // pace the otherwise-instant stream
Sse::new(stream).keep_alive(KeepAlive::default())
}
// Router::new().route("/sse", get(sse_handler)) // identical on axum 0.7 and 0.8
.map(Ok) is load-bearing: it converts Stream<Item = Event> into
Stream<Item = Result<Event, Infallible>>, which satisfies the
TryStream<Ok = Event> bound on Sse::new.
Pattern: build a multi-field Event
Each single-use builder method is called at most once per Event.
letevt = Event::default()
.event("user-joined") // EventSource: addEventListener("user-joined", ...)
.id("42") // becomes the client's lastEventId, resent on reconnect
.data("Alice joined the room"); // the visible payload
NEVER call .data, .event, .id, or .retry twice on the same Event:
each panics on the second call. Build one fresh Event per logical message.
Only .comment may be called repeatedly.
Pattern: fallible stream from a broadcast channel
When the stream genuinely can fail, map each item into Result<Event, E> where
E: Into<BoxError>.
use axum::response::sse::{Event, KeepAlive, Sse};
use tokio_stream::{wrappers::BroadcastStream, StreamExt as _};
use futures_util::Stream;
// rx: tokio::sync::broadcast::Receiver<String>, taken from shared Stateasyncfnevents(rx: tokio::sync::broadcast::Receiver<String>)
-> Sse<implStream<Item = Result<Event, axum::BoxError>>>
{
letstream = BroadcastStream::new(rx).map(|msg| {
msg.map(|text| Event::default().data(text)) // Ok branch -> Event
.map_err(axum::BoxError::from) // Err branch -> BoxError
});
Sse::new(stream).keep_alive(KeepAlive::new())
}
Pattern: JSON payload event
Event::json_data requires the json feature and returns Result because
serialization can fail.
A quiet stream that emits nothing for minutes looks dead to proxies and load
balancers, which then drop the idle connection. KeepAlive injects an invisible
comment record during idle gaps so the connection stays open and the client can
tell "live but quiet" from "dead".
use axum::response::sse::{Event, KeepAlive};
use std::time::Duration;
// default: 15-second interval, empty-comment payloadletka = KeepAlive::new();
// custom interval and a named-event payload instead of a bare commentletka = KeepAlive::new()
.interval(Duration::from_secs(10))
.event(Event::default().comment("ping"));
ALWAYS attach KeepAlive to a long-lived SSE stream. Lower the interval below
15 seconds only when a proxy has a shorter idle timeout.
Pattern: keep heavy work off the poll path
The SSE stream is polled on a Tokio worker thread. NEVER run blocking or
CPU-heavy work inside the stream's poll_next. ALWAYS do async work in a
tokio::spawn task and sync or CPU work in tokio::task::spawn_blocking, then
feed results through a channel and stream that channel as Events. See
references/examples.md for the channel-fed pattern.
Idle connection dropped by proxy; client cannot detect a dead stream
Attach KeepAlive
Call .data / .event / .id / .retry twice
Runtime panic
One builder chain per Event
Newline in .event / .id / .comment
Runtime panic
Only .data accepts newlines
Non-Send or non-'static stream (Rc, borrowed data)
Compile error on Sse::new
Own captured data; use Arc
Blocking work in the stream poll path
Stalls the Tokio worker
spawn / spawn_blocking + channel
Full anti-pattern analysis with WHY each fails: references/anti-patterns.md.
Reference Links
references/methods.md : complete API signatures for Sse, Event,
KeepAlive, the stream item type, feature flags, and BoxError.
references/examples.md : working version-annotated code, including the
verbatim official handler, a full router with main, the channel-fed
pattern, and the EventSource client side.
references/anti-patterns.md : the six SSE anti-patterns with root-cause
explanations and fixes.
Related skills:
axum-impl-websockets : the bidirectional alternative; use when the client
must also send messages.
axum-syntax-handlers : handler signatures, extractors, and return types.
