Axum Knowledge Patch

Covers Axum 0.8.0–0.8.5 (2025-01-01 through 2025-09-28). Claude Opus 4.6 knows Axum through 0.7.x. It is unaware of the 0.8 breaking changes and features below.

Index

Topic	Reference	Key features
Routing & handlers	references/routing-and-handlers.md	Path syntax /{param}, reset_fallback, method_not_allowed_fallback, CONNECT, Sync requirement
Extractors	references/extractors.md	No #[async_trait], OptionalFromRequestParts, Option<Json<T>>, Option<Multipart>
WebSockets & responses	references/websockets-and-responses.md	Utf8Bytes/Bytes messages, HTTP/2 WS, NoContent, SSE binary, ResponseAxumBodyLayer

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0.7 → 0.8 Migration — Breaking Changes

What changed	Before (0.7)	After (0.8)
Path params	/:id, /*path	/{id}, /{*path} (old syntax panics)
Extractor traits	#[async_trait] impl FromRequestParts	Native async fn (remove #[async_trait])
Option<T> extraction	Swallows all errors as None	Requires OptionalFromRequestParts; invalid → rejection
WS message types	String / Vec<u8>	Utf8Bytes / Bytes
WebSocket::close	Method on socket	Removed — send Message::Close explicitly
HTTP/2 WebSockets	Not supported	Use any(ws_handler) instead of get(ws_handler)
Handler trait bounds	Send	Send + Sync
Host extractor	axum::extract::Host	Moved to axum-extra
Serve::tcp_nodelay	Method on Serve	Removed — use serve::ListenerExt

Path Parameter Syntax (0.8.0)

The most common migration issue. Old colon/star syntax panics at runtime.

// Before (0.7)
Router::new().route("/users/:id/files/*path", get(handler));

// After (0.8)
Router::new().route("/users/{id}/files/{*path}", get(handler));

Escape literal braces with {{ / }}.

See references/routing-and-handlers.md for details.

Extractor Trait Migration (0.8.0)

Remove #[async_trait] from custom FromRequestParts / FromRequest impls — Axum 0.8 uses native async fn in traits (RPITIT):

// Before (0.7)
#[async_trait]
impl<S> FromRequestParts<S> for MyExtractor where S: Send + Sync {
    type Rejection = StatusCode;
    async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
        todo!()
    }
}

// After (0.8) — remove #[async_trait], signature unchanged
impl<S> FromRequestParts<S> for MyExtractor where S: Send + Sync {
    type Rejection = StatusCode;
    async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
        todo!()
    }
}

See references/extractors.md for OptionalFromRequestParts and optional extractors.

Optional Extractors (0.8.0–0.8.5)

Option<T> extraction changed: T must implement OptionalFromRequestParts (or OptionalFromRequest). Missing value → None, invalid value → rejection error.

Built-in support added incrementally:

Extractor	Option<T> support	Version
Json<T>	OptionalFromRequest	0.8.3
Extension<T>	OptionalFromRequestParts	0.8.3
Multipart	OptionalFromRequest	0.8.5

async fn handler(body: Option<Json<MyPayload>>) -> impl IntoResponse {
    match body {
        Some(Json(payload)) => { /* process payload */ }
        None => { /* no body sent */ }
    }
}

WebSocket Changes (0.8.0)

Message types changed from String/Vec<u8> to Utf8Bytes/Bytes:

// Before (0.7)
Message::Text(String)
Message::Binary(Vec<u8>)

// After (0.8)
Message::Text(Utf8Bytes)
Message::Binary(Bytes)

WebSocket::close removed — send close messages explicitly:

socket.send(Message::Close(None)).await.ok();

For HTTP/2 WebSocket support, use any() instead of get():

Router::new().route("/ws", any(ws_handler));

See references/websockets-and-responses.md for selected_protocol and more.

New APIs

API	Version	Description
Router::method_not_allowed_fallback	0.8.0	Custom handler when path matches but method doesn't
NoContent	0.8.0	Shortcut response for StatusCode::NO_CONTENT
routing::connect	0.8.0	CONNECT method support
Router::reset_fallback	0.8.4	Remove a fallback, reverting to default 404
WebSocketUpgrade::selected_protocol	0.8.4	Get the subprotocol selected during WS handshake
Event::bytes()	0.8.5	Binary data in SSE events
ResponseAxumBodyLayer	0.8.5	Map any body type to axum::body::Body

Quick Examples

use axum::response::NoContent;

// NoContent response
async fn delete_item() -> NoContent {
    NoContent
}

// Router with method_not_allowed_fallback and reset_fallback
let api = Router::new()
    .route("/items", get(list_items).post(create_item))
    .fallback(api_fallback);

let app = Router::new()
    .nest("/api", api.reset_fallback())
    .method_not_allowed_fallback(|| async { "Method not supported" })
    .fallback(global_fallback);

// SSE with binary data (0.8.5)
use axum::response::sse::{Event, Sse};
let event = Event::default().bytes(my_bytes);

// ResponseAxumBodyLayer — normalize body types from tower services (0.8.5)
use axum::middleware::ResponseAxumBodyLayer;
let app = Router::new()
    .route("/", get(handler))
    .layer(ResponseAxumBodyLayer::new());

Reference Files

File	Contents
routing-and-handlers.md	Path syntax, router methods, CONNECT, Sync requirement, serve changes
extractors.md	async_trait removal, OptionalFromRequestParts, optional extractor support
websockets-and-responses.md	WS message types, HTTP/2 WS, NoContent, SSE binary, ResponseAxumBodyLayer