actix/actix-web · critical

actix-http client only supports versions http/1.1 & http/2

Error message

actix-http client only supports versions http/1.1 & http/2

What it means

The AWC client `Connector::max_http_version()` only accepts `HTTP_11` or `HTTP_2`. At awc/src/client/connector.rs:336-343 any other `http::Version` (e.g. `HTTP_09`, `HTTP_10`, `HTTP_3`) hits `unimplemented!()` and panics during connector construction. This is a builder-time panic, so it typically crashes at startup.

Source

Thrown at awc/src/client/connector.rs:341

    /// ```
    #[cfg(feature = "rustls-0_23")]
    pub fn rustls_0_23(
        mut self,
        connector: std::sync::Arc<actix_tls::connect::rustls_0_23::reexports::ClientConfig>,
    ) -> Self {
        self.tls = OurTlsConnector::Rustls023(connector);
        self
    }

    /// Sets maximum supported HTTP major version.
    ///
    /// Supported versions are HTTP/1.1 and HTTP/2.
    pub fn max_http_version(mut self, val: http::Version) -> Self {
        let versions = match val {
            http::Version::HTTP_11 => vec![b"http/1.1".to_vec()],
            http::Version::HTTP_2 => vec![b"h2".to_vec(), b"http/1.1".to_vec()],
            _ => {
                unimplemented!("actix-http client only supports versions http/1.1 & http/2")
            }
        };
        self.tls = Connector::build_tls(versions);
        self
    }

    /// Sets the initial window size (in bytes) for HTTP/2 stream-level flow control for received
    /// data.
    ///
    /// The default value is 65,535 and is good for APIs, but not for big objects.
    pub fn initial_window_size(mut self, size: u32) -> Self {
        self.config.stream_window_size = size;
        self
    }

    /// Sets the initial window size (in bytes) for HTTP/2 connection-level flow control for
    /// received data.
    ///

View on GitHub (pinned to 937960ca67)

Solutions

1. Use `http::Version::HTTP_11` (HTTP/1.1 only) or `http::Version::HTTP_2` (HTTP/2 with HTTP/1.1 fallback).
2. If you need HTTP/3, note that AWC does not support it; choose a different client crate.

Example fix

// before
let connector = Connector::new()
    .max_http_version(http::Version::HTTP_10);

// after
let connector = Connector::new()
    .max_http_version(http::Version::HTTP_2);

// Guard before calling max_http_version.
fn assert_supported_version(v: http::Version) {
    assert!(
        matches!(v, http::Version::HTTP_11 | http::Version::HTTP_2),
        "AWC only supports HTTP/1.1 and HTTP/2"
    );
}
// assert_supported_version(http::Version::HTTP_2);

fn is_supported_version(v: http::Version) -> bool {
    matches!(v, http::Version::HTTP_11 | http::Version::HTTP_2)
}

When it happens

Trigger: Calling `.max_http_version(http::Version::HTTP_10)` or `HTTP_3` on a client `Connector`.

Common situations: Assuming the client supports HTTP/1.0 or HTTP/3, or passing a version constant by mistake.

Related errors

• All default headers must be added before cloning.
• Unsupported HTTP version: {:?}.
• Invalid header name
• Invalid header value
• cannot reuse response builder

AI-assisted analysis of actix/actix-web@937960ca67 (2026-08-06). Data as JSON: /data/errors/0f4d534b6d2e1518.json. Report an issue: GitHub.