tokio-rs/tokio · error · syn::Error

The runtime flavor `multi_thread` requires the `rt-multi-thr

Error message

The runtime flavor `multi_thread` requires the `rt-multi-thread` feature.

What it means

Companion to error 15, this message (entry.rs:213) fires when the user *explicitly* set `flavor = "multi_thread"` but the `rt-multi-thread` feature is disabled. The flavor choice and the enabled feature must agree.

Source

Thrown at tokio-macros/src/entry.rs:213

        let flavor = self.flavor.unwrap_or(self.default_flavor);

        let worker_threads = match (flavor, self.worker_threads) {
            (F::CurrentThread | F::Local, Some((_, worker_threads_span))) => {
                let msg = format!(
                    "The `worker_threads` option requires the `multi_thread` runtime flavor. Use `#[{}(flavor = \"multi_thread\")]`",
                    self.macro_name(),
                );
                return Err(syn::Error::new(worker_threads_span, msg));
            }
            (F::CurrentThread | F::Local, None) => None,
            (F::Threaded, worker_threads) if self.rt_multi_thread_available => {
                worker_threads.map(|(val, _span)| val)
            }
            (F::Threaded, _) => {
                let msg = if self.flavor.is_none() {
                    "The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled."
                } else {
                    "The runtime flavor `multi_thread` requires the `rt-multi-thread` feature."
                };
                return Err(syn::Error::new(Span::call_site(), msg));
            }
        };

        let start_paused = match (flavor, self.start_paused) {
            (F::Threaded, Some((_, start_paused_span))) => {
                let msg = format!(
                    "The `start_paused` option requires the `current_thread` runtime flavor. Use `#[{}(flavor = \"current_thread\")]`",
                    self.macro_name(),
                );
                return Err(syn::Error::new(start_paused_span, msg));
            }
            (F::CurrentThread | F::Local, Some((start_paused, _))) => Some(start_paused),
            (_, None) => None,
        };

        let unhandled_panic = match (flavor, self.unhandled_panic) {

View on GitHub (pinned to 625954f365)

Solutions

1. Add `rt-multi-thread` to tokio's features in `Cargo.toml`.
2. Or change the attribute to `flavor = "current_thread"` if multi-threading is not needed.

Example fix

# before — Cargo.toml
tokio = { version = "1", features = ["macros"] }
// src/main.rs: #[tokio::main(flavor = "multi_thread")] async fn main() {}

# after — Cargo.toml
tokio = { version = "1", features = ["rt-multi-thread", "macros"] }

When it happens

Trigger: Writing `#[tokio::main(flavor = "multi_thread")]` while `Cargo.toml` lists tokio without `rt-multi-thread`.

Common situations: Manually setting flavor to multi-thread on a dependency that was feature-gated down for size; CI using a minimal feature set.

Related errors

• The `worker_threads` option requires the `multi_thread` runt
• The default runtime flavor is `multi_thread`, but the `rt-mu
• The `start_paused` option requires the `current_thread` runt
• The `unhandled_panic` option requires the `current_thread` r
• `name` set multiple times.

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/ea6d2f63f7a5c632. Report an issue: GitHub.