What causes "`unhandled_panic` set multiple times." in tokio-rs/tokio?

Remove the duplicate `unhandled_panic`, keeping the single intended behavior.

tokio-rs/tokio · error · syn::Error

`unhandled_panic` set multiple times.

Error message

`unhandled_panic` set multiple times.

What it means

`set_unhandled_panic` (entry.rs:171) records the first `unhandled_panic = "..."` (one of `ignore` / `shutdown_runtime`, controlling behavior when a task panics) and rejects a second one. The option is only valid for the `current_thread` flavor.

Source

Thrown at tokio-macros/src/entry.rs:173

    fn set_crate_name(&mut self, name: syn::Lit, span: Span) -> Result<(), syn::Error> {
        if self.crate_name.is_some() {
            return Err(syn::Error::new(span, "`crate` set multiple times."));
        }
        let name_path = parse_path(name, span, "crate")?;
        self.crate_name = Some(name_path);
        Ok(())
    }

    fn set_unhandled_panic(
        &mut self,
        unhandled_panic: syn::Lit,
        span: Span,
    ) -> Result<(), syn::Error> {
        if self.unhandled_panic.is_some() {
            return Err(syn::Error::new(
                span,
                "`unhandled_panic` set multiple times.",
            ));
        }

        let unhandled_panic = parse_string(unhandled_panic, span, "unhandled_panic")?;
        let unhandled_panic =
            UnhandledPanic::from_str(&unhandled_panic).map_err(|err| syn::Error::new(span, err))?;
        self.unhandled_panic = Some((unhandled_panic, span));
        Ok(())
    }

    fn macro_name(&self) -> &'static str {
        if self.is_test {
            "tokio::test"
        } else {
            "tokio::main"
        }
    }

View on GitHub (pinned to 625954f365)

Solutions

Remove the duplicate `unhandled_panic`, keeping the single intended behavior.

Example fix

// before
#[tokio::main(flavor = "current_thread", unhandled_panic = "ignore", unhandled_panic = "shutdown_runtime")]
async fn main() {}

// after
#[tokio::main(flavor = "current_thread", unhandled_panic = "shutdown_runtime")]
async fn main() {}

Defensive patterns

Strategy: validation

Prevention

Set `unhandled_panic` at most once and only with `flavor = "current_thread"`.
Choose `ignore` to keep the runtime alive or `shutdown_runtime` to tear it down on task panic.
Document the chosen policy for your team.

When it happens

Trigger: Writing `#[tokio::main(unhandled_panic = "ignore", unhandled_panic = "shutdown_runtime")]`.

Common situations: Copy-paste; iterating on panic policy and forgetting to remove the previous setting.

Related errors

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/d423470905cbb82b. Report an issue: GitHub.