fullstack: server fn with `-> Result<SetHeader<SetCookie>>` always returns `Some(SetHeader { data: None })`

[jarjk]

Problem

fullstack: server fn with -> Result<SetHeader<SetCookie>> always returns Some(Ok(SetHeader { data: None })) to client, browser kinda receives the proper HttpError's message as response json, but with 500 as code on Err no matter what.

Steps To Reproduce

check out the example

dioxus/examples/07-fullstack/login_form.rs

Lines 75 to 90 in 2da0193

	/// In our `login` form, we'll return a `SetCookie` header if the login is successful.
	///
	/// This will set a cookie in the user's browser that can be used for subsequent authenticated requests.
	/// The `SetHeader::new()` method takes anything that can be converted into a `HeaderValue`.
	///
	/// We can set multiple headers by returning a tuple of `SetHeader` types, or passing in a tuple
	/// of headers to `SetHeader::new()`.
	#[post("/api/login")]
	async fn login(form: Form<LoginForm>) -> Result<SetHeader<SetCookie>> {
	// Verify the username and password. In a real application, you'd check these against a database.
	if form.0.username == "admin" && form.0.password == "password" {
	return Ok(SetHeader::new(format!("auth-demo={};", &*THIS_SESSION_ID))?);
	}
	HttpError::unauthorized("Invalid username or password")?
	}

Steps to reproduce the behavior:

• git clone https://github.com/dioxuslabs/dioxus
• add a pre on line 38, just like

  dioxus/examples/07-fullstack/login_form.rs

  Line 37 in 2da0193

  pre { "Response from locked API: {fetch_sensitive.value():?}"}

  but with fetch_login
• dx serve --example login_form
• check login thingy

Expected behavior

make the dioxus client receive the proper Result<_, _> to be able to handle the Err properly, display the message to the user if necessary

Screenshots

Environment:

• Dioxus version: 0.7.2 (35351eb)
• Rust version: 1.92.0 (ded5c06cf 2025-12-08)
• OS info: aarch64 macos 26.1, tested on x86_64 (arch) linux as well, most probably not os specific
• App platform: fullstack (web or desktop+server)