fix: set Content-Type header for server function errors (closes #4209) (#4215)

spencewenski · gbj · commit f06e0586860e · 2025-08-29T17:03:50.000-04:00
diff --git a/server_fn/src/error.rs b/server_fn/src/error.rs
@@ -568,7 +568,7 @@ pub trait FromServerFnError: std::fmt::Debug + Sized + 'static {
     /// Converts a [`ServerFnErrorErr`] into the application-specific custom error type.
     fn from_server_fn_error(value: ServerFnErrorErr) -> Self;
 
-    /// Converts the custom error type to a [`String`].
+    /// Serializes the custom error type to bytes, according to the encoding given by `Self::Encoding`.
     fn ser(&self) -> Bytes {
         Self::Encoder::encode(self).unwrap_or_else(|e| {
             Self::Encoder::encode(&Self::from_server_fn_error(
@@ -581,7 +581,7 @@ pub trait FromServerFnError: std::fmt::Debug + Sized + 'static {
         })
     }
 
-    /// Deserializes the custom error type from a [`&str`].
+    /// Deserializes the custom error type, according to the encoding given by `Self::Encoding`.
     fn de(data: Bytes) -> Self {
         Self::Encoder::decode(data).unwrap_or_else(|e| {
             ServerFnErrorErr::Deserialization(e.to_string()).into_app_error()
diff --git a/server_fn/src/lib.rs b/server_fn/src/lib.rs
@@ -307,16 +307,18 @@ pub trait ServerFn: Send + Sized {
                     .await
                     .map(|res| (res, None))
                     .unwrap_or_else(|e| {
-                        (
+                        let mut response =
                             <<Self as ServerFn>::Server as crate::Server<
                                 Self::Error,
                                 Self::InputStreamError,
                                 Self::OutputStreamError,
                             >>::Response::error_response(
                                 Self::PATH, e.ser()
-                            ),
-                            Some(e),
-                        )
+                            );
+                        let content_type =
+                    <Self::Error as FromServerFnError>::Encoder::CONTENT_TYPE;
+                        response.content_type(content_type);
+                        (response, Some(e))
                     });
 
             // if it accepts HTML, we'll redirect to the Referer
diff --git a/server_fn/src/middleware/mod.rs b/server_fn/src/middleware/mod.rs
@@ -72,6 +72,10 @@ mod axum {
             let inner = self.call(req);
             Box::pin(async move {
                 inner.await.unwrap_or_else(|e| {
+                    // TODO: This does not set the Content-Type on the response. Doing so will
+                    //  require a breaking change in order to get the correct encoding from the
+                    //  error's `FromServerFnError::Encoder::CONTENT_TYPE` impl.
+                    //  Note: This only applies to middleware errors.
                     let err =
                         ser(ServerFnErrorErr::MiddlewareError(e.to_string()));
                     Response::<Body>::error_response(&path, err)
@@ -149,6 +153,10 @@ mod actix {
             let inner = self.call(req);
             Box::pin(async move {
                 inner.await.unwrap_or_else(|e| {
+                    // TODO: This does not set the Content-Type on the response. Doing so will
+                    //  require a breaking change in order to get the correct encoding from the
+                    //  error's `FromServerFnError::Encoder::CONTENT_TYPE` impl.
+                    //  Note: This only applies to middleware errors.
                     let err =
                         ser(ServerFnErrorErr::MiddlewareError(e.to_string()));
                     ActixResponse::error_response(&path, err).take()
diff --git a/server_fn/src/response/actix.rs b/server_fn/src/response/actix.rs
@@ -5,7 +5,7 @@ use crate::error::{
 use actix_web::{
     http::{
         header,
-        header::{HeaderValue, LOCATION},
+        header::{HeaderValue, CONTENT_TYPE, LOCATION},
         StatusCode,
     },
     HttpResponse,
@@ -80,6 +80,12 @@ impl Res for ActixResponse {
         ))
     }
 
+    fn content_type(&mut self, content_type: &str) {
+        if let Ok(content_type) = HeaderValue::from_str(content_type) {
+            self.0.headers_mut().insert(CONTENT_TYPE, content_type);
+        }
+    }
+
     fn redirect(&mut self, path: &str) {
         if let Ok(path) = HeaderValue::from_str(path) {
             *self.0.status_mut() = StatusCode::FOUND;
diff --git a/server_fn/src/response/generic.rs b/server_fn/src/response/generic.rs
@@ -100,6 +100,13 @@ impl Res for Response<Body> {
             .unwrap()
     }
 
+    fn content_type(&mut self, content_type: &str) {
+        if let Ok(content_type) = HeaderValue::from_str(content_type) {
+            self.headers_mut()
+                .insert(header::CONTENT_TYPE, content_type);
+        }
+    }
+
     fn redirect(&mut self, path: &str) {
         if let Ok(path) = HeaderValue::from_str(path) {
             self.headers_mut().insert(header::LOCATION, path);
diff --git a/server_fn/src/response/http.rs b/server_fn/src/response/http.rs
@@ -60,6 +60,13 @@ impl Res for Response<Body> {
             .unwrap()
     }
 
+    fn content_type(&mut self, content_type: &str) {
+        if let Ok(content_type) = HeaderValue::from_str(content_type) {
+            self.headers_mut()
+                .insert(header::CONTENT_TYPE, content_type);
+        }
+    }
+
     fn redirect(&mut self, path: &str) {
         if let Ok(path) = HeaderValue::from_str(path) {
             self.headers_mut().insert(header::LOCATION, path);
diff --git a/server_fn/src/response/mod.rs b/server_fn/src/response/mod.rs
@@ -37,9 +37,14 @@ where
 
 /// Represents the response as created by the server;
 pub trait Res {
-    /// Converts an error into a response, with a `500` status code and the error text as its body.
+    /// Converts an error into a response, with a `500` status code and the error as its body.
     fn error_response(path: &str, err: Bytes) -> Self;
-
+    /// Set the `Content-Type` header for the response.
+    fn content_type(&mut self, #[allow(unused_variables)] content_type: &str) {
+        // TODO 0.9: remove this method and default implementation. It is only included here
+        //  to allow setting the `Content-Type` header for error responses without requiring a
+        //  semver-incompatible change.
+    }
     /// Redirect the response by setting a 302 code and Location header.
     fn redirect(&mut self, path: &str);
 }
@@ -103,6 +108,10 @@ impl Res for BrowserMockRes {
         unreachable!()
     }
 
+    fn content_type(&mut self, _content_type: &str) {
+        unreachable!()
+    }
+
     fn redirect(&mut self, _path: &str) {
         unreachable!()
     }
