fix: explicitly set Content-Type to text/plain for json serialization mode

Serialization mode:"json" uses Seroval special serialization format that is not valid JSON. Some runtimes (e.g. AWS Lambda) default to `application/json` when no `Content-Type` is set,  causing downstream `JSON.parse` errors such as:
`SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data.`

When using "json" serialization mode, explicitly set `Content-Type` to `text/plain` for server-function responses to prevent incorrect parsing by runtimes. This restores the 2.x behavior, is safer for downstream environments, and is recommended best practice.

Author: yinonburgansky

.changeset/blue-dots-march.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/start": patch
+---
+
+Fixed an issue where runtimes like AWS Lambda would default to `application/json` when no header was present, causing parsing errors in `json` serialization mode. To ensure consistent behavior, the `Content-Type` is now explicitly set to `text/plain`.

packages/start/src/runtime/server-handler.ts

@@ -154,6 +154,10 @@ async function handleServerFunction(h3Event: HTTPEvent) {
       setHeader(h3Event, "content-type", "text/javascript");
       return serializeToJSStream(instance, result);
     }
+    // Explicitly set the Content-Type to avoid runtimes (e.g., AWS Lambda)
+    // that default to `application/json`, which can break serialization
+    // when the SEROVAL output is not valid JSON.
+    setHeader(h3Event, "content-type", "text/plain");
     return serializeToJSONStream(result);
   } catch (x) {
     if (x instanceof Response) {
@@ -181,6 +185,10 @@ async function handleServerFunction(h3Event: HTTPEvent) {
         setHeader(h3Event, "content-type", "text/javascript");
         return serializeToJSStream(instance, x);
       }
+      // Explicitly set the Content-Type to avoid runtimes (e.g., AWS Lambda)
+      // that default to `application/json`, which can break serialization
+      // when the SEROVAL output is not valid JSON.
+      setHeader(h3Event, "content-type", "text/plain");
       return serializeToJSONStream(x);
     }
     return x;