From 77643e41c87b9e2a9dd896ad3e90311bef224ef5 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 08:53:42 +0800
Subject: [PATCH 01/20] add seroval json mode

---
 packages/start/src/config/index.ts            |   5 +
 packages/start/src/server/serialization.ts    | 253 ++++++++++++++++++
 .../src/server/server-functions-handler.ts    | 155 ++++-------
 packages/start/src/server/server-runtime.ts   | 183 +++----------
 4 files changed, 345 insertions(+), 251 deletions(-)
 create mode 100644 packages/start/src/server/serialization.ts

diff --git a/packages/start/src/config/index.ts b/packages/start/src/config/index.ts
index 4b1c82179..869aef0cf 100644
--- a/packages/start/src/config/index.ts
+++ b/packages/start/src/config/index.ts
@@ -21,6 +21,10 @@ export interface SolidStartOptions {
   routeDir?: string;
   extensions?: string[];
   middleware?: string;
+  serialization?: {
+    // This only matters for server function responses
+    mode?: 'js' | 'json';
+  };
 }
 
 const absolute = (path: string, root: string) =>
@@ -131,6 +135,7 @@ export function solidStart(options?: SolidStartOptions): Array<PluginOption> {
             "import.meta.env.START_APP_ENTRY": JSON.stringify(appEntryPath),
             "import.meta.env.START_CLIENT_ENTRY": JSON.stringify(handlers.client),
             "import.meta.env.START_DEV_OVERLAY": JSON.stringify(start.devOverlay),
+            "import.meta.env.SEROVAL_MODE": JSON.stringify(start.serialization?.mode || 'json'),
           },
           builder: {
             sharedPlugins: true,
diff --git a/packages/start/src/server/serialization.ts b/packages/start/src/server/serialization.ts
new file mode 100644
index 000000000..c157d157e
--- /dev/null
+++ b/packages/start/src/server/serialization.ts
@@ -0,0 +1,253 @@
+import {
+  crossSerializeStream,
+  deserialize,
+  Feature,
+  fromCrossJSON,
+  getCrossReferenceHeader,
+  type SerovalNode,
+  toCrossJSONStream,
+} from "seroval";
+import {
+  AbortSignalPlugin,
+  CustomEventPlugin,
+  DOMExceptionPlugin,
+  EventPlugin,
+  FormDataPlugin,
+  HeadersPlugin,
+  ReadableStreamPlugin,
+  RequestPlugin,
+  ResponsePlugin,
+  URLPlugin,
+  URLSearchParamsPlugin,
+} from "seroval-plugins/web";
+
+// TODO(Alexis): if we can, allow providing an option to extend these.
+const DEFAULT_PLUGINS = [
+  AbortSignalPlugin,
+  CustomEventPlugin,
+  DOMExceptionPlugin,
+  EventPlugin,
+  FormDataPlugin,
+  HeadersPlugin,
+  ReadableStreamPlugin,
+  RequestPlugin,
+  ResponsePlugin,
+  URLSearchParamsPlugin,
+  URLPlugin,
+];
+const MAX_SERIALIZATION_DEPTH_LIMIT = 64;
+const DISABLED_FEATURES = Feature.RegExp;
+
+/**
+ * Alexis:
+ *
+ * A "chunk" is a piece of data emitted by the streaming serializer.
+ * Each chunk is represented by a 32-bit value (encoded in hexadecimal),
+ * followed by the encoded string (8-bit representation). This format
+ * is important so we know how much of the chunk being streamed we
+ * are expecting before parsing the entire string data.
+ *
+ * This is sort of a bootleg "multipart/form-data" except it's bad at
+ * handling File/Blob LOL
+ *
+ * The format is as follows:
+ * ;0xFFFFFFFF;<string data>
+ */
+function createChunk(data: string): Uint8Array {
+  const encodeData = new TextEncoder().encode(data);
+  const bytes = encodeData.length;
+  const baseHex = bytes.toString(16);
+  const totalHex = "00000000".substring(0, 8 - baseHex.length) + baseHex; // 32-bit
+  const head = new TextEncoder().encode(`;0x${totalHex};`);
+
+  const chunk = new Uint8Array(12 + bytes);
+  chunk.set(head);
+  chunk.set(encodeData, 12);
+  return chunk;
+}
+
+export function serializeToJSStream(id: string, value: any) {
+  return new ReadableStream({
+    start(controller) {
+      crossSerializeStream(value, {
+        scopeId: id,
+        plugins: DEFAULT_PLUGINS,
+        onSerialize(data: string, initial: boolean) {
+          controller.enqueue(
+            createChunk(
+              initial ? `(${getCrossReferenceHeader(id)},${data})` : data,
+            ),
+          );
+        },
+        onDone() {
+          controller.close();
+        },
+        onError(error: any) {
+          controller.error(error);
+        },
+      });
+    },
+  });
+}
+
+export function serializeToJSONStream(value: any) {
+  return new ReadableStream({
+    start(controller) {
+      toCrossJSONStream(value, {
+        disabledFeatures: DISABLED_FEATURES,
+        depthLimit: MAX_SERIALIZATION_DEPTH_LIMIT,
+        plugins: DEFAULT_PLUGINS,
+        onParse(node) {
+          controller.enqueue(createChunk(JSON.stringify(node)));
+        },
+        onDone() {
+          controller.close();
+        },
+        onError(error) {
+          controller.error(error);
+        },
+      });
+    },
+  });
+}
+
+class SerovalChunkReader {
+  reader: ReadableStreamDefaultReader<Uint8Array>;
+  buffer: Uint8Array;
+  done: boolean;
+  constructor(stream: ReadableStream<Uint8Array>) {
+    this.reader = stream.getReader();
+    this.buffer = new Uint8Array(0);
+    this.done = false;
+  }
+
+  async readChunk() {
+    // if there's no chunk, read again
+    const chunk = await this.reader.read();
+    if (!chunk.done) {
+      // repopulate the buffer
+      const newBuffer = new Uint8Array(this.buffer.length + chunk.value.length);
+      newBuffer.set(this.buffer);
+      newBuffer.set(chunk.value, this.buffer.length);
+      this.buffer = newBuffer;
+    } else {
+      this.done = true;
+    }
+  }
+
+  async next(): Promise<
+    { done: true; value: undefined } | { done: false; value: string }
+  > {
+    // Check if the buffer is empty
+    if (this.buffer.length === 0) {
+      // if we are already done...
+      if (this.done) {
+        return {
+          done: true,
+          value: undefined,
+        };
+      }
+      // Otherwise, read a new chunk
+      await this.readChunk();
+      return await this.next();
+    }
+    // Read the "byte header"
+    // The byte header tells us how big the expected data is
+    // so we know how much data we should wait before we
+    // deserialize the data
+    const head = new TextDecoder().decode(this.buffer.subarray(1, 11));
+    const bytes = Number.parseInt(head, 16); // ;0x00000000;
+    // Check if the buffer has enough bytes to be parsed
+    while (bytes > this.buffer.length - 12) {
+      // If it's not enough, and the reader is done
+      // then the chunk is invalid.
+      if (this.done) {
+        throw new Error("Malformed server function stream.");
+      }
+      // Otherwise, we read more chunks
+      await this.readChunk();
+    }
+    // Extract the exact chunk as defined by the byte header
+    const partial = new TextDecoder().decode(
+      this.buffer.subarray(12, 12 + bytes),
+    );
+    // The rest goes to the buffer
+    this.buffer = this.buffer.subarray(12 + bytes);
+
+    // Deserialize the chunk
+    return {
+      done: false,
+      value: partial,
+    };
+  }
+
+  async drain(interpret: (chunk: string) => void) {
+    while (true) {
+      const result = await this.next();
+      if (result.done) {
+        break;
+      } else {
+        interpret(result.value);
+      }
+    }
+  }
+}
+
+export async function serializeToJSONString(value: any) {
+  const response = new Response(serializeToJSONStream(value));
+  return await response.text();
+}
+
+export async function deserializeFromJSONString(json: string) {
+  const blob = new Response(json);
+  return await deserializeJSONStream(blob);
+}
+
+export async function deserializeJSONStream(response: Response | Request) {
+  if (!response.body) {
+    throw new Error("missing body");
+  }
+  const reader = new SerovalChunkReader(response.body);
+  const result = await reader.next();
+  if (!result.done) {
+    const refs = new Map();
+
+    function interpretChunk(chunk: string): unknown {
+      const value = fromCrossJSON(JSON.parse(chunk) as SerovalNode, {
+        refs,
+        disabledFeatures: DISABLED_FEATURES,
+        depthLimit: MAX_SERIALIZATION_DEPTH_LIMIT,
+        plugins: DEFAULT_PLUGINS,
+      });
+      return value;
+    }
+
+    void reader.drain(interpretChunk);
+
+    return interpretChunk(result.value);
+  }
+  return undefined;
+}
+
+export async function deserializeJSStream(id: string, response: Response) {
+  if (!response.body) {
+    throw new Error("missing body");
+  }
+  const reader = new SerovalChunkReader(response.body);
+
+  const result = await reader.next();
+
+  if (!result.done) {
+    reader.drain(deserialize).then(
+      () => {
+        // @ts-ignore
+        delete $R[id];
+      },
+      () => {
+        // no-op
+      },
+    );
+    return deserialize(result.value);
+  }
+  return undefined;
+}
diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index 160672684..e5912aaa1 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -1,74 +1,21 @@
-import { getServerFnById } from "solidstart:server-fn-manifest";
 import { parseSetCookie } from "cookie-es";
 import { type H3Event, parseCookies } from "h3";
-import { crossSerializeStream, fromJSON, getCrossReferenceHeader } from "seroval";
-import {
-  CustomEventPlugin,
-  DOMExceptionPlugin,
-  EventPlugin,
-  FormDataPlugin,
-  HeadersPlugin,
-  ReadableStreamPlugin,
-  RequestPlugin,
-  ResponsePlugin,
-  URLPlugin,
-  URLSearchParamsPlugin,
-} from "seroval-plugins/web";
 import { sharedConfig } from "solid-js";
 import { renderToString } from "solid-js/web";
 import { provideRequestEvent } from "solid-js/web/storage";
+import { getServerFnById } from "solidstart:server-fn-manifest";
 
 import { getFetchEvent, mergeResponseHeaders } from "./fetchEvent.ts";
 import { createPageEvent } from "./handler.ts";
+import {
+  deserializeFromJSONString,
+  deserializeJSONStream,
+  serializeToJSONStream,
+  serializeToJSStream,
+} from "./serialization.ts";
 import type { FetchEvent, PageEvent } from "./types.ts";
 import { getExpectedRedirectStatus } from "./util.ts";
 
-function createChunk(data: string) {
-  const encodeData = new TextEncoder().encode(data);
-  const bytes = encodeData.length;
-  const baseHex = bytes.toString(16);
-  const totalHex = "00000000".substring(0, 8 - baseHex.length) + baseHex; // 32-bit
-  const head = new TextEncoder().encode(`;0x${totalHex};`);
-
-  const chunk = new Uint8Array(12 + bytes);
-  chunk.set(head);
-  chunk.set(encodeData, 12);
-  return chunk;
-}
-
-function serializeToStream(id: string, value: any) {
-  return new ReadableStream({
-    start(controller) {
-      crossSerializeStream(value, {
-        scopeId: id,
-        plugins: [
-          CustomEventPlugin,
-          DOMExceptionPlugin,
-          EventPlugin,
-          FormDataPlugin,
-          HeadersPlugin,
-          ReadableStreamPlugin,
-          RequestPlugin,
-          ResponsePlugin,
-          URLSearchParamsPlugin,
-          URLPlugin,
-        ],
-        onSerialize(data: string, initial: boolean) {
-          controller.enqueue(
-            createChunk(initial ? `(${getCrossReferenceHeader(id)},${data})` : data),
-          );
-        },
-        onDone() {
-          controller.close();
-        },
-        onError(error: any) {
-          controller.error(error);
-        },
-      });
-    },
-  });
-}
-
 export async function handleServerFunction(h3Event: H3Event) {
   const event = getFetchEvent(h3Event);
   const request = event.request;
@@ -99,26 +46,10 @@ export async function handleServerFunction(h3Event: H3Event) {
   if (!instance || h3Event.method === "GET") {
     const args = url.searchParams.get("args");
     if (args) {
-      const json = JSON.parse(args);
-      (json.t
-        ? (fromJSON(json, {
-            plugins: [
-              CustomEventPlugin,
-              DOMExceptionPlugin,
-              EventPlugin,
-              FormDataPlugin,
-              HeadersPlugin,
-              ReadableStreamPlugin,
-              RequestPlugin,
-              ResponsePlugin,
-              URLSearchParamsPlugin,
-              URLPlugin,
-            ],
-          }) as any)
-        : json
-      ).forEach((arg: any) => {
+      const result = (await deserializeFromJSONString(args)) as any[];
+      for (const arg of result) {
         parsed.push(arg);
-      });
+      }
     }
   }
   if (h3Event.method === "POST") {
@@ -129,21 +60,8 @@ export async function handleServerFunction(h3Event: H3Event) {
       contentType?.startsWith("application/x-www-form-urlencoded")
     ) {
       parsed.push(await event.request.formData());
-    } else if (contentType?.startsWith("application/json")) {
-      parsed = fromJSON(await event.request.json(), {
-        plugins: [
-          CustomEventPlugin,
-          DOMExceptionPlugin,
-          EventPlugin,
-          FormDataPlugin,
-          HeadersPlugin,
-          ReadableStreamPlugin,
-          RequestPlugin,
-          ResponsePlugin,
-          URLSearchParamsPlugin,
-          URLPlugin,
-        ],
-      });
+    } else {
+      parsed = (await deserializeJSONStream(event.request.clone())) as any[];
     }
   }
   try {
@@ -178,9 +96,12 @@ export async function handleServerFunction(h3Event: H3Event) {
     // handle no JS success case
     if (!instance) return handleNoJS(result, request, parsed);
 
-    h3Event.res.headers.set("content-type", "text/javascript");
-
-    return serializeToStream(instance, result);
+    if (import.meta.env.SEROVAL_MODE === "js") {
+      h3Event.res.headers.set("content-type", "text/javascript");
+      return serializeToJSStream(instance, result);
+    }
+    h3Event.res.headers.set("content-type", "text/plain");
+    return serializeToJSONStream(result);
   } catch (x) {
     if (x instanceof Response) {
       if (singleFlight && instance) {
@@ -189,28 +110,41 @@ export async function handleServerFunction(h3Event: H3Event) {
       // forward headers
       if ((x as any).headers) mergeResponseHeaders(h3Event, (x as any).headers);
       // forward non-redirect statuses
-      if ((x as any).status && (!instance || (x as any).status < 300 || (x as any).status >= 400))
+      if (
+        (x as any).status &&
+        (!instance || (x as any).status < 300 || (x as any).status >= 400)
+      )
         h3Event.res.status = (x as any).status;
       if ((x as any).customBody) {
         x = (x as any).customBody();
       } else if ((x as any).body === undefined) x = null;
       h3Event.res.headers.set("X-Error", "true");
     } else if (instance) {
-      const error = x instanceof Error ? x.message : typeof x === "string" ? x : "true";
+      const error =
+        x instanceof Error ? x.message : typeof x === "string" ? x : "true";
 
       h3Event.res.headers.set("X-Error", error.replace(/[\r\n]+/g, ""));
     } else {
       x = handleNoJS(x, request, parsed, true);
     }
     if (instance) {
-      h3Event.res.headers.set("content-type", "text/javascript");
-      return serializeToStream(instance, x);
+      if (import.meta.env.SEROVAL_MODE === "js") {
+        h3Event.res.headers.set("content-type", "text/javascript");
+        return serializeToJSStream(instance, x);
+      }
+      h3Event.res.headers.set("content-type", "text/plain");
+      return serializeToJSONStream(x);
     }
     return x;
   }
 }
 
-function handleNoJS(result: any, request: Request, parsed: any[], thrown?: boolean) {
+function handleNoJS(
+  result: any,
+  request: Request,
+  parsed: any[],
+  thrown?: boolean,
+) {
   const url = new URL(request.url);
   const isError = result instanceof Error;
   let statusCode = 302;
@@ -220,7 +154,10 @@ function handleNoJS(result: any, request: Request, parsed: any[], thrown?: boole
     if (result.headers.has("Location")) {
       headers.set(
         `Location`,
-        new URL(result.headers.get("Location")!, url.origin + import.meta.env.BASE_URL).toString(),
+        new URL(
+          result.headers.get("Location")!,
+          url.origin + import.meta.env.BASE_URL,
+        ).toString(),
       );
       statusCode = getExpectedRedirectStatus(result);
     }
@@ -237,7 +174,10 @@ function handleNoJS(result: any, request: Request, parsed: any[], thrown?: boole
           result: isError ? result.message : result,
           thrown: thrown,
           error: isError,
-          input: [...parsed.slice(0, -1), [...parsed[parsed.length - 1].entries()]],
+          input: [
+            ...parsed.slice(0, -1),
+            [...parsed[parsed.length - 1].entries()],
+          ],
         }),
       )}; Secure; HttpOnly;`,
     );
@@ -263,7 +203,7 @@ function createSingleFlightHeaders(sourceEvent: FetchEvent) {
   // 	useH3Internals = true;
   // 	sourceEvent.nativeEvent.node.req.headers.cookie = "";
   // }
-  SetCookies.forEach(cookie => {
+  SetCookies.forEach((cookie) => {
     if (!cookie) return;
     const { maxAge, expires, name, value } = parseSetCookie(cookie);
     if (maxAge != null && maxAge <= 0) {
@@ -284,7 +224,10 @@ function createSingleFlightHeaders(sourceEvent: FetchEvent) {
 
   return headers;
 }
-async function handleSingleFlight(sourceEvent: FetchEvent, result: any): Promise<Response> {
+async function handleSingleFlight(
+  sourceEvent: FetchEvent,
+  result: any,
+): Promise<Response> {
   let revalidate: string[];
   let url = new URL(sourceEvent.request.headers.get("referer")!).toString();
   if (result instanceof Response) {
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index 8bbabe40b..d2be79ef9 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -1,122 +1,20 @@
 // @ts-ignore - seroval exports issue with NodeNext
-import { join } from "pathe";
-import { deserialize, toJSONAsync } from "seroval";
-import {
-  CustomEventPlugin,
-  DOMExceptionPlugin,
-  EventPlugin,
-  FormDataPlugin,
-  HeadersPlugin,
-  ReadableStreamPlugin,
-  RequestPlugin,
-  ResponsePlugin,
-  URLPlugin,
-  URLSearchParamsPlugin,
-} from "seroval-plugins/web";
 import { type Component } from "solid-js";
-
-class SerovalChunkReader {
-  reader: ReadableStreamDefaultReader<Uint8Array>;
-  buffer: Uint8Array;
-  done: boolean;
-  constructor(stream: ReadableStream<Uint8Array>) {
-    this.reader = stream.getReader();
-    this.buffer = new Uint8Array(0);
-    this.done = false;
-  }
-
-  async readChunk() {
-    // if there's no chunk, read again
-    const chunk = await this.reader.read();
-    if (!chunk.done) {
-      // repopulate the buffer
-      let newBuffer = new Uint8Array(this.buffer.length + chunk.value.length);
-      newBuffer.set(this.buffer);
-      newBuffer.set(chunk.value, this.buffer.length);
-      this.buffer = newBuffer;
-    } else {
-      this.done = true;
-    }
-  }
-
-  async next(): Promise<any> {
-    // Check if the buffer is empty
-    if (this.buffer.length === 0) {
-      // if we are already done...
-      if (this.done) {
-        return {
-          done: true,
-          value: undefined,
-        };
-      }
-      // Otherwise, read a new chunk
-      await this.readChunk();
-      return await this.next();
-    }
-    // Read the "byte header"
-    // The byte header tells us how big the expected data is
-    // so we know how much data we should wait before we
-    // deserialize the data
-    const head = new TextDecoder().decode(this.buffer.subarray(1, 11));
-    const bytes = Number.parseInt(head, 16); // ;0x00000000;
-    // Check if the buffer has enough bytes to be parsed
-    while (bytes > this.buffer.length - 12) {
-      // If it's not enough, and the reader is done
-      // then the chunk is invalid.
-      if (this.done) {
-        throw new Error("Malformed server function stream.");
-      }
-      // Otherwise, we read more chunks
-      await this.readChunk();
-    }
-    // Extract the exact chunk as defined by the byte header
-    const partial = new TextDecoder().decode(this.buffer.subarray(12, 12 + bytes));
-    // The rest goes to the buffer
-    this.buffer = this.buffer.subarray(12 + bytes);
-
-    // Deserialize the chunk
-    return {
-      done: false,
-      value: deserialize(partial),
-    };
-  }
-
-  async drain() {
-    while (true) {
-      const result = await this.next();
-      if (result.done) {
-        break;
-      }
-    }
-  }
-}
-
-async function deserializeStream(id: string, response: Response) {
-  if (!response.body) {
-    throw new Error("missing body");
-  }
-  const reader = new SerovalChunkReader(response.body);
-
-  const result = await reader.next();
-
-  if (!result.done) {
-    reader.drain().then(
-      () => {
-        // @ts-ignore
-        delete $R[id];
-      },
-      () => {
-        // no-op
-      },
-    );
-  }
-
-  return result.value;
-}
+import {
+  deserializeJSONStream,
+  deserializeJSStream,
+  serializeToJSONStream,
+  serializeToJSONString,
+} from "./serialization";
 
 let INSTANCE = 0;
 
-function createRequest(base: string, id: string, instance: string, options: RequestInit) {
+function createRequest(
+  base: string,
+  id: string,
+  instance: string,
+  options: RequestInit,
+) {
   return fetch(base, {
     method: "POST",
     ...options,
@@ -127,20 +25,6 @@ function createRequest(base: string, id: string, instance: string, options: Requ
     },
   });
 }
-
-const plugins = [
-  CustomEventPlugin,
-  DOMExceptionPlugin,
-  EventPlugin,
-  FormDataPlugin,
-  HeadersPlugin,
-  ReadableStreamPlugin,
-  RequestPlugin,
-  ResponsePlugin,
-  URLSearchParamsPlugin,
-  URLPlugin,
-];
-
 async function fetchServerFunction(
   base: string,
   id: string,
@@ -154,15 +38,18 @@ async function fetchServerFunction(
       ? createRequest(base, id, instance, { ...options, body: args[0] })
       : args.length === 1 && args[0] instanceof URLSearchParams
         ? createRequest(base, id, instance, {
-            ...options,
-            body: args[0],
-            headers: { ...options.headers, "Content-Type": "application/x-www-form-urlencoded" },
-          })
+          ...options,
+          body: args[0],
+          headers: {
+            ...options.headers,
+            "Content-Type": "application/x-www-form-urlencoded",
+          },
+        })
         : createRequest(base, id, instance, {
-            ...options,
-            body: JSON.stringify(await Promise.resolve(toJSONAsync(args, { plugins }))),
-            headers: { ...options.headers, "Content-Type": "application/json" },
-          }));
+          ...options,
+          body: serializeToJSONStream(args),
+          headers: { ...options.headers, "Content-Type": "application/json" },
+        }));
 
   if (
     response.headers.has("Location") ||
@@ -172,7 +59,8 @@ async function fetchServerFunction(
     if (response.body) {
       /* @ts-ignore-next-line */
       response.customBody = () => {
-        return deserializeStream(instance, response);
+        // TODO check for serialization mode
+        return deserializeJSStream(instance, response);
       };
     }
     return response;
@@ -184,8 +72,11 @@ async function fetchServerFunction(
     result = await response.text();
   } else if (contentType && contentType.startsWith("application/json")) {
     result = await response.json();
+  } else if (import.meta.env.SEROVAL_MODE === "js") {
+    // TODO check for serialization mode
+    result = await deserializeJSStream(instance, response);
   } else {
-    result = await deserializeStream(instance, response);
+    result = await deserializeJSONStream(response);
   }
   if (response.headers.has("X-Error")) {
     throw result;
@@ -197,7 +88,8 @@ export function createServerReference(id: string) {
   let baseURL = import.meta.env.BASE_URL ?? "/";
   if (!baseURL.endsWith("/")) baseURL += "/";
 
-  const fn = (...args: any[]) => fetchServerFunction(`${baseURL}_server`, id, {}, args);
+  const fn = (...args: any[]) =>
+    fetchServerFunction(`${baseURL}_server`, id, {}, args);
 
   return new Proxy(fn, {
     get(target, prop, receiver) {
@@ -211,15 +103,16 @@ export function createServerReference(id: string) {
         const url = `${baseURL}_server?id=${encodeURIComponent(id)}`;
         return (options: RequestInit) => {
           const fn = async (...args: any[]) => {
-            const encodeArgs = options.method && options.method.toUpperCase() === "GET";
+            const encodeArgs =
+              options.method && options.method.toUpperCase() === "GET";
             return fetchServerFunction(
               encodeArgs
                 ? url +
-                    (args.length
-                      ? `&args=${encodeURIComponent(
-                          JSON.stringify(await Promise.resolve(toJSONAsync(args, { plugins }))),
-                        )}`
-                      : "")
+                (args.length
+                  ? `&args=${encodeURIComponent(
+                    await serializeToJSONString(args),
+                  )}`
+                  : "")
                 : `${baseURL}_server`,
               id,
               options,

From 05fde61c36a21775945626ec9c36eb04f9112440 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 09:00:33 +0800
Subject: [PATCH 02/20] Update server-runtime.ts

---
 packages/start/src/server/server-runtime.ts | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index d2be79ef9..d2a51f2a7 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -59,8 +59,10 @@ async function fetchServerFunction(
     if (response.body) {
       /* @ts-ignore-next-line */
       response.customBody = () => {
-        // TODO check for serialization mode
-        return deserializeJSStream(instance, response);
+        if (import.meta.env.SEROVAL_MODE === "js") {
+          return deserializeJSStream(instance, response);
+        }
+        return deserializeJSONStream(response);
       };
     }
     return response;
@@ -73,7 +75,6 @@ async function fetchServerFunction(
   } else if (contentType && contentType.startsWith("application/json")) {
     result = await response.json();
   } else if (import.meta.env.SEROVAL_MODE === "js") {
-    // TODO check for serialization mode
     result = await deserializeJSStream(instance, response);
   } else {
     result = await deserializeJSONStream(response);

From c06b5d0df8bdc3009aaa1caeb5ae230112531b58 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 09:06:24 +0800
Subject: [PATCH 03/20] Create plenty-geese-enter.md

---
 .changeset/plenty-geese-enter.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/plenty-geese-enter.md

diff --git a/.changeset/plenty-geese-enter.md b/.changeset/plenty-geese-enter.md
new file mode 100644
index 000000000..27c7c5457
--- /dev/null
+++ b/.changeset/plenty-geese-enter.md
@@ -0,0 +1,5 @@
+---
+"@solidjs/start": minor
+---
+
+seroval json mode

From 90f5706a339dfd0f3a6a04ad086e6c1c7d43ed36 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 09:43:12 +0800
Subject: [PATCH 04/20] Fix client-server comms

---
 .../src/server/server-functions-handler.ts    |  4 ++--
 packages/start/src/server/server-runtime.ts   | 21 ++++++++++++-------
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index e5912aaa1..c536b940f 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -60,7 +60,7 @@ export async function handleServerFunction(h3Event: H3Event) {
       contentType?.startsWith("application/x-www-form-urlencoded")
     ) {
       parsed.push(await event.request.formData());
-    } else {
+    } else if (contentType?.startsWith('text/plain')) {
       parsed = (await deserializeJSONStream(event.request.clone())) as any[];
     }
   }
@@ -96,11 +96,11 @@ export async function handleServerFunction(h3Event: H3Event) {
     // handle no JS success case
     if (!instance) return handleNoJS(result, request, parsed);
 
+    h3Event.res.headers.set("x-serialized", "true");
     if (import.meta.env.SEROVAL_MODE === "js") {
       h3Event.res.headers.set("content-type", "text/javascript");
       return serializeToJSStream(instance, result);
     }
-    h3Event.res.headers.set("content-type", "text/plain");
     return serializeToJSONStream(result);
   } catch (x) {
     if (x instanceof Response) {
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index d2a51f2a7..3c0f2a201 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -47,8 +47,11 @@ async function fetchServerFunction(
         })
         : createRequest(base, id, instance, {
           ...options,
-          body: serializeToJSONStream(args),
-          headers: { ...options.headers, "Content-Type": "application/json" },
+          // TODO(Alexis): move to serializeToJSONStream
+          body: await serializeToJSONString(args),
+          // duplex: 'half',
+          // body: serializeToJSONStream(args),
+          headers: { ...options.headers, "Content-Type": "text/plain" },
         }));
 
   if (
@@ -70,14 +73,16 @@ async function fetchServerFunction(
 
   const contentType = response.headers.get("Content-Type");
   let result;
-  if (contentType && contentType.startsWith("text/plain")) {
+  if (contentType?.startsWith("text/plain")) {
     result = await response.text();
-  } else if (contentType && contentType.startsWith("application/json")) {
+  } else if (contentType?.startsWith("application/json")) {
     result = await response.json();
-  } else if (import.meta.env.SEROVAL_MODE === "js") {
-    result = await deserializeJSStream(instance, response);
-  } else {
-    result = await deserializeJSONStream(response);
+  } else if (response.headers.get('x-serialized')) {
+    if (import.meta.env.SEROVAL_MODE === "js") {
+      result = await deserializeJSStream(instance, response);
+    } else {
+      result = await deserializeJSONStream(response);
+    }
   }
   if (response.headers.has("X-Error")) {
     throw result;

From 22ba63f716c05cf8de3b3f9fdba44053edc89fd3 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 09:43:17 +0800
Subject: [PATCH 05/20] Add tests

---
 apps/tests/src/e2e/server-function.test.ts    |  5 ++++
 .../tests/src/routes/server-function-ping.tsx | 23 +++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 apps/tests/src/routes/server-function-ping.tsx

diff --git a/apps/tests/src/e2e/server-function.test.ts b/apps/tests/src/e2e/server-function.test.ts
index 7a8131be9..edba86fcc 100644
--- a/apps/tests/src/e2e/server-function.test.ts
+++ b/apps/tests/src/e2e/server-function.test.ts
@@ -67,4 +67,9 @@ test.describe("server-function", () => {
     await page.goto("http://localhost:3000/generator-server-function");
     await expect(page.locator("#server-fn-test")).toContainText("¡Hola, Mundo!");
   });
+
+  test("should build with a server function ping", async ({ page }) => {
+    await page.goto("http://localhost:3000/server-function-ping");
+    await expect(page.locator("#server-fn-test")).toContainText('{"result":true}');
+  });
 });
diff --git a/apps/tests/src/routes/server-function-ping.tsx b/apps/tests/src/routes/server-function-ping.tsx
new file mode 100644
index 000000000..70f9d2617
--- /dev/null
+++ b/apps/tests/src/routes/server-function-ping.tsx
@@ -0,0 +1,23 @@
+import { createEffect, createSignal } from "solid-js";
+
+async function ping(value: string) {
+  "use server";
+
+  return await Promise.resolve(value);
+}
+
+export default function App() {
+  const [output, setOutput] = createSignal<{ result?: boolean }>({});
+
+  createEffect(async () => {
+    const value = `${Math.random() * 1000}`;
+    const result = await ping(value);
+    setOutput(prev => ({ ...prev, result: value === result }));
+  });
+
+  return (
+    <main>
+      <span id="server-fn-test">{JSON.stringify(output())}</span>
+    </main>
+  );
+}

From 450b7d0f924660816adb4baa47a6da291ceb69c1 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 10:06:47 +0800
Subject: [PATCH 06/20] Add new header

---
 packages/start/src/server/server-functions-handler.ts |  4 +++-
 packages/start/src/server/server-runtime.ts           | 10 +++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index c536b940f..4881e8bbc 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -60,7 +60,9 @@ export async function handleServerFunction(h3Event: H3Event) {
       contentType?.startsWith("application/x-www-form-urlencoded")
     ) {
       parsed.push(await event.request.formData());
-    } else if (contentType?.startsWith('text/plain')) {
+    } else if (contentType?.startsWith('application/json')) {
+      parsed = await event.request.json() as any[];
+    } else if (request.headers.has('x-serialized')) {
       parsed = (await deserializeJSONStream(event.request.clone())) as any[];
     }
   }
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index 3c0f2a201..723542a05 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -1,9 +1,9 @@
-// @ts-ignore - seroval exports issue with NodeNext
+
 import { type Component } from "solid-js";
 import {
   deserializeJSONStream,
   deserializeJSStream,
-  serializeToJSONStream,
+  // serializeToJSONStream,
   serializeToJSONString,
 } from "./serialization";
 
@@ -51,7 +51,11 @@ async function fetchServerFunction(
           body: await serializeToJSONString(args),
           // duplex: 'half',
           // body: serializeToJSONStream(args),
-          headers: { ...options.headers, "Content-Type": "text/plain" },
+          headers: {
+            ...options.headers,
+            "x-serialized": "true",
+            "Content-Type": "text/plain"
+          },
         }));
 
   if (

From abe6ec7f00b8eef0a74ab44ae5535e89e48d2c44 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sun, 4 Jan 2026 12:13:06 +0800
Subject: [PATCH 07/20] Update server-functions-handler.ts

---
 packages/start/src/server/server-functions-handler.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index 4881e8bbc..6152fc8bd 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -130,11 +130,11 @@ export async function handleServerFunction(h3Event: H3Event) {
       x = handleNoJS(x, request, parsed, true);
     }
     if (instance) {
+      h3Event.res.headers.set("x-serialized", "true");
       if (import.meta.env.SEROVAL_MODE === "js") {
         h3Event.res.headers.set("content-type", "text/javascript");
         return serializeToJSStream(instance, x);
       }
-      h3Event.res.headers.set("content-type", "text/plain");
       return serializeToJSONStream(x);
     }
     return x;

From a80bfbb27f06eb1e68de42c4d655f0ec79f16de8 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Mon, 5 Jan 2026 13:10:35 +0800
Subject: [PATCH 08/20] Clone requests/responses

---
 .../start/src/server/server-functions-handler.ts    |  9 +++++----
 packages/start/src/server/server-runtime.ts         | 13 +++++++------
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index 6152fc8bd..62aa27f69 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -52,18 +52,19 @@ export async function handleServerFunction(h3Event: H3Event) {
       }
     }
   }
-  if (h3Event.method === "POST") {
+  if (request.method === "POST") {
     const contentType = request.headers.get("content-type");
+    const clone = request.clone();
 
     if (
       contentType?.startsWith("multipart/form-data") ||
       contentType?.startsWith("application/x-www-form-urlencoded")
     ) {
-      parsed.push(await event.request.formData());
+      parsed.push(await clone.formData());
     } else if (contentType?.startsWith('application/json')) {
-      parsed = await event.request.json() as any[];
+      parsed = await clone.json() as any[];
     } else if (request.headers.has('x-serialized')) {
-      parsed = (await deserializeJSONStream(event.request.clone())) as any[];
+      parsed = (await deserializeJSONStream(clone)) as any[];
     }
   }
   try {
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index 723542a05..3a31cc2f7 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -67,25 +67,26 @@ async function fetchServerFunction(
       /* @ts-ignore-next-line */
       response.customBody = () => {
         if (import.meta.env.SEROVAL_MODE === "js") {
-          return deserializeJSStream(instance, response);
+          return deserializeJSStream(instance, response.clone());
         }
-        return deserializeJSONStream(response);
+        return deserializeJSONStream(response.clone());
       };
     }
     return response;
   }
 
   const contentType = response.headers.get("Content-Type");
+  const clone = response.clone();
   let result;
   if (contentType?.startsWith("text/plain")) {
-    result = await response.text();
+    result = await clone.text();
   } else if (contentType?.startsWith("application/json")) {
-    result = await response.json();
+    result = await clone.json();
   } else if (response.headers.get('x-serialized')) {
     if (import.meta.env.SEROVAL_MODE === "js") {
-      result = await deserializeJSStream(instance, response);
+      result = await deserializeJSStream(instance, clone);
     } else {
-      result = await deserializeJSONStream(response);
+      result = await deserializeJSONStream(clone);
     }
   }
   if (response.headers.has("X-Error")) {

From a39958a9bbb66454b5d63e73f0b6450b413e4d2f Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Thu, 15 Jan 2026 02:15:25 +0800
Subject: [PATCH 09/20] Add more formats

---
 .../src/server/server-functions-handler.ts    |  46 ++++--
 .../src/server/server-functions-shared.ts     |  15 ++
 packages/start/src/server/server-runtime.ts   | 138 ++++++++++++++----
 3 files changed, 160 insertions(+), 39 deletions(-)
 create mode 100644 packages/start/src/server/server-functions-shared.ts

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index 62aa27f69..0f7c22ce3 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -13,6 +13,7 @@ import {
   serializeToJSONStream,
   serializeToJSStream,
 } from "./serialization.ts";
+import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared.ts";
 import type { FetchEvent, PageEvent } from "./types.ts";
 import { getExpectedRedirectStatus } from "./util.ts";
 
@@ -43,7 +44,7 @@ export async function handleServerFunction(h3Event: H3Event) {
   let parsed: any[] = [];
 
   // grab bound arguments from url when no JS
-  if (!instance || h3Event.method === "GET") {
+  if (!instance || request.method === "GET") {
     const args = url.searchParams.get("args");
     if (args) {
       const result = (await deserializeFromJSONString(args)) as any[];
@@ -54,17 +55,38 @@ export async function handleServerFunction(h3Event: H3Event) {
   }
   if (request.method === "POST") {
     const contentType = request.headers.get("content-type");
+    const startType = request.headers.get(BODY_FORMAT_KEY);
     const clone = request.clone();
 
-    if (
-      contentType?.startsWith("multipart/form-data") ||
-      contentType?.startsWith("application/x-www-form-urlencoded")
-    ) {
-      parsed.push(await clone.formData());
-    } else if (contentType?.startsWith('application/json')) {
-      parsed = await clone.json() as any[];
-    } else if (request.headers.has('x-serialized')) {
-      parsed = (await deserializeJSONStream(clone)) as any[];
+    switch (true) {
+      case startType === BodyFormat.Seroval:
+        parsed = (await deserializeJSONStream(clone)) as any[];
+        break;
+      case startType === BodyFormat.String:
+        parsed.push(await clone.text());
+        break;
+      case startType === BodyFormat.File: {
+        const formData = await clone.formData();
+        parsed.push(formData.get(BODY_FORMAL_FILE));
+        break;
+      }
+      case startType === BodyFormat.FormData:
+      case contentType?.startsWith("multipart/form-data"):
+        parsed.push(await clone.formData());
+        break;
+      case startType === BodyFormat.URLSearchParams:
+      case contentType?.startsWith("application/x-www-form-urlencoded"):
+        parsed.push(new URLSearchParams(await clone.text()));
+        break;
+      case startType === BodyFormat.Blob:
+        parsed.push(await clone.blob());
+        break;
+      case startType === BodyFormat.ArrayBuffer:
+        parsed.push(await clone.arrayBuffer());
+        break;
+      case startType === BodyFormat.Uint8Array:
+        parsed.push(await clone.bytes());
+        break;
     }
   }
   try {
@@ -92,14 +114,14 @@ export async function handleServerFunction(h3Event: H3Event) {
           h3Event.res.status = result.status;
         if ((result as any).customBody) {
           result = await (result as any).customBody();
-        } else if (result.body == undefined) result = null;
+        } else if (result.body == null) result = null;
       }
     }
 
     // handle no JS success case
     if (!instance) return handleNoJS(result, request, parsed);
 
-    h3Event.res.headers.set("x-serialized", "true");
+    h3Event.res.headers.set(BODY_FORMAT_KEY, "true");
     if (import.meta.env.SEROVAL_MODE === "js") {
       h3Event.res.headers.set("content-type", "text/javascript");
       return serializeToJSStream(instance, result);
diff --git a/packages/start/src/server/server-functions-shared.ts b/packages/start/src/server/server-functions-shared.ts
new file mode 100644
index 000000000..f7c253dd8
--- /dev/null
+++ b/packages/start/src/server/server-functions-shared.ts
@@ -0,0 +1,15 @@
+
+export const BODY_FORMAT_KEY = "X-Start-Type";
+
+export const BODY_FORMAL_FILE = "__START__";
+
+export const enum BodyFormat {
+  Seroval = "0",
+  String = "1",
+  FormData = "2",
+  URLSearchParams = "3",
+  Blob = "4",
+  File = "5",
+  ArrayBuffer = "6",
+  Uint8Array = "7",
+}
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index 3a31cc2f7..b1496fd31 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -1,4 +1,3 @@
-
 import { type Component } from "solid-js";
 import {
   deserializeJSONStream,
@@ -6,6 +5,7 @@ import {
   // serializeToJSONStream,
   serializeToJSONString,
 } from "./serialization";
+import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared";
 
 let INSTANCE = 0;
 
@@ -25,6 +25,113 @@ function createRequest(
     },
   });
 }
+
+function getHeadersAndBody(body: any): {
+  headers?: HeadersInit;
+  body: BodyInit;
+} | undefined {
+  switch (true) {
+    case typeof body === "string":
+      return {
+        headers: {
+          "Content-Type": "text/plain",
+          [BODY_FORMAT_KEY]: BodyFormat.String,
+        },
+        body,
+      };
+    case body instanceof FormData:
+      return {
+        headers: {
+          "Content-Type": "multipart/form-data",
+          [BODY_FORMAT_KEY]: BodyFormat.FormData,
+        },
+        body,
+      };
+    case body instanceof URLSearchParams:
+      return {
+        headers: {
+          "Content-Type": "application/x-www-form-urlencoded",
+          [BODY_FORMAT_KEY]: BodyFormat.URLSearchParams,
+        },
+        body,
+      };
+    case body instanceof Blob:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.Blob,
+        },
+        body,
+      };
+    case body instanceof File: {
+      const formData = new FormData();
+      formData.append(BODY_FORMAL_FILE, body, body.name);
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.File,
+        },
+        body: new FormData(),
+      };
+    }
+    case body instanceof ArrayBuffer:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.ArrayBuffer,
+        },
+        body,
+      };
+    case body instanceof Uint8Array:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.Uint8Array,
+        },
+        body: new Uint8Array(body),
+      };
+    default:
+      return undefined;
+  }
+}
+
+async function initializeResponse(
+  base: string,
+  id: string,
+  instance: string,
+  options: RequestInit,
+  args: any[],
+) {
+  // No args, skip serialization
+  if (args.length === 0) {
+    return createRequest(base, id, instance, options);
+  }
+  // For single arguments, we can directly encode as body
+  if (args.length === 1) {
+    const body = args[0];
+    const result = getHeadersAndBody(body);
+    if (result) {
+      return createRequest(base, id, instance, {
+        ...options,
+        body: result.body,
+        headers: {
+          ...options.headers,
+          ...result.headers,
+        },
+      });
+    }
+  }
+  // Fallback to seroval
+  return createRequest(base, id, instance, {
+    ...options,
+    // TODO(Alexis): move to serializeToJSONStream
+    body: await serializeToJSONString(args),
+    // duplex: 'half',
+    // body: serializeToJSONStream(args),
+    headers: {
+      ...options.headers,
+      "Content-Type": "text/plain",
+      [BODY_FORMAT_KEY]: BodyFormat.Seroval,
+    },
+  });
+}
+
 async function fetchServerFunction(
   base: string,
   id: string,
@@ -32,31 +139,8 @@ async function fetchServerFunction(
   args: any[],
 ) {
   const instance = `server-fn:${INSTANCE++}`;
-  const response = await (args.length === 0
-    ? createRequest(base, id, instance, options)
-    : args.length === 1 && args[0] instanceof FormData
-      ? createRequest(base, id, instance, { ...options, body: args[0] })
-      : args.length === 1 && args[0] instanceof URLSearchParams
-        ? createRequest(base, id, instance, {
-          ...options,
-          body: args[0],
-          headers: {
-            ...options.headers,
-            "Content-Type": "application/x-www-form-urlencoded",
-          },
-        })
-        : createRequest(base, id, instance, {
-          ...options,
-          // TODO(Alexis): move to serializeToJSONStream
-          body: await serializeToJSONString(args),
-          // duplex: 'half',
-          // body: serializeToJSONStream(args),
-          headers: {
-            ...options.headers,
-            "x-serialized": "true",
-            "Content-Type": "text/plain"
-          },
-        }));
+
+  const response = await initializeResponse(base, id, instance, options, args);
 
   if (
     response.headers.has("Location") ||
@@ -82,7 +166,7 @@ async function fetchServerFunction(
     result = await clone.text();
   } else if (contentType?.startsWith("application/json")) {
     result = await clone.json();
-  } else if (response.headers.get('x-serialized')) {
+  } else if (response.headers.get(BODY_FORMAT_KEY)) {
     if (import.meta.env.SEROVAL_MODE === "js") {
       result = await deserializeJSStream(instance, clone);
     } else {

From a44176d1af00828d263ec0d99d0c3e429f843795 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Thu, 15 Jan 2026 02:17:41 +0800
Subject: [PATCH 10/20] Fix `File` encoding

---
 packages/start/src/server/server-runtime.ts | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index b1496fd31..46d07808c 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -55,13 +55,6 @@ function getHeadersAndBody(body: any): {
         },
         body,
       };
-    case body instanceof Blob:
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.Blob,
-        },
-        body,
-      };
     case body instanceof File: {
       const formData = new FormData();
       formData.append(BODY_FORMAL_FILE, body, body.name);
@@ -69,9 +62,16 @@ function getHeadersAndBody(body: any): {
         headers: {
           [BODY_FORMAT_KEY]: BodyFormat.File,
         },
-        body: new FormData(),
+        body: formData,
       };
     }
+    case body instanceof Blob:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.Blob,
+        },
+        body,
+      };
     case body instanceof ArrayBuffer:
       return {
         headers: {

From 8488a5ec9614aec10b764cf8c548ca2d38a67deb Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Thu, 15 Jan 2026 11:16:00 +0800
Subject: [PATCH 11/20] Update server-functions-handler.ts

---
 packages/start/src/server/server-functions-handler.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index 0f7c22ce3..c03340dae 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -153,7 +153,7 @@ export async function handleServerFunction(h3Event: H3Event) {
       x = handleNoJS(x, request, parsed, true);
     }
     if (instance) {
-      h3Event.res.headers.set("x-serialized", "true");
+      h3Event.res.headers.set(BODY_FORMAT_KEY, "true");
       if (import.meta.env.SEROVAL_MODE === "js") {
         h3Event.res.headers.set("content-type", "text/javascript");
         return serializeToJSStream(instance, x);

From 1b2464bde1846d261015b65b1ab5bb5a286cb7ba Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Thu, 15 Jan 2026 11:17:48 +0800
Subject: [PATCH 12/20] Update server-runtime.ts

---
 packages/start/src/server/server-runtime.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index 46d07808c..c0f4ec4d6 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -4,8 +4,8 @@ import {
   deserializeJSStream,
   // serializeToJSONStream,
   serializeToJSONString,
-} from "./serialization";
-import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared";
+} from "./serialization.ts";
+import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared.ts";
 
 let INSTANCE = 0;
 

From db926e9c50b9305f838032a571e377170df21300 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Fri, 16 Jan 2026 08:05:01 +0800
Subject: [PATCH 13/20] Fix `FormData` missing boundary

---
 packages/start/src/server/server-runtime.ts | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index c0f4ec4d6..cb2b889f2 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -42,7 +42,6 @@ function getHeadersAndBody(body: any): {
     case body instanceof FormData:
       return {
         headers: {
-          "Content-Type": "multipart/form-data",
           [BODY_FORMAT_KEY]: BodyFormat.FormData,
         },
         body,

From 4bbe94bbed08a8df42e1814507b8fdd4548d3640 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Fri, 16 Jan 2026 16:42:23 +0800
Subject: [PATCH 14/20] Add form-data test

---
 apps/tests/src/e2e/server-function.test.ts    |  5 ++++
 .../src/routes/server-function-form-data.tsx  | 26 +++++++++++++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 apps/tests/src/routes/server-function-form-data.tsx

diff --git a/apps/tests/src/e2e/server-function.test.ts b/apps/tests/src/e2e/server-function.test.ts
index edba86fcc..a4dbc3746 100644
--- a/apps/tests/src/e2e/server-function.test.ts
+++ b/apps/tests/src/e2e/server-function.test.ts
@@ -72,4 +72,9 @@ test.describe("server-function", () => {
     await page.goto("http://localhost:3000/server-function-ping");
     await expect(page.locator("#server-fn-test")).toContainText('{"result":true}');
   });
+
+  test("should build with a server function w/ form data", async ({ page }) => {
+    await page.goto("http://localhost:3000/server-form-data");
+    await expect(page.locator("#server-fn-test")).toContainText('{"result":true}');
+  });
 });
diff --git a/apps/tests/src/routes/server-function-form-data.tsx b/apps/tests/src/routes/server-function-form-data.tsx
new file mode 100644
index 000000000..d54ad095a
--- /dev/null
+++ b/apps/tests/src/routes/server-function-form-data.tsx
@@ -0,0 +1,26 @@
+import { createEffect, createSignal } from "solid-js";
+
+async function ping(value: FormData) {
+  "use server";
+  const file = value.get('example') as File;
+  return await file.text();
+}
+
+export default function App() {
+  const [output, setOutput] = createSignal<{ result?: boolean }>({});
+
+  createEffect(async () => {
+    const file = new File(['Hello, World!'], 'hello-world.txt');
+    const formData = new FormData();
+    formData.append('example', file);
+    const result = await ping(formData);
+    const value = await file.text();
+    setOutput(prev => ({ ...prev, result: value === result }));
+  });
+
+  return (
+    <main>
+      <span id="server-fn-test">{JSON.stringify(output())}</span>
+    </main>
+  );
+}

From 28064a6620e1dd4dc6f3c72b5db7fc25aeb3651e Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Fri, 16 Jan 2026 16:46:36 +0800
Subject: [PATCH 15/20] fix tests

---
 apps/tests/src/e2e/server-function.test.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/apps/tests/src/e2e/server-function.test.ts b/apps/tests/src/e2e/server-function.test.ts
index a4dbc3746..861495e3b 100644
--- a/apps/tests/src/e2e/server-function.test.ts
+++ b/apps/tests/src/e2e/server-function.test.ts
@@ -74,7 +74,7 @@ test.describe("server-function", () => {
   });
 
   test("should build with a server function w/ form data", async ({ page }) => {
-    await page.goto("http://localhost:3000/server-form-data");
+    await page.goto("http://localhost:3000/server-function-form-data");
     await expect(page.locator("#server-fn-test")).toContainText('{"result":true}');
   });
 });

From 4ddcff31a6ff167efcf645dcf6a81710ba08e79e Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Fri, 23 Jan 2026 15:02:13 +0800
Subject: [PATCH 16/20] Bump `seroval` to `1.5.0`

---
 packages/start/package.json |  2 +-
 pnpm-lock.yaml              | 16 ++++++++--------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/packages/start/package.json b/packages/start/package.json
index 9170468d3..af2bbca6c 100644
--- a/packages/start/package.json
+++ b/packages/start/package.json
@@ -56,7 +56,7 @@
     "path-to-regexp": "^8.2.0",
     "pathe": "^2.0.3",
     "radix3": "^1.1.2",
-    "seroval": "^1.4.1",
+    "seroval": "^1.5.0",
     "seroval-plugins": "^1.4.0",
     "shiki": "^1.26.1",
     "solid-js": "^1.9.9",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index d00c64b91..2d030c9f8 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -368,11 +368,11 @@ importers:
         specifier: ^1.1.2
         version: 1.1.2
       seroval:
-        specifier: ^1.4.1
-        version: 1.4.1
+        specifier: ^1.5.0
+        version: 1.5.0
       seroval-plugins:
         specifier: ^1.4.0
-        version: 1.4.0(seroval@1.4.1)
+        version: 1.4.0(seroval@1.5.0)
       shiki:
         specifier: ^1.26.1
         version: 1.26.1
@@ -4851,8 +4851,8 @@ packages:
     resolution: {integrity: sha512-RbcPH1n5cfwKrru7v7+zrZvjLurgHhGyso3HTyGtRivGWgYjbOmGuivCQaORNELjNONoK35nj28EoWul9sb1zQ==}
     engines: {node: '>=10'}
 
-  seroval@1.4.1:
-    resolution: {integrity: sha512-9GOc+8T6LN4aByLN75uRvMbrwY5RDBW6lSlknsY4LEa9ZmWcxKcRe1G/Q3HZXjltxMHTrStnvrwAICxZrhldtg==}
+  seroval@1.5.0:
+    resolution: {integrity: sha512-OE4cvmJ1uSPrKorFIH9/w/Qwuvi/IMcGbv5RKgcJ/zjA/IohDLU6SVaxFN9FwajbP7nsX0dQqMDes1whk3y+yw==}
     engines: {node: '>=10'}
 
   serve-placeholder@2.0.2:
@@ -10519,13 +10519,13 @@ snapshots:
     dependencies:
       seroval: 1.3.2
 
-  seroval-plugins@1.4.0(seroval@1.4.1):
+  seroval-plugins@1.4.0(seroval@1.5.0):
     dependencies:
-      seroval: 1.4.1
+      seroval: 1.5.0
 
   seroval@1.3.2: {}
 
-  seroval@1.4.1: {}
+  seroval@1.5.0: {}
 
   serve-placeholder@2.0.2:
     dependencies:

From 1fa3f7ca0ec523232608e7f9e955f8e4638e145e Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Sat, 24 Jan 2026 14:20:03 +0800
Subject: [PATCH 17/20] Bump `seroval-plugins`

---
 packages/start/package.json |  2 +-
 pnpm-lock.yaml              | 10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/packages/start/package.json b/packages/start/package.json
index af2bbca6c..40ea19875 100644
--- a/packages/start/package.json
+++ b/packages/start/package.json
@@ -57,7 +57,7 @@
     "pathe": "^2.0.3",
     "radix3": "^1.1.2",
     "seroval": "^1.5.0",
-    "seroval-plugins": "^1.4.0",
+    "seroval-plugins": "^1.5.0",
     "shiki": "^1.26.1",
     "solid-js": "^1.9.9",
     "source-map-js": "^1.2.1",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 2d030c9f8..508daf5c5 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -371,8 +371,8 @@ importers:
         specifier: ^1.5.0
         version: 1.5.0
       seroval-plugins:
-        specifier: ^1.4.0
-        version: 1.4.0(seroval@1.5.0)
+        specifier: ^1.5.0
+        version: 1.5.0(seroval@1.5.0)
       shiki:
         specifier: ^1.26.1
         version: 1.26.1
@@ -4841,8 +4841,8 @@ packages:
     peerDependencies:
       seroval: ^1.0
 
-  seroval-plugins@1.4.0:
-    resolution: {integrity: sha512-zir1aWzoiax6pbBVjoYVd0O1QQXgIL3eVGBMsBsNmM8Ukq90yGaWlfx0AB9dTS8GPqrOrbXn79vmItCUP9U3BQ==}
+  seroval-plugins@1.5.0:
+    resolution: {integrity: sha512-EAHqADIQondwRZIdeW2I636zgsODzoBDwb3PT/+7TLDWyw1Dy/Xv7iGUIEXXav7usHDE9HVhOU61irI3EnyyHA==}
     engines: {node: '>=10'}
     peerDependencies:
       seroval: ^1.0
@@ -10519,7 +10519,7 @@ snapshots:
     dependencies:
       seroval: 1.3.2
 
-  seroval-plugins@1.4.0(seroval@1.5.0):
+  seroval-plugins@1.5.0(seroval@1.5.0):
     dependencies:
       seroval: 1.5.0
 

From 5e08e6259e45b160112658b27afb84df39b895d8 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Mon, 2 Feb 2026 00:15:24 +0800
Subject: [PATCH 18/20] Add bidirectional encoding

---
 .../tests/src/routes/server-function-blob.tsx |  28 +++++
 packages/start/src/server/serialization.ts    |   2 +-
 .../src/server/server-functions-handler.ts    |  64 ++++-------
 .../src/server/server-functions-shared.ts     | 101 ++++++++++++++++++
 packages/start/src/server/server-runtime.ts   |  80 +-------------
 5 files changed, 153 insertions(+), 122 deletions(-)
 create mode 100644 apps/tests/src/routes/server-function-blob.tsx

diff --git a/apps/tests/src/routes/server-function-blob.tsx b/apps/tests/src/routes/server-function-blob.tsx
new file mode 100644
index 000000000..31de2cb38
--- /dev/null
+++ b/apps/tests/src/routes/server-function-blob.tsx
@@ -0,0 +1,28 @@
+import { createEffect, createSignal } from "solid-js";
+
+async function ping(value: Blob) {
+  "use server";
+  return value;
+}
+
+const blobURI = 'data:image/svg+xml;base64,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';
+
+export default function App() {
+  const [output, setOutput] = createSignal<{ result?: boolean }>({});
+
+  createEffect(async () => {
+    const request = await fetch(blobURI);
+    const blob = await request.blob();
+    const result = await ping(blob);
+    const value = await blob.text();
+    const test = await result.text();
+
+    setOutput(prev => ({ ...prev, result: value === test }));
+  });
+
+  return (
+    <main>
+      <span id="server-fn-test">{JSON.stringify(output())}</span>
+    </main>
+  );
+}
diff --git a/packages/start/src/server/serialization.ts b/packages/start/src/server/serialization.ts
index c157d157e..a0c269505 100644
--- a/packages/start/src/server/serialization.ts
+++ b/packages/start/src/server/serialization.ts
@@ -229,7 +229,7 @@ export async function deserializeJSONStream(response: Response | Request) {
   return undefined;
 }
 
-export async function deserializeJSStream(id: string, response: Response) {
+export async function deserializeJSStream(id: string, response: Request | Response) {
   if (!response.body) {
     throw new Error("missing body");
   }
diff --git a/packages/start/src/server/server-functions-handler.ts b/packages/start/src/server/server-functions-handler.ts
index c03340dae..b8e95888a 100644
--- a/packages/start/src/server/server-functions-handler.ts
+++ b/packages/start/src/server/server-functions-handler.ts
@@ -9,11 +9,10 @@ import { getFetchEvent, mergeResponseHeaders } from "./fetchEvent.ts";
 import { createPageEvent } from "./handler.ts";
 import {
   deserializeFromJSONString,
-  deserializeJSONStream,
   serializeToJSONStream,
   serializeToJSStream,
 } from "./serialization.ts";
-import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared.ts";
+import { BODY_FORMAT_KEY, BodyFormat, extractBody, getHeadersAndBody } from "./server-functions-shared.ts";
 import type { FetchEvent, PageEvent } from "./types.ts";
 import { getExpectedRedirectStatus } from "./util.ts";
 
@@ -54,40 +53,7 @@ export async function handleServerFunction(h3Event: H3Event) {
     }
   }
   if (request.method === "POST") {
-    const contentType = request.headers.get("content-type");
-    const startType = request.headers.get(BODY_FORMAT_KEY);
-    const clone = request.clone();
-
-    switch (true) {
-      case startType === BodyFormat.Seroval:
-        parsed = (await deserializeJSONStream(clone)) as any[];
-        break;
-      case startType === BodyFormat.String:
-        parsed.push(await clone.text());
-        break;
-      case startType === BodyFormat.File: {
-        const formData = await clone.formData();
-        parsed.push(formData.get(BODY_FORMAL_FILE));
-        break;
-      }
-      case startType === BodyFormat.FormData:
-      case contentType?.startsWith("multipart/form-data"):
-        parsed.push(await clone.formData());
-        break;
-      case startType === BodyFormat.URLSearchParams:
-      case contentType?.startsWith("application/x-www-form-urlencoded"):
-        parsed.push(new URLSearchParams(await clone.text()));
-        break;
-      case startType === BodyFormat.Blob:
-        parsed.push(await clone.blob());
-        break;
-      case startType === BodyFormat.ArrayBuffer:
-        parsed.push(await clone.arrayBuffer());
-        break;
-      case startType === BodyFormat.Uint8Array:
-        parsed.push(await clone.bytes());
-        break;
-    }
+    parsed.push(await extractBody('', false, request.clone()));
   }
   try {
     let result = await provideRequestEvent(event, async () => {
@@ -121,12 +87,18 @@ export async function handleServerFunction(h3Event: H3Event) {
     // handle no JS success case
     if (!instance) return handleNoJS(result, request, parsed);
 
-    h3Event.res.headers.set(BODY_FORMAT_KEY, "true");
-    if (import.meta.env.SEROVAL_MODE === "js") {
-      h3Event.res.headers.set("content-type", "text/javascript");
-      return serializeToJSStream(instance, result);
-    }
-    return serializeToJSONStream(result);
+      const body = getHeadersAndBody(result);
+      if (body) {
+        return new Response(body.body, {
+          headers: body.headers,
+        });
+      }
+      h3Event.res.headers.set(BODY_FORMAT_KEY, BodyFormat.Seroval);
+      if (import.meta.env.SEROVAL_MODE === "js") {
+        h3Event.res.headers.set("content-type", "text/javascript");
+        return serializeToJSStream(instance, result);
+      }
+      return serializeToJSONStream(result);
   } catch (x) {
     if (x instanceof Response) {
       if (singleFlight && instance) {
@@ -153,7 +125,13 @@ export async function handleServerFunction(h3Event: H3Event) {
       x = handleNoJS(x, request, parsed, true);
     }
     if (instance) {
-      h3Event.res.headers.set(BODY_FORMAT_KEY, "true");
+      const body = getHeadersAndBody(x);
+      if (body) {
+        return new Response(body.body, {
+          headers: body.headers,
+        });
+      }
+      h3Event.res.headers.set(BODY_FORMAT_KEY, BodyFormat.Seroval);
       if (import.meta.env.SEROVAL_MODE === "js") {
         h3Event.res.headers.set("content-type", "text/javascript");
         return serializeToJSStream(instance, x);
diff --git a/packages/start/src/server/server-functions-shared.ts b/packages/start/src/server/server-functions-shared.ts
index f7c253dd8..5a6f6624a 100644
--- a/packages/start/src/server/server-functions-shared.ts
+++ b/packages/start/src/server/server-functions-shared.ts
@@ -1,3 +1,4 @@
+import { deserializeJSONStream, deserializeJSStream } from "./serialization";
 
 export const BODY_FORMAT_KEY = "X-Start-Type";
 
@@ -13,3 +14,103 @@ export const enum BodyFormat {
   ArrayBuffer = "6",
   Uint8Array = "7",
 }
+
+export function getHeadersAndBody(body: any):
+  | {
+    headers?: HeadersInit;
+    body: BodyInit;
+  }
+  | undefined {
+  switch (true) {
+    case typeof body === "string":
+      return {
+        headers: {
+          "Content-Type": "text/plain",
+          [BODY_FORMAT_KEY]: BodyFormat.String,
+        },
+        body,
+      };
+    case body instanceof FormData:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.FormData,
+        },
+        body,
+      };
+    case body instanceof URLSearchParams:
+      return {
+        headers: {
+          "Content-Type": "application/x-www-form-urlencoded",
+          [BODY_FORMAT_KEY]: BodyFormat.URLSearchParams,
+        },
+        body,
+      };
+    case body instanceof File: {
+      const formData = new FormData();
+      formData.append(BODY_FORMAL_FILE, body, body.name);
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.File,
+        },
+        body: formData,
+      };
+    }
+    case body instanceof Blob:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.Blob,
+        },
+        body,
+      };
+    case body instanceof ArrayBuffer:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.ArrayBuffer,
+        },
+        body,
+      };
+    case body instanceof Uint8Array:
+      return {
+        headers: {
+          [BODY_FORMAT_KEY]: BodyFormat.Uint8Array,
+        },
+        body: new Uint8Array(body),
+      };
+    default:
+      return undefined;
+  }
+}
+
+export async function extractBody(instance: string, client: boolean, source: Request | Response) {
+  const contentType = source.headers.get("content-type");
+  const startType = source.headers.get(BODY_FORMAT_KEY);
+  const clone = source.clone();
+
+  switch (true) {
+    case startType === BodyFormat.Seroval:
+      if (client && import.meta.env.SEROVAL_MODE === "js") {
+        return await deserializeJSStream(instance, clone);
+      }
+      return await deserializeJSONStream(clone);
+    case startType === BodyFormat.String:
+      return await clone.text();
+    case startType === BodyFormat.File: {
+      const formData = await clone.formData();
+      return formData.get(BODY_FORMAL_FILE);
+    }
+    case startType === BodyFormat.FormData:
+    case contentType?.startsWith("multipart/form-data"):
+      return await clone.formData();
+    case startType === BodyFormat.URLSearchParams:
+    case contentType?.startsWith("application/x-www-form-urlencoded"):
+      return new URLSearchParams(await clone.text());
+    case startType === BodyFormat.Blob:
+      return await clone.blob();
+    case startType === BodyFormat.ArrayBuffer:
+      return await clone.arrayBuffer();
+    case startType === BodyFormat.Uint8Array:
+      return await clone.bytes();
+  }
+
+  throw new Error("Unknown body format");
+}
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index cb2b889f2..d31a13942 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -5,7 +5,7 @@ import {
   // serializeToJSONStream,
   serializeToJSONString,
 } from "./serialization.ts";
-import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat } from "./server-functions-shared.ts";
+import { BODY_FORMAL_FILE, BODY_FORMAT_KEY, BodyFormat, extractBody, getHeadersAndBody } from "./server-functions-shared.ts";
 
 let INSTANCE = 0;
 
@@ -26,70 +26,6 @@ function createRequest(
   });
 }
 
-function getHeadersAndBody(body: any): {
-  headers?: HeadersInit;
-  body: BodyInit;
-} | undefined {
-  switch (true) {
-    case typeof body === "string":
-      return {
-        headers: {
-          "Content-Type": "text/plain",
-          [BODY_FORMAT_KEY]: BodyFormat.String,
-        },
-        body,
-      };
-    case body instanceof FormData:
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.FormData,
-        },
-        body,
-      };
-    case body instanceof URLSearchParams:
-      return {
-        headers: {
-          "Content-Type": "application/x-www-form-urlencoded",
-          [BODY_FORMAT_KEY]: BodyFormat.URLSearchParams,
-        },
-        body,
-      };
-    case body instanceof File: {
-      const formData = new FormData();
-      formData.append(BODY_FORMAL_FILE, body, body.name);
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.File,
-        },
-        body: formData,
-      };
-    }
-    case body instanceof Blob:
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.Blob,
-        },
-        body,
-      };
-    case body instanceof ArrayBuffer:
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.ArrayBuffer,
-        },
-        body,
-      };
-    case body instanceof Uint8Array:
-      return {
-        headers: {
-          [BODY_FORMAT_KEY]: BodyFormat.Uint8Array,
-        },
-        body: new Uint8Array(body),
-      };
-    default:
-      return undefined;
-  }
-}
-
 async function initializeResponse(
   base: string,
   id: string,
@@ -158,20 +94,8 @@ async function fetchServerFunction(
     return response;
   }
 
-  const contentType = response.headers.get("Content-Type");
   const clone = response.clone();
-  let result;
-  if (contentType?.startsWith("text/plain")) {
-    result = await clone.text();
-  } else if (contentType?.startsWith("application/json")) {
-    result = await clone.json();
-  } else if (response.headers.get(BODY_FORMAT_KEY)) {
-    if (import.meta.env.SEROVAL_MODE === "js") {
-      result = await deserializeJSStream(instance, clone);
-    } else {
-      result = await deserializeJSONStream(clone);
-    }
-  }
+  const result = await extractBody(instance, true, clone);
   if (response.headers.has("X-Error")) {
     throw result;
   }

From db9eaf527ef91904ab3b151f3a0acf9e29bea53d Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Mon, 2 Feb 2026 00:22:01 +0800
Subject: [PATCH 19/20] Fix error

---
 packages/start/src/server/server-functions-shared.ts |  2 +-
 packages/start/src/server/server-runtime.ts          | 10 +++-------
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/packages/start/src/server/server-functions-shared.ts b/packages/start/src/server/server-functions-shared.ts
index 5a6f6624a..023e5f413 100644
--- a/packages/start/src/server/server-functions-shared.ts
+++ b/packages/start/src/server/server-functions-shared.ts
@@ -112,5 +112,5 @@ export async function extractBody(instance: string, client: boolean, source: Req
       return await clone.bytes();
   }
 
-  throw new Error("Unknown body format");
+  return undefined;
 }
diff --git a/packages/start/src/server/server-runtime.ts b/packages/start/src/server/server-runtime.ts
index d31a13942..9a9e91dc5 100644
--- a/packages/start/src/server/server-runtime.ts
+++ b/packages/start/src/server/server-runtime.ts
@@ -84,18 +84,14 @@ async function fetchServerFunction(
   ) {
     if (response.body) {
       /* @ts-ignore-next-line */
-      response.customBody = () => {
-        if (import.meta.env.SEROVAL_MODE === "js") {
-          return deserializeJSStream(instance, response.clone());
-        }
-        return deserializeJSONStream(response.clone());
+      response.customBody = async () => {
+        return await extractBody(instance, true, response.clone())
       };
     }
     return response;
   }
 
-  const clone = response.clone();
-  const result = await extractBody(instance, true, clone);
+  const result = await extractBody(instance, true, response.clone());
   if (response.headers.has("X-Error")) {
     throw result;
   }

From db436df86b70c7fea2165b67f3d4363e81ae9827 Mon Sep 17 00:00:00 2001
From: "Alexis H. Munsayac" <alexis.munsayac@gmail.com>
Date: Mon, 9 Feb 2026 16:51:34 +0800
Subject: [PATCH 20/20] Update server-functions-shared.ts

---
 packages/start/src/server/server-functions-shared.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/start/src/server/server-functions-shared.ts b/packages/start/src/server/server-functions-shared.ts
index 023e5f413..c399fef5a 100644
--- a/packages/start/src/server/server-functions-shared.ts
+++ b/packages/start/src/server/server-functions-shared.ts
@@ -1,4 +1,4 @@
-import { deserializeJSONStream, deserializeJSStream } from "./serialization";
+import { deserializeJSONStream, deserializeJSStream } from "./serialization.ts";
 
 export const BODY_FORMAT_KEY = "X-Start-Type";