diff --git a/.changeset/quiet-errors-wait.md b/.changeset/quiet-errors-wait.md
new file mode 100644
index 000000000..594247dce
--- /dev/null
+++ b/.changeset/quiet-errors-wait.md
@@ -0,0 +1,6 @@
+---
+"@solidjs/start": patch
+---
+
+Await asynchronous `serverFunctions.onError` handlers before serializing server function errors.
+Preserve the original error if the handler throws or rejects.
diff --git a/apps/tests/src/server-fn-error.ts b/apps/tests/src/server-fn-error.ts
index 3d3628dd5..b7ba7ff0d 100644
--- a/apps/tests/src/server-fn-error.ts
+++ b/apps/tests/src/server-fn-error.ts
@@ -8,7 +8,7 @@ import type { ServerFunctionErrorHandler } from "@solidjs/start/server";
  * Returning `undefined` sends whatever was thrown, which is what leaves the
  * other server-function error tests in this app seeing their own errors.
  */
-const onServerFunctionError: ServerFunctionErrorHandler = thrown => {
+const onServerFunctionError: ServerFunctionErrorHandler = async thrown => {
   if (thrown instanceof Error && thrown.message === "replace me") {
     return new Error("replaced by onError");
   }
diff --git a/packages/start/src/config/index.ts b/packages/start/src/config/index.ts
index 34e06da46..ea9a00428 100644
--- a/packages/start/src/config/index.ts
+++ b/packages/start/src/config/index.ts
@@ -157,14 +157,17 @@ export interface SolidStartOptions {
      * straight to its caller, and errors from API routes never reach it
      * either.
      *
-     * The export is called synchronously with the thrown value, and what it
-     * returns decides what the client sees:
+     * The handler may be asynchronous. SolidStart awaits its return value
+     * before serializing the response, allowing a monitoring service to flush
+     * first. The resolved value decides what the client sees:
      *
      * - `undefined` (or `null`) sends what was thrown, unchanged.
      * - A `Response` is passed through unchanged, which keeps a thrown
      *   `redirect()` working.
      * - Any other value is sent in place of what was thrown, and the client
      *   call rejects with it.
+     * - If the handler throws or rejects, what the server function threw is
+     *   sent unchanged.
      *
      * Control flow reaches the export the same way errors do, so a handler
      * that replaces everything it sees turns redirects into errors.
@@ -173,10 +176,6 @@ export interface SolidStartOptions {
      * properties, so an error meant to be safe to expose must not carry
      * internal detail.
      *
-     * The return value is not awaited, so make the export a plain function
-     * rather than an `async` one, and report failures with calls that do not
-     * need awaiting.
-     *
      * Type the export as `ServerFunctionErrorHandler` from
      * `@solidjs/start/server`. The module is bundled into the server only, so
      * it may import server-only code such as a monitoring SDK.
diff --git a/packages/start/src/fns/error-handler.ts b/packages/start/src/fns/error-handler.ts
index 6ba8d2ba8..1164f0baa 100644
--- a/packages/start/src/fns/error-handler.ts
+++ b/packages/start/src/fns/error-handler.ts
@@ -5,14 +5,15 @@ import onServerFunctionError from "solid-start:server-fn-error-handler";
  * it to type the default export of the module named by the
  * `serverFunctions.onError` option in `vite.config.ts`.
  *
- * Called synchronously, and the value it returns is not awaited, so it cannot
- * be an `async` function.
+ * The handler may be asynchronous. SolidStart awaits its return value before
+ * sending the error to the client. If the handler throws or rejects, SolidStart
+ * sends the original thrown value instead.
  *
  * @param thrown The value the server function threw. This is a `Response` when
  * the server function threw control flow such as a `redirect()`.
  * @returns `undefined` (or `null`) to send `thrown` unchanged, a `Response` to
- * pass control flow through untouched, or any other value to send in place of
- * `thrown`.
+ * preserve control flow, any other replacement value, or a promise resolving
+ * to any of those values.
  *
  * @example
  * ```ts
@@ -33,6 +34,11 @@ import onServerFunctionError from "solid-start:server-fn-error-handler";
  */
 export type ServerFunctionErrorHandler = (thrown: unknown) => unknown;
 
-export function applyServerFunctionErrorHandler(thrown: unknown): unknown {
-  return onServerFunctionError?.(thrown) ?? thrown;
+export async function applyServerFunctionErrorHandler(thrown: unknown): Promise<unknown> {
+  try {
+    const replacement = await onServerFunctionError?.(thrown);
+    return replacement ?? thrown;
+  } catch {
+    return thrown;
+  }
 }
diff --git a/packages/start/src/fns/handler.spec.ts b/packages/start/src/fns/handler.spec.ts
index f4a44324d..58594bb4c 100644
--- a/packages/start/src/fns/handler.spec.ts
+++ b/packages/start/src/fns/handler.spec.ts
@@ -224,6 +224,14 @@ describe("the configured server function error handler", () => {
     expect(h3Event.res.headers.get("X-Error")).toBe("replaced");
   });
 
+  it("awaits an asynchronous replacement", async () => {
+    configuredErrorHandler.current = async () => new Error("replaced asynchronously");
+
+    const h3Event = await callThrowing(new Error("boom"));
+
+    expect(h3Event.res.headers.get("X-Error")).toBe("replaced asynchronously");
+  });
+
   it("treats a Response the handler returns as control flow", async () => {
     configuredErrorHandler.current = () => new Response(null, { status: 403 });
 
@@ -233,6 +241,15 @@ describe("the configured server function error handler", () => {
     expect(h3Event.res.headers.get("X-Error")).toBe("true");
   });
 
+  it("treats a Response resolved by an asynchronous handler as control flow", async () => {
+    configuredErrorHandler.current = async () => new Response(null, { status: 403 });
+
+    const h3Event = await callThrowing(new Error("boom"));
+
+    expect(h3Event.res.status).toBe(403);
+    expect(h3Event.res.headers.get("X-Error")).toBe("true");
+  });
+
   it("keeps the original error when the handler returns nothing", async () => {
     configuredErrorHandler.current = () => undefined;
 
@@ -241,6 +258,24 @@ describe("the configured server function error handler", () => {
     expect(h3Event.res.headers.get("X-Error")).toBe("boom");
   });
 
+  it("keeps the original error when an asynchronous handler returns nothing", async () => {
+    configuredErrorHandler.current = async () => undefined;
+
+    const h3Event = await callThrowing(new Error("boom"));
+
+    expect(h3Event.res.headers.get("X-Error")).toBe("boom");
+  });
+
+  it("keeps the original error when an asynchronous handler rejects", async () => {
+    configuredErrorHandler.current = async () => {
+      throw new Error("handler failed");
+    };
+
+    const h3Event = await callThrowing(new Error("boom"));
+
+    expect(h3Event.res.headers.get("X-Error")).toBe("boom");
+  });
+
   it("leaves the response untouched when no handler is configured", async () => {
     const h3Event = await callThrowing(new Error("boom"));
 
diff --git a/packages/start/src/fns/handler.ts b/packages/start/src/fns/handler.ts
index 135ce497c..1c6d994f4 100644
--- a/packages/start/src/fns/handler.ts
+++ b/packages/start/src/fns/handler.ts
@@ -127,7 +127,7 @@ export async function handleServerFunction(h3Event: H3Event) {
     }
     return serializeToJSONStream(result);
   } catch (x) {
-    x = applyServerFunctionErrorHandler(x);
+    x = await applyServerFunctionErrorHandler(x);
     if (x instanceof Response) {
       if (singleFlight && instance) {
         x = await handleSingleFlight(event, x);