diff --git a/apps/tests/src/server-fn-error.ts b/apps/tests/src/server-fn-error.ts
index 25d08c3ff..3d3628dd5 100644
--- a/apps/tests/src/server-fn-error.ts
+++ b/apps/tests/src/server-fn-error.ts
@@ -1,8 +1,9 @@
 import type { ServerFunctionErrorHandler } from "@solidjs/start/server";
 
 /**
- * Wired up through `serverFunctions.onError`. Unlike `seroval-plugins.ts` this
- * module is bundled into the server only, so it may reach for server-only code.
+ * Wired up through `serverFunctions.onError`. Unlike `src/seroval-plugins.ts`,
+ * this module is bundled into the server only, so it may reach for server-only
+ * code.
  *
  * Returning `undefined` sends whatever was thrown, which is what leaves the
  * other server-function error tests in this app seeing their own errors.
diff --git a/packages/start/src/config/index.ts b/packages/start/src/config/index.ts
index b29609f2c..34e06da46 100644
--- a/packages/start/src/config/index.ts
+++ b/packages/start/src/config/index.ts
@@ -141,22 +141,45 @@ export interface SolidStartOptions {
   env?: EnvPluginOptions;
 
   /**
-   * Options controlling which files are processed as server functions
-   * (inclusion / exclusion filters for the `"use server"` transform).
+   * Options for server functions: which files the `"use server"` transform
+   * processes (inclusion / exclusion filters), and what happens when a server
+   * function throws.
    */
   serverFunctions?: Pick<ServerFunctionsOptions, "filter"> & {
     /**
-     * Path to a module whose default export is called with whatever a server
-     * function threw, before it is serialized into the response. Return a
-     * value to send it in place of what was thrown, or `undefined` to send the
-     * original.
+     * Path to a module whose default export handles whatever a server function
+     * throws, before it reaches the client. Use it to report failures to a
+     * monitoring service, or to replace an error carrying internal detail with
+     * one that is safe to send.
      *
-     * Naming the module here rather than registering a handler at runtime
-     * keeps the app in sole control of it: no dependency can reach into the
-     * running server and take over reporting.
+     * Only server function calls made over the network run through it. A
+     * server function called during rendering runs in process and throws
+     * straight to its caller, and errors from API routes never reach it
+     * either.
      *
-     * The module is bundled into the server only, so it may import server-only
-     * code such as a monitoring SDK.
+     * The export is called synchronously with the thrown value, and what it
+     * returns 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.
+     *
+     * Control flow reaches the export the same way errors do, so a handler
+     * that replaces everything it sees turns redirects into errors.
+     *
+     * Whatever is sent gets serialized to the client along with its own
+     * 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.
      *
      * @example "src/server-fn-error.ts"
      */
diff --git a/packages/start/src/fns/error-handler.ts b/packages/start/src/fns/error-handler.ts
index ce98ee4e2..6ba8d2ba8 100644
--- a/packages/start/src/fns/error-handler.ts
+++ b/packages/start/src/fns/error-handler.ts
@@ -1,8 +1,38 @@
 import onServerFunctionError from "solid-start:server-fn-error-handler";
 
+/**
+ * Handles whatever a server function threw, before it reaches the client. Use
+ * 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.
+ *
+ * @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`.
+ *
+ * @example
+ * ```ts
+ * // src/server-fn-error.ts
+ * import type { ServerFunctionErrorHandler } from "@solidjs/start/server";
+ * import { captureException } from "./your-monitoring-client";
+ *
+ * const onServerFunctionError: ServerFunctionErrorHandler = thrown => {
+ *   // redirect() throws a Response, so returning it preserves that control flow.
+ *   if (thrown instanceof Response) return thrown;
+ *
+ *   captureException(thrown); // or console.error, or any reporter
+ *   return new Error("Something went wrong");
+ * };
+ *
+ * export default onServerFunctionError;
+ * ```
+ */
 export type ServerFunctionErrorHandler = (thrown: unknown) => unknown;
 
-/** @internal */
 export function applyServerFunctionErrorHandler(thrown: unknown): unknown {
   return onServerFunctionError?.(thrown) ?? thrown;
 }