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Runtime Extensions

While most configuration in your Zuplo gateway is set on a per-route or per-policy basis, there are times when behaviors need to be modified globally. To plug into the global initialization of your gateway, create a file called zuplo.runtime.ts in the modules folder with the following code.


Any error thrown in the runtimeInit method will prevent the gateway from starting and yield a 500 error for all requests. Be sure to add only reliable code here and use try/catch as appropriate to handle any recoverable exceptions.

import { RuntimeExtensions } from "@zuplo/runtime";

export function runtimeInit(runtime: RuntimeExtensions) {
// Extensions go here

The name of the export must be runtimeInit and must conform to the above function signature.

The following configurations are available.

Custom Problem (Error) Response Formatter

Zuplo includes built-in error handling that returns errors in the format of the Problem Details for HTTP APIs proposed standard. This means that HTTP errors (or other exceptions) will return responses that look like the following.

"type": "",
"title": "Not Found",
"status": 404,
"detail": "Not Found",
"instance": "/not-a-path",
"trace": {
"timestamp": "2023-03-14T15:49:38.581Z",
"requestId": "05968b6d-6f82-4ae3-8e13-f92e0d0499c5",
"buildId": "a9b200a3-734c-413a-a1ae-ce171d53e5a7",
"rayId": "7a7daaf3bac2f325"

If you want to customize this format, you can configure the problemResponseFormat function and return a Response in the format of your choice.

import { RuntimeExtensions } from "@zuplo/runtime";

export function runtimeInit(runtime: RuntimeExtensions) {
runtime.problemResponseFormat = (
{ problem, statusText, additionalHeaders },
) => {
// Build the response body
const body = JSON.stringify(problem, null, 2);

// Send the response with headers and status
return new Response(body, {
status: problem.status,
headers: {
"content-type": "application/problem+json",

Hook: OnResponseSending

The OnResponseSending hook allows modification of the Response immediately before it is sent to the client. The hook provides the Request and Response and returns a Response. To modify the outgoing response create and return a new Response().

The example below shows modifying the response for a specific path.

import { RuntimeExtensions } from "@zuplo/runtime";

export function runtimeInit(runtime: RuntimeExtensions) {
runtime.addResponseSendingHook(async (response, latestRequest, context) => {
const url = new URL(request.url);
if (url.pathname === "/path-to-change-response") {
return new Response("New Response", response);
return response;

Hook: Context OnResponseSendingFinal

The OnResponseSendingFinal hook on ZuploContext fires immediately after the response is sent to the client. The Response in this hook is immutable and the body has been used. This hook is useful for custom performing various tasks like logging or analytics.

import { ZuploContext, ZuploRequest } from "@zuplo/runtime";

export async function pluginWithHook(
request: ZuploRequest,
context: ZuploContext,
policyName: string
) {
const cloned = request.clone();
async (response, latestRequest, context) => {
const body = await cloned.text();
await fetch("", {
method: "GET",

return request;

Plugin and Handler Extensions

Built-in and custom plugins and handlers can expose their own extensibility. The AWS Lambda handler exposes the ability to customize the event that is sent when invoking the Lambda function.

The example below shows how to use a route's custom property to set the path on the outgoing event to a custom value.

import { AwsLambdaHandlerExtensions, RuntimeExtensions } from "@zuplo/runtime";

export function runtimeInit(runtime: RuntimeExtensions) {
async (request, context, event: AwsLambdaEventV1) => {
event.path = context.custom.lambdaPath ?? event.path;
return event;