fix: only update content-length header if one was present in spy

cli/CHANGELOG.md

@@ -7,6 +7,7 @@ _Released 03/28/2023 (PENDING)_
 
  - Fixed a compatibility issue so that component test projects can use [Vite](https://vitejs.dev/) version 4.2.0 and greater. Fixes [#26138](https://github.com/cypress-io/cypress/issues/26138).
  - Changed the way that Git hashes are loaded so that non-relevant runs are excluded from the Debug page. Fixes [#26058](https://github.com/cypress-io/cypress/issues/26058).
+ - Fixed an issue where [`cy.intercept()`](https://docs.cypress.io/api/commands/intercept) added an additional `content-length` header to intercepted requests that did not set a `content-length` header on the original request. Fixes [#24407](https://github.com/cypress-io/cypress/issues/24407).
 
 **Misc:**
 

packages/net-stubbing/lib/server/middleware/request.ts

@@ -140,7 +140,7 @@ export const InterceptRequest: RequestMiddleware = async function () {
   request.req.body = req.body
 
   const mergeChanges = (before: CyHttpMessages.IncomingRequest, after: CyHttpMessages.IncomingRequest) => {
-    if (before.headers['content-length'] === after.headers['content-length']) {
+    if ('content-length' in before.headers && before.headers['content-length'] === after.headers['content-length']) {
       // user did not purposely override content-length, let's set it
       after.headers['content-length'] = String(Buffer.from(after.body).byteLength)
     }

packages/net-stubbing/test/unit/middleware-request-spec.ts

@@ -0,0 +1,80 @@
+import { expect } from 'chai'
+import sinon from 'sinon'
+import { InterceptRequest } from '../../lib/server/middleware/request'
+import { state as NetStubbingState } from '../../lib/server/state'
+
+describe('request', () => {
+  context('InterceptedRequest', () => {
+    // @see https://github.com/cypress-io/cypress/issues/24407
+    it('does not set the content-length header if the header was not there to begin with on the original request', async () => {
+      const socket = {
+        toDriver: sinon.stub(),
+      }
+      const state = NetStubbingState()
+
+      const beforeRequestData = {
+        body: 'stubbed_body',
+        proxiedUrl: 'https://foobar.com',
+        url: 'https://foobar.com',
+      }
+
+      const afterRequestData = {
+        ...beforeRequestData,
+        body: '',
+        headers: {},
+      }
+
+      // using a ES6 proxy to intercept the promise assignment on pendingEventHandlers.
+      // this way, we can resolve the event immediately
+      const pendingEventProxy = new Proxy(state.pendingEventHandlers, {
+        get (target, prop, receiver) {
+          // @ts-expect-error
+          return Reflect.get(...arguments)
+        },
+        set (obj, prop, value) {
+          // @ts-expect-error
+          const setProp = Reflect.set(...arguments)
+
+          // invoke the promise function immediately
+          if (typeof value === 'function') {
+            value({
+              changedData: afterRequestData,
+              stopPropagation: false,
+            })
+          }
+
+          return setProp
+        },
+      })
+
+      state.pendingEventHandlers = pendingEventProxy
+
+      const request = {
+        req: {
+          ...beforeRequestData,
+          headers: {},
+          matchingRoutes: [
+            {
+              id: '1',
+              hasInterceptor: true,
+              routeMatcher: {},
+            },
+          ],
+          pipe: sinon.stub(),
+        },
+        res: {
+          once: sinon.stub(),
+        },
+        socket,
+        debug: sinon.stub(),
+        netStubbingState: state,
+        next: sinon.stub(),
+        onError: sinon.stub(),
+        onResponse: sinon.stub(),
+      }
+
+      await InterceptRequest.call(request)
+      expect(request.req.headers['content-length']).to.be.undefined
+    })
+  })
+})