fix: res.send of cy.intercept doesn't override json-related content types

packages/driver/cypress/integration/commands/net_stubbing_spec.ts

@@ -2685,6 +2685,30 @@ describe('network stubbing', { retries: 2 }, function () {
       .wait('@get')
     })
 
+    // https://github.com/cypress-io/cypress/issues/17084
+    it('does not overwrite the json-related content-type header', () => {
+      cy.intercept('/json-content-type', (req) => {
+        req.on('response', (res) => {
+          res.send({
+            statusCode: 500,
+            headers: {
+              'content-type': 'application/problem+json',
+              'access-control-allow-origin': '*',
+            },
+            body: {
+              status: 500,
+              title: 'Internal Server Error',
+            },
+          })
+        })
+      })
+
+      fetch('/json-content-type')
+      .then((res) => {
+        expect(res.headers.get('content-type')).to.eq('application/problem+json')
+      })
+    })
+
     context('body parsing', function () {
       [
         'application/json',

packages/driver/src/cy/net-stubbing/events/response.ts

@@ -75,7 +75,14 @@ export const onResponse: HandlerFn<CyHttpMessages.IncomingResponse> = async (Cyp
         // arguments to res.send() are merged with the existing response
         const _staticResponse = _.defaults({}, staticResponse, _.pick(res, STATIC_RESPONSE_KEYS))
 
-        _.defaults(_staticResponse.headers, res.headers)
+        _staticResponse.headers = _.defaults({}, _staticResponse.headers, res.headers)
+
+        // https://github.com/cypress-io/cypress/issues/17084
+        // When a user didn't provide content-type,
+        // we remove the content-type provided by the server
+        if (!staticResponse.headers || !staticResponse.headers['content-type']) {
+          delete _staticResponse.headers['content-type']
+        }
 
         sendStaticResponse(requestId, _staticResponse)
 

packages/driver/src/cy/net-stubbing/static-response-utils.ts

@@ -5,6 +5,9 @@ import {
   BackendStaticResponseWithArrayBuffer,
   FixtureOpts,
 } from '@packages/net-stubbing/lib/types'
+import {
+  caseInsensitiveHas,
+} from '@packages/net-stubbing/lib/util'
 import $errUtils from '../../cypress/error_utils'
 
 // user-facing StaticResponse only
@@ -112,7 +115,16 @@ export function getBackendStaticResponse (staticResponse: Readonly<StaticRespons
       backendStaticResponse.body = staticResponse.body
     } else {
       backendStaticResponse.body = JSON.stringify(staticResponse.body)
-      _.set(backendStaticResponse, 'headers.content-type', 'application/json')
+
+      // There are various json-related MIME types. We cannot simply set it as `application/json`.
+      // @see https://www.iana.org/assignments/media-types/media-types.xhtml
+      if (
+        !backendStaticResponse.headers ||
+        (backendStaticResponse.headers &&
+          !caseInsensitiveHas(backendStaticResponse.headers, 'content-type'))
+      ) {
+        _.set(backendStaticResponse, 'headers.content-type', 'application/json')
+      }
     }
   }
 

packages/net-stubbing/lib/server/util.ts

@@ -16,6 +16,7 @@ import ThrottleStream from 'throttle'
 import MimeTypes from 'mime-types'
 import { CypressIncomingRequest } from '@packages/proxy'
 import { InterceptedRequest } from './intercepted-request'
+import { caseInsensitiveGet, caseInsensitiveHas } from '../util'
 
 // TODO: move this into net-stubbing once cy.route is removed
 import { parseContentType } from '@packages/server/lib/controllers/xhrs'
@@ -79,24 +80,6 @@ function _getFakeClientResponse (opts: {
   return clientResponse
 }
 
-const caseInsensitiveGet = function (obj, lowercaseProperty) {
-  for (let key of Object.keys(obj)) {
-    if (key.toLowerCase() === lowercaseProperty) {
-      return obj[key]
-    }
-  }
-}
-
-const caseInsensitiveHas = function (obj, lowercaseProperty) {
-  for (let key of Object.keys(obj)) {
-    if (key.toLowerCase() === lowercaseProperty) {
-      return true
-    }
-  }
-
-  return false
-}
-
 export function setDefaultHeaders (req: CypressIncomingRequest, res: IncomingMessage) {
   const setDefaultHeader = (lowercaseHeader: string, defaultValueFn: () => string) => {
     if (!caseInsensitiveHas(res.headers, lowercaseHeader)) {

packages/net-stubbing/lib/util.ts

@@ -0,0 +1,17 @@
+export const caseInsensitiveGet = function (obj, lowercaseProperty) {
+  for (let key of Object.keys(obj)) {
+    if (key.toLowerCase() === lowercaseProperty) {
+      return obj[key]
+    }
+  }
+}
+
+export const caseInsensitiveHas = function (obj, lowercaseProperty) {
+  for (let key of Object.keys(obj)) {
+    if (key.toLowerCase() === lowercaseProperty) {
+      return true
+    }
+  }
+
+  return false
+}