diff --git a/packages/driver/cypress/integration/commands/task_spec.js b/packages/driver/cypress/integration/commands/task_spec.js
index 62c2e4d1498..56c57d21d46 100644
--- a/packages/driver/cypress/integration/commands/task_spec.js
+++ b/packages/driver/cypress/integration/commands/task_spec.js
@@ -210,7 +210,7 @@ describe('src/cy/commands/task', () => {
           expect(lastLog.get('error')).to.eq(err)
           expect(lastLog.get('state')).to.eq('failed')
 
-          expect(err.message).to.eq(`\`cy.task('bar')\` failed with the following error:\n\nThe task 'bar' was not handled in the plugins file. The following tasks are registered: return:arg, arg:is:undefined, wait, create:long:file\n\nFix this in your plugins file here:\n${Cypress.config('pluginsFile')}`)
+          expect(err.message).to.eq(`\`cy.task('bar')\` failed with the following error:\n\nThe task 'bar' was not handled in the plugins file. The following tasks are registered: return:arg, cypress:env, arg:is:undefined, wait, create:long:file\n\nFix this in your plugins file here:\n${Cypress.config('pluginsFile')}`)
 
           done()
         })
diff --git a/packages/driver/cypress/integration/issues/18805_spec.js b/packages/driver/cypress/integration/issues/18805_spec.js
new file mode 100644
index 00000000000..d5e88f8574d
--- /dev/null
+++ b/packages/driver/cypress/integration/issues/18805_spec.js
@@ -0,0 +1,5 @@
+describe('CYPRESS env var', () => {
+  it('checks that the CYPRESS env var exists in the plugin file', () => {
+    cy.task('cypress:env').should('eq', 'true')
+  })
+})
diff --git a/packages/driver/cypress/plugins/index.js b/packages/driver/cypress/plugins/index.js
index 0aa63efb502..57b70ee98a0 100644
--- a/packages/driver/cypress/plugins/index.js
+++ b/packages/driver/cypress/plugins/index.js
@@ -38,6 +38,9 @@ module.exports = (on) => {
     'return:arg' (arg) {
       return arg
     },
+    'cypress:env' () {
+      return process.env['CYPRESS']
+    },
     'arg:is:undefined' (arg) {
       if (arg === undefined) {
         return 'arg was undefined'
diff --git a/packages/server/lib/environment.js b/packages/server/lib/environment.js
index e225a4292f8..09e21d3c9c0 100644
--- a/packages/server/lib/environment.js
+++ b/packages/server/lib/environment.js
@@ -25,6 +25,8 @@ const pkg = require('@packages/root')
 // or development as default
 const env = process.env['CYPRESS_INTERNAL_ENV'] || (process.env['CYPRESS_INTERNAL_ENV'] = pkg.env != null ? pkg.env : 'development')
 
+process.env['CYPRESS'] = 'true'
+
 const config = {
   // uses cancellation for automation timeouts
   cancellation: true,
diff --git a/packages/server/test/unit/environment_spec.js b/packages/server/test/unit/environment_spec.js
index 21c40beaefb..e130b9b7ebd 100644
--- a/packages/server/test/unit/environment_spec.js
+++ b/packages/server/test/unit/environment_spec.js
@@ -134,4 +134,10 @@ describe('lib/environment', () => {
       return expectedEnv('development')
     })
   })
+
+  context('it sets process.env.CYPRESS', () => {
+    it('sets CYPRESS=true when Cypress runs', () => {
+      expect(process.env['CYPRESS']).to.eq('true')
+    })
+  })
 })