diff --git a/api/src/routes/challenge.test.ts b/api/src/routes/challenge.test.ts
index b8ab313ffc6..03b3cad10a2 100644
--- a/api/src/routes/challenge.test.ts
+++ b/api/src/routes/challenge.test.ts
@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/naming-convention */
 /* eslint-disable @typescript-eslint/no-unsafe-member-access */
 /* eslint-disable @typescript-eslint/no-unsafe-assignment */
+import { omit } from 'lodash';
 import { challengeTypes } from '../../../config/challenge-types';
 import { devLogin, setupServer, superRequest } from '../../jest.utils';
 
@@ -37,6 +38,69 @@ const backendChallengeBody2 = {
   id: backendChallengeId2
 };
 
+// /modern-challenge-completed
+const HtmlChallengeId = '5dc174fcf86c76b9248c6eb2';
+const JsProjectId = '56533eb9ac21ba0edf2244e2';
+const multiFileCertProjectId = 'bd7158d8c242eddfaeb5bd13';
+
+const HtmlChallengeBody = {
+  challengeType: challengeTypes.html,
+  id: HtmlChallengeId
+};
+const JsProjectBody = {
+  challengeType: challengeTypes.jsProject,
+  id: JsProjectId,
+  files: [
+    {
+      contents: 'console.log("Hello There!")',
+      key: 'scriptjs',
+      ext: 'js',
+      name: 'script',
+      history: ['script.js']
+    }
+  ]
+};
+const multiFileCertProjectBody = {
+  challengeType: challengeTypes.multifileCertProject,
+  id: multiFileCertProjectId,
+  files: [
+    {
+      contents: '<h1>Multi File Project v1</h1>',
+      key: 'indexhtml',
+      ext: 'html',
+      name: 'index',
+      history: ['index.html']
+    },
+    {
+      contents: '.hello-there { general: kenobi; }',
+      key: 'stylescss',
+      ext: 'css',
+      name: 'styles',
+      history: ['styles.css']
+    }
+  ]
+};
+const updatedMultiFileCertProjectBody = {
+  challengeType: challengeTypes.multifileCertProject,
+  id: multiFileCertProjectId,
+  files: [
+    {
+      contents: '<h1>Multi File Project v2</h1>',
+      key: 'indexhtml',
+      ext: 'html',
+      name: 'index',
+      history: ['index.html']
+    },
+    {
+      contents: '.wibbly-wobbly { timey: wimey; }',
+      key: 'stylescss',
+      ext: 'css',
+      name: 'styles',
+      history: ['styles.css']
+    }
+  ]
+};
+
 describe('challengeRoutes', () => {
   setupServer();
   describe('Authenticated user', () => {
@@ -603,6 +667,254 @@ describe('challengeRoutes', () => {
         });
       });
     });
+
+    describe('/modern-challenge-completed', () => {
+      describe('validation', () => {
+        test('POST rejects requests without ids', async () => {
+          const response = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          });
+
+          expect(response.statusCode).toBe(400);
+          expect(response.body).toStrictEqual(
+            isValidChallengeCompletionErrorMsg
+          );
+        });
+
+        test('POST rejects requests without valid ObjectIDs', async () => {
+          const response = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send({ id: 'not-a-valid-id' });
+
+          expect(response.statusCode).toBe(400);
+          expect(response.body).toStrictEqual(
+            isValidChallengeCompletionErrorMsg
+          );
+        });
+      });
+
+      describe('handling', () => {
+        afterEach(async () => {
+          await fastifyTestInstance.prisma.user.updateMany({
+            where: { email: 'foo@bar.com' },
+            data: {
+              completedChallenges: [],
+              savedChallenges: [],
+              progressTimestamps: []
+            }
+          });
+        });
+
+        // HTML(0), JS(1), Modern(6), Video(11), The Odin Project(15)
+        test('POST accepts challenges without files present', async () => {
+          const now = Date.now();
+
+          const response = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send(HtmlChallengeBody);
+
+          const user = await fastifyTestInstance.prisma.user.findFirstOrThrow({
+            where: { email: 'foo@bar.com' }
+          });
+
+          expect(user).toMatchObject({
+            completedChallenges: [
+              {
+                id: HtmlChallengeId,
+                completedDate: expect.any(Number)
+              }
+            ]
+          });
+
+          const completedDate = user.completedChallenges[0]?.completedDate;
+          expect(completedDate).toBeGreaterThanOrEqual(now);
+          expect(completedDate).toBeLessThanOrEqual(now + 1000);
+
+          expect(response.statusCode).toBe(200);
+          expect(response.body).toStrictEqual({
+            alreadyCompleted: false,
+            points: 1,
+            completedDate,
+            savedChallenges: []
+          });
+        });
+
+        // JS Project(5), Multi-file Cert Project(14)
+        test('POST accepts challenges with files present', async () => {
+          const now = Date.now();
+
+          const response = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send(JsProjectBody);
+
+          const user = await fastifyTestInstance.prisma.user.findFirstOrThrow({
+            where: { email: 'foo@bar.com' }
+          });
+
+          const file = omit(JsProjectBody.files[0], 'history');
+
+          expect(user).toMatchObject({
+            completedChallenges: [
+              {
+                id: JsProjectId,
+                challengeType: JsProjectBody.challengeType,
+                files: [file],
+                completedDate: expect.any(Number)
+              }
+            ]
+          });
+
+          const completedDate = user.completedChallenges[0]?.completedDate;
+          expect(completedDate).toBeGreaterThanOrEqual(now);
+          expect(completedDate).toBeLessThanOrEqual(now + 1000);
+
+          expect(response.statusCode).toBe(200);
+          expect(response.body).toStrictEqual({
+            alreadyCompleted: false,
+            points: 1,
+            completedDate,
+            savedChallenges: []
+          });
+        });
+
+        test('POST accepts challenges with saved solutions', async () => {
+          const now = Date.now();
+
+          const response = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send(multiFileCertProjectBody);
+
+          const user = await fastifyTestInstance.prisma.user.findFirstOrThrow({
+            where: { email: 'foo@bar.com' }
+          });
+
+          const testFiles = multiFileCertProjectBody.files.map(
+            ({ history: _history, ...rest }) => rest
+          );
+
+          expect(user).toMatchObject({
+            needsModeration: true,
+            completedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                challengeType: multiFileCertProjectBody.challengeType,
+                files: testFiles,
+                completedDate: expect.any(Number),
+                isManuallyApproved: true
+              }
+            ],
+            savedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                lastSavedDate: expect.any(Number),
+                files: multiFileCertProjectBody.files
+              }
+            ]
+          });
+
+          const completedDate = user.completedChallenges[0]?.completedDate;
+          expect(completedDate).toBeGreaterThanOrEqual(now);
+          expect(completedDate).toBeLessThanOrEqual(now + 1000);
+
+          expect(response.statusCode).toBe(200);
+          expect(response.body).toStrictEqual({
+            alreadyCompleted: false,
+            points: 1,
+            completedDate,
+            savedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                lastSavedDate: completedDate,
+                files: multiFileCertProjectBody.files
+              }
+            ]
+          });
+        });
+
+        test('POST correctly handles multiple requests', async () => {
+          const resOriginal = await superRequest(
+            '/modern-challenge-completed',
+            {
+              method: 'POST',
+              setCookies
+            }
+          ).send(multiFileCertProjectBody);
+
+          await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send(HtmlChallengeBody);
+
+          const resUpdate = await superRequest('/modern-challenge-completed', {
+            method: 'POST',
+            setCookies
+          }).send(updatedMultiFileCertProjectBody);
+
+          const user = await fastifyTestInstance.prisma.user.findFirstOrThrow({
+            where: { email: 'foo@bar.com' }
+          });
+
+          const expectedProgressTimestamps = user.completedChallenges.map(
+            challenge => challenge.completedDate
+          );
+
+          const testFiles = updatedMultiFileCertProjectBody.files.map(file =>
+            omit(file, 'history')
+          );
+
+          expect(user).toMatchObject({
+            needsModeration: true,
+            completedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                challengeType: updatedMultiFileCertProjectBody.challengeType,
+                files: testFiles,
+                completedDate: expect.any(Number),
+                isManuallyApproved: true
+              },
+              {
+                id: HtmlChallengeId,
+                completedDate: expect.any(Number)
+              }
+            ],
+            savedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                lastSavedDate: expect.any(Number),
+                files: updatedMultiFileCertProjectBody.files
+              }
+            ],
+            progressTimestamps: expectedProgressTimestamps
+          });
+
+          expect(
+            resUpdate.body.savedChallenges[0].lastSavedDate
+          ).toBeGreaterThan(
+            // eslint-disable-next-line @typescript-eslint/no-unsafe-argument
+            resOriginal.body.savedChallenges[0].lastSavedDate
+          );
+
+          expect(resUpdate.statusCode).toBe(200);
+          expect(resUpdate.body).toStrictEqual({
+            alreadyCompleted: true,
+            points: 2,
+            completedDate: expect.any(Number),
+            savedChallenges: [
+              {
+                id: multiFileCertProjectId,
+                lastSavedDate: expect.any(Number),
+                files: updatedMultiFileCertProjectBody.files
+              }
+            ]
+          });
+        });
+      });
+    });
   });
 
   describe('Unauthenticated user', () => {
@@ -644,5 +956,14 @@ describe('challengeRoutes', () => {
 
       expect(response.statusCode).toBe(401);
     });
+
+    test('POST /modern-challenge-completed returns 401 status code with error message', async () => {
+      const response = await superRequest('/modern-challenge-completed', {
+        method: 'POST',
+        setCookies
+      });
+
+      expect(response?.statusCode).toBe(401);
+    });
   });
 });
diff --git a/api/src/routes/challenge.ts b/api/src/routes/challenge.ts
index 362f67c66f8..2bc84d9f893 100644
--- a/api/src/routes/challenge.ts
+++ b/api/src/routes/challenge.ts
@@ -1,13 +1,18 @@
 import { type FastifyPluginCallbackTypebox } from '@fastify/type-provider-typebox';
 import jwt from 'jsonwebtoken';
 import { uniqBy } from 'lodash';
-import { getChallenges } from '../utils/get-challenges';
-import { updateUserChallengeData } from '../utils/common-challenge-functions';
-import { formatValidationError } from '../utils/error-formatting';
-import { schemas } from '../schemas';
-import { getPoints, ProgressTimestamp } from '../utils/progress';
-import { JWT_SECRET } from '../utils/env';
 import { challengeTypes } from '../../../config/challenge-types';
+import { schemas } from '../schemas';
+import {
+  jsCertProjectIds,
+  multifileCertProjectIds,
+  updateUserChallengeData,
+  type CompletedChallenge
+} from '../utils/common-challenge-functions';
+import { JWT_SECRET } from '../utils/env';
+import { formatValidationError } from '../utils/error-formatting';
+import { getChallenges } from '../utils/get-challenges';
+import { ProgressTimestamp, getPoints } from '../utils/progress';
 import {
   canSubmitCodeRoadCertProject,
   createProject,
@@ -310,5 +315,69 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
     }
   );
 
+  fastify.post(
+    '/modern-challenge-completed',
+    {
+      schema: schemas.modernChallengeCompleted,
+      errorHandler(error, request, reply) {
+        if (error.validation) {
+          void reply.code(400);
+          return formatValidationError(error.validation);
+        } else {
+          fastify.errorHandler(error, request, reply);
+        }
+      }
+    },
+    async (req, reply) => {
+      try {
+        const { id, files, challengeType } = req.body;
+
+        const user = await fastify.prisma.user.findUniqueOrThrow({
+          where: { id: req.session.user.id }
+        });
+        const RawProgressTimestamp = user.progressTimestamps as
+          | ProgressTimestamp[]
+          | null;
+        const points = getPoints(RawProgressTimestamp);
+
+        const completedChallenge: CompletedChallenge = {
+          id,
+          files,
+          completedDate: Date.now()
+        };
+
+        if (challengeType === challengeTypes.multifileCertProject) {
+          completedChallenge.isManuallyApproved = true;
+          user.needsModeration = true;
+        }
+
+        if (
+          jsCertProjectIds.includes(id) ||
+          multifileCertProjectIds.includes(id)
+        ) {
+          completedChallenge.challengeType = challengeType;
+        }
+
+        const { alreadyCompleted, userSavedChallenges: savedChallenges } =
+          await updateUserChallengeData(fastify, user, id, completedChallenge);
+
+        return {
+          alreadyCompleted,
+          points: alreadyCompleted ? points : points + 1,
+          completedDate: completedChallenge.completedDate,
+          savedChallenges
+        };
+      } catch (error) {
+        fastify.log.error(error);
+        void reply.code(500);
+        return {
+          message:
+            'Oops! Something went wrong. Please try again in a moment or contact support@freecodecamp.org if the error persists.',
+          type: 'danger'
+        } as const;
+      }
+    }
+  );
+
   done();
 };
diff --git a/api/src/schemas.ts b/api/src/schemas.ts
index c15789bf3a8..0570117218a 100644
--- a/api/src/schemas.ts
+++ b/api/src/schemas.ts
@@ -423,5 +423,60 @@ export const schemas = {
         )
       })
     }
+  },
+  modernChallengeCompleted: {
+    body: Type.Object({
+      id: Type.String({ format: 'objectid', maxLength: 24, minLength: 24 }),
+      challengeType: Type.Number(),
+      files: Type.Optional(
+        Type.Array(
+          Type.Object({
+            contents: Type.String(),
+            key: Type.String(),
+            ext: Type.String(),
+            name: Type.String(),
+            history: Type.Array(Type.String())
+          })
+        )
+      )
+    }),
+    response: {
+      200: Type.Object({
+        completedDate: Type.Number(),
+        points: Type.Number(),
+        alreadyCompleted: Type.Boolean(),
+        savedChallenges: Type.Array(
+          Type.Object({
+            id: Type.String({
+              format: 'objectid',
+              maxLength: 24,
+              minLength: 24
+            }),
+            lastSavedDate: Type.Number(),
+            files: Type.Array(
+              Type.Object({
+                contents: Type.String(),
+                key: Type.String(),
+                ext: Type.String(),
+                name: Type.String(),
+                history: Type.Array(Type.String())
+              })
+            )
+          })
+        )
+      }),
+      400: Type.Object({
+        type: Type.Literal('error'),
+        message: Type.Literal(
+          'That does not appear to be a valid challenge submission.'
+        )
+      }),
+      500: Type.Object({
+        type: Type.Literal('danger'),
+        message: Type.Literal(
+          'Oops! Something went wrong. Please try again in a moment or contact support@freecodecamp.org if the error persists.'
+        )
+      })
+    }
   }
 };
diff --git a/api/src/utils/common-challenge-functions.ts b/api/src/utils/common-challenge-functions.ts
index b1831b059b8..b615beaa9cf 100644
--- a/api/src/utils/common-challenge-functions.ts
+++ b/api/src/utils/common-challenge-functions.ts
@@ -4,7 +4,7 @@ import { omit, pick } from 'lodash';
 import { challengeTypes } from '../../../config/challenge-types';
 import { getChallenges } from './get-challenges';
 
-const jsCertProjectIds = [
+export const jsCertProjectIds = [
   'aaa48de84e1ecc7c742e1124',
   'a7f4d8f2483413a6ce226cac',
   '56533eb9ac21ba0edf2244e2',
@@ -12,7 +12,7 @@ const jsCertProjectIds = [
   'aa2e6f85cab2ab736c9a9b24'
 ];
 
-const multifileCertProjectIds = getChallenges()
+export const multifileCertProjectIds = getChallenges()
   .filter(c => c.challengeType === challengeTypes.multifileCertProject)
   .map(c => c.id);
 
@@ -24,14 +24,14 @@ type SavedChallengeFile = {
   key: string;
   ext: string; // NOTE: This is Ext type in client
   name: string;
-  history?: string[];
+  history: string[];
   contents: string;
 };
 
 type SavedChallenge = {
   id: string;
   lastSavedDate: number;
-  files?: SavedChallengeFile[];
+  files: SavedChallengeFile[];
 };
 
 // TODO: Confirm this type - read comments below
@@ -55,7 +55,7 @@ type CompletedChallengeFile = {
   path?: string | null;
 };
 
-type CompletedChallenge = {
+export type CompletedChallenge = {
   id: string;
   solution?: string | null;
   githubLink?: string | null;