diff --git a/api/src/routes/challenge.test.ts b/api/src/routes/challenge.test.ts
index 239c023695d..1d999f0b008 100644
--- a/api/src/routes/challenge.test.ts
+++ b/api/src/routes/challenge.test.ts
@@ -362,7 +362,33 @@ describe('challengeRoutes', () => {
           expect(response.body).toStrictEqual(
             isValidChallengeCompletionErrorMsg
           );
-          expect(response.statusCode).toBe(400);
+          expect(response.statusCode).toBe(403);
+        });
+
+        it('POST rejects backendProject requests without URL githubLinks', async () => {
+          const response = await superPost('/project-completed').send({
+            id: id1,
+            challengeType: challengeTypes.backEndProject,
+            // Solution is allowed to be localhost for backEndProject
+            solution: 'http://localhost:3000'
+          });
+
+          expect(response.body).toStrictEqual(
+            isValidChallengeCompletionErrorMsg
+          );
+          expect(response.statusCode).toBe(403);
+
+          const response_2 = await superPost('/project-completed').send({
+            id: id1,
+            challengeType: challengeTypes.backEndProject,
+            solution: 'http://localhost:3000',
+            githubLink: 'not-a-valid-url'
+          });
+
+          expect(response_2.body).toStrictEqual(
+            isValidChallengeCompletionErrorMsg
+          );
+          expect(response_2.statusCode).toBe(403);
         });
 
         it('POST rejects CodeRoad/CodeAlly projects when the user has not completed the required challenges', async () => {
diff --git a/api/src/routes/challenge.ts b/api/src/routes/challenge.ts
index 7133b0ac3ee..52e9fa1dd44 100644
--- a/api/src/routes/challenge.ts
+++ b/api/src/routes/challenge.ts
@@ -2,6 +2,7 @@ import { type FastifyPluginCallbackTypebox } from '@fastify/type-provider-typebo
 import jwt from 'jsonwebtoken';
 import { uniqBy } from 'lodash';
 import { CompletedExam, ExamResults } from '@prisma/client';
+import isURL from 'validator/lib/isURL';
 
 import { challengeTypes } from '../../../shared/config/challenge-types';
 import { schemas } from '../schemas';
@@ -192,9 +193,28 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
       }
     },
     async (req, reply) => {
+      // TODO: considering validation is determined by `challengeType`, it should not come from the client
+      //       Determine `challengeType` by `id`
       const { id: projectId, challengeType, solution, githubLink } = req.body;
       const userId = req.user?.id;
 
+      // If `backEndProject`:
+      // - `solution` needs to exist, but does not have to be valid URL
+      // - `githubLink` needs to exist and be valid URL
+      if (challengeType === challengeTypes.backEndProject) {
+        if (!solution || !isURL(githubLink + '')) {
+          return void reply.code(403).send({
+            type: 'error',
+            message: 'That does not appear to be a valid challenge submission.'
+          });
+        }
+      } else if (solution && !isURL(solution + '')) {
+        return void reply.code(403).send({
+          type: 'error',
+          message: 'That does not appear to be a valid challenge submission.'
+        });
+      }
+
       try {
         const user = await fastify.prisma.user.findUniqueOrThrow({
           where: { id: userId },
@@ -221,6 +241,7 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
         const oldChallenge = user.completedChallenges?.find(
           ({ id }) => id === projectId
         );
+
         const updatedChallenge = {
           challengeType,
           solution,
diff --git a/api/src/schemas.ts b/api/src/schemas.ts
index 2b615003390..a108834b404 100644
--- a/api/src/schemas.ts
+++ b/api/src/schemas.ts
@@ -526,8 +526,8 @@ export const schemas = {
     body: Type.Object({
       id: Type.String({ format: 'objectid', maxLength: 24, minLength: 24 }),
       challengeType: Type.Optional(Type.Number()),
-      solution: Type.String({ format: 'url', maxLength: 1024 }),
-      // TODO(Post-MVP): require format: 'url' for githubLink
+      // The solution must be a valid URL only if it is a `backEndProject`.
+      solution: Type.String({ maxLength: 1024 }),
       githubLink: Type.Optional(Type.String())
     }),
     response: {
@@ -551,9 +551,14 @@ export const schemas = {
       }),
       403: Type.Object({
         type: Type.Literal('error'),
-        message: Type.Literal(
-          'You have to complete the project before you can submit a URL.'
-        )
+        message: Type.Union([
+          Type.Literal(
+            'You have to complete the project before you can submit a URL.'
+          ),
+          Type.Literal(
+            'That does not appear to be a valid challenge submission.'
+          )
+        ])
       }),
       500: Type.Object({
         message: Type.Literal(