diff --git a/client/utils/buildChallenges.js b/client/utils/buildChallenges.js
index 90ab93665a..a23c9aea1c 100644
--- a/client/utils/buildChallenges.js
+++ b/client/utils/buildChallenges.js
@@ -5,18 +5,13 @@ const {
   createChallenge,
   getChallengesDirForLang
 } = require('../../curriculum/getChallenges');
-const { dasherize, nameify } = require('../../utils/slugs');
 const { locale } = require('../config/env.json');
-const { blockNameify } = require('../../utils/block-nameify');
-
-const arrToString = arr =>
-  Array.isArray(arr) ? arr.join('\n') : _.toString(arr);
 
 exports.localeChallengesRootDir = getChallengesDirForLang(locale);
 
 exports.replaceChallengeNode = locale => {
   return async function replaceChallengeNode(fullFilePath) {
-    return prepareChallenge(await createChallenge(fullFilePath, null, locale));
+    return await createChallenge(fullFilePath, null, locale);
   };
 };
 
@@ -32,29 +27,7 @@ exports.buildChallenges = async function buildChallenges() {
 
   const builtChallenges = blocks
     .filter(block => !block.isPrivate)
-    .map(({ challenges }) => challenges.map(prepareChallenge))
+    .map(({ challenges }) => challenges)
     .reduce((accu, current) => accu.concat(current), []);
   return builtChallenges;
 };
-
-function prepareChallenge(challenge) {
-  challenge.name = nameify(challenge.title);
-  if (challenge.files) {
-    challenge.files = _.reduce(
-      challenge.files,
-      (map, file) => {
-        map[file.key] = {
-          ...file,
-          head: arrToString(file.head),
-          contents: arrToString(file.contents),
-          tail: arrToString(file.tail)
-        };
-        return map;
-      },
-      {}
-    );
-  }
-  challenge.block = dasherize(challenge.block);
-  challenge.superBlock = blockNameify(challenge.superBlock);
-  return challenge;
-}
diff --git a/curriculum/getChallenges.js b/curriculum/getChallenges.js
index abeaafce89..d563445834 100644
--- a/curriculum/getChallenges.js
+++ b/curriculum/getChallenges.js
@@ -1,5 +1,5 @@
 const path = require('path');
-const { findIndex } = require('lodash');
+const { findIndex, reduce, toString } = require('lodash');
 const readDirP = require('readdirp-walk');
 const { parseMarkdown } = require('../tools/challenge-md-parser');
 const fs = require('fs');
@@ -11,8 +11,10 @@ const {
 /* eslint-enable max-len*/
 const { COMMENT_TRANSLATIONS } = require('./comment-dictionary');
 
-const { dasherize } = require('../utils/slugs');
 const { isAuditedCert } = require('../utils/is-audited');
+const { dasherize, nameify } = require('../utils/slugs');
+const { createPoly } = require('../utils/polyvinyl');
+const { blockNameify } = require('../utils/block-nameify');
 
 const challengesDir = path.resolve(__dirname, './challenges');
 const metaDir = path.resolve(challengesDir, '_meta');
@@ -180,6 +182,43 @@ Trying to parse ${fullPath}`);
   challenge.template = template;
   challenge.time = time;
 
+  return prepareChallenge(challenge);
+}
+
+// gets the challenge ready for sourcing into Gatsby
+function prepareChallenge(challenge) {
+  challenge.name = nameify(challenge.title);
+  if (challenge.files) {
+    challenge.files = reduce(
+      challenge.files,
+      (map, file) => {
+        map[file.key] = {
+          ...file,
+          head: arrToString(file.head),
+          contents: arrToString(file.contents),
+          tail: arrToString(file.tail)
+        };
+        return map;
+      },
+      {}
+    );
+    // TODO: This should be something that can be folded into the above reduce
+    challenge.files = Object.keys(challenge.files)
+      .filter(key => challenge.files[key])
+      .map(key => challenge.files[key])
+      .reduce(
+        (files, file) => ({
+          ...files,
+          [file.key]: {
+            ...createPoly(file),
+            seed: file.contents.slice(0)
+          }
+        }),
+        {}
+      );
+  }
+  challenge.block = dasherize(challenge.block);
+  challenge.superBlock = blockNameify(challenge.superBlock);
   return challenge;
 }
 
@@ -255,3 +294,7 @@ function getBlockNameFromFullPath(fullFilePath) {
   const [, block] = fullFilePath.split(path.sep).reverse();
   return block;
 }
+
+function arrToString(arr) {
+  return Array.isArray(arr) ? arr.join('\n') : toString(arr);
+}
diff --git a/curriculum/test/test-challenges.js b/curriculum/test/test-challenges.js
index 3f4658301a..f8ab80c7ea 100644
--- a/curriculum/test/test-challenges.js
+++ b/curriculum/test/test-challenges.js
@@ -42,6 +42,7 @@ const {
 } = require('../../client/utils/challengeTypes');
 
 const { dasherize } = require('../../utils/slugs');
+const { sortFiles } = require('../../utils/sort-files');
 
 const { testedLang } = require('../utils');
 
@@ -50,7 +51,6 @@ const {
   buildJSChallenge
 } = require('../../client/src/templates/Challenges/utils/build');
 
-const { createPoly } = require('../../utils/polyvinyl');
 const { sortChallenges } = require('./utils/sort-challenges');
 
 const testEvaluator = require('../../client/config/test-evaluator').filename;
@@ -319,7 +319,6 @@ function populateTestsForLang({ lang, challenges, meta }) {
             });
           });
 
-          let { files = [] } = challenge;
           if (challengeType === challengeTypes.backend) {
             it('Check tests is not implemented.');
             return;
@@ -334,7 +333,6 @@ function populateTestsForLang({ lang, challenges, meta }) {
           // TODO: create more sophisticated validation now we allow for more
           // than one seed/solution file.
 
-          files = files.map(createPoly);
           it('Test suite must fail on the initial contents', async function() {
             this.timeout(5000 * tests.length + 1000);
             // suppress errors in the console.
@@ -344,7 +342,7 @@ function populateTestsForLang({ lang, challenges, meta }) {
             let testRunner;
             try {
               testRunner = await createTestRunner(
-                { ...challenge, files },
+                challenge,
                 '',
                 buildChallenge
               );
@@ -390,7 +388,7 @@ function populateTestsForLang({ lang, challenges, meta }) {
               it(`Solution ${index + 1} must pass the tests`, async function() {
                 this.timeout(5000 * tests.length + 1000);
                 const testRunner = await createTestRunner(
-                  { ...challenge, files },
+                  challenge,
                   solution,
                   buildChallenge
                 );
@@ -418,14 +416,10 @@ async function createTestRunner(
   solution,
   buildChallenge
 ) {
-  // TODO: solutions will need to be multi-file, too, with a fallback when there
-  // is only one file.
-  // we cannot simply use the solution instead of files, because the are not
-  // just the seed(s), they contain the head and tail code. The best approach
-  // is probably to separate out the head and tail from the files.  Then the
-  // files can be entirely replaced by the solution.
+  // fallback for single solution
+  const sortedFiles = sortFiles(files);
   if (solution) {
-    files[0].contents = solution;
+    files[sortedFiles[0].key].contents = solution;
   }
 
   const { build, sources, loadEnzyme } = await buildChallenge({