diff --git a/act/artifactcache/handler.go b/act/artifactcache/handler.go
index 29b15e8c..c5bb0376 100644
--- a/act/artifactcache/handler.go
+++ b/act/artifactcache/handler.go
@@ -325,10 +325,6 @@ func (h *Handler) openDB() (*bolthold.Store, error) {
 func (h *Handler) find(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
 	cred := credFromContext(r.Context())
 	keys := strings.Split(r.URL.Query().Get("keys"), ",")
-	// cache keys are case insensitive
-	for i, key := range keys {
-		keys[i] = strings.ToLower(key)
-	}
 	version := r.URL.Query().Get("version")
 
 	db, err := h.openDB()
@@ -371,8 +367,6 @@ func (h *Handler) reserve(w http.ResponseWriter, r *http.Request, _ httprouter.P
 		h.responseJSON(w, r, 400, err)
 		return
 	}
-	// cache keys are case insensitive
-	api.Key = strings.ToLower(api.Key)
 
 	cache := api.ToCache()
 	cache.Repo = cred.Repo
diff --git a/act/artifactcache/handler_test.go b/act/artifactcache/handler_test.go
index a3fe16ac..7cca691c 100644
--- a/act/artifactcache/handler_test.go
+++ b/act/artifactcache/handler_test.go
@@ -459,17 +459,20 @@ func TestHandler(t *testing.T) {
 		assert.Equal(t, contents[except], content)
 	})
 
-	t.Run("case insensitive", func(t *testing.T) {
+	t.Run("case preserved", func(t *testing.T) {
+		// Some actions (e.g. actions/setup-go, actions/setup-node) build cache keys that contain mixed-case fragments such as RUNNER_OS=Linux,
+		// then compare the cacheKey returned by the cache server to their original key with case-sensitive equality to decide whether the
+		// cache was a complete hit. The server must therefore preserve the original key case.
+
 		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d20"
-		key := strings.ToLower(t.Name())
+		key := strings.ToLower(t.Name()) + "_ABC"
 		content := make([]byte, 100)
 		_, err := rand.Read(content)
 		require.NoError(t, err)
-		uploadCacheNormally(t, base, key+"_ABC", version, content)
+		uploadCacheNormally(t, base, key, version, content)
 
 		{
-			reqKey := key + "_aBc"
-			resp, err := testClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, reqKey, version))
+			resp, err := testClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
 			require.NoError(t, err)
 			defer resp.Body.Close()
 			require.Equal(t, 200, resp.StatusCode)
@@ -480,7 +483,8 @@ func TestHandler(t *testing.T) {
 			}{}
 			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
 			assert.Equal(t, "hit", got.Result)
-			assert.Equal(t, key+"_abc", got.CacheKey)
+			assert.Equal(t, key, got.CacheKey)
+			assert.NotEqual(t, strings.ToLower(key), got.CacheKey)
 		}
 	})
 
@@ -643,7 +647,7 @@ func uploadCacheNormally(t *testing.T, base, key, version string, content []byte
 		}{}
 		require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
 		assert.Equal(t, "hit", got.Result)
-		assert.Equal(t, strings.ToLower(key), got.CacheKey)
+		assert.Equal(t, key, got.CacheKey)
 		archiveLocation = got.ArchiveLocation
 	}
 	{