diff --git a/build-system/tasks/report-test-status.js b/build-system/tasks/report-test-status.js
index 29d6ef9ae3ad4..41468811d45c6 100644
--- a/build-system/tasks/report-test-status.js
+++ b/build-system/tasks/report-test-status.js
@@ -136,7 +136,7 @@ async function postReport(type, action) {
           'Content-Type': 'application/json',
         },
       });
-      const body = await response.text();
+      const body = /** @type {string} */ (await response.text());
 
       log('Reported', cyan(`${type}/${action}`), 'to GitHub');
       if (body.length > 0) {
@@ -211,7 +211,11 @@ async function reportAllExpectedTests() {
  * Callback to the Karma.Server on('run_complete') event for simple test types.
  * Optionally takes an object containing test results if they were run.
  *
- * @param {?Karma.TestResults} results
+ * @param {?{
+ *   error: string|number,
+ *   failed: string|number,
+ *   success: string|number,
+ * }} results
  */
 async function reportTestRunComplete(results) {
   if (!results || results.error) {
diff --git a/build-system/tasks/serve.js b/build-system/tasks/serve.js
index 66ac0270ebfbc..23dbe8d9d0cf4 100644
--- a/build-system/tasks/serve.js
+++ b/build-system/tasks/serve.js
@@ -105,6 +105,18 @@ async function startServer(
     started = resolve;
   });
   setServeMode(modeOptions);
+  /**
+   * @type {{
+   *   name: string,
+   *   root: string,
+   *   host: string,
+   *   port: number,
+   *   https: string,
+   *   preferHttp1: boolean,
+   *   silent: boolean,
+   *   middleware: function[],
+   * }}
+   */
   const options = {
     name: 'AMP Dev Server',
     root: process.cwd(),
@@ -161,7 +173,7 @@ function resetServerFiles() {
 async function stopServer() {
   if (url) {
     connect.serverClose();
-    log(green('Stopped server at'), cyan(url));
+    log(green('Stopped server at'), cyan(/** @type {string} */ (url)));
     url = null;
   }
 }
diff --git a/build-system/tasks/test-report-upload.js b/build-system/tasks/test-report-upload.js
index d0ed6e48e5683..11e8301b3c136 100644
--- a/build-system/tasks/test-report-upload.js
+++ b/build-system/tasks/test-report-upload.js
@@ -120,7 +120,7 @@ async function sendCiKarmaReport(testType) {
       'failed to report results of type',
       cyan(testType),
       ': \n',
-      yellow(await response.text())
+      yellow(/** @type {string} */ (await response.text()))
     );
   }
 }
diff --git a/build-system/test-configs/jscodeshift/index.js b/build-system/test-configs/jscodeshift/index.js
index 3b50d3f18fe7d..2efb1f15194b9 100644
--- a/build-system/test-configs/jscodeshift/index.js
+++ b/build-system/test-configs/jscodeshift/index.js
@@ -27,14 +27,14 @@ const command = (args = []) =>
 
 /**
  * @param {Array<string>} args
- * @param {Object} opts
- * @return {string}
+ * @param {?Object=} opts
+ * @return {!Object}
  */
 const jscodeshift = (args = [], opts) => getOutput(command(args), opts);
 
 /**
  * @param {Array<string>} args
- * @param {?Object=} opts
+ * @param {Object} opts
  * @return {ReturnType<execScriptAsync>}
  */
 const jscodeshiftAsync = (args = [], opts) =>