diff --git a/src/host/main.js b/src/host/main.js
index 6031bce..6092f48 100644
--- a/src/host/main.js
+++ b/src/host/main.js
@@ -82,7 +82,7 @@ export async function hostMain(options) {
         emitter.on(HostMessage.STOP_RUNNER, () => resolve());
       }),
       timesOption,
-      baseUrl: serverDirectory.baseUrl,
+      baseUrl: new URL(serverDirectory.baseUrl.toString()),
       mock: runnerMockOptions({
         mock: runnerMock,
         mockOpenPage: runnerMockOpenPage,
diff --git a/src/runner/create-test-runner.js b/src/runner/create-test-runner.js
index 4d72573..4dde0d4 100644
--- a/src/runner/create-test-runner.js
+++ b/src/runner/create-test-runner.js
@@ -13,7 +13,7 @@ import { createATDriver } from './at-driver.js';
 /**
  * @param {object} options
  * @param {{hostname: string, port: number | string, pathname: string}} options.atDriverUrl
- * @param {AriaATCIShared.BaseURL} options.baseUrl
+ * @param {URL} options.baseUrl
  * @param {AriaATCIHost.Log} options.log
  * @param {Promise<void>} options.abortSignal
  * @param {AriaATCIRunner.MockOptions} [options.mock]
diff --git a/src/runner/driver-test-runner.js b/src/runner/driver-test-runner.js
index a50a93c..c438a1d 100644
--- a/src/runner/driver-test-runner.js
+++ b/src/runner/driver-test-runner.js
@@ -14,7 +14,7 @@ import { RunnerMessage } from './messages.js';
 export class DriverTestRunner {
   /**
    * @param {object} options
-   * @param {AriaATCIShared.BaseURL} options.baseUrl
+   * @param {URL} options.baseUrl
    * @param {AriaATCIHost.Log} options.log
    * @param {BrowserDriver} options.browserDriver
    * @param {ATDriver} options.atDriver
@@ -288,10 +288,8 @@ export class DriverTestRunner {
   }
 
   _appendBaseUrl(pathname) {
-    // protocol ends with a ':' and pathname starts with a '/'
-    const base = `${this.baseUrl.protocol}://${this.baseUrl.hostname}:${this.baseUrl.port}${this.baseUrl.pathname}`;
     const newPath = `${this.baseUrl.pathname ? `${this.baseUrl.pathname}/` : ''}${pathname}`;
-    return new URL(newPath, base);
+    return new URL(newPath, this.baseUrl.toString());
   }
 }
 
diff --git a/src/runner/mock-test-runner.js b/src/runner/mock-test-runner.js
index 31254fe..d93c891 100644
--- a/src/runner/mock-test-runner.js
+++ b/src/runner/mock-test-runner.js
@@ -16,7 +16,7 @@ import { validateKeysFromCommand } from './driver-test-runner.js';
 export class MockTestRunner {
   /**
    * @param {object} options
-   * @param {AriaATCIShared.BaseURL} options.baseUrl
+   * @param {URL} options.baseUrl
    * @param {AriaATCIHost.Log} options.log
    * @param {AriaATCIRunner.MockOptions} options.mock
    */
@@ -64,11 +64,10 @@ export class MockTestRunner {
    * @param {AriaATCIData.CollectedTest} task
    */
   async run(task) {
-    const base = `${this.baseUrl.protocol}://${this.baseUrl.hostname}:${this.baseUrl.port}${this.baseUrl.pathname}`;
     await this.openPage(
       new URL(
         `${this.baseUrl.pathname ? `${this.baseUrl.pathname}/` : ''}${task.target.referencePage}`,
-        base
+        this.baseUrl.toString()
       )
     );