Assert Like You Mean It: How To Validate Outcomes in Synthetic Tests

Most synthetic tools can already alert you when a page times out, fails to load, or throws a 4xx or 5xx error. But that is not enough. If you want synthetics to be a reliable signal in your observability toolbelt, you need to validate outcomes, not just page loads.

Assertions give your test the power to confirm that the workflow actually succeeded, not just that a page rendered.

This article explains Best Practice #2 in the Getting Synthetics Right Series: Assert Like You Mean It.

Key terms in Splunk browser synthetics

Before we dive in, here are a few terms you will see throughout this article:

• Step: A single action in a test, such as clicking a button, entering text, or loading a page.
• Transaction: A group of steps that represent a meaningful workflow, like login or checkout.
• Assertion: A validation step that confirms whether the expected outcome occurred (for example text is present, or an element is visible).

With those basics, we can now talk about how assertions fit into your tests.

Why assertions matter in browser synthetics

The word “assert” usually means to state a fact confidently. In the context of synthetic browser tests, an assertion is a step you add to your test that verifies whether the expected outcome occurred. It’s an explicit check.

Assertions can check for things like text, elements, or visibility states on the page. They can also confirm that error conditions did not appear. Think of them as truth checks that turn a basic page-load test into a workflow validation.

Examples

• After login → assert that the “Welcome” page loads and shows the user’s name.
• After submitting a form → assert that a confirmation message appears.
• Before clicking checkout → assert that the “Add to Cart” button is visible.

Without assertions, you are left with only surface-level checks: did the page load or not? With assertions, you gain confidence that what users expect has actually happened.

Why this matters

Many teams stop at navigation checks (click → load → pass/fail). But:

• A test can pass even when functionality is broken (false negative).
• A test can fail for irrelevant reasons (false positive).
• Responders waste time chasing noise.
• Real regressions slip through until customers report them.

Assertions flip the script. They make your synthetic failures actionable by ensuring they line up with actual business outcomes.

Putting it into practice: How to assert for strong synthetic tests

1. Be intentional about what you assert

Not every step needs an assertion, but every transaction should have at least one. Think about the definition of success from the end-user perspective.

• Login: the “overview” page is displayed and shows user-specific data.
• Add to cart: The item appears in the cart list.
• Report generation: The report name shows up in the dashboard.

2. Make precise assertions

Do not assert for something so generic that it could appear anywhere. For example, if you work for Buttercup Retail, the word “Buttercup” might show up in the header, footer, or every template. That is not proof the login worked.

Instead, assert for something unique to the outcome, such as:

• “Welcome, <username>” on the post-login page.
• An order number format (“Order ID ######”).
• A success icon or element that only appears after completion.

3. Choose the right type of assertion

Splunk Observability Cloud supports multiple assertion types. Each can be used to confirm (or negate) an outcome:

Pro-tip: Pick assertions that are both specific and stable. Use something unique to the transaction outcome, not generic text that might appear elsewhere on the page.

Assertions in Splunk run results

Splunk Observability Cloud highlights assertions directly in the run results filmstrip with assertion watermarks. You can also filter the playback by transaction, page, or step.

This is extremely useful when troubleshooting a failed test:

• If a login assertion fails, you can filter right to that transaction instead of scrubbing through the entire run.
• If a “Submit” button assertion fails, you can narrow the view to just that page and compare it against other runs.
• If multiple assertions are included in the same transaction, the watermarks make it clear which one triggered the failure.

The combination of step-level playback and assertion watermarks means you not only know a test failed, you know why it failed and exactly where to start investigating.

4. Use assertions before and after steps

Most people think of assertions as after-the-fact checks such as logging in and then asserting the overview page loads. That is important, but assertions can also be used before executing a step:

• Pre-validation (before a step): Ensure the page is in the right state before proceeding.

  • Example: Assert the “Add to Cart” button is visible before trying to click it.
  • Example: Assert the account balance element is present before submitting a transfer.

• Post-validation (after a step): Confirm that the action succeeded and the expected outcome is visible.

  • Example: After clicking “Submit,” assert the confirmation page loads and contains a confirmation number.
  • Example: After login, assert the “Welcome” message displays.

5. Link assertions to backend services

The best assertions validate not just the UI but the underlying service response.

• Assert that a search result is displayed because the API returned data.
• Assert that a confirmation ID is generated because the payment service responded.

Splunk makes this powerful by linking synthetic spans to APM traces, letting you drill into the backend call behind your assertion if it fails, making troubleshooting effortless.

Call to action: Asserting for success

Assertions are the difference between a test that runs and a test you can trust. By validating real outcomes at the right points, you avoid false confidence, cut noise, and give responders signals they can act on.

Next step: Review one of your synthetic browser tests. Does it only confirm navigation, or does it validate the actual business outcome? Add one meaningful assertion, pre or post, and watch how much more valuable the signal becomes.

Try it for yourself: Start your free trial of Splunk Observability Cloud and add outcome-based assertions to your tests today.