Preview TMDL Script Changes with the TMDL View in Power BI

Working with Tabular Model Definition Language (TMDL) provides a powerful way to define and manage your Power BI semantic models through scripts. But before applying any script, especially one copied or adapted from another source, it's crucial to understand its potential impact.

That’s where the TMDL View Preview comes in.

In this blog, we’ll walk you through how TMDL preview works, how it helps you understand script changes before applying them, and what to keep in mind while using it.

🧠 What Is TMDL Preview?

The TMDL View Preview is a feature that allows you to see a side-by-side comparison of your semantic model before and after executing a TMDL script, shown as a TMDL code diff.

This preview provides an easy-to-read visual of what will change, helping you avoid mistakes and better understand the implications of script modifications, before you commit them to your semantic model.

Detailed Video: https://youtu.be/WobpB3ZR3L0?si=Hv3D7yvQwt4v_7n8

🧪 How It Works

When working in the TMDL view and editing a script:

Click the Preview button located in the command bar.

A side-by-side comparison window appears in the right pane.

This feature is particularly helpful when you've pasted in scripts from documentation, blogs, or colleagues, and want to safely inspect what would be affected.

📝 Important Notes

While the preview is incredibly useful, there are some technical aspects and limitations to understand:

• 🔄 The preview is not a literal diff of the code you wrote. Instead, it performs a full semantic model diff, based on how your script would change the model.

• 🛑 The preview is read-only. You can’t edit the model from the preview screen directly.

• 🛠 If you make changes to the script after previewing, you have to click on Update preview.

🧭 Navigating the Preview Toolbar

In the upper-right corner of the preview pane, there’s a toolbar to help you navigate the diff efficiently:

• 🔁 Toggle between inline or side-by-side view.

• 👁 Show or hide unchanged regions.

• ❌ Close the preview pane when done.

This navigation flexibility helps you focus on what matters most, the parts of your model that will be impacted.

⚠️ Considerations When Using TMDL Preview

Keep these key points in mind:

1. 🧼 The TMDL view resets its configuration to default every time you preview a script. You may need to reconfigure your view after each preview.

2. ✅ A valid TMDL script is required to run the preview. If your script contains syntax errors or invalid definitions:

✅ Final Thoughts

The TMDL script preview feature is a must-use tool for anyone working with semantic models in Power BI using TMDL. It gives you the confidence and clarity needed to ensure that your changes are safe, correct, and as expected—before execution.

Whether you’re experimenting with new model changes, reviewing scripts from peers, or maintaining an enterprise-grade model, this feature will save time and reduce risk.

🔔 Have questions about using TMDL view or want help creating scripts? Feel free to connect with me on Anmol Malviya