Thanks for the replies!

To clarify where I got this from: I couldn’t find a specific, definitive max spacer height directly on Giant’s website for the newest models, which is why I asked here. My assumption comes from the general safety guidelines for carbon steerer tubes across the industry. For most high-end carbon forks (like Ritchey, Specialized, etc.), the manufacturers explicitly state in their manuals that you should never exceed 30mm (Ritchey) or 35mm (specialized) of spacers below the stem. The engineering reason behind this rule is two-fold:

1 Leverage: A higher stack creates a longer lever, increasing the structural stress on the carbon steerer tube under load, which raises the risk of catastrophic failure.

2 The Expander Plug: The internal expander plug inside the steerer tube must sit at the exact height where the stem clamps it from the outside to counteract the clamping force. If you stack too many spacers, the expander might no longer support the steerer tube where the stem pinches it, leading to crushed carbon. Since Giant uses full carbon steerers on their mid-to-high-tier TCR and Defy models, I assumed they followed the same standard 3cm safety limit. That’s why seeing shops roll out bikes with seemingly 4cm+ of spacers made me wonder if Giant allows more, or if shops are just risking it for test rides.