What a hydration mismatch is
In frameworks that render HTML on the server and then make it interactive in the browser, a process called hydration, a mismatch happens when the HTML the server produced does not match what the client expects to render. The framework notices the difference and warns, because it can cause visual glitches or broken behavior. Understanding what causes these mismatches, and fixing the underlying cause rather than suppressing the warning, is key to a correct app.
Time-dependent values
A common cause is values that differ between when the server renders and when the client does. For example, rendering the current time or a random value produces one result on the server and a different one in the browser moments later, causing a mismatch. The fix is to handle such values carefully, so the server and client agree, rather than generating something that inevitably differs between the two renders.
Browser-only APIs
Another frequent cause is using browser-only features during the initial render, things that exist in the browser but not on the server, like certain window or document properties. If your component depends on these during rendering, the server cannot produce the same output, causing a mismatch. The solution is to access browser-only features only after the component is running in the browser, not during the server render, so both environments produce consistent initial HTML.
Invalid markup
Invalid or improperly nested HTML can also cause hydration mismatches, because the browser may silently correct the markup, making what it has differ from what the server sent. For example, placing an element where the HTML rules do not allow it leads the browser to rearrange it. Ensuring your markup is valid and correctly structured removes this cause, since the browser then keeps the HTML as the server intended.
A debugging sequence
To diagnose a mismatch: read the warning, which often points to the component or element involved; then check that component for the common causes, time-dependent values, browser-only APIs used during render, and invalid markup. Narrow down which part differs between server and client. The goal is to find and fix the actual cause so server and client render the same thing, not to silence the warning, which only hides a real inconsistency.
The bottom line
A hydration mismatch means server-rendered HTML differs from what the client expects, commonly caused by time-dependent values, browser-only APIs used during render, or invalid markup. Diagnose it by following the warning to the component and checking for these causes, then fix the underlying inconsistency rather than suppressing the warning. Understanding this helps you resolve hydration issues properly instead of hiding them.
Related on Skillo
See also: What is hydration in web development? Explained, Next.js server components vs client components explained.
Sources
Published date reflects the original event date (2026-10-23). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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