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Fix React hydration errors

Hydration errors mean the server-rendered HTML did not match the first client render. The fix is to find the source of nondeterminism — dates, random values, browser-only APIs, or invalid HTML nesting.

WorkflowDifficulty: moderatemedium risk

Last verified · Updated May 22, 2026

A hydration error means the HTML React rendered on the server did not match the first client render. Track down the nondeterministic value or invalid nesting causing the mismatch — do not silence it by disabling SSR.

Symptoms

  • "Hydration failed because the server rendered HTML didn't match the client."
  • Text content or attributes flicker on first paint.
  • Errors that only appear in production SSR builds.

Likely causes

  • Rendering Date.now()/new Date()/Math.random() during render.
  • Reading window/localStorage during the initial render.
  • Invalid HTML nesting (e.g. <div> inside <p>).
  • Locale- or timezone-dependent formatting differing between server and client.

Diagnostic commands

shell
# Reproduce with a production-like SSR build$ npm run build && npm run start# Then open the page and read the first hydration warning's component stack
Test failure debuggingAI debugging prompt
I'm getting a React hydration mismatch on <route>. Here is the warning and component stack: <paste>. Inspect the named component and its children for nondeterministic render values (dates, random, window/localStorage) and invalid HTML nesting. Propose the smallest change that makes server and client render identically — prefer useEffect or suppressHydrationWarning only as a last resort, and explain the tradeoff.

Safety: Do not disable SSR or wrap large trees in suppressHydrationWarning to hide the real cause.

Safe fix workflow

  • Move nondeterministic values into useEffect so they only run on the client.
  • Guard browser-only APIs behind a mounted check.
  • Fix invalid HTML nesting flagged by the warning.
  • Re-run the production SSR build and confirm the warning is gone.

Escalation checklist

  • Mismatch persists after removing obvious nondeterminism
  • A third-party component renders differently across environments
  • The mismatch only reproduces under a specific locale/timezone

Related upgrade pages

Frequently asked questions

Is suppressHydrationWarning a real fix?

Only for intentionally dynamic content like timestamps on a single element. Using it to mask a structural mismatch hides bugs that will resurface.