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Error: Hydration Mismatch. Server Action state did not match between server and client.React 19 introduces 'useActionState' (formerly useFormState). Hydration mismatches occur when the server-rendered HTML for a form's pendency state (e.g., a 'Submitting...' spinner) does not match the initial client-side JS state, often due to micro-tasks firing too early.
React 19's useActionState handles loading internally. Avoid external state variables that can get out of sync.
const [state, action, isPending] = useActionState(fn, initial);If the mismatch is purely visual and non-breaking, use 'suppressHydrationWarning'.
<div suppressHydrationWarning>{new Date().toLocaleTimeString()}</div>In React 19, the hydration process has been made more resilient, but the introduction of Actions and the useActionState hook creates new edge cases for SSR (Server-Side Rendering).
Hydration mismatches in Server Actions often occur because the server renders the outcome of an action (e.g., a success message), but the client-side micro-task queue hasn't processed the corresponding state update yet. This results in a 'flicker' where the UI reverts to its initial state for a few milliseconds before correcting itself.
Search engines like Google Bot can now 'wait' for hydration to complete. If they see a mismatch between the rendered HTML and the hydrated state, it can lead to 'flickering' indexing results where the bot isn't sure which version of the content to rank. Always ensure your initial state is stable and derived from props rather than ephemeral local state.