Custom scalar? I hardly know her!

Apollo Client 4.3 introduces native custom scalar support, enabling developers to centrally define parsing, serialization, and type-detection logic for values such as dates and URLs, then use parsed types consistently in cache reads, writes, query variables, and complex input objects. The release also adds TypeScript improvements, including configurable cache typing through TypeOverrides and improved GraphQL Codegen types that narrow incremental query data more accurately when streamed or deferred results become complete. Supporting custom scalars required broader changes to incremental delivery behavior: Apollo Client now uses defer-aware cache reads and data pruning to return transformed cache values without exposing deferred fields before the network has delivered them. Cache feud detection was also redesigned so results are always written to the cache while repeated missing-field patterns prevent infinite refetches, improving consistency and addressing a longstanding refetch issue. Additional changes include a new custom-scalars code generation plugin, support for skipToken in useSubscription, stricter typing options for the from setting, and a minimum supported TypeScript version of 5.9.

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