A soft fork changes a blockchain's rules in a way that remains backward-compatible with older software. Blocks created under the new rules are still valid to old nodes, because the new rules are a stricter subset of the old ones. This lets the network upgrade without forcing every participant to update simultaneously. Soft forks avoid the chain-split risk of hard forks. The network transitions smoothly because older nodes continue accepting new blocks. Bitcoin's SegWit upgrade was a soft fork that changed how transaction signatures are stored, freeing up block space for more transactions without breaking older wallets. Ethereum's London upgrade added a fee-burn mechanism while keeping legacy clients operational. For developers, soft forks mean they can write code using new rules without abandoning users on old versions. For end users, the change is usually invisible.
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