Breaking Change: Correcting Custom Network Chain and Network ID Values MetaMask – Medium

Fixed Custom Network Chain and Network ID Values

Beginning in MetaMask 8.1, we are correcting the value that the MetaMask extension emits with the chainChanged event for some custom networks. This could cause issues for sites that are relying on the incorrect behavior.

As a dapp developer, you should do the following to handle this change:

  1. Always use the chainChanged event to detect network changes. The networkChanged event is deprecated.
  2. If you need to continue using the networkChanged event, ensure that your dapp expects it to emit with decimal string values. Previously, this event could have emitted with a hexadecimal string for some custom networks.


The MetaMask provider emits the chainChanged and networkChanged events when the user switches the network in MetaMask. Due to a bug, in cases where the user added a custom network and specified its chain ID, MetaMask emitted the specified chain ID with networkChanged, and a nonsense value with chainChanged.

Prior to MetaMask 8.1, the specified chain ID for a custom network could be a decimal or hexadecimal string. This meant that the networkChanged event could emit with decimal or hexadecimal string values for the affected custom networks.

For example, if the user entered a “chain ID” of 0x64 for a custom network and switched to it, the MetaMask provider would emit networkChanged with 0x64. Likewise, if the user entered a “chain ID” of 100, networkChanged would emit with 100.

In MetaMask 8.1 and later, these two events will emit the values according to the specification for all networks:

  • networkChanged will emit the network ID as a decimal string
  • chainChanged will emit the chain ID as a hexadecimal string

For a list of networks and their various IDs, see Note that all listed values are decimal numbers.


This particular bug goes back to the DAO hack and EIP-155, which added chain IDs to EVM-compatible blockchains so that wallets could ensure that a transaction was only valid on one chain, and not on both sides of a fork.

Initially, chain_id was often identical to network_id, and this conflation was baked into our codebase, resulting in many developers relying on our network ID values when they should have been relying on the chain ID. As a result, the networkChanged event would sometimes emit with the chain ID. You can read more about this history in this post by Pedro Gomes.

Fixing this issue brings our provider into tighter compatibility with the spec and the broader ecosystem. We hope that the impact is minimal, but please contact us if there is anything else we can do to help your app to continue operating smoothly.

Breaking Change: Correcting Custom Network Chain and Network ID Values was originally published in MetaMask on Medium, where people are continuing the conversation by highlighting and responding to this story.

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