What is evm-chain-id-format?

A set of rules for handling EVM network IDs in MetaMask Connect. EVM is the system used by Ethereum-compatible blockchains; these rules distinguish hexadecimal IDs used by wallets from decimal IDs used in CAIP-2 network labels.

It is for converting chain IDs, defining supported networks, calling network-switch functions, and creating CAIP-2 identifiers for Ethereum-compatible chains.

It prevents connection failures and runtime errors caused by passing network IDs in the wrong format.

evm-chain-id-format — Cursor rule for Cursor by MetaMask: A set of rules for handling EVM network IDs…

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add rules/metamask/metamask-connect-cursor-plugin/evm-chain-id-format

Clone the repo

git clone --depth 1 https://github.com/MetaMask/metamask-connect-cursor-plugin

Made for: Cursor.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/rules/metamask/metamask-connect-cursor-plugin/evm-chain-id-format.svg)](https://agentmods.dev/rules/metamask/metamask-connect-cursor-plugin/evm-chain-id-format)

Your own site

<a href="https://agentmods.dev/rules/metamask/metamask-connect-cursor-plugin/evm-chain-id-format"><img src="https://agentmods.dev/badge/rules/metamask/metamask-connect-cursor-plugin/evm-chain-id-format.svg" alt="Measured on agentmods" height="20"></a>

Per session 1,377 This file is loaded in full into every session.

When invoked 1,377 The same file — it is already loaded in full.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.01377	$0.01377
Opus 5	$0.00688	$0.00688
Sonnet 5	$0.00275	$0.00275
Haiku 4.5	$0.00138	$0.00138

Measured 5d ago against content hash af66d3ca14ad, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-05, from the pricing page.

Security

Grade A, and why

evm-chain-id-format scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 5d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

rules/evm-chain-id-format.mdc · 65 lines

How it starts

The opening of the file, as written. The whole thing — 65 lines — stays where its author put it; the contents beside it link to each section on GitHub.

EVM Chain ID Format

Hex String Requirement

Chain IDs in MetaMask Connect must always be hex strings: '0x1' not 1 or '1'
All chainIds arrays, supportedNetworks keys, and switchChain parameters expect hex format
Passing a number or decimal string will cause silent failures or runtime errors
Use '0x' + chainId.toString(16) to convert from decimal to hex

Common Chain IDs

Network	Decimal	Hex	CAIP-2 Scope
Ethereum Mainnet	1	`0x1`	`eip155:1`
Sepolia	11155111	`0xaa36a7`	`eip155:11155111`
Polygon	137	`0x89`	`eip155:137`
Arbitrum One	42161	`0xa4b1`	`eip155:42161`
Optimism	10	`0xa`	`eip155:10`
Base	8453	`0x2105`	`eip155:8453`
Avalanche C-Chain	43114	`0xa86a`	`eip155:43114`
BNB Smart Chain	56	`0x38`	`eip155:56`
Celo	42220	`0xa4ec`	`eip155:42220`
Linea	59144	`0xe708`	`eip155:59144`

CAIP-2 Conversion

EVM CAIP-2 format is eip155:<decimal-chainId> — always uses decimal, not hex
EVM RPC / EIP-1193 format uses hex strings (0x1)
Multichain invokeMethod scope uses CAIP-2 (eip155:1)
EVM client connect({ chainIds }) uses hex strings (['0x1'])
Convert: hex 0x89 → decimal 137 → CAIP-2 eip155:137

Auto-Included Chain

0x1 (Ethereum mainnet) is automatically included in the EVM client's connect() permission request even if you don't pass it in chainIds
It is not injected into api.supportedNetworks — that map must explicitly contain every chain you use (including mainnet), and createEVMClient throws if it is empty
All chains need valid RPC URLs in supportedNetworks
If you use Infura RPC URLs, make sure the needed chains are enabled for your Infura project/API key

Wagmi Connector

The wagmi MetaMask connector is imported from wagmi/connectors: import { metaMask } from 'wagmi/connectors' — it requires @metamask/connect-evm as a peer dependency
Use getInfuraRpcUrls({ infuraApiKey: 'API_KEY', chainIds?: Hex[] }) from @metamask/connect-evm to populate supportedNetworks — returns a hex-chain-ID-keyed map of Infura RPC URLs (e.g. { '0x1': 'https://...', '0x89': 'https://...' }); chainIds is optional and filters to specific hex chain IDs
The multichain equivalent in @metamask/connect-multichain is getInfuraRpcUrls({ infuraApiKey: 'API_KEY', caipChainIds?: string[] }) — returns a CAIP-2-keyed map (e.g. { 'eip155:1': 'https://...' }) and accepts CAIP-2 IDs for filtering

Read the full file on GitHub · 65 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

5d ago First seen · 65 lines · 1,377 tokens per session scan A af66d3ca14ad

Subscribe to this mod's changes

evm-chain-id-format is a cursor rule published in the GitHub repository MetaMask/metamask-connect-cursor-plugin (2 stars, last pushed 2mo ago), licensed MIT. It adds 1,377 tokens to every session, about $0.0069 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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