How to fix MetaMask "could not fetch chain ID" error on custom RPC?

Root Cause Analysis

1. The error commonly stems from mismatched chain ID values between the RPC endpoint and MetaMask’s internal validation layer.

2. Some custom RPCs return chain IDs in hexadecimal format while MetaMask expects decimal representation, triggering silent rejection.

3. Network timeout or incomplete response headers from the RPC server may cause MetaMask to abort chain ID retrieval before parsing completes.

4. Misconfigured CORS policies on self-hosted RPC nodes prevent MetaMask from reading response metadata required for chain identification.

5. Browser extensions like ad blockers or privacy shields interfere with JSON-RPC request/response payloads, corrupting chain ID extraction logic.

RPC Endpoint Validation Protocol

1. Manually invoke the RPC endpoint using curl: curl -X POST --data '{'jsonrpc':'2.0','method':'eth_chainId','params':[],'id':1}' -H 'Content-Type: application/json' https://rpc.maplestoryuniverse.io.

2. Confirm the response contains a valid hex-encoded chain ID (e.g., '0x7e8') and HTTP status code 200.

3. Verify that the same endpoint responds to eth_getBlockByNumber with non-null result to rule out partial node synchronization.

4. Cross-check the returned chain ID against the official registry at chainlist.org to detect deprecated or conflicting identifiers.

5. Test the same RPC URL in a clean Chrome profile with all extensions disabled to isolate interference sources.

MetaMask Configuration Safeguards

1. Enable “Allow custom chains” under Settings → Experimental Features — this flag unlocks non-standard chain ID acceptance.

2. Input chain ID strictly as a decimal integer (e.g., 2024), never as “0x7e8” or string “2024”, even if the RPC returns hex.

3. Set block explorer URL to a functional domain matching the network’s official explorer — broken links trigger fallback failures during chain initialization.

4. Disable Privacy Mode temporarily to ensure MetaMask can fetch token lists and network parameters without restriction.

5. After saving, click the network name dropdown and select “Switch to MapleChain” instead of relying on auto-detection.

Browser-Level Resolution Steps

1. Open Chrome DevTools (F12), navigate to Application → Clear Storage, and select only “Cache storage” and “Service workers” before clearing.

2. In chrome://settings/content/cookies, locate metamask.io and remove all associated data to eliminate stale session artifacts.

3. For Brave users, disable Shields entirely and grant MetaMask permission to run in incognito mode via brave://extensions.

4. Reset MetaMask account nonce by going to Settings → Advanced → “Reset Account”, which forces re-synchronization of pending transaction counters.

5. Avoid using Firefox with uBlock Origin enabled — its aggressive script filtering breaks MetaMask’s internal provider handshake sequence.

Frequently Asked Questions

Q: Why does MetaMask show “Invalid Chain ID” even when eth_chainId returns 0x7e8?A: MetaMask requires decimal input during network setup. Enter 2024, not 0x7e8 or '2024'.

Q: Can I use the same RPC URL for both Mainnet and Testnet configurations?A: No. Each network must have a unique chain ID and distinct RPC endpoint. Mixing them causes signature verification failure.

Q: Does enabling “Custom Chains” expose my wallet to phishing networks?A: Not inherently. The setting only permits manual addition; it does not auto-connect to unverified chains unless explicitly selected.

Q: Why does the error persist after clearing site data and resetting the account?A: Residual IndexedDB entries may retain corrupted network state. Delete “metamask” and “web3” databases manually via DevTools → Application → IndexedDB.