Description
What is this about?
MetaMask replaces certain fee-related parameters (eth_sendTransaction) with its own gas estimator just before showing the signing UI. These modified parameters are not surfaced back to the dApp or other extensions.
As a result, a dApp that needs to:
display the exact transaction hash that will be signed on a hardware wallet (e.g., Ledger shows the hash of the final tx), or
cache/analyze fees for analytics or compliance,
has no reliable way to access the final transaction object before signature.
The first moment the dApp can observe the real parameters is after MetaMask broadcasts the signed transaction via eth_sendRawTransaction.
This breaks flows where the dApp must pre-compute or display the same hash the user sees on their hardware wallet.
Scenario
As a user pairing MetaMask with an iOS companion app to enhance the Ledger signing experience, I want to preview the exact transaction hash (including gas details) so that the hash shown on the Ledger matches what I see on the companion app, providing full transparency before signature.
Design
Proposed solution:
Emit a browser event so dApps and companion extensions can read the prepared transaction just before signature.
Example event:
window.dispatchEvent(
new CustomEvent('metamask:preparedTx', { detail: txObject })
);Technical Details
- Hook into MetaMask’s internal flow just before calling the signing prompt.
- Package the prepared txObject with all final gas parameters and nonce.
- Emit a browser CustomEvent (e.g., metamask:preparedTx) with the txObject as the detail payload.
- Ensure the event does not interfere with security, permissions, or signing flow.
- Document the event shape and expected behavior for developers.
Threat Modeling Framework
n.a.
Acceptance Criteria
n.a.
Stakeholder review needed before the work gets merged
- Engineering (needed in most cases)
- Design
- Product
- QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- Security
- Legal
- Marketing
- Management (please specify)
- Other (please specify)
References
Similar issue in Rabby Wallet: RabbyHub/Rabby#2946