Core: Implement Gas Station Network for Sponsor Gas Fees on EVM Chains

[Smartdevs17]

Context

Gas fees on EVM chains prevent non-custodial adoption. Users need to hold native tokens (ETH, MATIC, etc.) just to pay gas, creating friction for new users.

Current Limitation/Problem

There is no gas sponsorship mechanism. Every transaction requires the sender to hold native gas tokens, which is a significant UX barrier for mainstream adoption.

Expected Outcome

A Gas Station Network (GSN) implementation that allows AgenticPay to sponsor gas fees for users on EVM chains, using meta-transactions with EIP-2771/EIP-712, with configurable sponsorship policies and budget controls.

Acceptance Criteria

• Implement forwarder contract (EIP-2771) on supported EVM chains
• Implement relay server that accepts signed meta-transactions and submits them
• Support gas price estimation with configurable multiplier
• Implement sponsorship budget management per merchant/app
• Rate limit sponsorship per wallet address
• Track gas sponsorship costs with billing to merchants
• Fallback to user-pays-gas when sponsorship budget exhausted
• Handle EIP-1559 transactions correctly

Technical Scope

• contracts/evm/gsn/ - GSN forwarder and relay contracts
• backend/src/services/gsn/relay-server.ts - meta-tx relay service
• backend/src/services/gsn/budget-manager.ts - sponsorship budget tracking
• Prisma: GasSponsorshipBudget, GasSponsorshipTx models
• Integration with ethers.js for contract interactions
• Redis for nonce management to prevent replay
• Edge cases: gas price spikes, budget exhaustion, relay failures, signature replay

[akintewe]

@akintewe has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I've been creating intricate forms and e-commerce systems for more than four years as a full-stack developer. I have designed inventory management systems for agricultural enterprises and have experience with image processing and large-scale state management. I have also worked on this project in the past.
My ETA is one hour, and I would love to work on this problem.

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This issue has been completed by @akintewe as part of the Stellar Wave Program's 6th Wave 🥳

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