feat: add Nexus implementation address to environment configuration

[sundayonah]

Biconomy V2 → Nexus migration – implementation steps

Goal: Migrate from Biconomy V2/Infinitism to Nexus (and MEE versioning).
Scope: Aggregator only (bundler/paymaster, factory, initCode, paymaster payload).
Refs: Account Migration Guide, V2→Nexus, MEE Versioning, Contracts and Audits.

---

Phase 1: Prep and config

Step	Task	Where
1.1	Choose MEE version (e.g. 2.1.0 or 2.2.1) and get Nexus implementation, bootstrap, and factory addresses per chain from Contracts and Audits	Docs + config/DB
1.2	Add Nexus addresses to config (env) or DB (network table); ensure EntryPoint remains correct for Nexus	config/, ent/ (if DB), utils/userop.go

---

Phase 2: Nexus initCode for new accounts (aggregator code)

Step	Task	Where
2.1	Add Nexus Account Factory ABI/binding; replace V2 factory 0x9406Cc6185a346906296840746125a0E44976454 and SimpleAccountFactory createAccount(owner, salt) with Nexus factory + Nexus init encoding (bootstrap + initData / initNexusWithDefaultValidator)	services/contracts/, utils/userop.go (~79–92)
2.2	Optional: add USE_NEXUS_ACCOUNTS (or similar) flag to toggle Nexus vs V2 path during rollout; remove V2 path after migration	Config + utils/userop.go

---

Phase 3: Paymaster payload (aggregator code)

Step	Task	Where
3.1	In SponsorUserOperation, replace Biconomy payload smartAccountInfo: { "name": "INFINITISM", "version": "1.0.0" } with Nexus/MEE payload per Biconomy paymaster API for Nexus	utils/userop.go (~143–176)
3.2	Confirm paymaster response parsing (e.g. paymasterAndData, gas limits) still works with Nexus response format	utils/userop.go

---

Phase 4: Bundler and endpoints

Step	Task	Where
4.1	Update DB so each Biconomy network’s BundlerURL and PaymasterURL point to Nexus bundler/paymaster endpoints for that chain; keep staging/production in sync	DB / scripts / scripts/db_data/dump.sql
4.2	If Nexus uses a different host, extend detectAAService and keep using the same biconomy code path for Sponsor/Send	utils/userop.go

---

Phase 5: Signing and paymaster account (aggregator)

Step	Task	Where
5.1	No change to SignUserOperation (same UserOp hash/signature for Nexus)	—
5.2	Confirm or make configurable the Biconomy paymaster account address in GetPaymasterAccount (currently hardcoded); use Nexus paymaster address per chain if different	utils/userop.go

---

Phase 6: Bundler send and receipt

Step	Task	Where
6.1	Ensure SendUserOperation (and any receipt/status calls) use same or updated params for Nexus bundler; adjust only if Biconomy docs require it	utils/userop.go

---

Phase 7: Contracts and tests

Step	Task	Where
7.1	Add Nexus factory binding/ABI; keep or remove SimpleAccountFactory when V2 path is removed	services/contracts/
7.2	Update utils/userop_test.go and test env for Nexus payload and URLs	utils/userop_test.go, utils/test/test.env

---

Phase 8: Existing V2 accounts (outside aggregator)

Step	Task	Where
8.1	Run one-time V2→Nexus migration (updateImplementation + initializeAccount) via script or in-app flow with user signer; aggregator continues to use same sender address after migration	Separate script (e.g. Node/TS with @biconomy/account + @biconomy/abstractjs) or frontend + small service

---

Phase 9: Deploy and docs

Step	Task
9.1	Deploy to staging; test new-account creation and sponsored UserOps; run migration for test V2 accounts if applicable
9.2	Sync production with same Nexus config and URLs before Feb 28, 2026
9.3	Reference Biconomy migration and MEE versioning docs in code/comments and this PR

---

Checklist (aggregator)

• Choose MEE version; add Nexus implementation, bootstrap, factory addresses (config or DB)
• Replace V2 factory + initCode with Nexus factory + initCode in InitializeUserOperation
• Update Biconomy paymaster payload from INFINITISM to Nexus in SponsorUserOperation
• Update BundlerURL and PaymasterURL for Biconomy networks in DB to Nexus endpoints
• (If needed) Make GetPaymasterAccount biconomy address configurable per chain
• Add Nexus factory ABI/binding; update tests and test env
• Plan/execute one-time V2→Nexus migration for existing accounts (outside aggregator)
• Staging + production deploy; document references

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/biconomy-v2-to-nexus

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Invalid placeholder address risks silent zero-address in production

Medium Severity

NEXUS_IMPLEMENTATION_ADDRESS is set to ************************* (25 asterisks) which is not a valid Ethereum address. Notably, Dockerfile.prod copies .env.example directly as the production .env file. Unlike the comparable ENTRY_POINT_CONTRACT_ADDRESS which contains an actual 0x-prefixed address, this placeholder will silently resolve to a zero address when passed to common.HexToAddress() once consuming code is added in subsequent phases. The value here needs to be either the real public Nexus implementation contract address or left empty with clear documentation.