How do I mock the ERC-4337 Bundler RPC API locally?

Install stunt, add a one-service block to stunt.yaml (adapter: embedded:erc4337-style), and run stunt up. You get a local, stateful stand-in for ERC-4337 Bundler RPC 0.7 — 2 routes over rest — with no remote account, no network, and no bills.

Is the ERC-4337 Bundler RPC mock data synthetic?

Yes — entirely. The adapter returns generated data only; stunt's adapter lint rejects real-looking secrets and PII at build time. It simulates the shape and behavior of ERC-4337 Bundler RPC (routes, status codes, pagination, webhooks), never real records.

← all adapters

erc4337-style

simulates ERC-4337 Bundler RPC 0.7

restadapter v0.1.02 routes

verification

VMVM tier — no SDK suite yet· surface 2 · not implemented 5 · differs 6

use it

# add to stunt.yaml
services:
  erc4337:
    adapter: embedded:erc4337-style

# then
$ stunt up

The embedded: source extracts this adapter from the binary — no git clone, fully offline.

how to mock the ERC-4337 Bundler RPC API locally

Install stunt, paste the service block above into stunt.yaml, run stunt up — then point your client at localhost instead of ERC-4337 Bundler RPC. It works offline, keeps state across requests and restarts, and every response is synthetic: the route shapes, status codes, pagination, and webhook delivery follow the real ERC-4337 Bundler RPC 0.7 surface — the data is fake, the behavior is real.

backing stores

useropsreceipts

api surface (2)

generated from the adapter manifest — exactly what boots, nothing more

2 routes

POST/

POST/paymaster/sign

VM-verified behaviors (7)

✓supportedEntryPoints, chainId, and the JSON-RPC envelope
✓estimateUserOperationGas validates the full v0.7 field set
✓sendUserOperation answers a deterministic hash and defaults the entry point
✓the op walks mempool -> bundled -> included on the virtual clock
✓simulate_fail reverts on inclusion with the AA95 reason
✓the paymaster signs the op into paymasterAndData
✓missing or invalid userOps are 400s

Not implemented (5)

No eth_chainId, eth_blockNumber, or eth_getBlockByNumber passthrough
No ERC-7677 paymaster RPC (pm_* methods); only local /paymaster/sign
No EntryPoint v0.6 support; v0.7 entrypoint only
No debug_bundler_* ops: sendBundleNow, clearState, dropUserOperation
No WebSocket transport or newHeads subscriptions

Differs from the real API (6)

gas estimates are deterministic fixed values; real bundlers differ per implementation
inclusion runs on a fixed clock: mempool 0-1s, bundled 1-3s, included at >=3s
eth_sendUserOperation accepts {simulate_fail:true} as a third params element
mock paymaster POST /paymaster/sign mints synthetic sponsorship signatures
userOp keeps the v0.6 paymasterAndData field though the adapter is EntryPoint-v0.7-only
Error envelopes use generic -32602 rather than AA-prefixed codes (-32500...)

full scorecard →

want more of the ERC-4337 Bundler RPC surface? adapters are YAML + Starlark — the authoring guide covers adding routes, fixtures, and state machines. Open an issue for anything big first, or just file a PR — merged routes ship in the next release.