chain-signer

A security suite for AI agents — the seatbelt that catches the dangerous thing BEFORE it happens. Three guards, each callable on its own (and as MCP tools), pairing with any wallet or identity stack:

• preflight(tx) — decode an unsigned transaction and flag drains before signing (unlimited/large approval, approve-all, token & NFT transferFrom, proxy upgrade, on-chain permit, approvals hidden in multicall incl. Uniswap router batches, EIP-7702 account delegation, will-revert).

• inspect_typed_data(td) — catch permit-phishing in an EIP-712 message before the agent signs it (ERC-2612, Uniswap Permit2, DAI-style permits).

• check_action(action, policy) — enforce allow/forbid + value/recipient limits before the agent acts.

All three fail safe and are guards, not guarantees. Also bundled: a non-custodial multi-chain wallet (burner, balance, send, swap) — the agent holds its own key and signs locally. No MetaMask, no account, no custody.

from chain_signer import assert_safe
assert_safe(tx)   # raises if the tx is a drain/unlimited-approval/revert — review before signing

Install

pip install chain-signer
export ETHERSCAN_API_KEY=...   # for live balance reads + broadcast (Etherscan v2)

Bitcoin/Solana support is optional: pip install "chain-signer[all]".

Quickstart (5 lines)

from chain_signer import burner, send_ether
from chain_signer.balance import get_balance

w = burner()                          # fresh throwaway wallet; the agent owns w.private_key
print(w.address, get_balance(w))      # live on-chain balance
send_ether(w, "0x...recipient", 0.001)  # auto nonce+gas, signed locally, broadcast

That's it — your agent just held a wallet and moved funds, no human in the loop.

Try it in 10 seconds (offline — no key, no funds, no network)

pip install chain-signer

from chain_signer import preflight
spender = "0x" + "22" * 20
tx = {"to": "0x" + "33" * 20, "data": "0x095ea7b3" + spender[2:].rjust(64, "0") + "f" * 64, "value": 0}
print(preflight(tx))   # ok=False — flags unlimited_approval before you'd ever sign

Full runnable demos are in the repo: examples/agent_safety_demo.py (all three guards stop three real attacks) and examples/quickstart.py (wallet) — clone to run them, or just import as above.

Safety preflight (the wedge)

Before an agent signs, hand the unsigned tx to preflight() — it decodes the calldata and returns the risks, or use assert_safe() to hard-stop on a HIGH flag. Offline, no network, never raises.

from chain_signer import preflight, assert_safe

# an unlimited-allowance approve() to a spender — the classic drain setup
tx = {"to": token, "data": "0x095ea7b3" + spender_padded + "f"*64, "value": 0}

report = preflight(tx)
# {'decoded': {...}, 'ok': False,
#  'risk_flags': [{'code': 'unlimited_approval', 'severity': 'HIGH',
#                  'detail': 'approve() grants an effectively-unlimited allowance ...'}]}

assert_safe(tx)          # raises ValueError on a HIGH flag; pass force=True to override
assert_safe(tx, sim=my_simulator)   # optional: also flag will-revert via your simulation hook

What it flags today: unlimited/large approval, increaseAllowance, setApprovalForAll, ERC-20 transferFrom + ERC-721/1155 safeTransferFrom (token & NFT drains), on-chain permit, proxy upgradeTo/upgradeToAndCall, approvals hidden inside multicall (all router variants, nested), EIP-7702 account delegation (the "wallet upgrade" drainer), large native value, opaque calldata, malformed calls, and will-revert (with a sim hook). Honest limits: it can't read intent it can't decode. A guard, not a guarantee.

Signed-message inspector (the off-chain half)

A drain doesn't need a transaction. A dApp can ask the agent to sign an EIP-712 message — most dangerously a permit granting an unlimited token allowance, which preflight (a tx check) can't see. inspect_typed_data() catches it before the agent signs:

from chain_signer import inspect_typed_data
report = inspect_typed_data(typed_data)   # the EIP-712 object you're about to sign
# ok=False, risk_flags=[{'code': 'unlimited_permit_signature', 'severity': 'HIGH', ...}]

Covers all three major permit shapes: ERC-2612, Uniswap Permit2 (PermitSingle/PermitBatch), and DAI-style (allowed: true). Offline, never raises.

Action-policy gate (inspect what the agent DOES)

Identity tells you who the agent is; it doesn't stop a bad action. check_action() enforces a policy on a proposed tool call before it runs — fail-safe (denies on unreadable input):

from chain_signer import check_action
policy = {"forbid_tools": ["bridge"], "max_value_wei": 10**18, "allow_recipients": [trusted_addr]}
r = check_action({"tool": "send", "args": {"to": addr, "value_wei": 5*10**18}}, policy)
# {'allowed': False, 'violations': [{'code': 'value_over_limit', ...}]}

All three guards are exposed as MCP tools (preflight, inspect_signature, check_action) — any agent runtime (Claude, Cursor, …) can call them directly, read-only, no key.

What's caught and what isn't — the honest threat-coverage map: docs/THREAT-COVERAGE.md.

What you get

• preflight(tx) / assert_safe(tx) — decode an unsigned tx and flag drain patterns before signing.

• inspect_typed_data(td) — flag permit-phishing in an EIP-712 message before the agent signs it.

• check_action(action, policy) — enforce allow/forbid + value/recipient limits before the agent acts.

• burner() — a fresh wallet for a one-off task; discard it when done.

• restore(key) — reload a wallet later from its exported private key (same key → same address).

• send_ether(w, to, amount) — send in ETH (not wei); nonce, gas, and broadcast handled for you.

• get_balance(w) — live balance from the chain (Etherscan v2 indexer, not a flaky public RPC).

• swap(...) — token swaps via 0x/Paraswap.

• Optional Solana + Bitcoin wallets via the [all] extra.

Non-custodial guarantee

The private key is generated/loaded locally, used only to sign, and never logged, returned, or stored by this library. You hold the key; we never touch your funds. That is the whole design.

Handling the key (read this)

w.private_key is the keys to the wallet. Treat it like a password:

• NEVER log it, print it in production, or write it into notes/memory/chat. Anyone who has it controls the funds.

• For a burner holding a few dollars this is low-stakes by design — but the rule still holds.

• To reuse a wallet later, store the key in a secret manager / env var, then restore(key).

• Better: export_encrypted(w, password) gives a password-protected keystore dict to store at rest; load_encrypted(keystore, password) brings the wallet back. Never store the raw key if you can store the keystore.

Signing idiom (note for web3.py users)

The wallet does not expose sign_transaction / sign_message methods. Signing is done by function helpers you pass the wallet to — e.g. send_ether(w, to, amount) signs and broadcasts, and sign_message(w, "text") returns an EIP-191 signature for auth / sign-in flows (recoverable via eth_account Account.recover_message).

CLI on PATH

pip install may warn that the chain-signer script dir isn't on your PATH. The library works regardless; to use the CLI directly, add that dir to PATH or run python -m chain_signer ....

Tool surface (for any AI / MCP / CLI)

chain_signer.mcp_server exposes list_tools() and call_tool(name, arguments). CLI:

python -m chain_signer list
python -m chain_signer call create_wallet '{"chain":"evm"}'

Responsible use

General-purpose, non-custodial tooling. You are responsible for using it within the laws and terms of service that apply to you. Not intended or marketed for any restricted or prohibited trading in your jurisdiction.

Notes

• Balances/broadcast use the Etherscan v2 indexer (authoritative), never a free public RPC.

• Low-level building blocks (tx.send, call_contract, explicit nonce/gas) remain available for advanced use.

Pay an x402 API in one call

from chain_signer import burner, sign_x402_payment
w = burner()
payload = sign_x402_payment(w, token=USDC, to=PAY_TO, value=1000, valid_before=EXPIRES, chain_id=8453)
# -> {"signature", "authorization"} ready for the x402 payment header. Signed locally, no prompt.

Builds + signs the EIP-3009 authorization x402 expects (the "exact" scheme). Your agent pays a paid API by itself — no password prompt, no signup, no custody.

Sign typed data (EIP-712) — for agent payments / x402

from chain_signer import burner, sign_typed_data
w = burner()
sig = sign_typed_data(w, domain, types, message)  # EIP-712; for x402 / EIP-3009 authorizations

Your agent can authorize a payment by signing typed data locally — no password prompt, no signup.

Run as an MCP server

chain-signer is also a Model Context Protocol (MCP) server, so MCP-aware agents can use it directly:

pip install chain-signer
chain-signer-mcp          # speaks MCP over stdio (JSON-RPC 2.0)

Exposes 6 tools: create_wallet, get_balance (balance), send, call_contract, swap, bridge.

Wire it into any MCP client (Claude Desktop, Cursor, etc.) by adding it to the client's mcpServers config:

{
  "mcpServers": {
    "chain-signer": {
      "command": "chain-signer-mcp",
      "env": { "ETHERSCAN_API_KEY": "your-key-for-live-balance-and-broadcast" }
    }
  }
}

That's all — the agent can now make a wallet, read balances, send, and swap as native tools. (ETHERSCAN_API_KEY is optional; needed only for live balance reads and broadcasting.)