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Technical overview

L1 actions use a phantom agent construction - a temporary signing identity created from your action’s hash. This provides privacy by hashing the actual data before signing.
The Hyperliquid Python SDK v0.18.0+ handles phantom agent construction internally. You don’t need to implement this complexity yourself.

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Key characteristics

PropertyValue
Chain ID1337 (NOT Arbitrum’s 42161)
Domain name”Exchange”
Version”1”
SerializationMsgpack binary format
Agent typePhantom agent from action hash
Source”a” for mainnet, “b” for testnet

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What is a phantom agent?

A phantom agent is a cryptographic construct that provides privacy for trading operations:
1

Serialize action

Action is serialized using msgpack binary format
2

Append metadata

Nonce and vault address (if applicable) are appended
3

Hash the data

Complete data is hashed with keccak256
4

Create agent object

Temporary “agent” object created with hash as connectionId
5

Sign via EIP-712

This phantom agent is signed using EIP-712 typed data

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Supported L1 actions

All L1 actions use the same signing method sign_l1_action() - only the action payload changes:

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Trading actions

  • order — Place new orders
  • cancel — Cancel orders by order ID
  • cancelByCloid — Cancel orders by client order ID
  • modify — Modify existing orders
  • batchModify — Modify multiple orders

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Position management

  • updateLeverage — Adjust leverage for positions
  • updateIsolatedMargin — Manage isolated margin

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Transfers

  • vaultTransfer — Transfer between vault accounts
  • subAccountTransfer — Transfer between sub-accounts

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Utility

  • scheduleCancel — Schedule order cancellation
  • noop — No operation (for testing)

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Complete implementation example

#!/usr/bin/env python3
"""
L1 Action Signing Example - Demonstrates phantom agent construction
SDK v0.18.0+
"""

from hyperliquid.exchange import Exchange
from hyperliquid.utils import constants
from hyperliquid.utils.signing import sign_l1_action, get_timestamp_ms
from eth_account import Account
import json

def main():
    # Load configuration
    with open('config.json') as f:
        config = json.load(f)
    
    wallet = Account.from_key(config['private_key'])
    print(f"Account: {wallet.address}")
    
    # Initialize exchange for sending signed actions
    exchange = Exchange(
        wallet=wallet,
        base_url=constants.MAINNET_API_URL
    )
    
    # Example 1: Place an order (SDK handles signing internally)
    order_result = exchange.order(
        coin="BTC",
        is_buy=True,
        sz=0.01,
        limit_px=50000,
        order_type={"limit": {"tif": "Gtc"}},
        reduce_only=False
    )
    print(f"Order result: {order_result}")
    
    # Example 2: Manual signing for understanding
    timestamp = get_timestamp_ms()
    
    # Create a minimal L1 action
    action = {"type": "noop"}
    
    # Sign with phantom agent construction
    signature = sign_l1_action(
        wallet,
        action,
        None,  # vault_address
        timestamp,
        None,  # expires_after
        True   # is_mainnet
    )
    
    print(f"\nPhantom agent signature generated:")
    print(f"  r: {signature['r']}")
    print(f"  s: {signature['s']}")
    print(f"  v: {signature['v']}")
    
    # Send the signed action to Hyperliquid
    result = exchange._post_action(action, signature, timestamp)
    print(f"\nResult: {result}")

if __name__ == "__main__":
    main()

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Action payload examples

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Place order

action = {
    "type": "order",
    "orders": [{
        "a": 0,  # Asset index (0 = BTC)
        "b": True,  # Buy = True, Sell = False
        "p": "50000",  # Price
        "s": "0.01",  # Size
        "r": False,  # Reduce-only
        "t": {"limit": {"tif": "Gtc"}}  # Order type
    }],
    "grouping": "na"
}

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Cancel order

action = {
    "type": "cancel",
    "cancels": [{
        "a": 0,  # Asset index
        "o": 123456  # Order ID
    }]
}

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Modify order

action = {
    "type": "batchModify",
    "modifies": [{
        "oid": 123456,  # Order ID
        "order": {
            "a": 0,
            "b": True,
            "p": "51000",  # New price
            "s": "0.01",
            "r": False,
            "t": {"limit": {"tif": "Gtc"}}
        }
    }]
}

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Update leverage

action = {
    "type": "updateLeverage",
    "asset": 0,
    "isCross": True,
    "leverage": 10
}

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Technical implementation details

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Phantom agent construction

def construct_phantom_agent(hash, is_mainnet):
    return {
        "source": "a" if is_mainnet else "b",
        "connectionId": hash
    }

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EIP-712 domain

domain = {
    "name": "Exchange",
    "version": "1",
    "chainId": 1337,
    "verifyingContract": "0x0000000000000000000000000000000000000000"
}

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Type definition

types = {
    "Agent": [
        {"name": "source", "type": "string"},
        {"name": "connectionId", "type": "bytes32"}
    ]
}

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TypeScript implementation

import { ethers } from 'ethers';
import msgpack from 'msgpack-lite';
import { keccak256 } from 'ethers/lib/utils';

async function signL1Action(
  privateKey: string,
  action: any,
  vaultAddress: string | null,
  nonce: number,
  isMainnet: boolean
): Promise<{ r: string; s: string; v: number }> {
  
  // Serialize action with msgpack
  const actionBytes = msgpack.encode(action);
  
  // Append vault address and nonce
  const data = Buffer.concat([
    actionBytes,
    vaultAddress ? Buffer.from(vaultAddress.slice(2), 'hex') : Buffer.alloc(20),
    Buffer.from(nonce.toString(16).padStart(16, '0'), 'hex')
  ]);
  
  // Hash to create connectionId
  const hash = keccak256(data);
  
  // EIP-712 domain
  const domain = {
    name: 'Exchange',
    version: '1',
    chainId: 1337,  // L1 actions always use 1337
    verifyingContract: '0x0000000000000000000000000000000000000000'
  };
  
  const types = {
    Agent: [
      { name: 'source', type: 'string' },
      { name: 'connectionId', type: 'bytes32' }
    ]
  };
  
  const value = {
    source: isMainnet ? 'a' : 'b',
    connectionId: hash
  };
  
  const wallet = new ethers.Wallet(privateKey);
  const signature = await wallet._signTypedData(domain, types, value);
  
  return ethers.utils.splitSignature(signature);
}

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Configuration file

Create a config.json file: 
{
    "private_key": "0x...",
    "account_address": "0x..."
}

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Common errors and solutions

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Wrong chain ID

Never use Arbitrum’s chain ID (42161) for L1 actions. Always use chain ID 1337.
# CORRECT
domain = {"chainId": 1337, ...}

# INCORRECT
domain = {"chainId": 42161, ...}  # This will fail!

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Invalid action format

Ensure action payloads match the expected structure: 
# CORRECT - proper structure
action = {
    "type": "order",
    "orders": [...],
    "grouping": "na"
}

# INCORRECT - missing required fields
action = {
    "type": "order",
    "orders": [...]
    # Missing "grouping" field
}

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Nonce issues

Always use current timestamp in milliseconds: 
from hyperliquid.utils.signing import get_timestamp_ms

# CORRECT
nonce = get_timestamp_ms()

# Also correct
import time
nonce = int(time.time() * 1000)

# INCORRECT
nonce = int(time.time())  # Missing milliseconds

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Testing your implementation

1

Start with noop

Test with a noop action first - it validates signing without any side effects
2

Use testnet

Test on testnet before mainnet: constants.TESTNET_API_URL or connect to a reliable Hyperliquid RPC endpoint
3

Verify signatures

Check that signatures have proper r, s, v components
4

Monitor responses

Successful actions return {"status": "ok"} or order details

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Best practices

DO
  • Use the official SDK when possible
  • Keep private keys secure (use environment variables)
  • Test thoroughly on testnet first
  • Handle errors gracefully
  • Use proper nonce management
DON’T
  • Hardcode private keys in code
  • Mix up chain IDs (1337 vs 0x66eee)
  • Reuse nonces across requests
  • Skip error handling
  • Use outdated SDK versions

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Summary

L1 actions use phantom agent construction for privacy - the actual data is hashed before signing. The SDK’s sign_l1_action handles all complexity internally, making it easy to implement trading operations securely.