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Hosted trading writes are authenticated by an EIP-712 typed-data signature computed locally on your machine. PMXT never sees your private key. The pmxt_api_key authenticates you to PMXT; the signature authenticates the order to the venue’s on-chain settlement contract.

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What the SDK does for you

Pass private_key to the exchange constructor and you’re done. The SDK auto-wraps it into a local signer (EthAccountSigner in Python, EthersSigner in TypeScript), builds the typed-data payload for the right venue, signs locally, and submits the signature. You never see the typed-data shape. 
import pmxt

client = pmxt.Polymarket(
    pmxt_api_key="pmxt_live_...",
    wallet_address="0xYourWallet...",
    private_key="0xYourPrivateKey...",
)

order = client.create_order(
    market_id="2eeb03dc-404b-41d5-bc57-6aeb37927ae6",
    outcome_id="a114f052-1fd1-4bcd-b9cf-de019db81b67",
    side="buy",
    order_type="market",
    amount=5.0,
    denom="usdc",
    slippage_pct=30.0,
)

import { Polymarket } from "pmxtjs";

const client = new Polymarket({
  pmxtApiKey: "pmxt_live_...",
  walletAddress: "0xYourWallet...",
  privateKey: "0xYourPrivateKey...",
});

const order = await client.createOrder({
  marketId: "2eeb03dc-404b-41d5-bc57-6aeb37927ae6",
  outcomeId: "a114f052-1fd1-4bcd-b9cf-de019db81b67",
  side: "buy",
  type: "market",
  amount: 5,
  denom: "usdc",
  slippage_pct: 30,
});

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Single-signature vs dual-signature flows

Most hosted writes are single-signature — one EIP-712 payload, surfaced as built.raw["typed_data"]. This covers:
  • Polymarket buys and sells — OrderParams on the Polygon PreFundedEscrow domain.
  • Opinion BUY — CrossChainOrderParams on the Polygon PreFundedEscrow domain (the BSC delivery leg needs no user signature; the user is the recipient).
  • Limitless buys and sells — ERC-7683 order on the Polygon escrow domain.
Opinion SELL is the one dual-signature case. The user gives up tokens custodied on BSC, so the SDK builds two payloads at once:
  • built.raw["typed_data"] — CrossChainSellPayParams on the Polygon PreFundedEscrow domain (the operator’s pay leg into the user’s Polygon balance).
  • built.raw["pull_typed_data"] — CrossChainSellPullParams on the BSC VenueEscrow domain (the operator’s pull leg against the user’s BSC outcome tokens).
The SDK signs both with the same EVM private key automatically — no extra wiring. The two domains are distinct (PreFundedEscrow on chainId 137, VenueEscrow on chainId 56) and each signature only authorizes its own chain’s leg.

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Why reads only need a wallet address

For reads (fetch_balance, fetch_positions, fetch_my_trades), no signature is required — the pmxt_api_key is enough. You can construct a hosted client with only pmxt_api_key and wallet_address and read hosted state for that wallet: 
import pmxt

read_only = pmxt.Polymarket(
    pmxt_api_key="pmxt_live_...",
    wallet_address="0xSomeOtherWallet...",
    # no private_key
)
balance = read_only.fetch_balance()  # works
Because reads don’t require a signature, anyone with the pmxt_api_key can read any wallet’s hosted state. Keep the key on the server. See the trust model.

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Advanced: bring your own signer

If your key lives in a hardware wallet, HSM, MPC service, or anything that isn’t a raw hex private key, skip private_key and inject a custom signer at construction. The SDK uses it transparently for every hosted write — you keep calling create_order as usual. The signer protocol is one method: sign_typed_data(typed_data: dict) -> hex (Python) or signTypedData(typedData): Promise<string> plus a readonly address (TypeScript). 
import pmxt

client = pmxt.Polymarket(
    pmxt_api_key="pmxt_live_...",
    wallet_address="0xYourWallet...",
    signer=my_custom_signer,  # exposes sign_typed_data(typed_data) -> hex
)

order = client.create_order(
    market_id="2eeb03dc-...",
    outcome_id="a114f052-...",
    side="buy",
    order_type="market",
    amount=5.0,
    denom="usdc",
    slippage_pct=30.0,
)

import { Polymarket } from "pmxtjs";

const client = new Polymarket({
  pmxtApiKey: "pmxt_live_...",
  walletAddress: "0xYourWallet...",
  signer: myCustomSigner,  // { address, signTypedData(typedData) -> Promise<hex> }
});

const order = await client.createOrder({
  marketId: "2eeb03dc-...",
  outcomeId: "a114f052-...",
  side: "buy",
  type: "market",
  amount: 5,
  denom: "usdc",
  slippage_pct: 30,
});
For Opinion SELL’s dual-signature flow, the same injected signer is invoked twice (once for the Polygon pay leg, once for the BSC pull leg) — no extra wiring. The SDK’s EthAccountSigner (Python) / EthersSigner (TypeScript) are reference implementations; ethers v6 supports Ledger out of the box, and for MPC see Fireblocks / Privy / Turnkey docs.