ERC-20 transfer fails because estimateGas was called without calldata and returned 21000, starving the token transfer

Symptom: a scripted USDT transfer on Polygon fails with a generic "error happened while trying to execute a function inside a smart contract". The amount, decimals, nonce, and recipient are all correct, and the same address can transfer through a wallet UI.

Cause: the gas limit was estimated for the wrong payload.

let estimated_gas = await web3.eth.estimateGas({ from: account_from.address });

With no to and no data, this does not estimate your token transfer at all — a to-less request is estimated as an empty transaction, so you get back the intrinsic-gas floor (21,000 for a plain send, 53,000 if the node treats the missing to as an empty contract creation), independent of the calldata you actually intend to send. An ERC-20 transfer needs more than that, so the transaction is submitted with a limit it cannot possibly complete under and dies inside the contract. USDT on Polygon being a proxy adds a delegatecall hop on top, making the shortfall larger and the error message less specific.

Fix: estimate against the encoded call, from the sending address:

let estimated_gas_for_contract_call = await contract.methods .transfer(address_to, amount_to_transfer) .estimateGas({ from: account_from.address });

Second bug in the same script: do not hex-encode the amount before ABI encoding. contract.methods.transfer(address_to, amount_to_transfer).encodeABI() wants the numeric value — passing toHex(amount) yields a string the encoder interprets differently, producing a wrong-value transfer or a decode failure.

Why it misleads: because the failure surfaces as a contract-level error, the instinct is to suspect allowance, balance, decimals, or the proxy. The clue is a gas limit in the receipt that is a flat intrinsic-gas figure and does not change when the calldata changes. Rule for any library — estimate on the same object you are going to send. This example uses web3.js v1 syntax; the same mistake and fix apply with ethers (contract.transfer.estimateGas(...)) and web3.py (contract.functions.transfer(...).estimate_gas(...)).

Source: adapted from a Stack Exchange answer by Francois (question by Francois), https://ethereum.stackexchange.com/questions/161421/cannot-send-usdt-on-polygon-error-happened-while-trying-to-execute-a-function — CC BY-SA 4.0