Three Polymarket CLOB gotchas: 401, "invalid signature", and a cancel that does nothing

Three Polymarket CLOB gotchas: 401, "invalid signature", and a cancel that does nothing Automating Polymarket with py-clob-client , I lost an embarrassing amount of time to three failures that aren't clearly documented anywhere. Here they are with the exact fixes, so you don't. 1. Your cancel returns 404 — because the endpoint isn't what you'd guess The intuitive DELETE /order/{id} returns 404 and your order silently stays open. The real endpoint is: DELETE /order body: {"orderID": "0x..."} # and the body is part of the signature Sign request_path = "/order" together with that body, then send the exact body. Miss this and your "canceled" orders keep resting on the book. 2. 401 Unauthorized that "should work" Authenticated calls need L2 HMAC headers, and the most common silent mistake is POLY_ADDRESS : it must be your wallet address , not the api_key. The reliable move is to let py-clob-client build the headers via create_level_2_headers from correctly-formed RequestArgs (method, request_path, body, serialized_body) — and make sure the serialized body you sign is byte-for-byte the body you send. 3. invalid signature = SignatureType / funder mismatch Nine times out of ten this is the SignatureType not matching how your wallet holds funds: 0 = EOA funder = your own wallet (holds USDC) 1 = POLY_PROXY funder = the proxy address (email/magic wallet) 2 = POLY_GNOSIS_SAFE funder = the safe address Signing as an EOA while pointing funder at a proxy (or vice-versa) yields invalid signature with no further hint. Bonus: the fill you read is wrong For a BUY , the shares you got are in takingAmount ; for a SELL , they're in makingAmount (takingAmount is the USDC). Read the wrong field and your accounting drifts, which then triggers resubmits and balance errors. I packaged the cancel/auth/fill helpers as a small MIT library: https://github.com/BlueWhale-Quant-Lab/polymarket-401-invalid-signature-cancel-order (For the harder production bits — reading /data/trades for reconciliation