400 Bad RequestDon't retry
invalid post-only order: order crosses book
A post-only (maker) order would immediately match. Adjust the price so it rests on the book.
A post-only order would match immediately, which post-only forbids. Move the price so the order rests on the book.
Retry? Don't retry
Sending the same request again returns the same error. Change the request first.
Any authenticated CLOB endpoint can return this.
Handling it in code
The CLOB API returns errors as {"error": "..."}. It also rewrites status codes by message, so branch on the text: anything containing "not found" becomes 404 and anything containing "unauthorized" becomes 401.
r = session.post(url, json=payload, headers=headers)
if r.status_code >= 400:
msg = r.json().get("error", "")
if "invalid post-only order: order crosses book" in msg:
raise ValueError(msg) # fix the request, don't retryRelated errors
For errors Polymarket doesn't document, like "order manager not ready" or 403s from the edge, see the full error guide.
Message and status from Polymarket's error reference, synced 2026-10-04.