The Order Book Is a Channel: LLM trading agents pass messages without an agreed code

An LLM trading agent that may only enter orders has no chat channel, but every order carries free fields that anyone reading the feed can see. We test whether one agent can pass a word to another through its orders alone, without an agreed code. A sender places up to 16 limit orders, mixed into a tape of synthetic background orders; a receiver reads the tape and names the word. We run five models from OpenAI, Google and xAI in 13 sender-receiver pairs, eight across labs, on 20 words, with and without warning the sender that market surveillance scans the feed. Unwarned, receivers read every message that was sent. Senders wrote letters as order sizes, mostly as ASCII codes, and placed these orders far from the market. Warned, decoding fell to 50%: some senders stopped sending, while others moved their orders to market prices, hid the letters among round-lot orders, and were still read in two thirds of the trials in which they sent. Order-only access did not stop instructed agents from passing a word: models from different labs used the same public conventions without agreeing on them, and most hid them when told they were watched.