Error: turn/start: turn/start failed: Input exceeds the maximum length of 1048576 characters.

Codex rejects user turns longer than 2^20 (1,048,576) characters with the following error:

Error: turn/start: turn/start failed: Input exceeds the maximum length of 1048576 characters.

Problem

GPT-5.4 supports up to 922k input tokens, but this character cap translates to roughly ~300k tokens — far below what the underlying model actually accepts.

Expected behavior

The per-turn input limit should reflect the model's actual context window, not an arbitrary character count that's ~3× more restrictive.

Steps to reproduce

1. Send a single user turn containing more than 1,048,576 characters.
2. Observe the turn/start failed error, even when the input is well within the model's token budget.