A codex thread that codex hands back via thread/resume already holds the conversation, but a
thread started fresh did not: a session that ran on another provider before /model switched to
openai-codex, a stored thread codex could not resume, or a thread retired mid-session (prompt
composition change, wedged client) answered the first turn blind. The prior user/assistant text,
tool names and tool-result previews (most recent 32K chars) now ride once on
thread/start.developerInstructions after the prompt composition; thread/resume never carries them,
and the recorded composition stays the bare prompt so the seed cannot make the next turn retire the
thread.
Direction from #26081 (first-turn seeding of the Hermes transcript); redone on the extracted
agent/codex_runtime.py path with the system prompt sent once (#115759) instead of duplicated.
Completes #26035 / #74712 (closed by #115759 for the prompt half; this is the history half).
Co-authored-by: LeonSGP43 <154585401+LeonSGP43@users.noreply.github.com>
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