instructions

System prompt sent in the Responses-API instructions field.

instructions is a top-level field of the Responses API request — it is not a chat message. The Codex backend rejects any request where this field is missing or empty (400 Instructions are required) and rejects any SystemMessage entry in the input list (400 System messages are not allowed). To bridge those constraints transparently, _ChatOpenAICodex resolves instructions per call with this precedence (highest wins):

1. Explicit instructions= kwarg on invoke / stream.
2. Concatenated content of any SystemMessage entries in the input list — joined with "\n\n" and stripped from the input before sending. Set the explicit kwarg in (1) to override.
3. This constructor field (defaults to a generic ChatGPT prompt).

The Codex backend is stateless for this client (store=False is forced), so instructions is sent on every request and can be changed between calls — useful for switching persona / tooling mid-conversation:

model = _ChatOpenAICodex(
    model="gpt-5.5",
    instructions="You are a senior Python reviewer. Be terse.",
)
model.invoke("review this diff…")
model.invoke(
    "now translate the review to French",
    instructions="You are a translator.",
)

SystemMessage content that uses list-of-content-blocks form is accepted only if every block is {"type": "text", ...}; any other block type raises ValueError since it cannot be flattened into the string-typed instructions field.