Make MultiAgentV2 spawned-agent context prompt configurable

What variant of Codex are you using?

CLI

What feature would you like to see?

Make the MultiAgentV2 spawned-agent context prompt configurable, or at least only inject it when history/context is actually forked.

Today codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs unconditionally appends SPAWN_AGENT_DEVELOPER_INSTRUCTIONS to every spawned child agent:

<spawned_agent_context>
... Treat the next user message as your assigned task, and use the forked history only as background context.
</spawned_agent_context>

The problem is that this prompt is added even when the child is spawned with no forked history (for example fork_turns = none, or when no forked context is otherwise provided).

That creates two issues:

• the prompt describes forked history even when there is none
• there is currently no config knob to disable or override this block for custom multi-agent workflows

It looks like later work improved surrounding spawn behavior (parent model/config inheritance, role-level fork defaults, prompt cache keys, etc.), but I could not find a way to:

• disable SPAWN_AGENT_DEVELOPER_INSTRUCTIONS
• replace it with a custom block
• gate it on whether forked context is actually present

A few reasonable options:

• only inject this prompt when fork_turns actually results in forked history/context
• add a config option to disable it
• add a config option to override the text
• split it into two variants: one for forked children, one for no-history children

Additional information

Current code path:

• codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs

Specifically, the block is appended unconditionally after spawn config is built, independent of fork_turns / fork mode.

This matters for users building custom multi-agent workflows because the injected prompt becomes part of the child agent's behavior contract even when it is semantically wrong for the spawn mode.