[FEATURE]: System prompt environment information plug-in API #31158

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I would like to contribute a core feature to OpenCode and would like to ask a contributor if it is OK.

I ask if it is possible to expose an API to modify system prompt environment information (this will benefit plugin authors).

Currently, the system prompt has a shape that is something like:

{agent prompt}\n{environment information}\n{instruction files}\n{skills}\n{system reminders/overrides}

(I am writing this from memory, it is probably in another order).

A non-source-code aware user can natively modify the system prompt by:

• changing the agent prompt
• creating, modifying, linking, and/or deleting instruction files
• creating new skills, removing old skills
  The user cannot:
• modify environment information
• disabled or modify system reminders

I am aware that plugins can directly modify the raw system prompt. Feel free to close this if the effort is not worth it (I can still create a fork regardless!).

Extra context: I wish to create a plugin that injects some stateful information into the system prompt and expose a tool for the LLM to modify this stateful information. It's useful for long coding pipelines where the current agent needs to know what stage they're currently at. This plugin can also be implemented without this feature by modifying the raw system prompt or dynamically modifying the agent prompt. I also wish to make this contribution to better understand OpenCode :)

I will comment on this thread (if it is not manually closed) when the PR is ready.