my-pi/extensions/system-prompt-cover.ts

import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";

import { readCoverFile, resolveCoverPath, spliceCover } from "./system-prompt-cover-config.ts";

export { COVER_FILE_NAME, readCoverFile, resolveCoverPath, spliceCover } from "./system-prompt-cover-config.ts";

/**
 * Replaces Pi's default system prompt preamble with the bundle-managed cover while
 * preserving every other prompt section in its original position.
 *
 * The cover is read on each `before_agent_start`, so edits to `system-prompt-cover.md` take
 * effect on the next turn of an already-running session rather than requiring a restart.
 *
 * Returning `systemPrompt` sets Pi's `forceSystemPrompt`, which is projected onto the
 * request at send time. That is deliberate: it is the only path that keeps the rendered
 * section order intact. The options-based alternative (`customPrompt`) would suppress
 * `tools`, `rules`, and `docs`, and any later re-injection cannot restore their original
 * positions. Pi still diffs and persists the structured sections in the transcript.
 */
export default function systemPromptCoverExtension(pi: ExtensionAPI): void {
	pi.on("before_agent_start", (event) => {
		const cover = readCoverFile(resolveCoverPath());
		if (cover === undefined) {
			return;
		}
		return { systemPrompt: spliceCover(event.systemPrompt, cover) };
	});
}