import type { HerdrCapabilityResolver } from "./capability.js";

/**
 * The tool loads on demand, so the model needs this note before it reaches for `herdr` or
 * `git worktree` in bash.
 */
export const HERDR_PROMPT_NOTE =
  "Use the herdr_worktree tool for Git worktrees, not herdr or git worktree in bash. Create a worktree when work needs its own checkout, for example parallel multi-agent work.";

interface BeforeAgentStartEvent {
  readonly systemPrompt?: unknown;
}

interface BeforeAgentStartContext {
  readonly cwd?: string;
  readonly signal?: AbortSignal;
}

interface BeforeAgentStartResult {
  readonly systemPrompt: string;
}

export interface HerdrPromptApi {
  on?(
    event: "before_agent_start",
    handler: (
      event: BeforeAgentStartEvent,
      context?: BeforeAgentStartContext,
    ) => Promise<BeforeAgentStartResult | undefined>,
  ): void;
}

export function registerHerdrGuidance(
  pi: HerdrPromptApi,
  capabilities: HerdrCapabilityResolver,
): void {
  pi.on?.("before_agent_start", async (event, context) => {
    const systemPrompt = event?.systemPrompt;
    // A host may chain several handlers, so the note must not be appended twice.
    if (typeof systemPrompt !== "string" || systemPrompt.includes(HERDR_PROMPT_NOTE)) {
      return undefined;
    }

    const capability = await capabilities.tryEnsure(context?.cwd ?? process.cwd(), context?.signal);
    if (!capability) {
      return undefined;
    }

    return { systemPrompt: `${systemPrompt}\n\n${HERDR_PROMPT_NOTE}` };
  });
}