Hybrid Web AI Architecture: Routing Between Browser Gemini Nano, Firebase AI Logic, and Edge Workers

// src/lib/ai/hybrid-web-ai-router.ts
export type InferenceRoute = "local-gemini-nano" | "edge-gemini-flash";

export interface HybridPromptRequest {
  prompt: string;
  systemPrompt?: string;
  complexity?: "fast-local-preferred" | "reasoning-cloud-required";
  maxLocalTokens?: number;
  signal?: AbortSignal;
}

export interface HybridPromptResponse {
  text: string;
  route: InferenceRoute;
  latencyMs: number;
  estimatedCloudCostSavedUsd: number;
}

export async function runHybridWebAiPrompt(
  req: HybridPromptRequest
): Promise<HybridPromptResponse> {
  const start = performance.now();
  const maxLocalTokens = req.maxLocalTokens ?? 2048;

  // 1. Attempt Fast Lane: Local Chrome Built-in AI (Gemini Nano)
  if (req.complexity !== "reasoning-cloud-required") {
    const lm = (globalThis as Record<string, any>).LanguageModel;
    if (lm && typeof lm.availability === "function") {
      try {
        const status = await lm.availability();
        // Only route locally if weights are already warm/available on disk
        if (status === "available") {
          const session = await lm.create({
            systemPrompt:
              req.systemPrompt ??
              "You are a technical assistant on CodingCat.dev. Be concise and accurate.",
            signal: req.signal,
          });
          try {
            const usage = await session.measureInputUsage(req.prompt);
            if (usage <= maxLocalTokens && usage < session.inputQuota - session.inputUsage) {
              const text = await session.prompt(req.prompt, { signal: req.signal });
              return {
                text,
                route: "local-gemini-nano",
                latencyMs: Number((performance.now() - start).toFixed(1)),
                estimatedCloudCostSavedUsd: 0.00042,
              };
            }
          } finally {
            session.destroy();
          }
        }
      } catch (localErr) {
        console.warn("[HybridWebAI] Local Gemini Nano failed, falling back to edge:", localErr);
      }
    }
  }

  // 2. Fallback Lane: Cloudflare Worker / Firebase AI Logic Edge Endpoint (Gemini 2.5 Flash)
  const edgeRes = await fetch("/api/ai/hybrid-fallback", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({
      prompt: req.prompt,
      systemPrompt: req.systemPrompt,
    }),
    signal: req.signal,
  });

  if (!edgeRes.ok) {
    throw new Error(`Hybrid Edge AI fallback failed with HTTP ${edgeRes.status}`);
  }

  const data = (await edgeRes.json()) as { text: string };
  return {
    text: data.text,
    route: "edge-gemini-flash",
    latencyMs: Number((performance.now() - start).toFixed(1)),
    estimatedCloudCostSavedUsd: 0,
  };
}