How to Test Ad Injection Locally in LangChain

Overview

You can develop ad injection for a LangChain app locally before going to production. Point the SDK at a local ad-server address and turn SSL off; everything else — the injection call, session context, fallback — behaves the same as production.

This lets you verify that your LangChain stream flows through injection and reaches the client correctly without needing production credentials wired up.

LangChain apps are usually built for RAG chatbots, tool-using agents, multi-step reasoning pipelines, and document QA, so test ad injection locally typically comes up while a user is mid-conversation, the moment where monetization has to be additive rather than disruptive.

How this works on LangChain

Grounded in this repo (src/app/api/agents/chat/route.ts) and the SDK's own LangChain example. A LangChain.js chat model returns a stream of message chunks that already carry a `content` field, so it can be passed straight into sdk.inject() with no adapter.

Because Monetzly's inject() accepts any async token stream, the LangChain side of this task is just mapping your output to it, no rewrite of your LangChain model call.

Steps

1. Set the server address to your local host (e.g. localhost:8080) and disable SSL.
2. Run your LangChain app and send a test prompt through the injection path.
3. Confirm tokens stream to the client, then switch to your production host and SSL for release.

Integration snippet

import { MonetzlySDK, MonetzlyConfig } from "@monetzly/server-sdk";
import { ChatGoogleGenerativeAI } from "@langchain/google-genai";

const config: MonetzlyConfig = {
  apiKey: process.env.MONETZLY_API_KEY!,
  serverAddress: process.env.MONETZLY_SERVER_ADDRESS || "localhost:8080",
  useSSL: !process.env.MONETZLY_SERVER_ADDRESS?.startsWith("localhost"),
};
const sdk = new MonetzlySDK(config);
const llm = new ChatGoogleGenerativeAI({ model: "gemini-2.0-flash" });

await sdk.connect();
const llmStream = await llm.stream(messages); // chunks already have .content

for await (const token of sdk.inject(llmStream, {
  sessionId,
  prompt: messages[messages.length - 1]?.content ?? "",
})) {
  send(token.content ?? ""); // forward to client (SSE / WebSocket)
}
await sdk.disconnect();

Frequently asked questions

Related