How to set up LLM analytics for Anthropic's Claude

Feb 06, 2025

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Tracking your Claude usage, costs, and latency is crucial to understanding how your users are interacting with your AI and LLM-powered features.

In this tutorial, we'll build a basic Next.js app, implement the Claude API, and capture these events automatically using PostHog's LLM observability product.

1. Creating a Next.js app

To showcase how to track important metrics, we create a simple app with the following:

  • A form with a textfield and button for user input
  • A dropdown to select different Anthropic models
  • An API route to call Claude and generate a response
  • A label to show Claude's output

First, ensure Node.js is installed (version 18.0 or newer) then run the following to create a new Next.js app. Say no to TypeScript, yes to app router, and the defaults for all the other options.

Terminal
npx create-next-app@latest claude-analytics

After creating your app, go into the newly created claude-analytics directory and install the PostHog Node SDK and ai package as well as Anthropic's JavaScript SDK.

Terminal
cd claude-analytics
npm install --save posthog-node @posthog/ai @anthropic-ai/sdk

Next, we'll create our frontend by replacing the placeholder code in app/page.js. Our frontend will be a simple form with an input, model selector, and response label. Each of these needs a state. We'll also set up an API call to /api/generate with the user's input and model.

JavaScript
// app/page.js
'use client'
import React, { useState } from 'react';


const models = [
  'claude-3-opus-20240229',
  'claude-3-sonnet-20240229',
  'claude-3-haiku-20240307'
];


export default function Home() {
  const [userInput, setUserInput] = useState('');
  const [claudeResponse, setClaudeResponse] = useState('');
  const [selectedModel, setSelectedModel] = useState(models[0]);


  const fetchClaudeResponse = async () => {
    try {
      setClaudeResponse('Generating...');


      const res = await fetch('/api/generate', {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
        },
        body: JSON.stringify({ input: userInput, model: selectedModel }),
      })
      const response = await res.json();
      setClaudeResponse(response.content);
    } catch (error) {
      setClaudeResponse(error.message);
    }
  };


  const handleInputChange = (event) => {
    setUserInput(event.target.value);
  };


  const handleModelChange = (event) => {
    setSelectedModel(event.target.value);
  };


  const handleSubmit = (event) => {
    event.preventDefault();
    fetchClaudeResponse();
  };


  return (
    <div style={{ display: 'flex', flexDirection: 'column', alignItems: 'center', justifyContent: 'center', minHeight: '100vh', gap: '20px' }}>
      <form onSubmit={handleSubmit}>
        <input
          type="text"
          value={userInput}
          onChange={handleInputChange}
          placeholder="Type your message"
        />
        <button type="submit">Send</button>
      </form>
      <select value={selectedModel} onChange={handleModelChange}>
        {models.map((model, index) => (
          <option key={index} value={model}>
            {model}
          </option>
        ))}
      </select>
      <label>Claude Response:</label>
      <label>{claudeResponse}</label>
    </div>
  );
};

Once updated, run npm run dev to see our app in action:

Basic web app

2. Adding and tracking the generate API route

In the app folder, create an api folder, a generate folder inside it, and then a route.js file in that. This is our /api/generate API route that calls the Claude API and returns the response.

Next, set up:

  1. The PostHog Node client using our project API key and API host which you can get from your project settings.
  2. The Anthropic client which requires an API key from your Anthropic console.

With both of these set up, we simply call the anthropic.messages.create method with the input and model then return the response. You can include a posthogDistinctId if you want to track specific users.

JavaScript
// app/api/generate/route.js
import { NextResponse } from 'next/server';
import { Anthropic } from '@posthog/ai';
import { PostHog } from 'posthog-node';


const phClient = new PostHog(
  '<ph_project_api_key>',
  { host: '<ph_api_client_host>' }
);


const anthropic = new Anthropic({
  apiKey: '<anthropic_api_key>',
  posthog: phClient,
});


export async function POST(request) {
  try {
    const body = await request.json();
    const { input, model } = body;


    const message = await anthropic.messages.create({
      model: model,
      max_tokens: 4096,
      messages: [{ role: "user", content: input }],
      posthogDistinctId: '<your_distinct_id>'
    });


    return NextResponse.json({
      content: message.content[0].text
    });


  } catch (error) {
    console.error('Anthropic API error:', error);
    return NextResponse.json(
      { error: 'There was an error processing your request' },
      { status: 500 }
    );
  }
}

Now, when we run npm run dev again and submit an input, we should see a response as well as the generation autocaptured into PostHog as a $ai_generation event.

Generated response

3. Viewing generations in PostHog

Once you generate a few responses, go to PostHog and enable the LLM observability feature preview. Once enabled, go to the LLM observability tab to get an overview of traces, users, costs, and more.

LLM observability dashboard

You can also go into more detail by clicking on the generations tab. This shows each generation as well as model, cost, token usage, latency, and more. You can even see the conversation input and output.

LLM generations list

From here, you can go further by filtering your LLM observability dashboard, use the $ai_generation event to create insights, A/B test models, and more.

Further reading

Questions? Ask Max AI.

It's easier than reading through 581 docs articles.

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