--- title: Read and stream a local AI response --- # Read and stream a local AI response Use Anthropic's unchanged Node SDK to read the same deterministic response normally and as a stream. This checks your application's response handling. The default response is a labeled stub; it does not measure model quality or make a real model call. Use Node 22.6 or newer, npm and registry access during installation. Start in an empty directory. No Anthropic account, real key or platform login is needed. ## Select the release Save these files. This example pins [@volter/twin-anthropic 1.0.3](https://world.volter.ai/twins/anthropic/~40volter~2Ftwin-anthropic/1~2E0~2E3) in the catalog. The catalog may select a newer default; keep the pins below for this example. Other versions do not inherit its execution scope. ```json file=package.json { "name": "local-ai-messages", "private": true, "type": "module", "dependencies": { "@anthropic-ai/sdk": "0.131.0" }, "devDependencies": { "@volter/world": "3.0.158", "@volter/world-core": "3.0.148", "@volter/twin-anthropic": "1.0.3" } } ``` ```text file=.env.example ANTHROPIC_API_KEY= ``` ```bash npm install ./node_modules/.bin/volter world init --name local-ai-messages --twins anthropic --source anthropic=@volter/twin-anthropic ``` Review the selected vendor and the package/version in `.volter/world.json`. Init creates a project-owned scenario file for this model twin. Keep the config and lockfile with your app. ## Consume the same response two ways The selected twin has its own model catalog, filtered by the World clock. A model listed on Anthropic's live service is not automatically available in an older twin release. This example uses `claude-sonnet-4-6`, listed as active in [Anthropic's model status](https://platform.claude.com/docs/en/about-claude/model-deprecations), with a frozen clock after its release. Keep your app's model choice explicit. ```js file=messages.mjs import assert from 'node:assert/strict'; import Anthropic from '@anthropic-ai/sdk'; assert.ok(process.env.VOLTER_WORLD, 'run through the World'); const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY, maxRetries: 0 }); const input = { model: 'claude-sonnet-4-6', max_tokens: 128, messages: [{ role: 'user', content: 'A deterministic local example' }] }; const response = await client.messages.create(input); const text = response.content.filter(block => block.type === 'text').map(block => block.text).join(''); assert.match(text, /\[twin-stub:/); assert.equal(response.role, 'assistant'); assert.equal(response.stop_reason, 'end_turn'); let streamed = ''; for await (const event of await client.messages.create({ ...input, stream: true })) { if (event.type === 'content_block_delta' && event.delta.type === 'text_delta') streamed += event.delta.text; } assert.equal(streamed, text); console.log('normal and streamed responses match the labeled local stub'); ``` ```bash ./node_modules/.bin/volter world up ./node_modules/.bin/volter world clock set 2026-03-01T12:00:00Z ./node_modules/.bin/volter world run -- node messages.mjs ./node_modules/.bin/volter world down ./node_modules/.bin/volter world up ./node_modules/.bin/volter world run -- node messages.mjs ./node_modules/.bin/volter world down ``` ```text normal and streamed responses match the labeled local stub Stopped local-ai-messages ``` These assertions cover the normal Messages response and text-delta stream with this SDK and input. They establish neither every API operation nor model reasoning. Stop/resume here repeats the calls; it does not claim a stored conversation. Your app stores its own conversation history. ## Author the answer your app needs The labeled stub is a starting point. To exercise an application's handling of a particular answer, stop your World and replace `.volter/handlers/anthropic.json` with this ordered rule: ```bash ./node_modules/.bin/volter world down ``` ```json file=.volter/handlers/anthropic.json { "handlers": [ { "id": "example-answer", "on": { "userTextIncludes": "A deterministic local example" }, "respond": { "text": "billing" } } ] } ``` Init already names that file in the Anthropic service's `execution.colocate.scenarioPath`. Inspect the generated config before booting; saving a different file does not select it. Restarting loads the edited file and retains this branch's state. Save this separate client; the earlier stub assertions intentionally do not accept an authored answer: ```js file=authored-message.mjs import Anthropic from '@anthropic-ai/sdk'; if (!process.env.VOLTER_WORLD) throw new Error('Run through the World.'); const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY, maxRetries: 0 }); const input = { model: 'claude-sonnet-4-6', max_tokens: 128, messages: [{ role: 'user', content: 'A deterministic local example' }] }; const answer = await client.messages.create(input); const text = answer.content.filter(block => block.type === 'text').map(block => block.text).join(''); let streamed = ''; for await (const event of await client.messages.create({ ...input, stream: true })) { if (event.type === 'content_block_delta' && event.delta.type === 'text_delta') streamed += event.delta.text; } const scenario = await fetch(`${process.env.ANTHROPIC_TWIN_URL}/twin/scenario`).then(r => r.json()); console.log(JSON.stringify({ text, streamed, scenario }, null, 2)); ``` ```bash ./node_modules/.bin/volter world up ./node_modules/.bin/volter world clock set 2026-03-01T12:00:00Z ./node_modules/.bin/volter world run -- node authored-message.mjs ./node_modules/.bin/volter world down ``` The two answers are `billing`; the scenario output names `example-answer` and its two matches. This exercises response handling, not whether a model would choose that label. For your own prompt, edit the matcher and response together. Unmatched calls fall back to a labeled stub; inspect `/twin/scenario` for misses rather than treating the stub as your authored decision. Handlers never fake a successful stored mutation. Use the vendor's own API for records an application's tool creates, and keep tool execution in the application. For one-time refusals and other matcher grammars, see [scenario handlers](../../docs/guides/shape-the-world-for-a-test.md). For an existing chat app, keep its SDK calls and [run its usual command inside the World](../../docs/guides/use-with-an-existing-app.md). [Inspect the World](../../docs/guides/inspect-a-world.md) to examine behavior and handler misses.