Setting Up Models

Generators need a model to call. The framework ships a model resolver that auto-detects providers from environment variables — for the common case you set one key and you're done.

This page covers the ten-minute path: pick a provider, set a key, run a flow. For deeper configuration (custom resolvers, fallback policy, prompt caching) see Models and Custom Model Resolver.

Choose a provider

Pick whichever you already have an account for. Any one is enough.

Provider	Environment variable	Install
Anthropic	`ANTHROPIC_API_KEY`	`pnpm add @ai-sdk/anthropic`
OpenAI	`OPENAI_API_KEY`	`pnpm add @ai-sdk/openai`
Google	`GOOGLE_GENERATIVE_AI_API_KEY`	`pnpm add @ai-sdk/google`
Vercel AI Gateway	`AI_GATEWAY_API_KEY`	`pnpm add @ai-sdk/gateway`
OpenRouter	`OPENROUTER_API_KEY`	`pnpm add @openrouter/ai-sdk-provider`

A gateway key (Vercel, OpenRouter) makes that gateway's whole catalog available — useful if you want to try several providers without managing each key separately. Direct keys take priority over gateways when both exist for the same provider.

Set the key

Drop it in your shell, a .env file, or your hosting provider's secret store:

export ANTHROPIC_API_KEY=sk-ant-...

If you're using Next.js, the standard .env.local works:

.env.local
ANTHROPIC_API_KEY=sk-ant-...

That's the whole configuration step for the default setup. The framework will detect the key when it boots.

What `preset/small` resolves to

The quick-start uses model: "preset/small". A preset is a named list of models tried in order. The resolver picks the first one whose provider has a working key:

small = [
  "openai/gpt-5.4-mini",
  "anthropic/claude-haiku-4-5",
  "google/gemini-3-flash",
]

If only ANTHROPIC_API_KEY is set, preset/small resolves to claude-haiku-4-5. If Anthropic is down, the resolver retries, then falls back to the next available model in the list. Your generator code doesn't change.

The full list of built-in presets (tiny, small, medium, large, thinking-small, thinking-medium, thinking-large) is documented in Models — Built-in Presets.

Override a preset

You can replace any built-in preset or define a new one when you create the resolver:

app/api/flows/[...path]/route.ts
import { createFlowApiRouter, createFlowRegistry } from "@flow-state-dev/server";
import { createModelResolver } from "@flow-state-dev/core/models";
import chatFlow from "@/flows/hello-chat/flow";

const registry = createFlowRegistry();
registry.register(chatFlow);

const router = createFlowApiRouter({
  registry,
  modelResolver: createModelResolver({
    presets: {
      // Override `small` to prefer Anthropic
      small: { models: ["anthropic/claude-haiku-4-5", "openai/gpt-5.4-mini"] },

      // Add a new preset
      coding: {
        models: ["anthropic/claude-opus-4-6", "openai/gpt-5.4"],
        defaults: { maxTokens: 8192 },
      },
    },
  }),
});

export const GET = router.GET;
export const POST = router.POST;
export const DELETE = router.DELETE;

Use the new preset like any other:

const coder = generator({ name: "coder", model: "preset/coding", /* ... */ });

Use a specific model directly

Skip presets if you want exactly one model:

const chat = generator({
  name: "chat",
  model: "anthropic/claude-sonnet-4-6",
  prompt: "...",
});

The format is provider/model-id, or gateway/provider/model-id for gateway routing ("vercel/openai/gpt-5.4").

Plug in a custom resolver

If you need to control provider construction yourself — for example, to inject a pre-built Anthropic client with custom HTTP middleware — pass the resolver explicit provider instances:

import { createAnthropic } from "@ai-sdk/anthropic";
import { createModelResolver } from "@flow-state-dev/core/models";

const anthropic = createAnthropic({
  apiKey: process.env.ANTHROPIC_API_KEY,
  // any custom config
});

const modelResolver = createModelResolver({
  providers: { anthropic },
});

For full control (custom retry policy, gateway instances, provider preference ordering) see Custom Model Resolver.

Verify it works

The CLI is the fastest way to confirm your key is wired up:

fsdev run hello-chat chat -i '{"message": "Say hi."}'

You should see streaming text in your terminal. If it fails with a provider error, check that the env var is exported in the shell where you ran the command.

Choose a provider​

Set the key​

What preset/small resolves to​

Override a preset​

Use a specific model directly​

Plug in a custom resolver​

Verify it works​

What to read next​