> ## Documentation Index
> Fetch the complete documentation index at: https://phaseo.app/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Dynamic turns

> Change models, instructions, sampling parameters, and tools between agent turns.

Use dynamic values when later model turns should respond to application context or earlier tool results. A value can be fixed or computed from the current turn.

## Choose a model at runtime

```typescript theme={null}
type ResearchContext = {
  fast: boolean;
};

const agent = createAgent<string, string, ResearchContext>({
  id: "research-agent",
  model: ({ context }) =>
    context?.fast ? "phaseo/free" : "anthropic/claude-sonnet-4",
  instructions: ({ numberOfTurns }) =>
    `Research turn ${numberOfTurns}. Cite the evidence you use.`,
});
```

The agent definition accepts dynamic `model`, `models`, `instructions`, `temperature`, `maxOutputTokens`, and `topP` values.

## Update application context from a tool

Tools can store information for later turns with `context.setContext()`:

```typescript theme={null}
const chooseMode = defineTool({
  id: "choose-mode",
  async execute(input: { fast: boolean }, context) {
    context.setContext({ fast: input.fast });
    return { selected: input.fast ? "fast" : "thorough" };
  },
});
```

Keep context small and serializable when a run may be persisted and continued later.

## Override only the next turn

Use `nextTurnParams` when a tool should change the immediately following request without permanently changing the agent definition:

```typescript theme={null}
const broadenSearch = defineTool({
  id: "broaden-search",
  nextTurnParams: {
    temperature: 0.3,
    maxOutputTokens: 2_000,
  },
  async execute() {
    return { broadened: true };
  },
});
```

Next-turn values can change the model, instructions, temperature, maximum output tokens, top-p sampling, or available tools.

## Related guides

* [Stop conditions](./agent-sdk-stop-conditions)
* [State and approval](./agent-sdk-state-and-approval)
* [Outputs and retries](./agent-sdk-run-controls)