[FEAT]: add missing prepareStep callback from AI SDK

[sapkra]

Is your feature request related to a problem? Please describe.

i need this to add or remove tools from steps to control what the agent can do in each step

e.g.

// Step 0: model may call data tools (toolChoice: auto).
// Step 1+: force text-only — no more tool calls. The output schema
// then ensures the text is valid structured JSON. This guarantees a
// response even when step 0 called tools, without any tool-as-output hack.
prepareStep: ({ steps }) => (steps.length > 0 ? { toolChoice: 'none' } : {}),

Describe alternatives you've considered

No response

Additional context

No response

Describe the thing to improve

https://ai-sdk.dev/docs/ai-sdk-core/tools-and-tool-calling#preparestep-callback

prepareStep is missing in Agent options

[ravyg]

Looking into this — prepareStep already passes through when provided in per-call options (e.g. agent.generateText({ prepareStep: ... })), because the AI SDK options are spread into the underlying generateText/streamText call. But it's not available as a top-level AgentOptions property, so you can't set a default at agent creation time.

Plan: add prepareStep to AgentOptions, store it on the Agent instance, and merge it with per-call prepareStep (per-call overrides agent-level). This way you can do:

const agent = new Agent({
  name: "my-agent",
  model: openai("gpt-4o"),
  prepareStep: ({ steps }) => (steps.length > 0 ? { toolChoice: "none" } : {}),
  // ...
});

Will open a PR shortly.

[ravyg]

@omeraplak @sapkra PR is up: #1192 please take look.

Includes 6 tests, changeset, and migration guide entry. Per-call prepareStep overrides the agent-level default, following the same pattern as stopWhen. Happy to iterate based on feedback.