# Migrate from VNext to standard APIs

As of `v0.20.0` for `@mastra/core`, the following changes apply.

## Legacy APIs (AI SDK v4)

The original methods have been renamed and maintain backward compatibility with **AI SDK v4** and `v1` models.

- `.stream()` → `.streamLegacy()`
- `.generate()` → `.generateLegacy()`

## Standard APIs (AI SDK v5)

These are now the current APIs with full **AI SDK v5** and `v2` model compatibility.

- `.streamVNext()` → `.stream()`
- `.generateVNext()` → `.generate()`

## Migration paths

If you're already using `.streamVNext()` and `.generateVNext()` use find/replace to change methods to `.stream()` and `.generate()` respectively.

If you're using the old `.stream()` and `.generate()`, decide whether you want to upgrade or not. If you don't, use find/replace to change to `.streamLegacy()` and `.generateLegacy()`.

Choose the migration path that fits your needs:

### Keep using AI SDK v4 models

- Rename all your `.stream()` and `.generate()` calls to `.streamLegacy()` and `.generateLegacy()` respectively.

> No further changes required.

### Keep using AI SDK v5 models

- Rename all your `.streamVNext()` and `.generateVNext()` calls to `.stream()` and `.generate()` respectively.

> No further changes required.

### Upgrade from AI SDK v4 to v5

- Bump all your model provider packages by a major version.

> This will ensure that they're all v5 models now. Follow the guide below to understand the key differences and update your code accordingly.

## Key differences

The updated `.stream()` and `.generate()` methods differ from their legacy counterparts in behavior, compatibility, return types, and available options. This section highlights the most important changes you need to understand when migrating.

### Model version support

**Legacy APIs**

- `.generateLegacy()`
- `.streamLegacy()`

Only support **AI SDK v4** models (`specificationVersion: 'v1'`)

**Standard APIs**

- `.generate()`
- `.stream()`

Only support **AI SDK v5** models (`specificationVersion: 'v2'`)

> This is enforced at runtime with clear error messages.

### Return types

**Legacy APIs**

- `.generateLegacy()` Returns: `GenerateTextResult` or `GenerateObjectResult`

- `.streamLegacy()` Returns: `StreamTextResult` or `StreamObjectResult`

See the following API references for more information:

- [Agent.generateLegacy()](https://mastra.ai/reference/agents/generateLegacy)
- [Agent.streamLegacy()](https://mastra.ai/reference/streaming/agents/streamLegacy)

**Standard APIs**

- `.generate()`

  - `format: 'mastra'` (default): Returns `MastraModelOutput.getFullOutput()`
  - `format: 'aisdk'`: Returns `AISDKV5OutputStream.getFullOutput()`
  - Internally calls `.stream()` and awaits `.getFullOutput()`

- `.stream()`

  - `format: 'mastra'` (default): Returns `MastraModelOutput<OUTPUT>`
  - `format: 'aisdk'`: Returns `AISDKV5OutputStream<OUTPUT>`

See the following API references for more information:

- [Agent.generate()](https://mastra.ai/reference/agents/generate)
- [Agent.stream()](https://mastra.ai/reference/streaming/agents/stream)

### Format control

#### Legacy APIs

No `format` option: Always return AI SDK v4 types

```typescript
// Mastra native format (default)
const result = await agent.stream(messages)
```

#### Standard APIs

Use `format` option to choose output:

- `'mastra'` (default)
- `'aisdk'` (AI SDK v5 compatible)

```typescript
// AI SDK v5 compatibility
const result = await agent.stream(messages, {
  format: 'aisdk',
})
```

### New options in standard APIs

The following options are available in the standard `.stream()` and `generate()`, but **NOT** in their legacy counterparts:

- `format` - Choose between 'mastra' or 'aisdk' output format:

  ```typescript
  const result = await agent.stream(messages, {
    format: 'aisdk', // or 'mastra' (default)
  })
  ```

- `system` - Custom system message (separate from instructions).

  ```typescript
  const result = await agent.stream(messages, {
    system: 'You are a helpful assistant',
  })
  ```

- `structuredOutput` - Enhanced structured output with model override and custom options.

  - `jsonPromptInjection` - Used to override the default behaviour of passing response\_format to the model. This will inject context into the prompt to coerce the model into returning structured outputs.

  - `model` - If a model is added this will create a sub agent to structure the response from the main agent. The main agent will call tools and return text, and the sub agent will return an object that conforms to your schema provided. This is a replacement for `experimental_output`.

  - `errorStrategy` - Determines what happens when the output doesn’t match the schema:

    - 'warn' - log a warning
    - 'error' - throw an error
    - 'fallback' - return a fallback value you provide

    ```typescript
    const result = await agent.generate(messages, {
      structuredOutput: {
        schema: z.object({
          name: z.string(),
          age: z.number(),
        }),
        model: 'openai/gpt-5.4', // Optional model override for structuring
        errorStrategy: 'fallback',
        fallbackValue: { name: 'unknown', age: 0 },
        instructions: 'Extract user information', // Override default structuring instructions
      },
    })
    ```

- `stopWhen` - Flexible stop conditions (step count, token limit, etc).

  ```typescript
  const result = await agent.stream(messages, {
    stopWhen: ({ steps, totalTokens }) => steps >= 5 || totalTokens >= 10000,
  })
  ```

- `providerOptions` - Provider-specific options (e.g., OpenAI-specific settings)

  ```typescript
  const result = await agent.stream(messages, {
    providerOptions: {
      openai: {
        store: true,
        metadata: { userId: '123' },
      },
    },
  })
  ```

- `onChunk` - Callback for each streaming chunk.

  ```typescript
  const result = await agent.stream(messages, {
    onChunk: chunk => {
      console.log('Received chunk:', chunk)
    },
  })
  ```

- `onError` - Error callback.

  ```typescript
  const result = await agent.stream(messages, {
    onError: error => {
      console.error('Stream error:', error)
    },
  })
  ```

- `onAbort` - Abort callback.

  ```typescript
  const result = await agent.stream(messages, {
    onAbort: () => {
      console.log('Stream aborted')
    },
  })
  ```

- `activeTools` - Specify which tools are active for this execution.

  ```typescript
  const result = await agent.stream(messages, {
    activeTools: ['search', 'calculator'], // Only these tools will be available
  })
  ```

- `abortSignal` - AbortSignal for cancellation.

  ```typescript
  const controller = new AbortController()
  const result = await agent.stream(messages, {
    abortSignal: controller.signal,
  })

  // Later: controller.abort();
  ```

- `prepareStep` - Callback before each step in multi-step execution.

  ```typescript
  const result = await agent.stream(messages, {
    prepareStep: ({ step, state }) => {
      console.log('About to execute step:', step)
      return {
        /* modified state */
      }
    },
  })
  ```

- `requireToolApproval` - Require approval for all tool calls.

  ```typescript
  const result = await agent.stream(messages, {
    requireToolApproval: true,
  })
  ```

### Legacy options that moved

- `temperature` and other `modelSettings`.

  Unified in `modelSettings`

  ```typescript
  const result = await agent.stream(messages, {
    modelSettings: {
      temperature: 0.7,
      maxTokens: 1000,
      topP: 0.9,
    },
  })
  ```

- `resourceId` and `threadId`.

  Moved to memory object.

  ```typescript
  const result = await agent.stream(messages, {
    memory: {
      resource: 'user-123',
      thread: 'thread-456',
    },
  })
  ```

### Deprecated or removed options

- `experimental_output`

  Use `structuredOutput` instead to allow for tool calls and an object return.

  ```typescript
  const result = await agent.generate(messages, {
    structuredOutput: {
      schema: z.object({
        summary: z.string(),
      }),
      model: 'openai/gpt-5.4',
    },
  })
  ```

- `output`

  The `output` property is deprecated in favor of `structuredOutput`, to achieve the same results, omit the model and only pass `structuredOutput.schema`, optionally add `jsonPromptInjection: true` if your model doesn't natively support `response_format`.

  ```typescript
  const result = await agent.generate(messages, {
    structuredOutput: {
      schema: z.object({
        name: z.string(),
      }),
    },
  })
  ```

- `memoryOptions`

  Use `memory` instead.

  ```typescript
  const result = await agent.generate(messages, {
    memory: {},
  })
  ```

### Type changes

**Legacy APIs**

- `CoreMessage[]`

See the following API references for more information:

- [Agent.generateLegacy()](https://mastra.ai/reference/agents/generateLegacy)
- [Agent.streamLegacy()](https://mastra.ai/reference/streaming/agents/streamLegacy)

**Standard APIs**

- `ModelMessage[]`

  `toolChoice` uses the AI SDK v5 `ToolChoice` type.

  ```typescript
  type ToolChoice<TOOLS extends Record<string, unknown>> =
    | 'auto'
    | 'none'
    | 'required'
    | {
        type: 'tool'
        toolName: Extract<keyof TOOLS, string>
      }
  ```

See the following API references for more information:

- [Agent.generate()](https://mastra.ai/reference/agents/generate)
- [Agent.stream()](https://mastra.ai/reference/streaming/agents/stream)