Error AI_TypeValidationError while using gemini-2.0-flash-lite-preview-02-05

### Description

After 7 or 8 interactions in the same session in a chat with the Gemini 2.0 Flash Lite model, I get this error. I have tried using fallbacks to switch models, added security filters to the Gemini models, and also used TypeValidationError to catch this type of error, but it still appears both in the console and on the frontend. However, I can continue interacting with the model in the chat. As seen at the end of the log, the model displays the result correctly, but I keep getting an AI_TypeValidationError in the console and frontend, which I can't get rid of.

```
Stream error occurred: {
  error: [Error [AI_TypeValidationError]: Type validation failed: Value: {"candidates":[{"content":{"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":7208,"candidatesTokenCount":30,"totalTokenCount":7238},"modelVersion":"gemini-2.0-flash-lite-preview-02-05"}.
  Error message: [
    {
      "code": "invalid_type",
      "expected": "array",
      "received": "undefined",
      "path": [
        "candidates",
        0,
        "content",
        "parts"
      ],
      "message": "Required"
    }
  ]] {
    value: {
      candidates: [Array],
      usageMetadata: [Object],
      modelVersion: 'gemini-2.0-flash-lite-preview-02-05'
    },
    [cause]: [Error [ZodError]: [
      {
        "code": "invalid_type",
        "expected": "array",
        "received": "undefined",
        "path": [
          "candidates",
          0,
          "content",
          "parts"
        ],
        "message": "Required"
      }
    ]] {
      issues: [ [Object] ],
      addIssue: [Function (anonymous)],
      addIssues: [Function (anonymous)]
    }
  }
}
Stream finished {
  id: 'aitxt-A4CF2atIQPaD4Xe39hdvLrBE',
  timestamp: 2025-02-18T16:40:32.058Z,
  modelId: 'gemini-2.0-flash-lite-preview-02-05',
  headers: {
    'alt-svc': 'h3=":443"; ma=2592000,h3-29=":443"; ma=2592000',
    'content-disposition': 'attachment',
    'content-type': 'text/event-stream',
    date: 'Tue, 18 Feb 2025 16:40:31 GMT',
    server: 'scaffolding on HTTPServer2',
    'server-timing': 'gfet4t7; dur=278',
    'transfer-encoding': 'chunked',
    vary: 'Origin, X-Origin, Referer',
    'x-content-type-options': 'nosniff',
    'x-frame-options': 'SAMEORIGIN',
    'x-xss-protection': '0'
  },
  messages: [
    {
      role: 'assistant',
      content: [Array],
      id: 'msg-JLHrX8vXcIM2wXtetezBVL4K'
    },
    {
      role: 'tool',
      id: 'msg-gnszRCPWM0DIGIUj2LnJUAlY',
      content: [Array]
    },
    {
      role: 'assistant',
      content: [Array],
      id: 'msg-gp0xXx1WNcepO1G6UBnxKl1m'
    }
  ]
}
```

### Code example

```
return streamText({
    model: createFallback({
      models: [
        provider.languageModel("gemini-2-flash-lite"),
        provider.languageModel("gemini-2-flash"),
        provider.languageModel("gpt-4o-mini"),
      ],
      shouldRetryThisError: (error: Error) => {
        if (TypeValidationError.isInstance(error)) {
          return true;
        }

        const defaultHandler = error.message.toLowerCase();
        if (defaultHandler.includes('api key') ||
          defaultHandler.includes('no such') ||
          error.name === 'AI_APICallError' ||
          error.name === 'AI_NoSuchModelError') {
          console.log('Retrying due to API or model error');
          return true;
        }

        return false;
      },
      onError: (error, modelId) => {
        if (!(error.name === 'AI_TypeValidationError' &&
          error.message.includes('candidates') &&
          error.message.includes('content.parts'))) {
          console.error(`\nError with model ${modelId}:`, error, "\n");
        }
      },
      retryAfterOutput: false,
      modelResetInterval: 120000,
    }),
    system: agent(),
    messages: convertToCoreMessages(messages),
    maxSteps: 5,
    temperature: 0,
    experimental_telemetry: { isEnabled: true },
    experimental_transform: smoothStream({ chunking: "word" }),
    experimental_activeTools: ALL_TOOLS,
    tools: {
      behavior,
      event,
      form,
      media,
      report,
      navigation,
    },
    experimental_repairToolCall: async ({
      toolCall,
      tools,
      parameterSchema,
      error,
    }) => {
      if (NoSuchToolError.isInstance(error)) {
        return null;
      }

      const tool = tools[toolCall.toolName as keyof typeof tools];

      const { object: repairedArgs } = await generateObject({
        model: openai('gpt-4o-mini', { structuredOutputs: true }),
        schema: tool.parameters,
        prompt: [
          `The model tried to call the tool "${toolCall.toolName}"` +
          ` with the following arguments:`,
          JSON.stringify(toolCall.args),
          `The tool accepts the following schema:`,
          JSON.stringify(parameterSchema(toolCall)),
          'Please fix the arguments.',
        ].join('\n'),
      });

      return { ...toolCall, args: JSON.stringify(repairedArgs) };
    },
  }).toDataStreamResponse();
```

### AI provider

_No response_

### Additional context

_No response_

Labels: bug
State: closed