// previous step messages
const messages: ModelMessage[] = [
  {
    role: "system", content: systemPrompt
  },
  {
    role: "user", content: userPrompt
  },
  {
    role: "assistant",
    content: [
      {
        type: "reasoning",
        text: "",
        providerOptions: {
          openai: {
            itemId: "rs_025dc37d353024330068dfe8bf3d9081939cf194d9b4f8c260",
            reasoningEncryptedContent: null
          }
        }
      },
      {
        type: "tool-call",
        toolCallId: "call_fVPGEmWa2xTGtMTJ3kVYOFYY",
        toolName: "doSomething",
        input: {
          someParam: "some params"
        },
        providerOptions: {
          openai: {
            itemId: "fc_025dc37d353024330068dfe8c3edf481938456f8be119a4b1b"
          }
        }
      }
    ]
  }
];

// after executing the tool call
messages.push({
  role: 'tool',
  content: [
    {
      type: 'tool-result',
      toolCallId: 'call_fVPGEmWa2xTGtMTJ3kVYOFYY',
      toolName: 'doSomething',
      output: {
        type: 'json',
        value: {
          a: "xxx",
          b: undefined // this causes validation error in the next step
        }
      },
    }
  ],
});

// execute next step
const result = streamText({
  model: openai('gpt-5'),
  messages,
  tools: toolSet,
})

export const jsonValueSchema: z.ZodType<JSONValue> = z.lazy(() =>
  z.union([
    z.null(),
    z.string(),
    z.number(),
    z.boolean(),
    z.record(z.string(), jsonValueSchema),
    z.array(jsonValueSchema),
  ]),
);