Less accuracy on complex schemas with new JSON schema conversion

[defless]

Since I have migrated to the v5, I can observe a drop of accuracy on complex generations using zod 4 schemas.
I have migrated from z.lazy to new getter method and get rid of all .transforms as the internal zod method to convert zod to json does not accepts them (while zod-to-json-schema was fine with them).

The problem is probably coming from my schema definition, but they worked very well on the v4 with the same models.
Maybe the conversion of json to zod via a third party library was producing better outputs ?

Don't want to open an issue as it's probably one of my own mistake, but if someone face the same problem please let me know.

[lgrammel]

We are using the zod4 internal tojsonschema for v4 schemas and you are right this might impact it.

As a workaround, you could try using the jsonSchema helper and implement a factory function that uses the zod v4 version of the zod-to-json-schema (there is a fork of the library). would be curious if you are getting better results with that and if the json schemas are different.

[defless]

I have tried the zod-to-json-schema fork . The results are different between the two conversion methods, but neither is much better than the other (I have also tried with openai and xai for both methods).

I guess I'll have to rework my schemas to be more accurate and maybe less complex.

import type { JSONSchema7 } from '@ai-sdk/provider';
import { z } from 'zod/v4';
import { jsonSchema } from 'ai';
import zodToJsonSchema from  '@alcyone-labs/zod-to-json-schema';

const json = jsonSchema(
  zodToJsonSchema(myComplexZodSchema, {
    $refStrategy: 'root',
    target: 'jsonSchema7',
  }) as JSONSchema7,
  {
    validate: async value => {
      const result = await myComplexZodSchema.safeParseAsync(value);

      return result.success
        ? { success: true, value: result.data }
        : { success: false, error: result.error };
    },
  },
);

[IrakliJani]

I also ran into this issue when upgrading from v4 to v5.

In v4, setting structuredOutput when creating a provider automatically enabled strict JSON schema validation. The official migration guide for v5 mentions that structured outputs are now enabled by default (https://ai-sdk.dev/docs/migration-guides/migration-guide-5-0#structured-outputs-default), but apparently this no longer implies strict tool calls, and unfortunately the guide doesn’t mention this change.

I was able to re-enable strict mode for tool calls by passing providerOptions to streamText for OpenAI like this:

streamText({
  // ...
  providerOptions: {
    openai: {
      strictJsonSchema: true,
    },
  },
  // ...
});

I think this is a regression and that strictJsonSchema should always default to true.

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