feat(provider/google): support JSON Schema for structured outputs via useResponseJsonSchema

[onatozmenn]

Background

Fixes #6494.

Gemini structured outputs currently go out as generationConfig.responseSchema, which is a subset of the OpenAPI 3.0 spec rather than JSON Schema. That subset does not cover unions or records, so z.union / z.record schemas either fail with errors like response_schema.properties[occupation].type: must be specified or force users to disable structured outputs entirely (structuredOutputs: false), losing constrained decoding.

Gemini 2.5 and later support submitting JSON Schema through generationConfig.responseJsonSchema, which is mutually exclusive with responseSchema and requires responseMimeType (API reference, guide).

Summary

Adds a useResponseJsonSchema provider option to @ai-sdk/google:

• when true, the response format schema is sent unchanged as generationConfig.responseJsonSchema instead of being converted to the OpenAPI subset for generationConfig.responseSchema
• the two fields are never sent together, and responseMimeType keeps being set as before
• defaults to false, so existing requests are unchanged
• structuredOutputs: false still wins and suppresses both fields

This unblocks unions, records and recursive schemas for object generation on Gemini 2.5+ without giving up structured outputs.

const { output } = await generateText({
  model: google('gemini-2.5-flash'),
  providerOptions: {
    google: {
      useResponseJsonSchema: true,
    } satisfies GoogleLanguageModelOptions,
  },
  output: Output.object({
    schema: z.object({
      name: z.string(),
      contact: z.union([
        z.object({ type: z.literal('email'), value: z.string() }),
        z.object({ type: z.literal('phone'), value: z.string() }),
      ]),
    }),
  }),
  prompt: 'Generate an example person for testing.',
});

The schema is forwarded as-is (including $schema), matching how the official js-genai SDK routes JSON Schema payloads to this field.

Contributor Credit

• @namuorg for reporting Support JSON schema for Gemini 2.5 #6494

End-to-End Verification

Verified through the provider request-body tests, which assert the exact JSON sent to :generateContent:

• useResponseJsonSchema: true produces generationConfig.responseJsonSchema with the untouched anyOf union schema and no responseSchema
• useResponseJsonSchema: true combined with structuredOutputs: false sends neither schema field
• all pre-existing structured-output snapshots are unchanged, confirming the default path is untouched

pnpm --filter @ai-sdk/google exec vitest --config vitest.node.config.js --run src/google-language-model.test.ts → 177 passed. Type check and oxlint on the touched files are clean.

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Future Work

The Gemini API exposes the same escape hatch for tool inputs via FunctionDeclaration.parametersJsonSchema. Wiring that up would let tool input schemas use unions and records too; it was left out here to keep this change focused on structured outputs.