vercel/ai

Summary

When using OpenAI models (gpt-5.2-codex, gpt-4.1) via AI Gateway, the model passes empty string "" for optional tool parameters instead of omitting them or passing undefined.

This bug is specific to OpenAI models. Other providers like Anthropic and xAI handle optional parameters correctly.

Environment

• AI SDK: 6.0.64
• @ai-sdk/gateway: 3.0.29
• Node.js: v22.18.0

Affected Models

Model	Result
openai/gpt-5.2-codex	❌ BUG - passes ""
openai/gpt-4.1	❌ BUG - passes ""
anthropic/claude-sonnet-4.5	✅ OK - passes undefined
xai/grok-3	✅ OK - passes undefined

Minimal Reproducible Example

import { generateText, tool } from 'ai';
import { gateway } from '@ai-sdk/gateway';
import { z } from 'zod/v4';

const greetTool = tool({
  description: 'Greet a user. If nickname is not provided, use their full name.',
  inputSchema: z.object({
    fullName: z.string().describe('The full name of the user'),
    nickname: z.string().optional().describe('Optional nickname to use instead'),
  }),
  execute: async ({ fullName, nickname }) => {
    console.log('nickname:', JSON.stringify(nickname));
    console.log('nickname === "":', nickname === '');           // OpenAI: true, Others: false
    console.log('nickname === undefined:', nickname === undefined); // OpenAI: false, Others: true
    return { greeting: `Hello, ${nickname || fullName}!` };
  },
});

await generateText({
  model: gateway('openai/gpt-4.1'), // BUG: passes nickname: ""
  // model: gateway('anthropic/claude-sonnet-4.5'), // OK: passes nickname: undefined
  prompt: 'Greet the user "John Smith". Do not use any nickname.',
  tools: { greet: greetTool },
});

Expected Behavior

When the user explicitly says "do not use any nickname", the nickname parameter should be:

• undefined, OR
• Omitted from the tool call arguments

Actual Behavior

OpenAI models consistently pass nickname: "" (empty string):

# OpenAI models (gpt-5.2-codex, gpt-4.1)
nickname: ""
nickname === "": true
nickname === undefined: false

# Other models (claude-sonnet-4.5, grok-3)
nickname: undefined
nickname === "": false
nickname === undefined: true