[BUG] Claude Code sends invalid tool schemas with `required: null`

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code sends invalid tool schemas with required: null

Summary

Claude Code CLI sends tool schemas where the required field is null instead of an empty array [] when tools have no required fields. This violates JSON Schema specification and causes strict validators (like AI Proxies and OpenAI-compatible endpoints) to reject requests with 400 Bad Request.

Error Message

API Error: 400 tools.67.custom.input_schema.required: Input should be a valid list

Expected Behavior

when a tool has no required fields, Claude Code should send:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...},
          "required": []  // Empty array, not null
        }
      }
    }
  ]
}

Actual Behavior

Claude Code sends:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...},
          "required": null  // Invalid - should be []
        }
      }
    }
  ]
}

Or omits the field entirely:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...}
          // "required" field missing entirely
        }
      }
    }
  ]
}

Impact

• Breaks proxying through strict JSON Schema validators
• Incompatible with AI Gateway's
• Incompatible with OpenAI-compatible API endpoints
• The Anthropic API itself is lenient and accepts this, but downstream systems don't

Environment

• Claude CLI: 2.1.112
• Runtime: Bun 1.3.13
• Proxy: Kong AI Gateway 3.14.0

Related

Similar issue with tool schema validation affects any strict JSON Schema compliant system downstream of Claude Code CLI.

What Should Happen?

Expected Behavior

when a tool has no required fields, Claude Code should send:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...},
          "required": []  // Empty array, not null
        }
      }
    }
  ]
}

Actual Behavior

Claude Code sends:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...},
          "required": null  // Invalid - should be []
        }
      }
    }
  ]
}

Or omits the field entirely:

{
  "tools": [
    {
      "name": "example",
      "custom": {
        "input_schema": {
          "type": "object",
          "properties": {...}
          // "required" field missing entirely
        }
      }
    }
  ]
}

Error Messages/Logs

## Error Message

API Error: 400 tools.67.custom.input_schema.required: Input should be a valid list

Steps to Reproduce

Steps to Reproduce

1. Configure Claude Code CLI to proxy through an AI Gateway (or any JSON Schema validator)
2. Run a command with Claude Code that uses tools
3. Claude Code sends a POST request with tools[N].custom.input_schema.required: null
4. The proxy/validator rejects the request

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.112

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_