[BUG] API Error: 400 JSON schema is invalid. It must match JSON Schema draft 2020-12

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [x] Other: custom MCP server<!-- specify -->
• Claude CLI version: 1.0.35 (Claude Code)<!-- output of claude --version -->
• Operating System: macOS <!-- e.g. macOS 14.3, Windows 11, Ubuntu 22.04 -->
• Terminal: Warp<!-- e.g. iTerm2, Terminal App -->

Bug Description

<!-- A clear and concise description of the bug -->

Receiving this error when invoking tool. I can list tools fine and I can invoke tools using mcp-inspector.

Steps to Reproduce

Tool call returns this response (which seems ok to me):

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Title: evcc REST-API\nVersion: latest"
      }
    ]
  }
}

Expected Behavior

An indication why this error triggers, or no error

Additional Context

This doesn't happen when the tool call response contains schema information:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Title: evcc REST-API\nVersion: latest"
      }
    ],
    "schema": {
      "properties": {},
      "type": "object"
    },
    "usage": "call info <json-args>",
    "next_steps": [
      "list",
      "schema info"
    ]
  }
}

but it's not clear how this relates to tools....custom.input_schema

The tool definition itself as returned by tool/list looks correct:

      {
        "annotations": {},
        "description": "Show API metadata: title, version, description, and terms of service.",
        "inputSchema": {
          "properties": {},
          "type": "object"
        },
        "name": "info"
      },