MCP: tool inputSchema includes $schema field, causing Anthropic API 400 errors

[xuanyunhui]

Bug Description

The Playwright MCP server's toMcpTool function generates tool inputSchema that includes a top-level "$schema": "https://json-schema.org/draft/2020-12/schema" field. This causes Anthropic API to reject all tool calls with a 400 error.

Root Cause

In packages/playwright-core/src/tools/utils/mcp/tool.ts, the toMcpTool function calls zod.toJSONSchema() without stripping the $schema field:

export function toMcpTool(tool: ToolSchema<any>): mcpServer.Tool {
  return {
    name: tool.name,
    description: tool.description,
    inputSchema: zod.toJSONSchema(tool.inputSchema) as mcpServer.Tool['inputSchema'],
  };
}

Zod v4's toJSONSchema() automatically adds "$schema": "https://json-schema.org/draft/2020-12/schema" to the output. While this is valid JSON Schema, the MCP protocol's tool.inputSchema is a JSON Schema subset and should not include the top-level $schema declaration.

Impact

• Anthropic API returns: 400 API Error: "tools.N.custom.input_schema: JSON schema is invalid"
• One invalid tool schema breaks ALL tool calls in the session, not just the affected tool
• Affects users connecting via mcp-remote, Claude Code, Claude Desktop, or any client that forwards schemas to Anthropic API

Suggested Fix

export function toMcpTool(tool: ToolSchema<any>): mcpServer.Tool {
  const jsonSchema = zod.toJSONSchema(tool.inputSchema);
  delete jsonSchema.$schema;
  return {
    name: tool.name,
    description: tool.description,
    inputSchema: jsonSchema as mcpServer.Tool['inputSchema'],
  };
}

Per the MCP specification, inputSchema defaults to JSON Schema 2020-12 when $schema is omitted. Removing the field is both safe and compliant.

Environment

• Playwright version: latest (Zod v4.3.6 bundled)
• MCP client: Claude Code / Claude Desktop via mcp-remote

[pavelfeldman]

MCP protocol and Anthropic tool calling convention are two different things. It is client's responsibility to convert one to another and strip the schema.