Feature: Strict JSON Schema validation with actionable error messages in CLI mode

[andylbrummer]

Problem

When using npx @modelcontextprotocol/inspector --cli --method tools/list, the inspector silently accepts invalid JSON Schema constructs that MCP clients like Claude Code reject. This makes debugging MCP server issues extremely difficult.

Example: Invalid schema accepted by inspector but rejected by Claude Code

The Go SDK's jsonschema package generates true for interface{} types:

{
  "name": "info",
  "outputSchema": {
    "type": "object",
    "properties": {
      "data": true,  // Invalid - should be a type definition
      "topic": { "type": "string" }
    }
  }
}

The inspector's tools/list returns this without any warning, but Claude Code rejects it with:

[ERROR] MCP server "my-server" Failed to fetch tools: [
  {
    "code": "custom",
    "path": ["tools", 7, "outputSchema", "properties", "data"],
    "message": "Invalid input"
  }
]

Other problematic patterns not caught:

• "type": ["null", "boolean"] - nullable type syntax
• Bare true for any-type fields (from Go's interface{})
• Missing required type fields

Proposed Solution

Add a --strict flag (or make it default) that validates tool schemas against JSON Schema Draft-07/2020-12 and reports actionable errors:

$ npx @modelcontextprotocol/inspector --cli --method tools/list --strict ./my-server

Error: Invalid JSON Schema in tool "info"
  Path: outputSchema.properties.data
  Issue: Bare 'true' is not a valid type definition
  Suggestion: Use {"type": "object", "additionalProperties": true} for dynamic types

Error: Invalid JSON Schema in tool "get_tree"  
  Path: inputSchema.properties.show_ids.type
  Issue: Array type syntax ["null", "boolean"] may not be supported by all clients
  Suggestion: Use {"type": "boolean"} with "nullable": true, or make field optional

Benefits

1. Faster debugging - Catch schema issues before deploying to MCP clients
2. Better DX - Clear error messages with fix suggestions
3. Ecosystem health - Encourage well-formed schemas across MCP servers

Related

• outputSchema does not fully support JSON Schema #552 - outputSchema does not fully support JSON Schema (similar validation gap)
• Support JSON Schema allOf, oneOf, etc. #496 - Support JSON Schema allOf, oneOf, etc.