Invalid JSON Schema: component_builder tool has array without items definition

[MadGraph]

Bug Description

When using the Webflow MCP server with Factory Droid (and likely other MCP clients), the component_builder tool fails validation with:

Error: 400 Invalid schema for function 'webflow___component_builder': 
In context=('properties', 'actions', 'items', 'properties', 'component_schema', 'properties', 'slots', 'items', 'properties', 'children'), 
array schema missing items.

Root Cause

The JSON Schema for the component_builder tool has a children field of type array that is missing the required items property. According to JSON Schema specification, arrays must define their items schema.

Expected Schema Structure

{
  "children": {
    "type": "array",
    "items": { ... }  // <-- This is missing
  }
}

Impact

This causes the MCP server to fail initialization in any MCP client that performs strict JSON Schema validation. In Factory Droid Missions, this blocks the scrutiny/validation phase entirely.

Environment

• MCP Client: Factory Droid CLI
• Webflow MCP: Remote server (https://mcp.webflow.com/sse)
• Date: April 2026

Workaround

Users must remove/disable the Webflow MCP to proceed with their workflows.

Related

PR #106 fixed similar validation issues with Zod schema ordering - this may be another instance of the same pattern.

[viratatwebflow]

Looks like Factory Droid CLI has some issue. on my end the schema looks like this. can you ask agent to write full input schema?

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "properties": {
    "actions": {
      "items": {
        "additionalProperties": false,
        "properties": {
          "build_label": {
            "description": "A label to identify this build action.",
            "type": "string"
          },
          "action_type": {
            "description": "The type of insertion. insert_in_element: insert component as child of parent_element_id. insert_in_slot: insert component into a slot of a component instance identified by parent_element_id.",
            "enum": ["insert_in_element", "insert_in_slot"],
            "type": "string"
          },
          "parent_element_id": {
            "additionalProperties": false,
            "description": "The id of the parent element (for insert_in_element) or the component instance (for insert_in_slot). e.g id:{component:123,element:456}.",
            "properties": {
              "component": {
                "description": "The component id of the element to perform action on.",
                "type": "string"
              },
              "element": {
                "description": "The element id of the element to perform action on.",
                "type": "string"
              }
            },
            "required": ["component", "element"],
            "type": "object"
          },
          "creation_position": {
            "description": "The position to insert the component. append/prepend insert as child. before/after insert as sibling adjacent to the target element.",
            "enum": ["append", "prepend", "before", "after"],
            "type": "string"
          },
          "component_schema": {
            "additionalProperties": false,
            "description": "The component schema to insert. Use name to specify which component, and optionally slots to populate child components in the instance's slots.",
            "properties": {
              "name": {
                "description": "The name of the component to insert.",
                "type": "string"
              },
              "slots": {
                "description": "Slots to populate with child components.",
                "items": {
                  "additionalProperties": false,
                  "properties": {
                    "name": {
                      "description": "The name of the slot.",
                      "type": "string"
                    },
                    "children": {
                      "description": "Array of ComponentSchema objects (same shape: { name, slots? }).",
                      "type": "array"
                    }
                  },
                  "required": ["name", "children"],
                  "type": "object"
                },
                "type": "array"
              }
            },
            "required": ["name"],
            "type": "object"
          },
          "slot_name": {
            "description": "The slot name to insert the component into. Required when action_type is insert_in_slot.",
            "type": "string"
          },
          "return_component_info": {
            "description": "Whether to return the component instance info after insertion.",
            "type": "boolean"
          }
        },
        "required": ["build_label", "action_type", "parent_element_id", "creation_position", "component_schema"],
        "type": "object"
      },
      "type": "array"
    },
    "context": {
      "description": "Explain why you are calling this tool (15-25 words, third-person, no sensitive info).",
      "type": "string"
    },
    "siteId": {
      "description": "The ID of the site.",
      "type": "string"
    }
  },
  "required": ["actions", "context", "siteId"],
  "type": "object"
}

[MadGraph]

Thanks for checking! Actually, the schema you posted confirms the issue:

"children": {
  "description": "Array of ComponentSchema objects (same shape: { name, slots? }).",
  "type": "array"
}

According to JSON Schema spec (draft-07 which you're using), when type: "array" is specified, the items keyword is required to define what the array contains. Without it, validators that strictly follow the spec will reject the schema.

The fix would be:

"children": {
  "description": "Array of ComponentSchema objects (same shape: { name, slots? }).",
  "type": "array",
  "items": {
    "$ref": "#/definitions/ComponentSchema"
  }
}

Or if recursive refs aren't supported:

"children": {
  "description": "Array of ComponentSchema objects (same shape: { name, slots? }).",
  "type": "array",
  "items": {
    "type": "object"
  }
}

Some MCP clients (like Cursor) may be lenient and skip this validation, but Factory Droid and Anthropic's Claude API enforce strict JSON Schema compliance, which is why this surfaces there.

Ref: https://json-schema.org/understanding-json-schema/reference/array#items

[viratatwebflow]

Gotcha! the above schema is fetch via Claude client. I guess primary issue is with any. we will check for a fix

[MadGraph]

Hey @MadGraph - just checking if you've had a chance to test with the latest MCP server version? The schema fix might have been deployed already.

[MadGraph]

@MadGraph ping - testing webhook integration 🧪