Problem

Virtual MCP Server composite tool definitions currently use a non-standard simplified parameter format in YAML configuration, which diverges from the MCP specification and creates confusion.

Current Behavior

The YAML configuration expects a simplified format:

composite_tools:
  - name: "my_tool"
    parameters:
      param1:
        type: "string"
        default: "value"
      param2:
        type: "integer"
        default: 42

This is then transformed by pkg/vmcp/config/yaml_loader.go:516-531 into full JSON Schema format before being passed to the workflow engine.

Expected Behavior (MCP Standard)

According to the MCP specification, tool inputSchema should be standard JSON Schema:

{
  "type": "object",
  "properties": {
    "param1": {"type": "string", "default": "value"},
    "param2": {"type": "integer", "default": 42}
  },
  "required": ["param2"]
}

The YAML equivalent should be:

composite_tools:
  - name: "my_tool"
    parameters:
      type: object
      properties:
        param1:
          type: string
          default: "value"
        param2:
          type: integer
          default: 42
      required: ["param2"]

Why This Matters

1. Non-standard: The simplified format is not documented in any standard (MCP spec, JSON Schema spec, etc.)
2. Confusion: Users familiar with JSON Schema will be surprised by the format
3. Limited expressiveness: Cannot specify required fields, additionalProperties, validation constraints, etc.
4. Inconsistency: The proposal doc THV-2106-virtual-mcp-server.md:442-443 shows JSON Schema format, but the implementation doesn't support it
5. Transformation overhead: Unnecessary conversion layer in yaml_loader.go

Evidence

Proposal doc (THV-2106-virtual-mcp-server.md:442-443):

parameters:
  pr_number: {type: "integer"}

WorkflowDefinition (pkg/vmcp/composer/composer.go:49):

Parameters map[string]any  // Expects full JSON Schema

YAML Loader (pkg/vmcp/config/yaml_loader.go:161):

Parameters  map[string]map[string]any  // Forces simplified format

Transformer (pkg/vmcp/server/workflow_converter.go:50-68):

// Manually builds JSON Schema from simplified format
params = map[string]any{
    "type":       "object",
    "properties": properties,
}

Proposed Solution

1. Change yaml_loader.go:161 to accept full JSON Schema:

   Parameters  map[string]any `yaml:"parameters"`  // Accept full JSON Schema

2. Remove transformation code in workflow_converter.go:50-68 - just pass through

3. Add validation that parameters is valid JSON Schema (has type: object, properties, etc.)

4. Update demo configs and docs to show standard JSON Schema format

Workaround

For now, use the simplified format as shown in demo-vmcp-composite-minimal.yaml:28-32:

parameters:
  base_url:
    type: "string"
    description: "Base URL"
    default: "https://example.com"

References

• MCP Spec: https://modelcontextprotocol.io/specification/2025-06-18/server/tools
• Code: pkg/vmcp/config/yaml_loader.go:516-531
• Code: pkg/vmcp/server/workflow_converter.go:50-68
• Proposal: docs/proposals/THV-2106-virtual-mcp-server.md:442-443