helix.mcp: Skill for Claude Code

MCP Wire-Format Trim

When to use

• You need to reduce tools/list token/byte cost without changing tool behavior.
• You are auditing MCP metadata bloat in McpServerTool attributes or auto-generated schemas.

Pattern 1: default-annotation audit

1. Verify SDK defaults from the exact package version in use (ModelContextProtocol.Core source or reflection).
2. Only remove attribute properties whose explicit value matches the SDK default.
3. In SDK 1.3.0, the important defaults are:
   • OpenWorld = true
   • ReadOnly = false
   • Idempotent = false
   • Destructive = true
   • UseStructuredContent = false
4. Consequence: OpenWorld=true is removable noise, but Destructive=false must stay.

Pattern 2: drop outputSchema while preserving wire payload

Use this only for tiny results where schema carries little value and you must keep the actual tool-call payload stable.

1. Change the tool method return type to CallToolResult (or Task<CallToolResult>).
2. Remove UseStructuredContent = true from the [McpServerTool] attribute so the SDK stops advertising outputSchema.
3. Return CallToolResult manually with both:
   • Content = [new TextContentBlock { Text = JsonSerializer.Serialize(value, McpJsonUtilities.DefaultOptions) }]
   • StructuredContent = JsonSerializer.SerializeToElement(value, McpJsonUtilities.DefaultOptions)
4. This keeps the tool-call JSON payload/structured content intact while shrinking tools/list.

Pattern 2b: minimal object outputSchema via an empty marker type (SDK 2.x)

Use when a tool must keep UseStructuredContent = true — because SDK 2.x picks the structuredContent envelope from the schema's objectness — but the detailed schema is not worth its fixed-context rent.

The rule that makes this safe: AIFunctionMcpServerTool.ShouldWrapValueForLegacyWire asks only whether the schema's type is object (or ["object","null"]). It never reads properties or required, and the C# SDK does no runtime validation of structuredContent against outputSchema. So {"type":"object"} is functionally equivalent to a full property mirror, at ~17 B instead of hundreds.