MCP Structured Output Validator

CodingIntermediate

Generates outputSchema definitions for MCP tools so responses include validated structuredContent alongside human-readable content.

Trigger/mcp-output-schema

Frequencyper tool

MCP Server Author? Use /mcp-output-schema to add type-safe outputs to your tools

Integration Developer? Ensure downstream systems can parse tool results reliably

MCPStructured OutputJSON SchemaValidation

How It Works

/mcp-output-schema [tool-name] run

↓

Phase 1: analyze tool output

output-analysis

Infer schema from current tool responses

schema-gen

Generate outputSchema definition

validator

Create runtime validation layer

↓

Wire schema into tool definition

↓

✓ Tool with outputSchema + structuredContent validation

Skill Code

# MCP Structured Output Validator
## Trigger: /mcp-output-schema [tool-name]

When invoked:

1. Define outputSchema on tool registration:
```typescript
server.tool(
  "get-weather",
  {
    description: "Get current weather for a city",
    inputSchema: {
      type: "object",
      properties: {
        city: { type: "string" }
      },
      required: ["city"]
    },
    // NEW: declare structured output schema
    outputSchema: {
      type: "object",
      properties: {
        temperature: { type: "number" },
        unit: { type: "string", enum: ["celsius", "fahrenheit"] },
        condition: { type: "string" },
        humidity: { type: "number" }
      },
      required: ["temperature", "unit", "condition"]
    }
  },
  async ({ city }) => {
    const data = await fetchWeather(city);
    return {
      // Human-readable content (always present)
      content: [{
        type: "text",
        text: `It's ${data.temp}°C and ${data.condition} in ${city}`
      }],
      // Machine-readable structured output (validated against outputSchema)
      structuredContent: {
        temperature: data.temp,
        unit: "celsius",
        condition: data.condition,
        humidity: data.humidity
      }
    };
  }
);
```

2. Validation behavior:
 - Server SDK auto-validates structuredContent against outputSchema
 - Schema violations throw at runtime (fail-fast)
 - structuredContent MUST be present when outputSchema is defined
 - content[] is still required for human-readable fallback

3. Client consumption:
```typescript
const result = await client.callTool("get-weather", { city: "Seoul" });
// Type-safe access to structured data
const temp = result.structuredContent.temperature; // number
const condition = result.structuredContent.condition; // string
```

Copy and paste into your CLAUDE.md to start using immediately.

How MCP Structured Output Validator Works

MCP Structured Output Validator analyzes existing tool response patterns to infer and generate an outputSchema, then adds a runtime validation layer ensuring structuredContent conforms to the declared schema. Guarantees dual output — human-readable content[] alongside machine-readable structuredContent.

When to Use MCP Structured Output Validator

Most powerful when downstream systems need to programmatically consume MCP tool results reliably, or when you need type-safe guarantees on tool response shapes.

Key Strengths

Auto-infers and generates outputSchema definitions
Runtime schema validation with fail-fast behavior
Dual human-readable and machine-readable output
Ensures downstream integration stability

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