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Core shape

app.tool("name", {
  description: "Human-friendly description",
  input,
  before: ({ input, ctx, request, tool }) => {},
  handler: ({ input, ctx, request, tool }) => {},
  after: ({ input, result, ctx, request, tool }) => {},
  annotations: {
    readOnlyHint: true,
  },
});

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Important fields

  • description is shown in tool listings
  • input provides schema parsing and JSON Schema generation
  • before is tool-local lifecycle before middleware
  • handler does the actual work
  • after runs after successful handler/middleware completion
  • annotations exposes MCP tool hints

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Event object recap

  • input = parsed tool input
  • ctx = request-scoped mutable state
  • request = transport metadata
  • tool = resolved tool name
  • result = only available in after