Prebuilt, parameterized, interactive HTML widgets for MCP Apps

Techthos released Gadget, a Go library that lets MCP servers ship prebuilt, parameterized, interactive HTML widgets such as data tables, card grids, and forms as self-contained embedded resources. The widgets implement the official MCP Apps extension spec (io.modelcontextprotocol/ui, 2026-01-26) and render in compliant hosts including Claude, ChatGPT, VS Code, Cursor, Goose, and Postman. The library is pre-release with unstable APIs.

Prebuilt, parameterized, interactive HTML widgets for MCP Apps https://modelcontextprotocol.io/extensions/apps/overview — in Go, out of the box. gadget lets an MCP server ship CRUD-style UI — data tables, card grids, forms — as fully self-contained HTML template resources: inline CSS, inline JavaScript, zero external files, everything embedded in your single Go binary. Widgets speak the official MCP Apps extension io.modelcontextprotocol/ui , spec 2026-01-26 and render in any compliant host Claude, ChatGPT, VS Code, Cursor, Goose, Postman, … . Status: pre-release.APIs are not stable yet. Table and Form widgets rendered by the examples/harness fake host — light and host dark themes. CardList lays a collection out as a card grid same filter/sort/pagination/selection as Table ; Card renders a single record. package main import "context" "net/http" "github.com/modelcontextprotocol/go-sdk/mcp" "github.com/techthos/gadget" "github.com/techthos/gadget/gosdk" func main { table := &gadget.Table{ URI: "ui://myapp/users", Title: "Users", Columns: gadget.Column{ gadget.Text "name", "Name" , gadget.Number "balance", "Balance", "currency:EUR" , gadget.Badge "status", "Status", map string gadget.BadgeVariant{ "active": gadget.BadgeSuccess, } , }, Filterable: true, PageSize: 10, } server := mcp.NewServer &mcp.Implementation{Name: "myapp"}, gosdk.EnableUI nil type in struct{} type out struct { Rows map string any json:"rows" } gosdk.AddWidgetToolFor server, table, &mcp.Tool{Name: "list users", Description: "List users in a table."}, func context.Context, mcp.CallToolRequest, in mcp.CallToolResult, out, error { rows, := gadget.RowsOf loadUsers return nil, out{Rows: rows}, nil } h := mcp.NewStreamableHTTPHandler func http.Request mcp.Server { return server }, nil http.ListenAndServe ":8080", h } Ask a connected assistant to "list the users" and it renders an interactive, host-themed table — sortable, filterable, paginated — inside the chat. Table : typed columns text/number/date/badge/link/actions , client-side sort/filter/pagination, row selection with bulk actions, per-row actions → MCP tool calls, inline destructive-action confirmation, empty/loading states. Form : 10 field types, native + inline client validation, submit as a tool call, server-side field errors mapped inline, prefill for edit flows. Host-aware theming : --gadget- design tokens defaulting to host-injected CSS variables Claude/ChatGPT look automatic , Theme struct overrides, dark mode. Locale-aware : numbers/dates formatted via Intl with the host's locale and time zone. SDK-agnostic core + adapter for the official go-sdk https://github.com/modelcontextprotocol/go-sdk ; the core works with any Go MCP implementation. Self-contained by construction : documents satisfy the spec's default locked-down CSP; no CDN, no network, no files on disk. Widget reference /Techthos/gadget/blob/main/docs/widgets.md — Table, Form, actions, data contract Theming /Techthos/gadget/blob/main/docs/theming.md — tokens, host variables, dark mode Architecture /Techthos/gadget/blob/main/docs/architecture.md — rendering model, runtime, security examples/demo — complete MCP server streamable HTTP or -stdio : list/edit/save/delete/archive users. Point MCPJam or any MCP Apps host at http://localhost:8080/mcp . examples/harness — a fake MCP Apps host in one HTML page: renders widgets in a sandboxed iframe, answers the JSON-RPC handshake, logs all traffic, simulates tool results/errors and theme changes. go run ./examples/harness , open http://localhost:8090 . The TypeScript/CSS runtime lives in ui/ and is bundled with esbuild into internal/assets/dist/ committed, go:embed -ed — consumers never need Node . make assets npm ci + build the runtime bundle make test go test ./... + vitest make verify-dist fail if committed dist doesn't match ui/ sources Golden-file tests: go test ./ -update regenerates testdata/golden/ . MIT