gadget

Prebuilt, parameterized, interactive HTML widgets for MCP Apps — 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.}

Quickstart

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.

Features

• 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; 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.

Documentation

• Widget reference — Table, Form, actions, data contract
• Theming — tokens, host variables, dark mode
• Architecture — rendering model, runtime, security

Examples

• 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.

Development

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/.

License

MIT