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 carousels,
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.

gadget Table widget: sortable, filterable, paginated data table with typed columns, badges, row selection and per-row actions

The same Table widget rendered in the host's dark theme gadget Form widget: labelled fields, validation and submit/cancel actions

_{Table and Form widgets rendered by the examples/harness fake host — light and host dark themes.}

gadget CardList widget: a collection rendered as a horizontally scrolling strip of cards with title, subtitle, status badge, typed label/value fields, filter, sort, selection and per-card actions

gadget Card widget: a single record rendered as a detail card with a status badge, label/value fields and actions

_{CardList lays a collection out as a horizontally scrolling card carousel that fits a narrow chat pane (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