_model: project
---
schema_version: 1
---
project_id: xzbt-ngn
---
title: XZBT-NGN Exhibit Engine
---
summary:

A generic single-exhibit host for XZBT Exhibit Contract 5.3: session negotiation, descriptor-driven control discovery, presentation-surface rendering, state inspection, and set/invoke operations, with no exhibit-specific vocabulary in the host.
---
status: active
---
started: 2026-09-13
---
author: cgcha
---
repository_url: https://git.labyricorn.com/Labyricorn/XZBT-NGN
---
default_branch: main
---
tags: xzbt, xzbt-ngn, exhibit-engine, contract-5-3, multi-surface, javascript
---
body: |
  XZBT-NGN is the host, orchestration and integration environment for exhibits that
  speak the XZBT Exhibit Contract. It does not replace an exhibit; it extends one.
  Everything an exhibit offers is discovered through the contract's target surface
  and rendered generically, so no exhibit-specific vocabulary is hard-coded
  anywhere in the host.

  ## Completed work

  The contract-boundary MVP is implemented and verified. A single unmodified host
  negotiates a session, requests `describe`, renders discovered capabilities and
  targets grouped by the exhibit's own categories, reads authoritative state,
  submits set and invoke operations with full descriptor validation, displays
  events with source and sequence, tracks registry and state revisions, and
  recovers from synchronization loss. That same unmodified host drives five
  maintained reference exhibits -- Aquarium, Planetarium, Haunted House, Museum
  Gallery and SciFi-XZBT -- with no fixture-specific logic.

  Contract 5.3 multi-surface support is complete. The host discovers presentation
  surfaces from `describe`, resolves their URLs against the supplying exhibit's
  document URL inside a same-origin boundary, and renders each in a local pane
  that carries no contract session of its own. Museum Gallery exercises three
  surfaces; SciFi-XZBT declares a console surface plus an Observation surface that
  attaches to the console's single authority and never creates a second one.

  Verification is automated and repeatable: 154 tests pass under `node --test`
  with no npm dependencies and no build step, and the loopback server exposes only
  `public/`, `src/` and `test-fixtures/` while denying paths that escape them.

  ## Formal design work

  This repository is the canonical home of the XZBT Exhibit Contract Specification
  (5.3 current; 5.2 retained as its historical predecessor, additively superseded),
  the XZBT Exhibit Authoring Guide, and the NGN Implementation Plan. The
  multi-surface architectural model was specified in full before implementation
  and fixes the Step 6 / Step 7 boundary at the schema level: a surface descriptor
  may name a document the host can load, and may never name a display transport,
  casting protocol, network endpoint or device class.

  ## Future work

  Step 7 -- display endpoints, meaning where a surface is physically shown -- is
  specified by contrast in the multi-surface model but is not started. Casting and
  remote displays, MIDI, MCP, webhooks, scenario authoring, recording, telemetry
  connectors, integration adapters, packaging, multi-exhibit orchestration and
  persistence all remain unimplemented; the host keeps in-memory state only. Five
  non-blocking follow-up items requiring sustained human observation remain open
  from the Step 6 closure.