2026-08-22 03:00:57 +00:00
40 changed files with 1286 additions and 44 deletions
@@ -40,7 +40,8 @@ pnpm infrastructure:config
  artifact the manifest declares, via `@ekklesia/module-ui-build` (ADR 0023) — module authors write
  no bundler configuration. Styling is React inline style objects only for now, and no host mounts a
  UI entrypoint yet.
- Module UI reaches its host only through `@ekklesia/module-ui-sdk`, and asks the host for its own
+- Module UI reaches its host only through `@ekklesia/module-ui-sdk` — never
+  `@ekklesia/module-ui-host`, which is the host-side loader — and asks the host for its own
  API by relative path — never by constructing `/api/v1/modules/<id>/...`, which is Ekklesia's
  private deployment detail and would break standalone hosting.
 - The containing application shell, navigation, global theme, and platform policy belong to Core and
@@ -287,9 +287,24 @@ from source, validate it with `parseModuleUiExport`, mount it into a real DOM, a
 output, and unmount it — so a UI that cannot be produced fails CI, and no generated file is ever
 committed. See [ADR 0023](docs/architecture/decisions/0023-module-ui-build-pipeline.md).

-**Still no host.** A module UI can be authored, built, and tested; **nothing serves one to a user
-yet.** The Dashboard shell, navigation, a design system, a stylesheet loading contract, and the
-standalone host do not exist.
+Phase 6B pass 3 closes the last two foundational gaps. **The browser boundary now actually holds**:
+`@ekklesia/module-ui-sdk` had one type-only import of `@ekklesia/contracts`, whose barrel pulled
+Drizzle's types — and with them Node's globals — into every module UI program, so `process.env`
+typechecked inside browser-only code. The SDK now has **no dependencies at all**, and owns a plain
+browser `ModuleUiPrincipal` instead of Core's Zod-branded server type; a compile test proves that
+supported UI source compiles and that `process` does not. **And there is now a generic host loader**:
+`@ekklesia/module-ui-host` imports a published artifact from a browser URL, validates its id and
+version against the manifest *before* mounting, mounts it into a host-owned container, and returns an
+idempotent `unmount()`. A module may never depend on it — it is host infrastructure, and the boundary
+checker enforces that. See
+[ADR 0024](docs/architecture/decisions/0024-browser-safe-ui-sdk-and-host-loader.md).
+
+**Still no host, and one specific reason why.** Core resolves `entrypoints.ui` to a path on the
+*server's filesystem*; a browser cannot import that. **Ekklesia has no route that serves an installed
+module's artifact at a browser URL**, so nothing can mount an installed module UI yet — the loader is
+complete, asset publication is not, and that is the next pass's first job. The Dashboard shell,
+navigation, a design system, a stylesheet loading contract, and the standalone host also do not
+exist.

 **Security model.** Installed modules are trusted compiled backend code that runs arbitrary
 JavaScript in the API process and shares its PostgreSQL role — table prefixes, manifests, permission
@@ -87,6 +87,14 @@ components, no layout, no design system, no router.

 ### The host context is deliberately four members

+> **Partial correction — see [ADR 0024](0024-browser-safe-ui-sdk-and-host-loader.md).**
+> `principal` is no longer typed as Core's `EkklesiaPrincipal`. That import made this package depend
+> on `@ekklesia/contracts`, whose barrel pulled Drizzle's types — and Node's globals — into every
+> browser program that imported the SDK. The context now uses a browser-owned `ModuleUiPrincipal`
+> with the same shape and an unbranded `userId`, and the SDK has no dependencies at all. The four
+> members and everything else in this section are unchanged.
+
+
 ```ts
 interface ModuleUiHostContext {
  readonly moduleId: string;
@@ -170,6 +170,13 @@ through two independent paths.

 ## Consequences

+> **Cross-reference — see [ADR 0024](0024-browser-safe-ui-sdk-and-host-loader.md).**
+> The Node-globals leak this ADR recorded under "Browser TypeScript stays scoped to UI source" is
+> now diagnosed and closed: it was a single type-only import of `@ekklesia/contracts` in the UI SDK,
+> whose barrel reached Drizzle's types. The SDK now has no dependencies, and a compile test proves a
+> module UI file importing it cannot use `process`. Everything else in this ADR is unchanged.
+
+
 A module author can now write React and TypeScript, run their normal build, and get exactly the
 artifact their manifest declares — with no bundler configuration of their own. The end-to-end path
 from ADR 0021's static field through ADR 0022's contract to a mounted, unmountable UI is closed and
@@ -0,0 +1,199 @@
+# 0024: Browser-safe module UI SDK and the generic host loader
+
+- Status: Accepted
+- Date: 2026-08-21
+- Extends: [0022](0022-module-ui-authoring-and-host-contract.md),
+  [0023](0023-module-ui-build-pipeline.md)
+- Supersedes (partial): [0022](0022-module-ui-authoring-and-host-contract.md) — only its decision to
+  type `ModuleUiHostContext.principal` as Core's `EkklesiaPrincipal`. Every other part of that ADR —
+  the mount-function ABI, the four-member context, module-relative API access, and the trust model —
+  remains accepted and unchanged.
+
+## Context
+
+Two gaps stand between the module UI contract and a host that could actually use it.
+
+**The browser boundary had a hole.** ADR 0023 recorded it honestly: a module UI file compiled with
+`"types": []` should not see Node globals, yet `process.env` typechecked. The cause was never
+diagnosed beyond "transitively, through dependency type graphs."
+
+**Nothing loads a module UI.** ADR 0022 defined `parseModuleUiExport` for a host to call, and ADR
+0023 made the artifact buildable, but no code imports an artifact, checks its identity, mounts it, or
+tears it down. Every future host — the Ekklesia Dashboard and a standalone host alike — needs that
+lifecycle, and it must not be invented twice.
+
+## Decision
+
+### The leak was one type-only import, and it is removed by deleting the dependency
+
+Bisected against the real type graph rather than inferred: `@ekklesia/module-ui-sdk` imported
+`type { EkklesiaPrincipal } from "@ekklesia/contracts"`. Importing `contracts/identity` alone is
+clean, and importing Zod alone is clean — but `@ekklesia/contracts` exposes a single barrel entry,
+so that one import pulled in `module-database`, whose Drizzle types re-introduce Node's global
+declarations. A browser file importing the SDK therefore inherited `process`.
+
+The runtime was never affected: the import was type-only and the emitted `host-context.js` is
+`export {};`. This was purely a type-graph leak — and a type-graph leak is still a real boundary
+failure, because it silently permits code that cannot run in the target environment.
+
+The fix is not a narrower import path. `@ekklesia/module-ui-sdk` now has **no dependencies at all**,
+so there is no graph to leak through and no future contracts change can reopen this. That is the
+property worth having: the boundary holds because the package is empty of external references, not
+because today's import happens to resolve to something clean.
+
+### A browser principal is not a server principal
+
+`ModuleUiPrincipal` is now defined by the SDK: `{ kind: "anonymous" }` or
+`{ kind: "authenticated", userId: string }`. The shape ADR 0022 chose is preserved; only the brand is
+dropped.
+
+That is a deliberate contract change, not a convenience. Core's `EkklesiaUserId` is a Zod-branded
+string, and the brand encodes a *server-side parsing* guarantee that a browser cannot produce without
+running Zod — which ADR 0022 already identified as the reason `permissions.has` takes a plain string.
+A browser-facing identity should be plain and serializable; a server type that drags an ORM's types
+behind it is not a browser contract merely because it is convenient to reuse.
+
+The two remain structurally compatible: a branded string is assignable to `string`, so an Ekklesia
+host passes its own `EkklesiaPrincipal` straight into the context with no conversion, while a
+standalone host builds the same object literal from whatever identity system it has. No field was
+added. Nothing server-internal is exposed to gain type reuse.
+
+### The generic loader lives in its own package
+
+`@ekklesia/module-ui-host` (new shared role `module-ui-host`) owns the load-and-mount lifecycle. It
+is not part of `@ekklesia/module-ui-sdk`, because the two have opposite audiences: the SDK is bundled
+into **every module artifact**, so putting host-only loader code there would ship it inside every
+module that never runs it and blur the author/host split. `pnpm boundaries` enforces the separation
+in both directions — the `module-ui-host` role is deliberately absent from the roles a module may
+depend on, and the loader itself may depend only on the SDK (plus dev-time `tooling`), never on a
+module, an app, a surface, or the contracts barrel.
+
+Dependency direction is therefore: module UI → SDK; host → loader → SDK. Never module → loader, and
+never SDK → anything.
+
+### A browser module URL is not a filesystem path, and this is the pass's central distinction
+
+Core's discovery resolves `entrypoints.ui` to an **absolute path on the server's disk**
+(`ResolvedModuleEntrypoints.ui`). A browser cannot import that. A loader that accepted one would look
+correct in a server-side test and fail in every real host.
+
+So `loadModuleUi` takes a `url` — a browser-loadable module URL — and this package deliberately does
+not know how one is obtained. Turning an installed artifact into a URL a browser may fetch is **asset
+publication**: a separate, host-specific concern. Ekklesia will need a trusted route that serves an
+installed module's compiled artifact; a standalone host may publish the same file from a CDN, a
+static directory, or its own bundle. Making the loader own publication would weld it to one host and
+recreate exactly the coupling ADR 0022 exists to prevent.
+
+**Ekklesia has no such route today.** The loader is complete; publication is deferred and is the
+first thing the next pass must build. Nothing here should be read as "a host can mount an installed
+module now" — it cannot, because it has no way to obtain a URL.
+
+### Dynamic import is injectable, and never `eval`
+
+The default importer is `(url) => import(url)` with a non-analyzable specifier, which esbuild and
+Rollup leave as a run-time import rather than resolving at build time. That is required, not
+incidental: installed modules are unknown when a host is built, and a statically-resolved import
+would either fail or force a hardcoded registry of module imports — defeating independently
+installable modules entirely.
+
+`importModule` is injectable so a host whose bundler needs a specific escape hatch (Vite's
+`@vite-ignore`, webpack's `webpackIgnore`) supplies a one-line importer instead of this package
+guessing. It is deliberately **not** implemented with `new Function` or `eval`: those defeat static
+analysis reliably but require `unsafe-eval` in a Content-Security-Policy, and loading a module should
+never cost a host its CSP.
+
+### Identity is verified before anything mounts
+
+The loader calls the SDK's existing `parseModuleUiExport(namespace, { id, version })` — it does not
+reimplement shape or identity checking. A missing default export, a malformed definition, a wrong
+module id, or a wrong version throws `ModuleUiContractError` with its specific code, **before**
+`mount` is called, so a stale or mismatched bundle can never render or execute module logic.
+
+Contract violations propagate unwrapped; the loader adds only two codes of its own,
+`MODULE_UI_IMPORT_FAILED` and `MODULE_UI_MOUNT_FAILED`, whose messages carry no URL or caught text.
+The originating error is attached as `cause` for host diagnostics.
+
+### Lifecycle: idempotent unmount, no hidden state, host owns the container
+
+`loadModuleUi` returns a `MountedModuleUi` that owns its own cleanup. There is no cache, registry, or
+singleton, so a module may be mounted into several containers at once and unmounting one leaves the
+others untouched.
+
+`unmount()` is idempotent: the first call runs the module's cleanup, later calls do nothing, and it
+is safe when the module returned no cleanup at all. An instance is marked unmounted *before* cleanup
+runs, so a throwing cleanup surfaces to the host exactly once and never re-runs.
+
+A failed `mount` registers no cleanup, so no handle can claim a failed module is mounted. The loader
+never mutates the container — not on success, not on failure, not on unmount. The host owns the
+container; the module owns its contents; deciding what a half-rendered failure should look like is
+the host's call, not the loader's.
+
+### Mounting executes trusted code in the host's page
+
+Importing a module UI artifact runs ordinary same-realm JavaScript: it can reach the DOM, globals,
+and anything else the page can. **The loader is not a sandbox and does not become one by validating
+identity.** Identity validation prevents mounting the *wrong* module; it says nothing about whether a
+module is safe.
+
+Publishing an artifact must never be read as evidence that it is safe. Only installed, reviewed
+modules may be published or mounted under the current trust model, and asset publication must be
+restricted accordingly when it is built. Server APIs remain the sole authority for authorization: a
+mounted module UI has gained no privilege it did not already have.
+
+## Alternatives
+
+- **Narrowing the import to a `@ekklesia/contracts` subpath.** Rejected. It would work today —
+  `identity` alone is clean — but it makes the browser boundary depend on which types a backend
+  package happens to co-locate, and any future edit could reopen the hole silently.
+- **A new `@ekklesia/client-contracts` package.** Rejected: only one package originates these types,
+  and the loader receives them transitively through the SDK. A package for two interfaces would be
+  structure without a second consumer to justify it.
+- **Keeping `EkklesiaPrincipal` and adding Node types to the UI tsconfig.** Rejected outright: that
+  hides the error instead of fixing the graph, and hands browser code a `process` that does not
+  exist at run time.
+- **Putting the loader in `@ekklesia/module-ui-sdk`.** Rejected: it would ship host-only code inside
+  every module artifact and erase the author/host distinction the roles exist to express.
+- **A loader that accepts Core's resolved filesystem path.** Rejected as the most tempting wrong
+  answer: it would test green on a server and fail in every browser, and it would quietly make the
+  loader responsible for asset publication.
+- **Implementing publication here to make the loader "complete".** Rejected: it is host-specific, it
+  needs an authorization decision about who may fetch an artifact, and bundling it in would couple
+  the generic loader to Ekklesia. Deferred explicitly instead of half-built.
+- **`new Function("return import(u)")` to defeat bundler analysis.** Rejected: requires `unsafe-eval`
+  in a CSP. An injectable importer achieves the same result without that cost.
+- **Throwing on a second `unmount()`.** Rejected: teardown races are ordinary in UI hosts, and an
+  idempotent unmount lets a host clean up without tracking whether it already did.
+- **Clearing the container on mount failure.** Rejected: the loader promises never to mutate the
+  host's container, and a single unconditional rule is easier to reason about than one with an
+  exception.
+
+## Consequences
+
+A module author importing the supported SDK now provably cannot reach Node globals — proven by
+compiling both a supported fixture and a deliberately-failing one under the exact project a module
+uses. And a host implementer has one narrow, tested lifecycle to call instead of writing dynamic
+import, identity checking, and teardown themselves.
+
+The cost is that this is still infrastructure without a consumer. **Ekklesia cannot mount an
+installed module UI**, because nothing serves an installed artifact at a browser URL. The loader is
+proven against `data:` URLs built from real artifacts, which is a genuine module URL rather than a
+filesystem path — but a real host needs a real route.
+
+`ModuleUiPrincipal` is also a public type change. Nothing outside this repository consumes it yet, so
+the migration cost is zero today, and an Ekklesia host's `EkklesiaPrincipal` remains assignable to it
+without conversion.
+
+## Deferred
+
+Not implemented or promised here: **module UI asset publication** — the trusted route that turns an
+installed module's compiled artifact into a browser URL, including who may fetch it — which is the
+prerequisite for any real host; the Dashboard shell, navigation, or grouping; a design system, theme
+tokens, or component library; stylesheet and asset loading; React runtime sharing; a standalone
+application host; module installation, marketplace, or hot loading; and every real feature module.
+
+`entrypoints.jobs` still accepts source TypeScript where other executable entrypoints require
+compiled output. This pass did not centralize compiled-entrypoint validation, so that inconsistency
+is unchanged and still open. The permission-id typing gap (`permissions.has` takes a plain string, so
+a typo silently hides a control) is likewise unchanged: the browser-safe split did not produce a
+clean answer for it, and inventing one would have meant shipping generated constants this phase does
+not need.
@@ -32,3 +32,4 @@ ADRs capture durable choices that affect more than one package or phase.
 - [0021: Modules as independently hostable products](0021-modules-as-hostable-products.md) — Accepted
 - [0022: Module UI authoring model and host contract](0022-module-ui-authoring-and-host-contract.md) — Accepted
 - [0023: Module UI build pipeline](0023-module-ui-build-pipeline.md) — Accepted
+- [0024: Browser-safe module UI SDK and the generic host loader](0024-browser-safe-ui-sdk-and-host-loader.md) — Accepted
@@ -206,6 +206,20 @@ styling is inline style objects only, and a stylesheet import fails the build ra
 asset nothing declares. Building remains an authoring step — Core never builds, and discovery still
 only validates the artifact a module shipped. The Dashboard shell, a UI runtime, a design system, a
 stylesheet loading contract, the standalone host, and every feature module remain future work.
+
+**Phase 6B pass 3 makes the browser boundary real and adds the generic host loader.**
+[ADR 0024](decisions/0024-browser-safe-ui-sdk-and-host-loader.md) removes the UI SDK's last
+dependency — one type-only import of the contracts barrel that pulled Drizzle's types, and Node's
+globals, into browser programs — so the SDK now depends on nothing and defines its own plain
+`ModuleUiPrincipal`. `@ekklesia/module-ui-host` is the loader both Ekklesia and a future standalone
+host will use: dynamic import from a browser URL, identity validated against the manifest before
+mounting, a host-owned container the loader never mutates, and an idempotent unmount. Modules may not
+depend on it.
+
+The distinction that shapes the next pass: Core resolves `entrypoints.ui` to a **server filesystem
+path**, and a browser needs a **URL**. Turning an installed artifact into a browser URL is asset
+publication — host-specific, deliberately outside the loader, and not implemented. Until it exists,
+no host can mount an installed module UI.
 Restart-based discovery, explicit operator-invoked migrations, and the trusted compiled-module model
 are all unchanged.

@@ -28,7 +28,12 @@ These rules map platform ownership to filesystem and package boundaries. The exe
  ([ADR 0023](decisions/0023-module-ui-build-pipeline.md)). An unsupported framework aliased behind an allowed name is resolved and rejected by its
  real target.
 - Module UI source is subject to the identical layer rules as module backend source: it may not
-  import Core, an app, a surface, a builder, or another module. Core still owns the containing
+  import Core, an app, a surface, a builder, or another module. It also may not import
+  `@ekklesia/module-ui-host`: that package is host infrastructure, and a module bundling the loader
+  would blur the author/host split ([ADR 0024](decisions/0024-browser-safe-ui-sdk-and-host-loader.md)).
+- `@ekklesia/module-ui-sdk` has no dependencies at all, and `@ekklesia/module-ui-host` depends only
+  on it. Both are browser-safe by construction: neither can pull a backend type graph — and with it
+  Node's global declarations — into browser code. Core still owns the containing
  application shell and all platform policy, and module UI reaches the host only through
  `@ekklesia/module-ui-sdk`.
 - A surface never imports Core or a feature module. It uses public API/client contracts.
@@ -76,6 +76,23 @@ The contract does leave room for a future sandboxed tier without a redesign: `mo
 context)` passes an element and a plain-data context rather than shared object graphs, so a host
 could implement it against an isolated realm. Nothing does today.

+### Loading a module UI executes trusted code in the host page
+
+`@ekklesia/module-ui-host` dynamically imports a module's compiled artifact from a URL and mounts it
+([ADR 0024](../architecture/decisions/0024-browser-safe-ui-sdk-and-host-loader.md)). Once imported,
+that artifact is ordinary same-realm JavaScript: it can reach the DOM, `window`, and anything else
+the page can. **The loader is not a sandbox.** It validates that the artifact is the module the host
+expected — the right id, the right version, a well-formed default export — and refuses to mount
+anything else, but that is an integrity check against a *stale or mismatched* bundle. It is not a
+judgement about whether a module is safe, and it must never be described as one.
+
+The publication side carries the real obligation, and it is not built yet. When Ekklesia gains a
+route that serves installed module artifacts as browser URLs, **the existence of an artifact on disk
+must not be what makes it servable.** Only installed, reviewed modules may be published or mounted
+under this trust model. A module UI that mounts has gained no privilege it did not already have —
+its server half already runs in the API process with the API's database authority — and server APIs
+remain the sole authority for authorization regardless of what a browser loaded.
+
 ### Client-side permission checks are not a security boundary

 A host may tell a module UI what permissions it believes the viewer holds
@@ -231,7 +231,7 @@ independent.
 | --- | --- |
 | `moduleId` | Your module's id, so you never re-derive it from a URL |
 | `api` | `request(path, init)` against **your own** module API — see below |
-| `principal` | Who is viewing (`anonymous` or `authenticated`), for presentation |
+| `principal` | Who is viewing: `ModuleUiPrincipal`, either `{ kind: "anonymous" }` or `{ kind: "authenticated", userId }` |
 | `permissions` | `has(permissionId)` — a presentation hint, never a security check |

 Navigation, host theme and design tokens, module configuration, organization context, and
@@ -332,12 +332,13 @@ not match `.tsx`, and a UI project covers the browser half:
 }
 ```

-`"types": []` is deliberate: UI source should not reach for Node globals. Be aware this is a
-convention rather than an airtight guard — TypeScript still pulls global declarations in
-transitively through dependency type graphs, so `process.env` currently typechecks in a UI file that
-imports `@ekklesia/module-ui-sdk`. Nothing catches it at build time either: only
-`process.env.NODE_ENV` is substituted, so any other Node global reaches the browser and fails there.
-Treat "no Node globals in UI source" as a rule you keep, not one the compiler keeps for you.
+`"types": []` is deliberate, and as of
+[ADR 0024](../architecture/decisions/0024-browser-safe-ui-sdk-and-host-loader.md) it actually holds:
+`process.env` in a UI file fails to compile even though the file imports `@ekklesia/module-ui-sdk`.
+It did not hold previously — the SDK imported `@ekklesia/contracts`, whose barrel pulled Drizzle's
+types and with them Node's globals into every module UI program. The SDK now has **no dependencies at
+all**, so importing it cannot drag a backend type graph into your browser code, and a compile test in
+that package proves both directions.

 `noEmit` is also deliberate — TypeScript only checks types here; esbuild produces the artifact.

@@ -382,9 +383,11 @@ Layer rules are unchanged: module UI source may not import Core, an app, a surfa
 another module. An unsupported framework aliased behind an allowed name — `"react": "npm:vue@3"` —
 is resolved and rejected by its real target.

-> **Still no host.** You can now author, build, and test a module UI end to end. **Nothing serves it
-> to a user yet** — the Dashboard shell does not exist, and neither does the standalone host. Your
-> artifact is proven by your own tests, not mounted in production.
+> **Still no host.** You can author, build, and test a module UI end to end, and
+> `@ekklesia/module-ui-host` can load and mount a built artifact from a browser URL. **Nothing serves
+> your artifact at such a URL yet** — Ekklesia has no module-asset publication route, and the
+> Dashboard shell and standalone host do not exist. Your artifact is proven by tests, not mounted in
+> production. See [How a host loads your UI](#how-a-host-loads-your-ui).

 ### What this does and does not do today

@@ -398,6 +401,49 @@ Core also never *builds* anything. Building is an authoring and packaging step y
 discovers the compiled artifact you shipped and validates it. Installing a module does not trigger a
 build, and neither does starting the API.

+### How a host loads your UI
+
+You do not write this — a host does — but knowing the shape explains what your artifact must satisfy.
+`@ekklesia/module-ui-host` is the generic loader both Ekklesia and a future standalone host use
+([ADR 0024](../architecture/decisions/0024-browser-safe-ui-sdk-and-host-loader.md)):
+
+```text
+compiled artifact (dist/ui/index.js)
+        ↓  asset publication — host-specific, NOT YET IMPLEMENTED for Ekklesia
+browser-loadable module URL
+        ↓  loadModuleUi({ source, container, context })
+dynamic import → parseModuleUiExport → identity checked against the manifest
+        ↓
+mount(container, context)
+        ↓
+MountedModuleUi.unmount()   // idempotent
+```
+
+```ts
+const mounted = await loadModuleUi({
+  source: { moduleId: "example", version: "0.1.0", url: publishedUrl },
+  container,
+  context,
+});
+await mounted.unmount();
+```
+
+What that means for you as a module author:
+
+- **Your `id` and `version` must match your manifest.** They are checked *before* `mount` runs, so a
+  stale artifact is rejected rather than rendered.
+- **Return an unmount function if you acquire anything.** The host calls it once; calling it again is
+  a no-op. Returning nothing is fine if you acquired nothing.
+- **If your `mount` throws, nothing is registered as mounted** and the host is told the mount failed.
+  The loader does not clear your container — the host owns it and decides what a failure looks like.
+- **Your UI may be mounted more than once**, into different containers, if your implementation
+  supports it. Each mount gets its own cleanup.
+
+> **Asset publication is the missing piece.** Core resolves `entrypoints.ui` to a path on the
+> *server's filesystem*; a browser cannot import that. Something must serve your compiled artifact at
+> a URL first, and Ekklesia has no such route yet. Until it does, no host can mount an installed
+> module's UI — the loader is ready, the plumbing to reach it is not.
+
 ### What a module UI will never own

 A module UI consumes platform policy and never redefines it.
@@ -0,0 +1,35 @@
+{
+  "name": "@ekklesia/module-ui-host",
+  "version": "0.1.0",
+  "type": "module",
+  "files": [
+    "dist"
+  ],
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.js"
+    }
+  },
+  "scripts": {
+    "build": "tsc --project tsconfig.build.json",
+    "lint": "biome lint .",
+    "prepack": "pnpm --workspace-root exec turbo run build --filter=@ekklesia/module-ui-host",
+    "test": "vitest run",
+    "typecheck": "tsc --noEmit --project tsconfig.json && tsc --noEmit --project tsconfig.test.json"
+  },
+  "dependencies": {
+    "@ekklesia/module-ui-sdk": "workspace:*"
+  },
+  "devDependencies": {
+    "@biomejs/biome": "2.5.3",
+    "@ekklesia/module-ui-build": "workspace:*",
+    "@types/node": "24.13.3",
+    "happy-dom": "20.0.11",
+    "typescript": "7.0.2",
+    "vitest": "4.1.10"
+  },
+  "ekklesia": {
+    "layer": "module-ui-host"
+  }
+}
@@ -0,0 +1 @@
+export * from "./load-module-ui.js";
@@ -0,0 +1,156 @@
+import {
+  type ModuleUiDefinition,
+  type ModuleUiHostContext,
+  type ModuleUiUnmount,
+  parseModuleUiExport,
+} from "@ekklesia/module-ui-sdk";
+
+/**
+ * How a host obtains a module's UI artifact.
+ *
+ * `url` is a **browser-loadable module URL**, not a filesystem path. That distinction is the whole
+ * reason this type exists. Core's module discovery resolves `entrypoints.ui` to an absolute path on
+ * the server's disk (`ResolvedModuleEntrypoints.ui`); a browser cannot import that, and a host that
+ * passed one here would fail at run time in a way no test on a server would catch. Turning an
+ * installed artifact into a URL a browser may fetch is **asset publication**, a separate concern
+ * this package deliberately does not own and Ekklesia does not implement yet (ADR 0024).
+ *
+ * `moduleId` and `version` are the identity the host already read from the module's validated
+ * manifest. They are checked against the artifact before anything is mounted.
+ */
+export interface ModuleUiSource {
+  readonly moduleId: string;
+  readonly version: string;
+  readonly url: string;
+}
+
+/**
+ * How the artifact is actually imported.
+ *
+ * Injectable because bundlers disagree about dynamic `import()`. The default is a plain
+ * `import(url)` with a non-analyzable specifier, which esbuild and Rollup leave as a run-time import
+ * rather than trying to resolve at build time — the behaviour a host needs, since installed modules
+ * are not known when the host is built. A host whose bundler needs a specific escape hatch (Vite's
+ * `/* @vite-ignore *\/`, webpack's `webpackIgnore`) supplies its own one-line importer instead of
+ * this package guessing.
+ *
+ * Deliberately not implemented with `new Function` or `eval`: those defeat static analysis reliably
+ * but require `unsafe-eval` in a Content-Security-Policy, and a host should not have to weaken its
+ * CSP to load a module.
+ */
+export type ModuleUiImporter = (url: string) => Promise<unknown>;
+
+const defaultImporter: ModuleUiImporter = (url) => import(url);
+
+export interface LoadModuleUiOptions {
+  readonly source: ModuleUiSource;
+  /** The element the module renders into. The host owns it; the module owns its contents. */
+  readonly container: Element;
+  /** Supplied whole by the host. This package forwards it and never adds to it. */
+  readonly context: ModuleUiHostContext;
+  readonly importModule?: ModuleUiImporter;
+}
+
+/** One mounted instance. Each `loadModuleUi` call returns its own; nothing is shared or cached
+ * here, so a module may be mounted into several containers at once if its own implementation
+ * supports it. */
+export interface MountedModuleUi {
+  readonly moduleId: string;
+  readonly version: string;
+  /** Whether cleanup is still pending. Becomes `false` after the first {@link unmount}. */
+  readonly mounted: boolean;
+  /**
+   * Runs the module's cleanup, if it returned one. Idempotent: the first call cleans up, later
+   * calls do nothing. Safe when the module returned no cleanup function.
+   */
+  unmount(): Promise<void>;
+}
+
+export type ModuleUiHostErrorCode = "MODULE_UI_IMPORT_FAILED" | "MODULE_UI_MOUNT_FAILED";
+
+const errorMessages: Readonly<Record<ModuleUiHostErrorCode, string>> = Object.freeze({
+  MODULE_UI_IMPORT_FAILED: "A module UI artifact could not be imported from its published URL.",
+  MODULE_UI_MOUNT_FAILED: "A module UI artifact failed while mounting.",
+});
+
+/**
+ * Carries a stable code and a fixed message, never the artifact URL or a caught cause, matching
+ * every other Ekklesia error boundary. The originating error is attached as `cause` for a host's
+ * own diagnostics, and is never part of the message.
+ */
+export class ModuleUiHostError extends Error {
+  readonly code: ModuleUiHostErrorCode;
+
+  constructor(code: ModuleUiHostErrorCode, options?: { readonly cause?: unknown }) {
+    super(errorMessages[code], options);
+    this.name = "ModuleUiHostError";
+    this.code = code;
+    this.stack = `${this.name}: ${this.message}`;
+  }
+}
+
+/**
+ * Imports a published module UI artifact, verifies it is the module the host expected, and mounts
+ * it into the host's container.
+ *
+ * **This executes trusted first-party code in the host's own page.** A module UI artifact is
+ * ordinary same-realm JavaScript once imported: it can reach the DOM, globals, and anything else the
+ * page can. Nothing here is a sandbox, and publishing an artifact must never be read as evidence
+ * that it is safe — only installed, reviewed modules should ever be published or mounted. Server
+ * APIs remain the sole authority for authorization; a module UI that is mounted has gained no
+ * privilege it did not already have (ADR 0022, ADR 0024).
+ *
+ * Nothing is mounted until the artifact's declared identity matches the manifest identity the host
+ * passed in, so a stale or mismatched bundle fails before it can render or run `mount`.
+ */
+export async function loadModuleUi(options: LoadModuleUiOptions): Promise<MountedModuleUi> {
+  const { container, context, source } = options;
+  const importModule = options.importModule ?? defaultImporter;
+
+  let namespace: unknown;
+  try {
+    namespace = await importModule(source.url);
+  } catch (error) {
+    throw new ModuleUiHostError("MODULE_UI_IMPORT_FAILED", { cause: error });
+  }
+
+  // The one contract parser, reused rather than reimplemented: it validates the default export's
+  // shape and that its id/version match what the host expected, and throws `ModuleUiContractError`
+  // with a specific code when either fails. That error propagates unwrapped — a malformed artifact
+  // is a contract violation, not a host failure.
+  const definition: ModuleUiDefinition = parseModuleUiExport(namespace, {
+    id: source.moduleId,
+    version: source.version,
+  });
+
+  let cleanup: ModuleUiUnmount | undefined;
+  try {
+    const result = await definition.mount(container, context);
+    if (typeof result === "function") {
+      cleanup = result;
+    }
+  } catch (error) {
+    // No cleanup is registered for a mount that never completed, so the returned handle can never
+    // claim a failed module is mounted. The container is deliberately left untouched: the host owns
+    // it and decides what a failed mount should look like.
+    throw new ModuleUiHostError("MODULE_UI_MOUNT_FAILED", { cause: error });
+  }
+
+  let mounted = true;
+  return {
+    moduleId: definition.id,
+    version: definition.version,
+    get mounted() {
+      return mounted;
+    },
+    async unmount() {
+      if (!mounted) {
+        return;
+      }
+      // Marked unmounted before running cleanup, so a throwing cleanup surfaces to the host once
+      // and a second call never re-runs it.
+      mounted = false;
+      await cleanup?.();
+    },
+  };
+}
@@ -0,0 +1,38 @@
+import { readFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { describe, expect, it } from "vitest";
+
+const packageDirectory = path.dirname(path.dirname(fileURLToPath(import.meta.url)));
+
+/**
+ * The host loader runs in a browser too, so it inherits the same rule as the authoring SDK: it must
+ * not reach a backend type graph. Its only dependency is the module-author contract (ADR 0024).
+ */
+describe("module UI host loader stays browser-safe", () => {
+  it("declares exactly one runtime dependency: the module UI SDK", async () => {
+    const manifest = JSON.parse(
+      await readFile(path.join(packageDirectory, "package.json"), "utf8"),
+    ) as { dependencies?: Record<string, string>; peerDependencies?: Record<string, string> };
+
+    expect(Object.keys(manifest.dependencies ?? {})).toEqual(["@ekklesia/module-ui-sdk"]);
+    expect(Object.keys(manifest.peerDependencies ?? {})).toEqual([]);
+  });
+
+  it("imports nothing beyond the module UI SDK in its source", async () => {
+    const source = await readFile(path.join(packageDirectory, "src", "load-module-ui.ts"), "utf8");
+    const external = [...source.matchAll(/from\s+"([^"]+)"/gmu)]
+      .map(([, specifier]) => specifier)
+      .filter((specifier) => specifier !== undefined && !specifier.startsWith("."));
+
+    expect([...new Set(external)]).toEqual(["@ekklesia/module-ui-sdk"]);
+  });
+
+  it("never names a Node built-in or an Ekklesia URL prefix", async () => {
+    const source = await readFile(path.join(packageDirectory, "src", "load-module-ui.ts"), "utf8");
+
+    expect(source).not.toContain('"node:');
+    // The loader forwards a host-supplied URL; it must never construct one.
+    expect(source).not.toMatch(/["'`]\/api\/v1\//u);
+  });
+});
@@ -0,0 +1,14 @@
+{
+  "manifestVersion": 4,
+  "id": "example",
+  "name": "Example",
+  "version": "0.1.0",
+  "compatibility": {
+    "platform": "^0.1.0",
+    "moduleSdk": "^0.1.0"
+  },
+  "entrypoints": {
+    "server": "./dist/server/index.js",
+    "ui": "./dist/ui/index.js"
+  }
+}
@@ -0,0 +1,21 @@
+import { defineModuleUi } from "@ekklesia/module-ui-sdk";
+
+/**
+ * A real module UI compiled by the canonical pipeline. It imports the public SDK so the built
+ * artifact proves the whole loop closes: SDK source -> esbuild bundle -> browser URL -> host loader.
+ *
+ * React is deliberately absent — the module template already proves the React path end to end, and
+ * omitting it keeps this artifact small enough to serve as a `data:` URL.
+ */
+export default defineModuleUi({
+  id: "example",
+  version: "0.1.0",
+  async mount(container, context) {
+    const response = await context.api.request("records");
+    const label = context.principal.kind === "authenticated" ? context.principal.userId : "guest";
+    container.textContent = `${context.moduleId}:${response.status}:${label}`;
+    return () => {
+      container.textContent = "";
+    };
+  },
+});
@@ -0,0 +1,309 @@
+// @vitest-environment happy-dom
+import { ModuleUiContractError, type ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { describe, expect, it } from "vitest";
+import { loadModuleUi, ModuleUiHostError, type ModuleUiImporter } from "../src/index.js";
+
+/**
+ * A browser-loadable URL for a module built from source text.
+ *
+ * `data:` URLs are genuine module URLs that both browsers and Node import natively, so these tests
+ * exercise the loader's **default** `import(url)` against a real URL rather than a filesystem path.
+ * That matters: Core resolves `entrypoints.ui` to a server filesystem path, and a loader proven only
+ * against such a path would not prove anything a browser host could rely on (ADR 0024).
+ */
+function moduleUrl(source: string): string {
+  return `data:text/javascript;base64,${Buffer.from(source, "utf8").toString("base64")}`;
+}
+
+const validArtifact = (options: { id?: string; version?: string; cleanup?: boolean } = {}) => `
+export default {
+  id: ${JSON.stringify(options.id ?? "announcements")},
+  version: ${JSON.stringify(options.version ?? "1.2.0")},
+  mount(container, context) {
+    globalThis.__mounts ??= [];
+    globalThis.__mounts.push({ moduleId: context.moduleId, container });
+    container.textContent = "mounted:" + context.moduleId;
+    void context.api.request("posts");
+    ${
+      options.cleanup === false
+        ? ""
+        : `return () => { globalThis.__unmounts ??= []; globalThis.__unmounts.push(context.moduleId); container.textContent = ""; };`
+    }
+  },
+};
+`;
+
+function hostContext(requested: string[], moduleId = "announcements"): ModuleUiHostContext {
+  return {
+    moduleId,
+    api: {
+      async request(path) {
+        requested.push(path);
+        return new Response(null, { status: 200 });
+      },
+    },
+    principal: { kind: "anonymous" },
+    permissions: { has: () => true },
+  };
+}
+
+function container(): Element {
+  const element = document.createElement("div");
+  document.body.append(element);
+  return element;
+}
+
+const source = { moduleId: "announcements", version: "1.2.0" };
+
+describe("generic module UI host loader", () => {
+  it("imports a browser URL with the default importer, mounts, and reports identity", async () => {
+    const element = container();
+    const requested: string[] = [];
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext(requested),
+    });
+
+    expect(mounted.moduleId).toBe("announcements");
+    expect(mounted.version).toBe("1.2.0");
+    expect(mounted.mounted).toBe(true);
+    expect(element.textContent).toBe("mounted:announcements");
+  });
+
+  it("passes the host's own container and context straight through", async () => {
+    const element = container();
+    const requested: string[] = [];
+    const context = hostContext(requested);
+
+    await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context,
+    });
+
+    const mounts = (globalThis as { __mounts?: { moduleId: string; container: Element }[] })
+      .__mounts;
+    expect(mounts?.at(-1)?.container).toBe(element);
+    expect(mounts?.at(-1)?.moduleId).toBe("announcements");
+  });
+
+  it("keeps the module's API calls relative — the loader adds no URL knowledge", async () => {
+    const element = container();
+    const requested: string[] = [];
+
+    await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext(requested),
+    });
+
+    expect(requested).toEqual(["posts"]);
+    expect(requested.join(" ")).not.toContain("/api/v1/modules");
+  });
+
+  it("runs the module's cleanup on unmount without touching the host container", async () => {
+    const element = container();
+    const parent = element.parentElement;
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext([]),
+    });
+    await mounted.unmount();
+
+    expect(mounted.mounted).toBe(false);
+    expect(element.textContent).toBe("");
+    // The host owns the container; unmounting a module never removes it.
+    expect(element.parentElement).toBe(parent);
+    expect(document.body.contains(element)).toBe(true);
+  });
+
+  it("is idempotent: a second unmount does nothing", async () => {
+    const element = container();
+    const before = ((globalThis as { __unmounts?: string[] }).__unmounts ?? []).length;
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext([]),
+    });
+    await mounted.unmount();
+    await mounted.unmount();
+    await mounted.unmount();
+
+    const after = ((globalThis as { __unmounts?: string[] }).__unmounts ?? []).length;
+    expect(after - before).toBe(1);
+    expect(mounted.mounted).toBe(false);
+  });
+
+  it("unmounts safely when the module returned no cleanup function", async () => {
+    const element = container();
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact({ cleanup: false })) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    await expect(mounted.unmount()).resolves.toBeUndefined();
+    expect(mounted.mounted).toBe(false);
+  });
+
+  it("marks an instance unmounted even when its cleanup throws, so it never re-runs", async () => {
+    const element = container();
+    const artifact = `
+      export default {
+        id: "announcements", version: "1.2.0",
+        mount() { return () => { globalThis.__cleanupCalls = (globalThis.__cleanupCalls ?? 0) + 1; throw new Error("cleanup exploded"); }; },
+      };`;
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(artifact) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    await expect(mounted.unmount()).rejects.toThrow("cleanup exploded");
+    expect(mounted.mounted).toBe(false);
+    await expect(mounted.unmount()).resolves.toBeUndefined();
+    expect((globalThis as { __cleanupCalls?: number }).__cleanupCalls).toBe(1);
+  });
+
+  it("supports two independent instances in two containers", async () => {
+    const first = container();
+    const second = container();
+    const url = moduleUrl(validArtifact());
+
+    const a = await loadModuleUi({
+      source: { ...source, url },
+      container: first,
+      context: hostContext([]),
+    });
+    const b = await loadModuleUi({
+      source: { ...source, url },
+      container: second,
+      context: hostContext([]),
+    });
+
+    expect(first.textContent).toBe("mounted:announcements");
+    expect(second.textContent).toBe("mounted:announcements");
+
+    await a.unmount();
+
+    // Each handle owns its own cleanup state; unmounting one leaves the other alone.
+    expect(a.mounted).toBe(false);
+    expect(b.mounted).toBe(true);
+    expect(first.textContent).toBe("");
+    expect(second.textContent).toBe("mounted:announcements");
+  });
+
+  describe("rejects an artifact before mounting it", () => {
+    it("when the module id does not match the manifest", async () => {
+      const element = container();
+      const url = moduleUrl(validArtifact({ id: "calendar" }));
+
+      await expect(
+        loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
+      ).rejects.toMatchObject({ code: "MODULE_UI_IDENTITY_MISMATCH" });
+      expect(element.textContent).toBe("");
+    });
+
+    it("when the version does not match the manifest", async () => {
+      const element = container();
+      const url = moduleUrl(validArtifact({ version: "1.1.0" }));
+
+      await expect(
+        loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
+      ).rejects.toMatchObject({ code: "MODULE_UI_IDENTITY_MISMATCH" });
+      expect(element.textContent).toBe("");
+    });
+
+    it("when there is no default export", async () => {
+      const element = container();
+      const url = moduleUrl("export const ui = {};");
+
+      await expect(
+        loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
+      ).rejects.toMatchObject({ code: "MODULE_UI_EXPORT_MISSING" });
+    });
+
+    it("when the default export is not a module UI definition", async () => {
+      const element = container();
+      const url = moduleUrl('export default { id: "announcements", version: "1.2.0" };');
+
+      const failure = loadModuleUi({
+        source: { ...source, url },
+        container: element,
+        context: hostContext([]),
+      });
+      await expect(failure).rejects.toBeInstanceOf(ModuleUiContractError);
+      await expect(failure).rejects.toMatchObject({ code: "MODULE_UI_DEFINITION_INVALID" });
+    });
+  });
+
+  it("reports an unreachable artifact as an import failure, with the cause attached", async () => {
+    const element = container();
+
+    try {
+      await loadModuleUi({
+        source: { ...source, url: "data:text/javascript,this is not(valid javascript" },
+        container: element,
+        context: hostContext([]),
+      });
+      expect.unreachable("expected the import to fail");
+    } catch (error) {
+      expect(error).toBeInstanceOf(ModuleUiHostError);
+      expect((error as ModuleUiHostError).code).toBe("MODULE_UI_IMPORT_FAILED");
+      expect((error as ModuleUiHostError).message).not.toContain("data:");
+      expect((error as ModuleUiHostError).cause).toBeDefined();
+    }
+  });
+
+  it("reports a throwing mount as a mount failure and registers no cleanup", async () => {
+    const element = container();
+    const url = moduleUrl(`
+      export default {
+        id: "announcements", version: "1.2.0",
+        mount(container) { container.textContent = "partial"; throw new Error("mount exploded"); },
+      };`);
+
+    try {
+      await loadModuleUi({
+        source: { ...source, url },
+        container: element,
+        context: hostContext([]),
+      });
+      expect.unreachable("expected mount to fail");
+    } catch (error) {
+      expect(error).toBeInstanceOf(ModuleUiHostError);
+      expect((error as ModuleUiHostError).code).toBe("MODULE_UI_MOUNT_FAILED");
+      expect((error as ModuleUiHostError).message).not.toContain("exploded");
+      expect((error as { cause?: Error }).cause?.message).toBe("mount exploded");
+    }
+    // No handle exists, so nothing can claim the module is mounted. The container is left exactly
+    // as the module left it: the host owns it and decides what a failed mount looks like.
+    expect(element.textContent).toBe("partial");
+  });
+
+  it("accepts a host-supplied importer for bundlers that need their own escape hatch", async () => {
+    const element = container();
+    const seen: string[] = [];
+    const importModule: ModuleUiImporter = async (url) => {
+      seen.push(url);
+      return { default: { id: "announcements", version: "1.2.0", mount: () => undefined } };
+    };
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: "https://assets.example.test/modules/announcements/ui.js" },
+      container: element,
+      context: hostContext([]),
+      importModule,
+    });
+
+    expect(seen).toEqual(["https://assets.example.test/modules/announcements/ui.js"]);
+    expect(mounted.mounted).toBe(true);
+  });
+});
@@ -0,0 +1,121 @@
+// @vitest-environment happy-dom
+import { readFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { buildModuleUi } from "@ekklesia/module-ui-build";
+import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { beforeAll, describe, expect, it } from "vitest";
+import { loadModuleUi } from "../src/index.js";
+
+const fixtureDirectory = path.join(
+  path.dirname(fileURLToPath(import.meta.url)),
+  "fixtures",
+  "artifact-module",
+);
+
+let artifactUrl: string;
+let artifactSource: string;
+
+/** An Ekklesia-shaped host: module routes live under a Core-owned prefix. */
+function ekklesiaContext(requested: string[]): ModuleUiHostContext {
+  return {
+    moduleId: "example",
+    api: {
+      async request(pathname) {
+        requested.push(`/api/v1/modules/example/${pathname.replace(/^\//u, "")}`);
+        return new Response(null, { status: 200 });
+      },
+    },
+    principal: { kind: "authenticated", userId: "user-123" },
+    permissions: { has: () => true },
+  };
+}
+
+/** A standalone host: one module, a different origin, a completely different URL shape, no Core. */
+function standaloneContext(requested: string[]): ModuleUiHostContext {
+  return {
+    moduleId: "example",
+    api: {
+      async request(pathname) {
+        requested.push(`https://example.test/api/${pathname.replace(/^\//u, "")}`);
+        return new Response(null, { status: 200 });
+      },
+    },
+    principal: { kind: "anonymous" },
+    permissions: { has: () => false },
+  };
+}
+
+/**
+ * Proves the loader consumes exactly the browser ESM artifact `@ekklesia/module-ui-build` produces
+ * (ADR 0023) — no second output format, no checked-in build output. The artifact is rebuilt from
+ * source each run and served through a `data:` URL, a real module URL rather than a filesystem path.
+ */
+describe("loading a real compiled artifact", () => {
+  beforeAll(async () => {
+    const result = await buildModuleUi({ moduleDirectory: fixtureDirectory });
+    artifactSource = await readFile(result.outputFile, "utf8");
+    artifactUrl = `data:text/javascript;base64,${Buffer.from(artifactSource, "utf8").toString("base64")}`;
+  }, 60_000);
+
+  it("produces a self-contained artifact with no bare imports for a host to resolve", () => {
+    expect(artifactSource).not.toMatch(/^import\s.*from\s*["'][^.]/mu);
+    expect(artifactSource).toContain("export {");
+    expect(artifactSource).not.toContain("/api/v1/modules");
+  });
+
+  it("mounts the real artifact through the generic loader and unmounts it", async () => {
+    const container = document.createElement("div");
+    const requested: string[] = [];
+
+    const mounted = await loadModuleUi({
+      source: { moduleId: "example", version: "0.1.0", url: artifactUrl },
+      container,
+      context: ekklesiaContext(requested),
+    });
+
+    expect(container.textContent).toBe("example:200:user-123");
+    await mounted.unmount();
+    expect(container.textContent).toBe("");
+    expect(mounted.mounted).toBe(false);
+  });
+
+  it("still rejects the real artifact when the host expects a different version", async () => {
+    await expect(
+      loadModuleUi({
+        source: { moduleId: "example", version: "0.2.0", url: artifactUrl },
+        container: document.createElement("div"),
+        context: ekklesiaContext([]),
+      }),
+    ).rejects.toMatchObject({ code: "MODULE_UI_IDENTITY_MISMATCH" });
+  });
+
+  it("runs the same artifact under an Ekklesia host and a standalone host", async () => {
+    const ekklesiaRequests: string[] = [];
+    const standaloneRequests: string[] = [];
+    const ekklesiaContainer = document.createElement("div");
+    const standaloneContainer = document.createElement("div");
+    const source = { moduleId: "example", version: "0.1.0", url: artifactUrl };
+
+    const inEkklesia = await loadModuleUi({
+      source,
+      container: ekklesiaContainer,
+      context: ekklesiaContext(ekklesiaRequests),
+    });
+    const inStandalone = await loadModuleUi({
+      source,
+      container: standaloneContainer,
+      context: standaloneContext(standaloneRequests),
+    });
+
+    // One artifact, one loader, two hosts. Every difference lives in the context the host supplied:
+    // the module asked for "records" both times and resolved nothing itself.
+    expect(ekklesiaRequests).toEqual(["/api/v1/modules/example/records"]);
+    expect(standaloneRequests).toEqual(["https://example.test/api/records"]);
+    expect(ekklesiaContainer.textContent).toBe("example:200:user-123");
+    expect(standaloneContainer.textContent).toBe("example:200:guest");
+
+    await inEkklesia.unmount();
+    await inStandalone.unmount();
+  });
+});
@@ -0,0 +1,13 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": true,
+    "declarationMap": true,
+    "noEmit": false,
+    "outDir": "dist",
+    "rootDir": "src",
+    "sourceMap": true
+  },
+  "include": ["src/**/*.ts"],
+  "exclude": ["src/**/*.test.ts", "tests"]
+}
@@ -0,0 +1,8 @@
+{
+  "extends": "../typescript-config/browser.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "types": []
+  },
+  "include": ["src/**/*.ts"]
+}
@@ -0,0 +1,8 @@
+{
+  "extends": "../typescript-config/browser.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "types": ["node", "vitest/globals"]
+  },
+  "include": ["tests/**/*.ts"]
+}
@@ -16,13 +16,11 @@
    "lint": "biome lint .",
    "prepack": "pnpm --workspace-root exec turbo run build --filter=@ekklesia/module-ui-sdk",
    "test": "vitest run",
-    "typecheck": "tsc --noEmit --project tsconfig.json"
-  },
-  "dependencies": {
-    "@ekklesia/contracts": "workspace:*"
+    "typecheck": "tsc --noEmit --project tsconfig.json && tsc --noEmit --project tsconfig.test.json"
  },
  "devDependencies": {
    "@biomejs/biome": "2.5.3",
+    "@types/node": "24.13.3",
    "typescript": "7.0.2",
    "vitest": "4.1.10"
  },
@@ -1,4 +1,33 @@
-import type { EkklesiaPrincipal } from "@ekklesia/contracts";
+/**
+ * Who is viewing, as a browser sees it.
+ *
+ * This is deliberately **not** Core's `EkklesiaPrincipal`, and the difference is the point. That
+ * type's `userId` is a Zod-branded string whose brand only means something to server-side parsing,
+ * and reaching for it made this package import `@ekklesia/contracts` — a backend barrel whose type
+ * graph pulls Drizzle, and with it Node's global declarations, back into every module UI program
+ * (measured: a UI file importing this package could typecheck `process.env`). A browser contract
+ * that drags a database ORM's types behind it is not a browser contract.
+ *
+ * So the shape is preserved and the brand is dropped: plain, serializable, and exactly what a UI
+ * needs to decide what to render. An Ekklesia host can pass its own `EkklesiaPrincipal` straight in
+ * — a branded string is assignable to `string`, so the two remain structurally compatible without
+ * this package knowing anything about the server's representation. A standalone host builds the
+ * same object literal from whatever identity system it has.
+ *
+ * Nothing server-internal belongs here. A field is added only when module UI provably needs it to
+ * render something.
+ */
+export interface ModuleUiAnonymousPrincipal {
+  readonly kind: "anonymous";
+}
+
+export interface ModuleUiAuthenticatedPrincipal {
+  readonly kind: "authenticated";
+  /** Opaque to module UI: an identifier to correlate with, never to parse or display verbatim. */
+  readonly userId: string;
+}
+
+export type ModuleUiPrincipal = ModuleUiAnonymousPrincipal | ModuleUiAuthenticatedPrincipal;

 /**
 * Everything a host hands a module's UI, and the complete list of what module UI may assume exists.
@@ -10,7 +39,8 @@ import type { EkklesiaPrincipal } from "@ekklesia/contracts";
 * than to add. See ADR 0022.
 *
 * Nothing in this object is an Ekklesia implementation. Every member is satisfiable by a standalone
- * host that never runs Core, which is what makes one module UI hostable by both.
+ * host that never runs Core, which is what makes one module UI hostable by both. This package has
+ * no dependencies at all, so importing it cannot pull a backend type graph into a browser program.
 */
 export interface ModuleUiHostContext {
  /** The module this UI belongs to, as the host knows it. Supplied so a module never has to hardcode
@@ -18,9 +48,9 @@ export interface ModuleUiHostContext {
  readonly moduleId: string;
  /** The only supported way for module UI to reach its own backend. See {@link ModuleUiApiClient}. */
  readonly api: ModuleUiApiClient;
-  /** Who is viewing, for presentation decisions only. Reuses Core's own principal type so a host and
-   * a module UI cannot disagree about its shape. */
-  readonly principal: EkklesiaPrincipal;
+  /** Who is viewing, for presentation decisions only. See {@link ModuleUiPrincipal} for why this is
+   * a browser-owned shape rather than Core's server principal type. */
+  readonly principal: ModuleUiPrincipal;
  /** Presentation-only permission hints. See {@link ModuleUiPermissions}. */
  readonly permissions: ModuleUiPermissions;
 }
@@ -0,0 +1,48 @@
+import { execFile } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { promisify } from "node:util";
+import { describe, expect, it } from "vitest";
+
+const run = promisify(execFile);
+const packageDirectory = path.dirname(path.dirname(fileURLToPath(import.meta.url)));
+const fixtureDirectory = path.join(packageDirectory, "tests", "browser-boundary");
+const typescript = path.join(packageDirectory, "node_modules", ".bin", "tsc");
+
+async function typecheck(project: string): Promise<{ code: number; output: string }> {
+  try {
+    await run(typescript, ["--noEmit", "--project", path.join(fixtureDirectory, project)]);
+    return { code: 0, output: "" };
+  } catch (error) {
+    const failure = error as { code?: number; stdout?: string; stderr?: string };
+    return { code: failure.code ?? 1, output: `${failure.stdout ?? ""}${failure.stderr ?? ""}` };
+  }
+}
+
+/**
+ * The browser boundary, tested the way a module author experiences it rather than by inspecting the
+ * dependency graph.
+ *
+ * Both fixtures compile under the *exact* project a module uses for UI source —
+ * `browser-react.json` with `"types": []`. Before this package dropped its `@ekklesia/contracts`
+ * dependency, the negative fixture below compiled successfully: that import reached a backend
+ * barrel whose Drizzle types re-introduced Node's global declarations, so `process.env` typechecked
+ * inside browser-only code (ADR 0024).
+ */
+describe("module UI SDK browser boundary", () => {
+  it("compiles supported UI source that imports the public SDK", async () => {
+    const result = await typecheck("tsconfig.supported.json");
+
+    expect(result.output).toBe("");
+    expect(result.code).toBe(0);
+  }, 60_000);
+
+  it("refuses Node globals in UI source that imports the public SDK", async () => {
+    const result = await typecheck("tsconfig.node-global.json");
+
+    expect(result.code).not.toBe(0);
+    // Specifically because `process` does not exist — not because of an unrelated error.
+    expect(result.output).toContain("Cannot find name 'process'");
+    expect(result.output).toContain("node-global.tsx");
+  }, 60_000);
+});
@@ -0,0 +1,6 @@
+// The negative case. Importing the public SDK must NOT make Node globals visible. If this file
+// ever typechecks, the browser boundary has regressed and `browser-boundary.test.ts` fails.
+import type { ModuleUiHostContext } from "../../src/index.js";
+
+export type Context = ModuleUiHostContext;
+export const leaked: string | undefined = process.env.NODE_ENV;
@@ -0,0 +1,24 @@
+// A module author's supported UI source: it imports the public SDK and must typecheck cleanly
+// under the exact browser project a module uses (`browser-react.json`, `"types": []`).
+import {
+  defineModuleUi,
+  type ModuleUiHostContext,
+  type ModuleUiPrincipal,
+} from "../../src/index.js";
+
+function describePrincipal(principal: ModuleUiPrincipal): string {
+  return principal.kind === "authenticated" ? principal.userId : "anonymous";
+}
+
+export default defineModuleUi({
+  id: "example",
+  version: "0.1.0",
+  mount(container: Element, context: ModuleUiHostContext) {
+    container.textContent = describePrincipal(context.principal);
+    void context.api.request("records");
+    void context.permissions.has("example.records.read");
+    return () => {
+      container.textContent = "";
+    };
+  },
+});
@@ -0,0 +1,5 @@
+{
+  "extends": "../../../typescript-config/browser-react.json",
+  "compilerOptions": { "noEmit": true, "types": [] },
+  "include": ["node-global.tsx"]
+}
@@ -0,0 +1,5 @@
+{
+  "extends": "../../../typescript-config/browser-react.json",
+  "compilerOptions": { "noEmit": true, "types": [] },
+  "include": ["supported.tsx"]
+}
@@ -18,29 +18,30 @@ async function sourceFiles(): Promise<string[]> {
 * way; `import type` is erased at build time, a plain `import` is not.
 */
 describe("module UI SDK stays browser-safe", () => {
-  it("imports nothing from another workspace package at runtime", async () => {
+  it("imports nothing from another package at all, at type level or runtime", async () => {
+    // Stronger than "type-only imports are fine". A type-only import is erased at runtime but still
+    // drags the imported package's *type graph* into every consuming program — which is exactly how
+    // Node globals reached browser-only module UI before ADR 0024. This package now imports nothing
+    // external, so there is no graph to drag.
    for (const file of await sourceFiles()) {
      const source = await readFile(path.join(sourceDirectory, file), "utf8");
-      const workspaceImports = [
-        ...source.matchAll(/^import\s+(.*?)\s+from\s+"(@ekklesia\/[^"]+)"/gmu),
-      ];
+      const externalImports = [...source.matchAll(/^(?:import|export)\s[^;]*?from\s+"([^"]+)"/gmu)]
+        .map(([, specifier]) => specifier)
+        .filter((specifier) => specifier !== undefined && !specifier.startsWith("."));

-      for (const [, clause, specifier] of workspaceImports) {
-        expect(
-          clause?.startsWith("type "),
-          `${file} imports ${specifier} at runtime; it must be a type-only import`,
-        ).toBe(true);
-      }
+      expect(externalImports, `${file} imports outside this package`).toEqual([]);
    }
  });

-  it("declares no runtime dependency other than the type-only contracts package", async () => {
+  it("declares no dependencies whatsoever", async () => {
    const manifestPath = path.join(sourceDirectory, "..", "package.json");
    const manifest = JSON.parse(await readFile(manifestPath, "utf8")) as {
      dependencies?: Record<string, string>;
+      peerDependencies?: Record<string, string>;
    };

-    expect(Object.keys(manifest.dependencies ?? {})).toEqual(["@ekklesia/contracts"]);
+    expect(Object.keys(manifest.dependencies ?? {})).toEqual([]);
+    expect(Object.keys(manifest.peerDependencies ?? {})).toEqual([]);
  });

  it("does not depend on a UI framework, so the contract stays framework-neutral", async () => {
@@ -61,5 +62,9 @@ describe("module UI SDK stays browser-safe", () => {
    for (const framework of ["react", "react-dom", "vue", "svelte", "preact"]) {
      expect(declared).not.toContain(framework);
    }
+    // Backend graphs must not reappear through a development dependency either.
+    for (const backend of ["drizzle-orm", "zod", "semver", "@ekklesia/contracts"]) {
+      expect(declared).not.toContain(backend);
+    }
  });
 });
@@ -6,8 +6,7 @@
    "noEmit": false,
    "outDir": "dist",
    "rootDir": "src",
-    "sourceMap": true,
-    "types": []
+    "sourceMap": true
  },
  "include": ["src/**/*.ts"],
  "exclude": ["src/**/*.test.ts", "tests"]
@@ -2,7 +2,7 @@
  "extends": "../typescript-config/browser.json",
  "compilerOptions": {
    "noEmit": true,
-    "types": ["vitest/globals"]
+    "types": []
  },
-  "include": ["src/**/*.ts", "tests/**/*.ts"]
+  "include": ["src/**/*.ts"]
 }
@@ -0,0 +1,8 @@
+{
+  "extends": "../typescript-config/browser.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "types": ["node", "vitest/globals"]
+  },
+  "include": ["tests/**/*.ts"]
+}
@@ -183,15 +183,39 @@ importers:
        specifier: 4.1.10
        version: 4.1.10(@types/[email protected])([email protected])([email protected](@types/[email protected])([email protected])([email protected])([email protected])([email protected]))

-  packages/module-ui-sdk:
+  packages/module-ui-host:
    dependencies:
-      '@ekklesia/contracts':
+      '@ekklesia/module-ui-sdk':
        specifier: workspace:*
-        version: link:../contracts
+        version: link:../module-ui-sdk
    devDependencies:
      '@biomejs/biome':
        specifier: 2.5.3
        version: 2.5.3
+      '@ekklesia/module-ui-build':
+        specifier: workspace:*
+        version: link:../module-ui-build
+      '@types/node':
+        specifier: 24.13.3
+        version: 24.13.3
+      happy-dom:
+        specifier: 20.0.11
+        version: 20.0.11
+      typescript:
+        specifier: 7.0.2
+        version: 7.0.2
+      vitest:
+        specifier: 4.1.10
+        version: 4.1.10(@types/[email protected])([email protected])([email protected](@types/[email protected])([email protected])([email protected])([email protected])([email protected]))
+
+  packages/module-ui-sdk:
+    devDependencies:
+      '@biomejs/biome':
+        specifier: 2.5.3
+        version: 2.5.3
+      '@types/node':
+        specifier: 24.13.3
+        version: 24.13.3
      typescript:
        specifier: 7.0.2
        version: 7.0.2
@@ -152,6 +152,7 @@ const SHARED_PACKAGE_ROLES = new Set([
  "contracts",
  "design-tokens",
  "module-sdk",
+  "module-ui-host",
  "module-ui-sdk",
  "shared",
  "surface-sdk",
@@ -254,7 +255,9 @@ function boundaryRule(from: Location, to: Location): string | undefined {

  if (
    from.layer === "shared" &&
-    ["builder-sdk", "module-sdk", "module-ui-sdk", "surface-sdk"].includes(from.role ?? "") &&
+    ["builder-sdk", "module-sdk", "module-ui-host", "module-ui-sdk", "surface-sdk"].includes(
+      from.role ?? "",
+    ) &&
    ["app", "builder", "core", "module", "surface"].includes(to.layer)
  ) {
    return "sdks-do-not-depend-on-implementations";
@@ -270,6 +273,13 @@ function boundaryRule(from: Location, to: Location): string | undefined {
    ) {
      return "sdks-do-not-depend-on-implementations";
    }
+    // The host loader consumes the module-author contract — that is its whole job — plus dev-time
+    // `tooling` (it builds a real artifact in its own tests through the canonical pipeline rather
+    // than reimplementing one). It may not reach further: never a module, an app, a surface, or the
+    // backend contracts barrel, which is what keeps it browser-safe (ADR 0024).
+    if (from.role === "module-ui-host" && !["module-ui-sdk", "tooling"].includes(to.role ?? "")) {
+      return "sdks-do-not-depend-on-implementations";
+    }
    if (from.role === "builder-sdk" && !["contracts", "surface-sdk"].includes(to.role ?? "")) {
      return "sdks-do-not-depend-on-implementations";
    }
@@ -50,6 +50,7 @@ describe("repository dependency boundaries", () => {
    ["module-ui-aliased-framework", "modules-do-not-own-ui"],
    ["module-ui-imports-core", "modules-use-public-platform-contracts"],
    ["module-ui-bundler-plugin", "modules-do-not-own-ui"],
+    ["module-imports-ui-host", "modules-use-public-platform-contracts"],
    ["module-imports-ui-package", "modules-use-public-platform-contracts"],
    ["cross-package-relative", "workspace-imports-use-public-exports"],
    ["cycle", "no-circular-source-dependencies"],
@@ -121,6 +122,18 @@ describe("repository dependency boundaries", () => {
    expect(result.violations).toEqual([]);
  });

+  it("keeps the host loader out of module packages", async () => {
+    // `@ekklesia/module-ui-host` is host infrastructure. A module that depended on it would bundle
+    // loader code into its own artifact and blur the author/host split (ADR 0024), so the
+    // `module-ui-host` role is deliberately absent from the roles a module may depend on.
+    const result = await checkBoundaries(path.join(fixtures, "module-imports-ui-host"));
+
+    expect(result.violations.map((violation) => violation.rule)).toEqual([
+      "modules-use-public-platform-contracts",
+      "modules-use-public-platform-contracts",
+    ]);
+  });
+
  it("still rejects a module reaching for a bundler plugin ecosystem", async () => {
    // Module authors do not configure a bundler; `@ekklesia/module-ui-build` owns that entirely.
    const result = await checkBoundaries(path.join(fixtures, "module-ui-bundler-plugin"));
@@ -0,0 +1,14 @@
+{
+  "name": "@ekklesia/module-calendar",
+  "version": "0.0.0",
+  "type": "module",
+  "dependencies": {
+    "@ekklesia/module-ui-host": "workspace:*"
+  },
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "ekklesia": {
+    "layer": "module"
+  }
+}
@@ -0,0 +1 @@
+export const moduleId = "calendar";
@@ -0,0 +1,3 @@
+import { loadModuleUi } from "@ekklesia/module-ui-host";
+
+export const misuse = loadModuleUi;
@@ -0,0 +1,11 @@
+{
+  "name": "@ekklesia/module-ui-host",
+  "version": "0.0.0",
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "ekklesia": {
+    "layer": "module-ui-host"
+  }
+}
@@ -0,0 +1 @@
+export function loadModuleUi(): void {}