Compose Preview Server

The server behind compose-preview serve: catalog hosting, live render sessions, the playground, and the browser viewer surfaces. Its history was extracted from yschimke/compose-ai-tools; the CLI remains there and launches this repository's distribution.

Archives on each GitHub release, and nothing on Maven Central.

The Compose/Wasm frontend is not an asset of these releases. It is built and released by yschimke/compose-ui-builder as compose-preview-ui-builder-web-<v>.zip on that repository's own releases, and this build resolves it from there — the server archive carries an unpacked copy either way, and the standalone zip is for serving the editor yourself or pointing an existing serve at with --ui-builder-dir.

Nothing here is a Maven coordinate. Six modules used to publish — compose-preview-serve, compose-preview-mcp, and four more that existed on Central only because the first one's POM named them — and the arrangement cost more than it bought. A project dependency reaches a published POM as a coordinate, so adding one to :server meant publishing the dependency too; getting that wrong shipped compose-preview-server:ui-builder-export-jvm:unspecified in 3.1.0 and an unresolvable compose-preview-serve from 3.3.0 through 3.8.0. Six releases nobody could resolve, for transitives nobody wanted.

The one library consumer was compose-ai-tools' :cli, which compiled two wire-drift tests against compose-preview-serve. Those tests launch the distribution now (compose-ai-tools#5436) — the artifact serve runs anyway, so they check the wire that actually ships. Released coordinates up to and including 3.24.0 stay resolvable on Central; there simply will not be new ones.

render-host exists because rendering a packed bundle and reading a preview timeline out of git open no sockets, and a caller doing only that should not link ktor-server-*, jmdns and kotlin-reflect to do it. It is published from compose-ai-tools, not from here: that is where everything it depends on lives, and offline behaviour belongs in that layer (#180). It used to be this repository's :render-host module, publishing as compose-preview-render-host; that coordinate stays resolvable at its final 2.x for anyone pinned to it, and the new one is on compose-ai-tools' version line. The measured before/after and the transitives it deliberately cannot drop are recorded in its build file there, and checkRenderHostIsServerFree moved with it.

The UI builder is a second repository — yschimke/compose-ui-builder — and :server consumes it as releases. Its :ui-builder-runtime owns authoritative persistent design state, exact catalog validation and revision-pinned export orchestration there; :server supplies HTTP/authentication and adapts its narrow render request onto the render host. The runtime therefore has no Ktor, daemon/render-host, MCP or Compose UI dependency, while the offline render host has no UI-builder protocol or service edge. Three jars and a BOM come from Maven Central at composeai-ui-builder in gradle/libs.versions.toml, and the editor archive from that repository's GitHub release; -PcomposeUiBuilderDir swaps all four for a checkout when working on both at once.

The build is intentionally repository-independent. Compose Preview implementation artifacts resolve from Maven Central at the version in gradle/libs.versions.toml; wire contracts resolve separately from compose-preview-contracts. There is no mavenLocal() repository and no shared version catalog outside this repository. A composite build is opt-in only — -PlocalBuilds=tools,daemon,contracts for the upstreams, -PcomposeUiBuilderDir for the UI builder — and for unreleased contract or generator changes an explicit local dependency manifest selects artifacts compiled from local checkouts. Leaving those options unset retains the released dependency graph.

Run the distribution on Java 17 or newer. The UI builder's PNG/SVG export needs Java 21.

The two numbers are one difference, and it is worth stating plainly rather than leaving to be inferred from build files. The server itself, mcp serve, and every artifact this repository publishes target Java 17, because compose-ai-tools' CLI compiles against them on a 17 toolchain and its serve / mcp serve commands launch this distribution's start script as a separate process, resolving java from JAVA_HOME/PATH. Only the UI builder's renderer is above that floor: the design it rasterizes is drawn by a Compose preview compiled for Java 21, packaged into the render bundle, and unpacked into a daemon that runs on the server's own JVM.

On an older JVM the server still starts and everything else works; the UI builder loses PNG and SVG export and says so at startup, naming the version it found and the one it needs. Point JAVA_HOME at a Java 21 JDK to get it back, or pass --ui-builder-state-dir none to run without the UI builder at all. The published container image (deploy/image/Dockerfile) is built on Temurin 21 and clears both.

Both floors are declared once, as java-server and java-ui-builder in gradle/libs.versions.toml, which is also where the reasoning lives. :server:checkServerJvmFloor fails the build if anything above java-server reaches the distribution's classpath, and the render bundle carries java-ui-builder as data so the startup message cannot drift from the bytes it describes.

./gradlew check needs one JDK: 17. The UI builder's frontend lane compiles at 21 in its own repository, and the only lane here that still wants both JDKs is visual-harness, which builds that repository's Wasm distributions from a checkout. Gradle finds a 21 installed anywhere it already scans (/usr/lib/jvm, SDKMAN, asdf, jabba); if it cannot, the failure is No matching toolchains found for requested specification: {languageVersion=21} and the fix is to install one, not to lower the target.

./gradlew check ktfmtCheckAll
npm --prefix serve-web ci
npm --prefix serve-web run verify

The independently installable visual harness lives in preview-harness/. The experimental Compose/Wasm frontend lives in wasm-ui/. The UI builder itself — editor, renderer, artwork and the Jetcaster fixtures — lives in yschimke/compose-ui-builder; this build resolves its editor archive, runtime and export from that repository's releases, and the harness builds the renderer and fixture distributions from a checkout of it. The server distribution packages the editor at /ui-builder/; the existing catalog-scoped /wasm/<system>/ preview application remains a distinct feature and route. The editor route opens an interactive Wasm editor around the frozen Jetcaster design; clean benchmark modes remain available to the independent visual harness. The editor mounts an exact retained renderer runtime under /ui-builder/runtime/<runtimeId>/ in a sandboxed iframe and receives measured node/slot geometry without placing editor overlays in the Compose tree.

The Remote Compose authoring extension is behind the default-off compile-time option -PuiBuilderRemoteCompose=true. It spans two repositories — the MCP adapter's half is generated here, and the server's half is baked into the UI-builder export at its build time, so enabling the server half means building against a UI-builder checkout with the option (-PcomposeUiBuilderDir=…). See feature scope and verification.

The server can expose every hosted catalog through one aggregate Streamable HTTP MCP endpoint at /mcp. Enable it with --agent-grants --catalog-mcp; published resources require a short-lived preview grant, while made-to-order renders and structured data products require a live grant. The endpoint is separate from the stateful UI-builder authoring MCP surface, but both use the same authenticated user approval and revocation flow. See the catalog MCP design and setup guide.

A portable bundle can publish an XR preview as a version-one SpatialScene document and its panel textures:

previews/<preview-id>.spatial/scene.json
previews/<preview-id>.spatial/<panel>.png

The viewer opens these scenes in an orbitable Three.js/WebGL stage and offers Enter VR when the browser exposes an immersive-vr WebXR session. WebXR requires a secure context, so a headset must reach the server over HTTPS (localhost remains suitable for desktop WebGL development). Uploaded bundles may carry JSON scene documents and PNG/JPEG/WebP textures only; all assets are served from the scene's same-origin /spatial/ route.

Build and run the standalone distribution with:

./gradlew :server:distTar
tar -xzf server/build/distributions/compose-preview-server-*.tar.gz
./compose-preview-server-*/bin/compose-preview-server help

The binary has five commands, and help lists them:

Flags may still be passed with no command in front of them: compose-preview-server --module app is exactly compose-preview-server serve --module app, and stays supported.

ui needs a build host — the compose-preview binary — because discovering and building a local Gradle project is work this server asks for over a pipe rather than doing itself. The builder's palette is a packaged design-system catalog; what ui adds is the project, by pointing --ui-builder-components at the module's discovered components.json so the Compose export generates call sites for your composables. From a checkout with the CLI installed:

compose-preview-server ui --module app

design is the client half of that lane, and the one command here that starts no server: it talks to one that is already up — a local serve, or a deployment — and writes a design's pixels or its generated source to a file, which is what a session otherwise re-invents as a curl into /mcp, a jq to unwrap the envelope and a base64 -d (#529):

export COMPOSE_PREVIEW_TOKEN=...          # or let the command ask a human to approve a grant
compose-preview-server design list --server https://preview.coo.ee
compose-preview-server design render spotify-wear-widget -o cover.png
compose-preview-server design export spotify-wear-widget -o Widget.kt
compose-preview-server design get spotify-wear-widget > design.json

The credential is read from $COMPOSE_PREVIEW_TOKEN (or the older $COMPOSE_PREVIEW_UI_BUILDER_TOKEN that scripts/ui-builder/design-sync.mjs reads), never from a flag. With neither set — or after a restart has dropped the grant — the command runs the server's own device-code flow: it prints an approval link and a code, waits for a human, and carries on. A refused export prints the generator's own diagnostics to stderr and exits non-zero, writing nothing, so it composes in CI.

--local is the half of that command which needs no server at all: it runs the same generator, compiler and render daemon a server would, here, against a catalog bundle on disk — and says why a frame is missing rather than only that it is, which the wire reply cannot (#551). That is what makes a broken host debuggable: design get captures its document (a read, not the render lane under suspicion) and the file replays anywhere.

compose-preview-server design get spotify-wear-widget --server https://preview.coo.ee > doc.json
compose-preview-server design render --document doc.json --local \
  --catalog wear-m3.bundle --assets ./assets -o replay.png
compose-preview-server design export --document doc.json --local   # the generated Kotlin, no server

A local render prints the classpath it resolved, the daemon opener it built and the compiler's own diagnostics; --components <catalog>=<components.json> names the record a record-driven catalog's call sites are proven against, exactly as serve --ui-builder-components does.

A signed-in person can browse every design the service permits them to read at /ui-builder/designs. The page separates owned designs from designs shared by somebody else, shows the exact service reason when one cannot open, and lets an owner manage sharing inline. It is actor-scoped and has no connection to the all-designs operator surface at /admin/ui-builder.

A served catalog's own composables can also be offered inside the builder's catalogs as a component pack (--ui-builder-packs confetti-mobile=mobile,confetti-wear=wear), switched on by an author from the editor's settings; see docs/design/UI_BUILDER_COMPONENT_PACKS.md.

Releases attach that distribution to the GitHub release, then build the production ghcr.io/yschimke/compose-preview-host image. The canonical Docker and preview.coo.ee configuration lives in deploy/image and deploy/preview.coo.ee.

One lane in release.yml: build compose-preview-server-<v>.tar.gz, compose-preview-mcp-<v>.tar.gz and compose-preview-ui-builder-web-<v>.zip, attach them to the GitHub release, then build the compose-preview-host image.

The GitHub release stays a draft until that lane has succeeded, so a failed build never leaves a tag whose assets do not exist.

There used to be a second, skippable Maven lane, with a release:no-maven label and a publish_maven workflow input to turn it off. Both are gone with the publication — every release is now what that label used to ask for.

checkServeModuleBoundary walks the resolved runtime classpath, transitives included. It rejects project dependencies, renderer/daemon implementations, the Gradle plugin, and any unregistered ee.schimke.composeai coordinate. Update its positive allowlist only when a reviewed dependency floor change is intentional.

The source package remains ee.schimke.composeai.cli.serve for binary/source continuity. A package rename is independent of repository ownership and is not part of the extraction.