Compose Preview

Render @Preview composables to PNG without Android Studio (Android via Robolectric, CMP Desktop via Skia). CLI, Gradle plugin and renderers ship from compose-ai-tools; the MCP server from compose-preview-server.

Render first

For a request to render or show a preview, follow the short loop below. For a requested guidelines review, use the review checklist to choose the local, hosted or UI Builder lane before invoking tools. Published catalogs and server-homed designs use hosted tools first; a missing local checkout does not require cloning, Gradle or a cloud environment. If tools are unavailable, report blocked evidence and follow the integration's setup guidance.

With the local MCP server attached (compose-preview-server 3.78.0+), its initialize instructions say how to call it: render_preview preview=<FunctionName>, no exploring or registering first, no hand-built mocks. Follow them. This skill adds the budget: a routine render is at most three tool calls, with no base64 in the reply and images only when you need to see them.

Your first call is the render. Don't grep for the preview or read its source first: the server finds it by function name. Look things up only if the render fails.

1. Render. If your client reads local files (Claude Code, Codex, Gemini

CLI, OpenCode, Antigravity), pass inline=false. The result is pngPath, sha256, dimensions and changed, with no image tokens. If it lists otherMatches, render one only if the person asked about it. If it carries a `variantChoice` whose `message` says to ask (several previews matched and the host showed no chooser), the person picks, not you: reply with the rendered match and every entry in choices as a numbered list, ask which one they meant, and stop. Don't pick one or shorten the list to "other variants are available". A grid result or a declined choice needs no question.

1. Look only when you need to. Read pngPath with your file reader

before you describe or judge the UI; that is what the person sees. Skip the read when changed or sha256 already answers the question, such as "did my edit land?". Describe only what you saw. If you can't view images here, say so plainly.

1. Reply briefly, with pngPath so the person can open the same image.

Don't write "the image above": in a terminal or print-mode harness the person doesn't see images your tools return. Point at pngPath instead.

Library components come from the catalog. When the person names a library component (Material 3, Wear M3 EdgeButton, …) rather than one of their own previews, use the hosted catalog (compose-catalogs): catalog_list_previews, then read the published snapshot with resources/read. Request live access only to change knobs with catalog_render_preview. Never add preview files to the person's project to show a library component unless they ask for one. See catalog-mcp.md.

Sweeps use hashes. For more than one render (variants, devices, font scales, locales, a before/after check), pass observe=hash or use render_matrix with contactSheet: false, which returns per-cell hashes only. Fetch pixels only for the final screen or the cells whose hash moved, or that you must judge by eye: one render_preview per such cell (its overrides or uri, inline=false) and one read of its pngPath.

Design guidelines review. When asked to review a preview or catalog design against Android design guidance, read design-guidelines.md. It covers keyless agent review, published results and the CLI's compose-preview review loop, including explicit surface selection, required pictures and incomplete coverage. It is in the default skill bundle; the optional PR-review bundle is not required.

Hand multi-render reviews to `design-reviewer`. If a design-reviewer subagent is available (the compose-agent-plugins plugins ship one), delegate accessibility, font-scale, round-device and other matrix checks to it so the images stay out of your context, and relay its verdict and paths. Give it the subject, home/revision, catalog rules and requested coverage. Without one, run the same checklist yourself and report that delegation was unavailable. For a routine hash sweep, keep contactSheet: false. A requested visual guidelines comparison needs pixels: inspect its required frames even in your own context when no reviewer exists. Prefer only the relevant individual images or a bounded comparison sheet, and never claim visual coverage from hashes alone.

After a source edit, call render_preview again. notify_file_changed is optional. If the result says it is stale, make exactly one more call with force: {"reason": "<why>"}. Never run ./gradlew, clean, or delete build/ dirs to chase a render; if a forced render is still wrong, read mcp.md § Troubleshooting (start with compose-preview mcp doctor).

Details on request. Add details: ["a11y", "layout"] only when the person asks about accessibility or layout. observe=png returns inline pixels for clients that can't read files.

No MCP tool? Use the CLI: compose-preview show --json --filter <Name>, then read the entry's pngPath. Stale: --force=<reason>, once. See cli.md. On either path, only output from the render tools is a render: never a hand-built HTML, CSS or SVG mock. Report a failed render with its error.

Not previewable (takes a ViewModel or injected service)? Propose extracting a stateless inner composable and preview that (state-hoisting.md).

Install / update (only if missing)

Check with compose-preview --version. If it's missing, run this skill's stub, bash <this skill's dir>/scripts/compose-preview --version: its first run installs the CLI and puts it on PATH. Every other route, updating, and the Gradle plugin are in setup.md; MCP wiring (the local server and the one hosted URL) is in mcp.md.

Working with platform skills

An official platform skill, such as Android's Wear Compose Material 3 skill, owns API, dependency and migration choices. This skill only renders and verifies:

• Defer. Follow the platform skill for which component, API or version to

use. Don't treat this skill's references as a second API guide.

• Changes are expected. During a migration a changed hash or image is

information, not a regression. Only a render failure or a new accessibility error blocks. Don't restore old screenshots or tune the UI to match them.

• Mind the catalog version. A hosted catalog renders one fixed library

version, which may differ from the project's libs.versions.toml. See catalog-mcp.md § Choosing a component.

Migration workflow. Before editing, find_previews_for_file for each file and record a baseline with render_preview observe=hash. Make the platform skill's edits, re-render, and report failures and new accessibility errors (get_preview_data kind=a11y/atf) as blockers. Show a before/after for one to three key screens, and check a small round device and the largest font scale on Wear. Details: wear-ui.md § Verification workflow.

Reference index

Read only what the task needs.

CI and PR review live in opt-in sibling skills: compose-preview-ci (baselines, PR-comment Actions) and compose-preview-review (base vs. head render and diff). To add one, see setup.md § Which skills.