Visionaire Engine

Which rule, which file, which line — and why it wins.

The problem: Something looks off. You screenshot it, explain it, the LLM guesses wrong, you re-explain.

The solution: Visionaire reads the live page and hands the LLM the exact rule, file, and line. Right fix, first try.

You shouldn't have to write a paragraph to explain a 2px margin bug — and now you don't. Less explaining, more fixing: built for developers, vibe coders, and anyone shipping site design changes with an LLM in the loop.

Status: v0.7 — 28 tools, 435 tests (252 unit + 183 end-to-end on real Chrome), a 24-case seeded-bug benchmark (npm run bench), verified live against wordpress.org.

• v0.7 — the verification layer: assert_visual (a 17-type assertion grammar — PASS/FAIL verdicts with measured pixels, offending uids, and re-runnable named suites), visual_diff (pixel diff vs a mockup or recorded baseline, divergent regions mapped to element uids), impact_preview (blast radius + sandboxed dry-run before editing a shared selector), diagnose (ranked "why is this broken" culprits with measured evidence), responsive_sweep (one call → per-viewport verdict matrix), capture_proof (before/after evidence bundles with a verdict delta); the verify-after-edit harness for Claude Code and Cursor (npx visionaire-engine init-harness); style_diff { capture_pixels } baselines; check_alignment deprecated in favor of assert_visual

• v0.6 — the pixel-perfect pack: check_alignment (group alignment / gap-rhythm / grid / pixel-snap audit) and pick_color (actual painted-pixel sampling + WCAG contrast verdicts)

• v0.5: inject_css — the live fix loop (trial a fix on the page, see what changed, converge, write source once); navigate { bypassCache } for stale-stylesheet hard reloads; blast-radius + scoped-fix reporting on explain_styles (change the button, not all buttons)

• v0.4 (field-report items): interact to drive the UI into a state and inspect it; measure_element for sub-pixel glyph/text-ink centering; an evaluate escape hatch; element-scoped crops/zoom on annotated_screenshot; match:"any" / visibleOnly:false on find_elements; zero-config cold-start Chrome discovery

• v0.3: the time dimension — event-listener attribution, animation diagnosis, source-attributed interaction timelines

• Hardened for untrusted pages: prompt-injection sanitization, fail-fast watchdog, dialog auto-dismiss

The problem, in the wild

AI coding agents are structurally blind to what they render. They emit CSS as text and never see the pixels that come back — so they guess, insist the guess worked, and loop. The community has a name for it now: "CSS gaslighting." Visionaire gives the agent deterministic eyes on the real render — measured geometry, the actual cascade winner, PASS/FAIL verdicts — so the loop ends. If you want the problem in other people's words first, start here:

• Why AI Sucks At Front End — Adam Argyle on why models can't reason about layout they can't perceive.

  "It's an LLM, not a rendering engine! It's notoriously bad at math, and throwing screenshots at it means very little. It's stabbing in the dark."

• Is AI Causing a Repeat of Frontend's Lost Decade? — Mauro Bieg on the wider cost of AI-generated frontend.

  "AI is enabling lots of AI slop — but this doesn't mean we don't still need people who know what they're doing."

Related MCP server: browser-inspector-mcp

The gap

Ask an LLM to fix a visual bug today and it gets one of two incomplete pictures: pixels, with no link back to code, or code, with no rendering truth about what's actually winning on screen. Existing browser MCPs make this worse for design work specifically — they ship accessibility snapshots that deliberately strip out all styling. What's missing is explanation and attribution:

• Which CSS rule wins the cascade for this property, and why did the others lose?

• Which file, which line does the winner live in — or which Elementor widget control, or which Customizer entry?

• Why is this element invisible, misaligned, or the wrong size?

Visionaire answers those questions with zero AI inside — everything is computed deterministically from the Chrome DevTools Protocol plus closed rulesets. The fuzzy part (matching "the button under the hero looks off" to an actual element) stays with the calling LLM, which gets uid-keyed snapshots, search tools, and annotated screenshots to do that cheaply.

What the output looks like

Live against wordpress.org:

why color = rgb(255, 255, 255):
  WINNER  [class*=wp-block] .wp-block-button__link { color: var(--wp--custom--button--color--text) }  spec(0,2,0)
    → themes/wporg-parent-2021/build/style.css:499  [line | theme: wporg-parent-2021 — edit themes/wporg-parent-2021/build/style.css]
  lost (specificity)  :root :where(.wp-element-button, .wp-block-button__link) { color: #fff }  spec(0,1,0)
    → global-styles-inline-css:2  [db-entity | Global Styles — Site Editor → Styles (theme.json / wp_global_styles)]
  lost (origin)  a:-webkit-any-link { color: -webkit-link }  spec(0,1,1)
    → user-agent stylesheet

Winner, losers with the decisive loss reason, and an honest edit pointer for each — including WordPress-aware answers like "Site Editor → Styles" instead of a useless path to a generated file.

Quick start

Requires Node ≥ 20 and Chrome/Chromium installed.

Fastest path — register straight from npm with Claude Code (no clone, no build):

claude mcp add visionaire -- npx -y visionaire-engine

Or run from a clone (for development or a pinned local build):

git clone https://github.com/mi60dev/visionaire-engine && cd visionaire-engine
npm install && npm run build
claude mcp add visionaire -- node "$PWD/dist/index.js"

Using GitHub Copilot, Cursor, Claude Desktop, Google Antigravity, or another client? See docs/clients.md for a copy-paste config for each, plus browser-install help for Linux/WSL/Docker.

Run it from your project's root directory — Visionaire is at its best when the agent has both the running site and its source on disk, so it can cross-reference the two. Ground before you search: take a page_snapshot (or read the source) to get real element names instead of guessing selectors. If a selector matches nothing, the error suggests the closest real ids/classes on the page.

Then, in a session:

1. connect { url: "https://your-site.com" } — launches Chrome (or { browserUrl: "http://127.0.0.1:9222" } to attach to your real, logged-in browser)

2. page_snapshot {} — a uid-keyed census of what's visible; target elements by their uid, not invented selectors

3. explain_styles { uid: "e17", property: "margin-bottom" } — cascade verdict with file:line

Try it without an MCP client:

npm run demo                                              # bundled fixture
npm run demo -- https://wordpress.org --selector "a.wp-block-button__link"

The 28 tools

Session & grounding — get connected and find the right element without guessing.

Explanation — the "why," with a receipt.

Pixel-level checks — new in v0.6.

Interaction & time — states, not just snapshots.

Fixing & verifying

Verification & proof — new in v0.7.

Full reference: docs/tools.md

The verify loop (stop the CSS gaslighting)

An agent edits a stylesheet, reads its own diff, and declares "now the cards are equal height" — without ever seeing a rendered pixel. Visionaire gives your agent deterministic eyes on rendered truth. The loop:

1. Preview shared-class blast radius → impact_preview.

2. Edit the smallest change.

3. Assert your claim → assert_visual (or re-run a named suite_id). You get PASS/FAIL + the actual measured pixels + the offending element uids.

4. Diagnose any FAIL → diagnose returns the ranked culprit with evidence.

5. Sweep responsive → responsive_sweep returns a per-viewport verdict matrix.

6. Prove it → capture_proof bundles before/after screenshots + verdict delta.

Real output — the same suite before and after a fix:

{
 "verdict": "FAIL",
 "summary": "1 assertion: 0 PASS, 1 FAIL — registered as suite 'cards' (re-run with just {\"suite_id\":\"cards\"})",
 "results": [
  { "type": "equal_height", "verdict": "FAIL", "id": "cards-equal",
    "measured": { "values": [412, 388], "unit": "px", "delta": 24, "tolerance_px": 1 },
    "offending_uids": ["e1", "e2"] }
 ],
 "truncated": false, "suite_id": "cards"
}

…fix the CSS, re-run with just { "suite_id": "cards" }:

{
 "verdict": "PASS",
 "summary": "1 assertion: 1 PASS, 0 FAIL",
 "results": [
  { "type": "equal_height", "verdict": "PASS", "id": "cards-equal",
    "measured": { "values": [412, 412], "unit": "px", "delta": 0, "tolerance_px": 1 } }
 ],
 "truncated": false, "suite_id": "cards"
}

Run npx visionaire-engine init-harness from your project root to wire the included Claude Code hooks (or Cursor rule) so the agent physically cannot end a turn claiming "it's fixed" without a verification pass on record. How the markers, hooks, and Stop gate work: docs/harness.md.

Documentation

• docs/clients.md — install in Claude, Copilot, Cursor, Antigravity, and other MCP clients

• docs/tools.md — tool-by-tool reference with real examples

• docs/harness.md — the verify-after-edit harness (hooks, markers, init-harness)

• docs/architecture.md — how the deterministic pipeline works

• docs/wordpress.md — WordPress origin resolution guide

• docs/development.md — building, testing, extending

Design principles

1. No internal LLM — deterministic, cacheable, testable, host-agnostic.

2. Fuzzy grounding belongs to the calling LLM; we make it cheap.

3. Complement the incumbent browser MCPs (same uid idiom), don't compete.

4. Honesty ladder on every attribution: line > file > db-entity > component > generated > unknown.

5. Token-budgeted output — a dossier is 300–800 tokens, never a dump.

Security posture

Visionaire is pointed at arbitrary, untrusted pages, so it treats page content as hostile:

• Prompt-injection defense. Page-derived strings (element text, class names, ids, attribute values) are sanitized at the single choke point where they enter tool output — collapsed to one line, stripped of control and bidirectional-override characters, and length-capped. A page cannot smuggle instruction-shaped text formatted as a "system message" toward the calling LLM; such content can only appear as an inert, quoted, truncated fragment.

• Fail-fast, never hang. Every tool call is wrapped in a watchdog (default 60s, VISIONAIRE_TOOL_TIMEOUT_MS to override; pick_element/record_interaction get their declared wait plus slack). A wedged browser returns an actionable error telling you to connect again, instead of blocking the client.

• No dead-locking dialogs. Page alert()/confirm()/prompt() calls are auto-dismissed — otherwise they would block every evaluate-family CDP call indefinitely.

Visionaire never executes page-authored code as instructions; it only reads and attributes. The calling LLM should still treat tool output as data about a page, not as commands.

Known limitations

• Chromium-only (CDP is the only path to matched-rule source locations; getMatchedCSSRules was removed from browsers years ago).

• @layer: unlayered-vs-layered ordering is exact; ordering between two different layer chains is a deterministic proxy (CDP doesn't expose layer declaration order).

• Some CDP fields we rely on (specificity, layers) are experimental; the engine feature-detects them and falls back (e.g. to its own specificity parser), and a contract smoke test in test/e2e.test.ts fails loudly if a Chrome update breaks the core protocol shape (the experimental fields are logged as present/absent).

Support

Visionaire is free and open source. If it saves you time and you'd like to help keep development going, you can chip in on Ko-fi or Patreon. Completely optional, and genuinely appreciated. Bug reports and field notes help just as much — see CONTRIBUTING.md.

License

Apache License 2.0 — free for everyone, including commercial use, with a patent grant. Copyright © 2026 mi60dev (NOTICE).

Built and maintained by @mi60dev. Contributions welcome under the Apache-2.0 terms — see CONTRIBUTING.md.