A coding agent writes the CSS and says it is done. It never sees the result. Nothing hands it a
number, so the loop cannot close: the agent guesses, you open the page, you send it back, it
guesses again. Every round costs you the one thing the agent was supposed to save.
The same gap exists without an agent, when a teammate says a section is finished and nobody in
the room has a measurement. The difference is that a person can at least look.
pixelpact reads the reference page and writes down what it actually renders: sizes, colors,
spacing, typography, hover and focus states, animation keyframes, design tokens. That file is
the contract. Point pixelpact at the implementation and it answers with one line per
property that drifted, which is precisely what an agent can act on.
npx pixelpact extract https://reference.example.com -o contract.json
npx pixelpact check contract.json http://localhost:3000
Works with
| MCP clients | Playwright | GitHub Actions | Any framework | Figma |
|---|
| Claude Code, Cursor, anything that speaks stdio MCP | the engine it drives | one comment on the pull request | it reads the DOM, not your stack | a frame as the reference, when you want one |
If a browser can render it, pixelpact can measure it.
For coding agents
An agent that writes UI code cannot tell whether it succeeded. pixelpact-mcp gives it the
measurement, so the loop closes without a person in the middle: extract the contract once,
then let the agent check its own work, read the deviation list, fix, and check again.
{
"mcpServers": {
"pixelpact": { "command": "npx", "args": ["-y", "pixelpact-mcp"] }
}
}
Tools exposed: extract_contract, check_implementation, diff_pixels,
read_contract_summary.
Why this is not visual regression testing
Percy, Chromatic, Applitools, BackstopJS and Pixeleye all compare your page against a baseline
that you approved earlier. That model works once the page already looks right and you want to
keep it that way. While you are still building toward a design, there is no baseline to
compare with, and the first run of a regression tool simply records whatever you produced.
| Visual regression tools | pixelpact |
|---|
| Compares against | a snapshot you approved earlier | the reference design itself |
| Useful when | the UI is already correct | the UI is being built |
| First run on a new page | records, cannot judge | measures against the reference |
| Answer you get | an image diff to inspect by eye | a value per property, with a delta |
| Fits an autonomous agent | needs a human to approve the diff | the numbers close the loop |
The two models are complementary. Use a regression tool to keep a finished page finished, and
pixelpact to get it finished in the first place.
Problems pixelpact solves
| Without pixelpact | With pixelpact |
|---|
| ❌ A coding agent writes CSS, declares it done, and a person has to open the page to find out that it is not. | ✅ The agent calls the MCP server, reads the deviations, fixes them and checks again. No person in the middle. |
| ❌ Someone says the section is finished, you disagree, and neither of you has a number. The louder opinion wins. | ✅ Every value in the reference is asserted against your page. What failed is listed with expected, actual and the difference. |
| ❌ A regression tool has nothing to compare a brand new page against, so its first run records whatever you happened to build. | ✅ The reference is the baseline from the first minute. Nothing has to be approved before the tool is useful. |
| ❌ Hover, focus and animation values are almost never reviewed, because checking them by hand is slow and boring. | ✅ They are part of the contract, so they are measured on every run like any other value. |
| ❌ An image diff tells you that something changed and leaves you to hunt for what. | ✅ Deviations name the element, the property, the expected value and the measured one. |
| ❌ The design lives in a file nobody opens during code review. | ✅ The contract is committed JSON, so a pull request shows exactly which values moved. |
Install
pnpm add -D pixelpact playwright
pnpm exec playwright install chromium
playwright is a peer dependency, so the browser download stays under your control and a
project that already has Playwright installs nothing extra. Node 22.12 or newer is required.
Quickstart
1. Extract the contract from the reference.
npx pixelpact extract https://reference.example.com \
--selector "main" \
--viewport desktop,mobile \
--screenshots .pixelpact/shots \
-o contract.json
2. Measure your implementation.
npx pixelpact check contract.json http://localhost:3000 --viewport desktop
3. Fix what it reports, then run it again. The command exits 0 when everything is inside
tolerance and 1 when it is not, so it drops straight into a script or a CI job.
What a contract looks like
A contract is plain JSON, readable and diffable, with no proprietary format and no service
behind it.
{
"version": 1,
"source": { "type": "url", "value": "https://reference.example.com" },
"root": "main",
"extractedAt": "2026-09-06T01:20:44.812Z",
"viewports": [{ "name": "desktop", "width": 1440, "height": 900 }],
"tokens": { "--brand-600": "rgb(11, 114, 133)" },
"keyframes": { "fade-up": [{ "offset": "0%", "css": "opacity: 0" }] },
"byViewport": {
"desktop": {
"documentHeight": 4218,
"elements": [
{
"selector": "main > header > a.cta",
"tag": "a",
"text": "Get started",
"box": { "x": 120, "y": 32, "w": 148, "h": 44 },
"styles": {
"background-color": "rgb(11, 114, 133)",
"font-size": "16px",
"border-radius": "8px"
},
"hover": { "background-color": "rgb(8, 90, 105)" },
"focus": { "outline": "2px solid rgb(11, 114, 133)" }
}
]
}
}
}
Because it is a file, you can commit it, review it in a pull request, hand it to another
developer, or hand it to an agent.
What a check prints
pixelpact check FAILED
target http://localhost:4173/impl.html
reference http://localhost:4173/ref.html
viewport desktop 1440x900
elements 14 matched, 0 missing of 14
checks 1056 passed, 10 failed (99.1% of 1066)
deviations (10)
SELECTOR PROPERTY EXPECTED ACTUAL DIFF
body > main > h1 font-size 48px 44px 4px
body > main > a box.width 117.75px 109.75px 8px
body > main > a padding-right 24px 20px 4px
body > main > a padding-left 24px 20px 4px
body > main > a background-color rgb(11, 114, 133) rgb(37, 99, 235) 60.1 (color)
body > main > a border-top-left-radius 8px 4px 4px
body > main > a border-top-right-radius 8px 4px 4px
body > main > a border-bottom-left-ra... 8px 4px 4px
body > main > a border-bottom-right-r... 8px 4px 4px
body > main > a focus.outline rgb(11, 114, 133) s... rgb(37, 99, 235) so... differs
That is a real run against two copies of one page with four declarations changed. Four edits
produce ten measured deviations, because padding moves the box width and one border-radius
shorthand sets four corners. Run the same check against the reference itself and all 1066
assertions pass, which is the property that matters: a passing check has to mean something.
Add --json to get the same report as a data structure, which is what CI jobs and agents read.
What a side by side run shows
check says which values moved. diff says how many pixels moved. Neither tells a person
where to look. side splits both pages into sections, puts them next to each other, and boxes
what differs.
npx pixelpact side https://reference.example.com http://localhost:3000 --widths 1440,390
# SECTION WIDTH VERDICT DIFF
01 hero 1440px PASS 0.000%
02 features 1440px FAIL 0.675%
.pixelpact/side/1440/02-features.png
03 pricing 1440px PASS 0.000%
04 foot 1440px FAIL 1.265%
.pixelpact/side/1440/04-foot.png
That is a real run against the two files in examples/side, which are
copies of one page where the card gap and the corner radius were changed. Reproduce it with:
npx pixelpact side "file://$PWD/examples/side/reference.html" \
"file://$PWD/examples/side/implementation.html" --widths 1440
This is the command to run before telling anyone that a page is finished.
Features
Contract from the reference
Reads the page you are building toward and writes down every value it renders. Nothing to approve first. | Numbers, not opinions
Each deviation carries the expected value, the measured value and the difference between them. | States, not just layout
Interactive elements are hovered and focused, and only the properties that actually change are stored. |
Tokens and keyframes
Custom properties on the root element and named animation keyframes travel inside the contract. | Pixel comparison
When every value passes and it still looks wrong, compare the screenshots and get a percentage. | Agent ready
An MCP server exposes the same measurements, so a coding agent can close its own loop. |
Pull request checks
A composite Action runs the check and keeps one comment on the pull request up to date. | Plain JSON
The contract is a file you can read, diff and review. No service, no account, nothing to log into. | Framework agnostic
It measures the rendered DOM, so React, Vue, Svelte and hand written HTML are all the same to it. |
Commands
| Command | What it does |
|---|
pixelpact extract <url> | Reads the reference and writes a contract file |
pixelpact check <contract> <url> | Measures an implementation, prints deviations, sets exit code |
pixelpact diff <contract> <url> | Pixel comparison against the screenshot stored in the contract |
pixelpact side <reference> <url> | Section by section side by side images with the differences boxed |
Shared flags cover the browser context (--viewport, --selector, --wait, --timeout,
--locale, --timezone, --channel, --headful), the output (--out, --json,
--quiet), and the parts of extraction you may want to cap (--max-elements, --max-states,
--mask). Run pixelpact <command> --help for the full list.
Exit codes
| Code | Meaning |
|---|
0 | Everything inside tolerance |
1 | Deviations found, or the pixel threshold was exceeded |
2 | Usage error, for example a bad flag or a contract file that is missing |
3 | Runtime failure, for example no browser available or the page would not load |
Programmatic use
import { extract, check, formatCheckReport, writeContract } from 'pixelpact-core'
const contract = await extract({
url: 'https://reference.example.com',
selector: 'main',
screenshotDir: '.pixelpact/shots',
})
await writeContract('contract.json', contract)
const report = await check(contract, { url: 'http://localhost:3000' })
console.log(formatCheckReport(report, { color: true }))
if (!report.ok) process.exitCode = 1
Everything is typed, and the contract and report shapes are validated at the boundary, so a
malformed file fails with a readable message instead of a stack trace.
In CI
The Action measures a preview deployment against the contract committed in the repository and
keeps a single pull request comment up to date instead of adding one per push.
- uses: jamalkamaladdin/pixelpact/action@v0
with:
contract: contract.json
url: ${{ steps.preview.outputs.url }}
viewport: desktop
tolerance: 1
It needs permissions: pull-requests: write and no secret beyond the automatic
GITHUB_TOKEN. Every input, every output and a complete workflow are in
action/README.md.
Reading a Figma frame
extract recognises a Figma url and reads the frame through the REST API. No browser is
launched for this step.
export FIGMA_TOKEN=figd_...
npx pixelpact extract "https://www.figma.com/design/KEY/Name?node-id=12-345" -o contract.json
npx pixelpact check contract.json http://localhost:3000
A Figma layer has no CSS selector, so a Figma contract binds to your markup through
data-contract attributes. Name the element after the layer and matching stops depending on
how the design happened to nest its frames:
<a class="btn btn-primary" data-contract="Hero/CTA">Get started</a>
Anything with no match is reported as missing rather than guessed from tag names.
Published styles come across as tokens: a color style as its color, a text style as a css font
shorthand such as 600 60px/72px Geist, an effect style as a box shadow. A fill that is a
gradient or an image has no single css value, so it is left out and named in the warnings
rather than approximated.
How it works
- Playwright opens the reference at each requested viewport, waits for fonts, network and
paint to settle, and dismisses cookie overlays.
- A single function is evaluated inside the page. It walks the DOM under your root selector
and records the computed style of every visible element, plus custom properties, keyframes,
and the geometry of each box.
- Interactive elements are hovered and focused, and only the properties that actually change
are stored, so a contract stays small enough to read.
- Checking repeats step 2 against your implementation, matches elements by
data-contract attribute, then by selector, then by tag and text, and compares property by
property. Lengths use a pixel tolerance, colors use a perceptual distance, and animations
are compared by name and timing rather than by string equality.
Repository layout
| Package | Published as | What it is |
|---|
packages/core | pixelpact-core | Extraction, checking, reporting |
packages/cli | pixelpact | The pixelpact command |
packages/mcp | pixelpact-mcp | MCP server for coding agents |
action | used from GitHub | Action for pull request checks |
FAQ
How does an agent use it? Install pixelpact-mcp, point the MCP client at it, extract the
contract once, then let the agent call check_implementation after every edit and read the
deviation table it gets back.
How is this different from Percy or Chromatic? They compare your page against a snapshot
you approved earlier, which assumes the page is already right. pixelpact compares it against
the reference, which is what you actually have while you are still building. The two fit
together: pixelpact to get the page correct, a regression tool to keep it that way.
Do I need a baseline? No. The reference is the baseline, and that is the entire point.
My markup does not match the reference structure. Will anything match? Element matching
tries the data-contract attribute first, then the selector, then the tag and its text. Put
data-contract="hero-cta" on your element and matching stops depending on how the reference
happened to nest its divs.
The page has content that changes on every load. Will a check ever pass? Mask it.
--mask ".carousel" keeps a region out of the pixel comparison, and --max-elements stops a
long page from producing a contract nobody can read.
Which browsers? Chromium through Playwright. Running a matrix across engines is out of
scope on purpose: this tool measures agreement with a design, not differences between browsers.
Does a passing check mean the page is correct? It means every value in the contract matched
inside tolerance. Elements that exist in your page but not in the reference are not flagged,
because the contract only describes what the reference contains. Run diff as well when
nothing extra is allowed.
Status
Everything described above is built and every number shown came from a real run: extraction
from a live page and from Figma, checking, the pixel diff, the side by side images, the MCP
server and the Action.
Version 0.3. Settled enough to use, not settled enough to promise: the contract format will
gain fields before 1.0. If a value you need is missing from it, open an issue and say which
one, because that is the fastest way for it to appear.
Contributing
Development setup, the commands, and the pull request flow are in
CONTRIBUTING.md. Security reports go through
SECURITY.md.
License
MIT © Jamal Kamaladdin
