Pixellint README

Pixellint

Pixellint is a spec-first validator for pixels, postbacks, conversion API payloads, and other measurement artifacts. It runs in a terminal, in CI, and in agents, and every finding carries a stable id, a severity, an evidence level, and the document it came from.

Broken pixels cost attribution, QA time, and campaign money. The tooling that exists is fragmented: vendor-specific helpers, enterprise tag auditors, and schema linters that know nothing about ad tech. Pixellint is the open validation layer underneath all of that.

$ pixellint validate url 'https://www.facebook.com/tr?ev=Purchse&em=buyer@example.com'
rulepack: core
  ok
rulepack: vendor/meta (vendor: meta)
  error   vendor.meta.param.id.missing   `id` is required on Meta Pixel requests but is not present. It is the numeric Pixel ID from Events Manager.
    fix: Set `id` to the numeric Pixel ID shown in Events Manager.
    docs: https://developers.facebook.com/docs/meta-pixel/get-started
  warning vendor.meta.param.ev.invalid   `ev` is `Purchse`, which is not one of the documented values: PageView, AddPaymentInfo, ...
    fix: Use a standard event name, or confirm the custom event is registered in Events Manager.
    docs: https://developers.facebook.com/docs/meta-pixel/reference
  error   vendor.meta.pii.unhashed_email A parameter carries what looks like a raw email address. Meta requires customer information to be normalized and SHA-256 hashed before it is sent.
    fix: Normalize the value, hash it with SHA-256, and send the hex digest instead of the plain address.
    docs: https://developers.facebook.com/docs/meta-pixel/advanced/advanced-matching

2 error(s), 1 warning(s), 0 info message(s) across 2 rulepack(s).

Server-side events are checked in the body, one event at a time:

$ pixellint validate json '{"data":[{"event_name":"Purchase","event_time":1770000000000,
    "action_source":"website","user_data":{"em":"buyer@example.com"}}]}'
rulepack: vendor/meta-conversions-api (vendor: meta)
  error   vendor.meta-conversions-api.body.event_time.invalid `event_time` must be at most 10 digits, but `1770000000000` has 13. Meta documents it as a Unix timestamp in seconds...
    fix: Send seconds, not milliseconds: divide a JavaScript `Date.now()` by 1000 and floor it.
  error   vendor.meta-conversions-api.body.purchase_requires_value_and_currency A `Purchase` event is missing `custom_data.value` or `custom_data.currency`.
  error   vendor.meta-conversions-api.body.website_requires_source_url The event is marked `action_source: website` but carries no `event_source_url`.

Install

cargo install pixellint          # CLI
cargo install pixellint-mcp      # MCP server
npm install pixellint            # library, WASM-backed

Or paste a URL into the playground at pixellint.org, which runs the same engine in your browser. Artifacts you test may be stored; see privacy. Vendor contracts are at pixellint.org/packs/. Guides for pixels, conversion APIs, and consent are at pixellint.org/docs/.

Start with the contracts people actually hit:

• Conversion API validator
• Pixel not firing
• Conversions API not working
• What is a conversion API
• Reddit CAPI
• DART Floodlight tags
• Facebook Pixel Helper vs Network
• Pinterest standard events

Use it

QA

Validate an inline artifact, a file with @path, or stdin with -:

pixellint validate url 'https://px.ads.linkedin.com/collect?pid=123456&fmt=gif'
pixellint validate postback @conversion-endpoint.txt --json
printf '%s\n' 'https://example.com/tracking?cb=[CACHEBUSTING]' | pixellint validate vast - --state template

Callers that already extracted many URLs (Vastlint, an HTML adapter) pass them as a document. Identical values validate once:

pixellint validate-many @extracted.json --json

Callers that know which artifacts belong to each ad session can supply explicit groups with original zero-based row indexes:

pixellint validate-sessions @sessions.json --json

The initial IAS UVP profile checks external-ID consistency within each group and uniqueness across groups. Its report preserves ordinary document findings and adds relationship coverage, original spans and provenance. See docs/SESSION_SCHEMA.md for grouping, typed skips and local resource limits.

extracted.json is the wrapper in docs/MULTI_ARTIFACT_SCHEMA.md, or a JSON array of URL strings. Pixellint does not parse VAST, HTML, or GTM.

vast means one tracking URL extracted from VAST, not a VAST XML document. It enables the IAB VAST macro-name checks on that URL.

Use request for a complete HTTP capture. It links endpoint, method, headers, query fields and body so the destination's transport and payload rules run together:

pixellint validate request @captured-request.json --json

The capture has string url and method, a headers object or list of { "name": "...", "value": "..." } entries. Use an optional raw string body for text, or body_base64 for captured binary bytes. The shared engine decodes identity and gzip entities within explicit local limits. See docs/HTTP_REQUEST_SCHEMA.md for examples and validation limits. A bare URL passed as request keeps its existing behavior.

If the artifact is still a template with unexpanded macros, say so, and macro rules adjust:

pixellint validate url 'https://example.com/pixel?cb=[CACHEBUSTING]' --state template

CI

pixellint validate exits 0 when the artifact is clean or only produced warnings, 1 when any error-severity finding is present, and 2 on a usage or input problem.

- uses: aleksUIX/pixellint@v0.31.12
  with:
    path: fixtures/conversion-pixel.txt
    kind: url

version selects the CLI release (auto follows the action's own v* tag). Linux and macOS runners, x86_64 and aarch64. The Action downloads a prebuilt tarball from GitHub Releases.

Or install the CLI:

- name: Validate tracking artifacts
  run: |
    cargo install pixellint
    pixellint validate url @fixtures/conversion-pixel.txt

Agents

cargo install pixellint-mcp

pixellint-mcp speaks MCP over stdio and exposes three tools. It does not send artifacts; there is no hosted MCP on pixellint.org. Playground artifacts on pixellint.org may be stored; see pixellint.org/privacy.

• MCP Registry name: mcp-name: io.github.aleksUIX/pixellint

• list_rulepacks

• list_vendors, optionally filtered by category or attributing a single host

• validate_artifact, taking artifact_kind, artifact, and optional claimed_vendor, expansion_state, rulepacks, except_rulepacks

Responses include the structured findings, the detected vendors, and a severity summary, so an agent can act on the result without parsing prose.

Node

The npm wrapper requires Node.js 18 or newer. Its ESM entry uses node:module to load the Node-target WASM build. For a browser integration, build pixellint-wasm for a browser target and supply a browser loader, as the playground does.

import { validate, isOk } from "pixellint";

const summary = validate("https://www.facebook.com/tr?ev=Purchase");
isOk(summary); // false, the pixel id is missing

Library

use pixellint_core::{ArtifactKind, Engine, ExpansionState, ValidationOptions, ValidationRequest};

let engine = Engine::default();
let request = ValidationRequest {
    artifact_kind: ArtifactKind::Url,
    artifact: "https://www.facebook.com/tr?id=1234567890123456&ev=PageView".to_string(),
    claimed_vendor: None,
    expansion_state: ExpansionState::Unknown,
};

let summary = engine.validate(&request, &ValidationOptions::default())?;
assert!(summary.is_ok());

Rulepacks

core runs on every URL-like artifact. Vendor packs run only when the artifact targets their endpoints, so you get vendor checks without asking for them, and nothing fires on an endpoint it does not understand. A conversion API body has no endpoint to go by, so those packs claim it by the shape of the payload.

The table shows selected built-in packs. Run pixellint list-rulepacks --json for the complete inventory in your installed build.

Vendor directory

There are 166 vendor rulepacks across 87 vendors. The vendor directory covers additional hosts by attribution: 134 vendor rows and 326 hosts, so an unrecognized pixel still gets a name. Full inventory: docs/STANDARDS.md.

The per-pack depth audit records reviewed sources, implemented checks and remaining requirements. Pack counts and passing fixtures do not certify complete vendor specification coverage. Private protocols, unsupported request representations, account settings and request history remain explicit coverage limits.

$ pixellint validate url 'https://trc.taboola.com/actions?a=1'
rulepack: directory (vendor: taboola)
  info  directory.no_rulepack_coverage  This endpoint belongs to Taboola (native). No Pixellint rulepack covers it, so only the core checks ran. The `vendor/taboola` rulepack covers other Taboola endpoints, not this one.

Directory entries make one claim, that a host belongs to a vendor. They carry no parameter contracts, the finding is always info, and attribution never changes an exit code. See docs/VENDOR_DIRECTORY.md.

pixellint list-vendors
pixellint list-vendors --json

Select packs per run:

pixellint validate url "$ARTIFACT" --rulepack core
pixellint validate url "$ARTIFACT" --except vendor/meta
pixellint list-rulepacks --json

Full rule inventory with citations: docs/STANDARDS.md.

Evidence levels

Findings say where their authority comes from, and you can hold packs to different standards accordingly:

• normative: a formal standard such as the WHATWG URL Standard or an RFC
• official_vendor: a parameter contract the vendor publishes, with the URL
• official_template: vendor-published templates or SDK behavior
• ecosystem_reference: consistent real-world behavior the vendor generates in its own UI but does not document
• heuristic: Pixellint's judgment, labeled as such

The manifest loader refuses to compile a pack that claims official_vendor without citing documentation.

Custom rulepacks

Rulepacks are data, not code. First-party vendor packs are written in the same JSON format you can write for an internal endpoint:

{
  "id": "custom/acme",
  "display_name": "Acme internal pixel",
  "description": "Contract for the internal conversion endpoint.",
  "vendor": "acme",
  "source_level": "heuristic",
  "match": { "hosts": ["px.acme.example"], "path_prefixes": ["/collect"] },
  "params": [
    { "name": "aid", "requirement": "required", "format": { "kind": "integer" } },
    { "name": "ev", "requirement": "required", "format": { "kind": "enum", "values": ["view", "purchase"] } }
  ]
}

pixellint validate url 'https://px.acme.example/collect?ev=purchase' --rulepack-file acme.json

The format is documented in docs/RULEPACK_SCHEMA.md.

Private pixels that only need a name, not a parameter contract, belong in a directory overlay:

pixellint validate url 'https://px.acme.example/collect' --directory-file extra.json

The overlay uses the same entry shape as the built-in directory. New hosts only; it cannot steal a first-party host. Contribute an upstream host through a PR: CONTRIBUTING.md.

What Pixellint does not do

• It does not extract artifacts from documents. vast accepts a tracking URL, not XML. html, js, and gtm are not validation kinds; the CLI exits 2 if you pass them. Callers such as Vastlint parse VAST, HTML, or GTM containers and hand Pixellint the URLs they found. pixellint validate-many wraps those extracted URLs. The document result model is in docs/MULTI_ARTIFACT_SCHEMA.md.
• It does not fire requests or check whether an endpoint responds.
• It does not auto-fix. Findings carry fix hints; applying them is on you.

Evidence

Every rule is backed by tests: unit tests in crates/pixellint-core/src/, a golden corpus in fixtures/ with one directory per rulepack, and integration tests that drive the real CLI binary and the real MCP stdio transport.

cargo test --workspace

Benchmark

In-process engine speed over the golden corpus, D1-shaped synthetic pixels, and large validate-many batches:

cargo run -p pixellint-bench --release -- --quick
cargo run -p pixellint-bench --release
cargo run -p pixellint-bench --release -- --mode load --heavy

See bench/README.md.

Docs

• Standards and rule inventory
• Vendor directory
• npm package
• Rulepack manifest schema
• Architecture
• Roadmap
• Multi-artifact output model
• Explicit ad-session input and output

License

Apache-2.0. Pixellint is not affiliated with or endorsed by any vendor named in its rulepacks; see NOTICE.

Local HAR 1.2 request capture import and offline replay are available through pixellint import-har, pixellint validate-har, Rust, npm and WASM. See HAR request replay and capture availability for header policies, omitted-body handling and recorded-clock validation.