VASTlint

VASTlint

Created by Aleksander Sekowski.

This repository contains the validation engine, CLI, bindings and gRPC reference server. Chrome and IDE extensions, MCP applications and frontend runtime packages are maintained separately in vastlint-tools.

Website & web validator: VAST tag validator Paste a VAST tag and get results in your browser, no install required.

A high-performance VAST XML validator built on a pure-Rust core. Checks ad tags against the IAB Tech Lab VAST specification so you don't have to read it. Over $30 billion in annual CTV and video ad spend flows through VAST XML, and malformed tags are one of the most common causes of lost impressions, broken tracking, and revenue discrepancies between platforms. There is no widely adopted open-source tool that validates VAST XML against the full IAB specification across all published versions.

The VASTlint tools repository ships a native MCP server, making VAST validation available as a callable tool from Claude, Cursor, the AAMP Buyer Agent SDK, or any MCP-compatible client. Connect to vastlint.org/mcp and call validate_vast, validate_vast_url, inspect_vast, list_rules, explain_rule, or fix_vast. Each tool returns structured JSON with rule IDs, XPath locations, and spec references.

Bindings for realtime ad pipelines: vastlint-go (embedded WebAssembly by default, optional native cgo engine), vastlint-java (gRPC client for JVM ad servers), vastlint-erlang (Elixir/Erlang DirtyCpu NIF), and a WASM npm package for Node.js and browsers. All bindings use the same Rust validation core. The CLI daemon also supports OTP port integrations.

Need a copy-paste frontend starting point? See the React drop-in example in the tools examples.

Validates VAST documents against:

• IAB Tech Lab VAST 2.0, 3.0, 4.0, 4.1, 4.2, and 4.3 — structural rules derived from the published XSD schemas (W3C REC-xmlschema-1) and spec prose (RFC 2119 normative key words)
• W3C XML 1.0 well-formedness — malformed documents are rejected before any spec rule runs
• RFC 3986 URI syntax — all URL fields
• IANA Media Types — MIME types on MediaFile, InteractiveCreativeFile, Mezzanine, and ClosedCaptionFile
• ISO 4217 currency codes — Pricing elements
• Ad-ID registry format — UniversalAdId
• IAB Tech Lab SIMID 1.0, 1.0.1, 1.1, 1.2 — XML envelope for and nonlinear (the IAB-sanctioned VPAID replacement). Creative fetch, frame headers, and the postMessage handshake live in the VAST tester, not in vastlint-core or default check.
• IAB Tech Lab OMID / Open Measurement compatibility in VAST — vendor format, duplicate vendor detection, OMID resource semantics, verification tracking validation, and pre-4.1 extension-carried compatibility blocks

246 rules across required fields, schema validation, structural correctness, security, consistency, deprecated features, ambiguous usage, value formats, SIMID validation, OMID validation, VMAP 1.0, and DAAST 1.0. Rules marked with $ have direct revenue impact - use vastlint check --fail-on-warning in CI to catch them before they reach production. See common errors for the ones that cost real money. New to VASTlint? Start with the tutorial.

The CTV Addendum 2024 is supported across its legacy extension encodings, SIMID creative extensions on 4.x, and Icon regs. Supplied payloads are validated by default.

Full rule reference with examples and fix instructions: VAST error rule reference · RULES.md

How rules are derived: Rule derivation methodology · METHODOLOGY.md

For adjacent checks, RTBlint validates OpenRTB bid requests and responses. Pixellint validates pixels, postbacks, tracking URLs, and conversion API payloads.

Enterprise readiness

Zero runtime dependencies in the core. vastlint-core has three compile-time dependencies (quick-xml, url, phf) and no runtime dependencies whatsoever — no async runtime, no regex engine, no schema interpreter. Rules are compiled Rust functions. There is no transitive dependency graph to audit, no CVE surface to track, and no supply chain to compromise at runtime.

Verifiable build provenance. All release artifacts are signed with SLSA Build Level 2 provenance via GitHub's native attestation store. Every binary, library, .vsix, and npm package can be verified cryptographically against the exact source commit that produced it. No developer machine is ever involved in producing release artifacts. SLSA L3 (hermetic, isolated signing) is in progress.

No data retention by default — and full self-hosting available. The VS Code extension and Chrome page-scan path process XML locally. Pasting a tag in the Chrome popup opens the hosted tester; that path follows the website policy. The CLI does not send tags unless you pass --contribute-sample. The hosted tester, inspector, validator, and MCP server at vastlint.org/mcp may store a redacted copy of tags you submit, so the rules can be improved; see vastlint.org/privacy. Local vastlint-mcp over stdio does not send tags. The RapidAPI /validate API and the gRPC sidecar still validate ephemerally. See Telemetry and sample contribution and PRIVACY.md for the Chrome extension policy.

For teams that require on-premise processing or air-gapped deployments, VASTlint runs entirely self-hosted: the CLI image (FROM scratch), the vastlint-grpc sidecar (aleksuix/vastlint-grpc:0.16.0), or the pre-built static musl binary. The Rust core has no network code: no callbacks, no telemetry, no license checks. The sidecar exposes partner tallies on /metrics (port 9090); scrape them yourself.

Apache 2.0 licensed. No CLA, no dual-license commercial upsell, no usage-based restrictions. Fork it, vendor it, embed it, redistribute it.

Dependency update automation. Dependabot monitors Cargo, npm, and GitHub Actions dependencies weekly and opens PRs automatically. Combined with cargo audit on every CI push and CodeQL static analysis on every push and PR, the dependency surface stays current without manual tracking.

Auditable. OpenSSF Scorecard runs weekly and publishes a public score. CII Best Practices badge covers vulnerability reporting, CI, fuzzing, and code review requirements. The Security Advisory channel provides a private disclosure path with a 48-hour acknowledgement SLA.

Fuzz-tested continuously. Three libFuzzer targets run on every CI push against the core validator and auto-fix engine. See the Fuzzing section below.

Performance

Benchmarked on Apple M4 (10-core), production-realistic VAST tags (17–44 KB):

A typical OpenRTB bid cycle takes 100–300 ms; validation adds less than 2.1% of that budget even on the heaviest tags. An SSAI pipeline doing 1,000 stitches/sec spends more time on DNS than on validating the VAST response.

No async runtime, no regex engine, no schema interpreter. Rules are compiled Rust functions. Three dependencies: quick-xml, url, and phf (compile-time hash maps).

Install

cargo install vastlint

CLI crate on crates.io: crates.io/crates/vastlint

Or download a pre-built binary from the releases page.

Docker

Pull the image from Docker Hub:

docker pull aleksuix/vastlint

Validate a file:

docker run --rm -v "$(pwd)":/data aleksuix/vastlint check /data/tag.xml

Pipe from stdin:

cat tag.xml | docker run --rm -i aleksuix/vastlint check -

JSON output:

docker run --rm -v "$(pwd)":/data aleksuix/vastlint check /data/tag.xml --format json

Validate a whole directory:

docker run --rm -v "$(pwd)/tags":/data aleksuix/vastlint check /data/*.xml

The image is built FROM scratch - a fully-static musl binary with no OS layer. Compressed size is under 5 MB. Cold-start to first result is under 10 ms.

Usage

# validate a file

vastlint check tag.xml

validate multiple files

vastlint check *.xml

read from stdin

cat tag.xml | vastlint check -

JSON output (one object per file, newline-delimited)

vastlint check tag.xml --format json

suppress colours

vastlint check tag.xml --no-color

exit 0 even on errors (useful in some CI setups)

vastlint check tag.xml --no-fail

opt in to anonymous usage telemetry (see Telemetry section below)

vastlint check tag.xml --telemetry

override the VAST version used for validation (ignores the version= attribute)

vastlint check tag.xml --vast-version 4.2

replace template macros before validation so URL