How do I validate a VAST tag online?

Paste your VAST tag (VAST XML) or drop a .xml file into the vastlint web validator at vastlint.org/validate. It runs spec-derived validation across IAB VAST 2.0–4.3 and returns a structured report in under a millisecond. Nothing is stored.

How do I validate VAST XML online?

Paste your VAST XML or drop a .xml file into the vastlint web validator at vastlint.org/validate. It runs spec-derived validation across IAB VAST 2.0–4.3 and returns a structured report in under a millisecond. Nothing is stored.

How do I validate VAST XML in a CI/CD pipeline?

Install the vastlint CLI via Homebrew (brew install aleksUIX/tap/vastlint) or Cargo (cargo install vastlint). Run vastlint validate tag.xml — exit code 0 means valid, exit code 1 means errors found. Use --format json for machine-readable output.

What does VAST XML validation check?

vastlint applies spec-derived checks from the published IAB VAST XSD schemas. Key checks include required elements (Impression, Duration, MediaFile), duration format (HH:MM:SS), URL validity, version consistency, deprecated features like VPAID, duplicate impressions, and skipOffset values.

What do VAST validation severity levels mean?

Error means the tag violates a MUST requirement in the spec and is likely to cause a render failure. Warning means the tag violates a SHOULD requirement or uses deprecated syntax and may cause issues on some players. Info is an advisory — the tag is valid but could be improved.

Can I validate VAST XML in Rust or Go?

Yes. vastlint-core is a zero-dependency Rust library on crates.io that validates production-size VAST tags in 363 µs to 2.1 ms on a single core. A Go binding wraps the same Rust core via CGo with no Rust toolchain required.

Validate VAST XML online

Short answer: if you already have the XML, paste it into the VAST validator for the fastest spec check. If you only have a live VAST URL or wrapper endpoint, start with the VAST Tag Tester first so you can fetch the response before validating it.

A VAST tag — also called a VAST XML document — is the IAB Tech Lab standard response that tells a video player how to find, display, and track an ad. A single malformed field, missing <Impression>, bad duration format, or deprecated VPAIDpattern can silently kill an impression. This guide covers the fastest way to validate VAST XML online plus the CLI and in-process options for pipelines.

Choose the right VAST debugging tool

Use the validator for pasted XML and pure spec checks, the tester for live VAST URLs and creative preview, and the inspector for hop-by-hop wrapper debugging.

Open validator →Test a live tag URL →Inspect wrapper hops →

Option 1: Online VAST validator (fastest)

The quickest way is to paste your tag into the vastlint web validator. It runs spec-derived validation across IAB VAST 2.0, 3.0, 4.0, 4.1, 4.2, and 4.3 and returns a structured report in under a millisecond. Nothing is stored.

If you do not have the XML yet and only have a live VAST URL or wrapper endpoint, start with the VAST Tag Tester or the VAST Inspector first. The tester is better for creative preview and click-tracking review; the inspector is better for hop-by-hop wrapper-chain debugging.

Open vastlint.org/validate
Paste your VAST tag or drop an .xml file
Press ⌘↵ or click Validate
Review errors, warnings, and advisory messages (each linked to the rule that fired)

Option 2: CLI (for CI/CD pipelines)

Install the vastlint CLI with Homebrew or Cargo and validate any file or stream from the command line. This is the standard approach for validating tags in a build pipeline or pre-flight check.

# Homebrew (macOS / Linux)
brew install aleksUIX/tap/vastlint

# Cargo
cargo install vastlint

# Validate a file
vastlint validate tag.xml

# Validate from stdin (pipe from curl, etc.)
curl -s https://example.com/vast.xml | vastlint validate -

# Exit code 0 = valid, 1 = errors found
# Use --format json for machine-readable output
vastlint validate tag.xml --format json

Option 3: Rust library (in-process, zero overhead)

For ad servers, SSPs, and DSPs that need inline validation at bid time, embed vastlint-core directly. It validates production-size VAST tags (17–44 KB) in 363 µs to 2.1 ms on a single core.

# Cargo.toml
[dependencies]
vastlint-core = "0.4"

use vastlint_core::validate;

let result = validate(vast_xml_bytes);
for issue in &result.issues {
    println!("{}: {} (line {})", issue.severity, issue.message, issue.line);
}

Option 4: Go binding

A Go binding wraps the same Rust core via CGo. Use it in Go-based ad servers without spawning a subprocess.

import vastlint "github.com/aleksUIX/vastlint-go"

result, err := vastlint.Validate(xmlBytes)
if err != nil {
    log.Fatal(err)
}
for _, issue := range result.Issues {
    fmt.Printf("%s: %s\n", issue.Severity, issue.Message)
}

What VAST validation checks

vastlint applies spec-derived checks from the published IAB VAST XSD schemas. Key checks include:

Required elements: <Impression>, <Duration>, <MediaFile> (for inline ads), <VASTAdTagURI> (for wrappers)
Duration format: must be HH:MM:SS or HH:MM:SS.mmm
URL validity: all tracking pixels, media URLs, and click-through URLs
Version consistency: elements from VAST 4.x used in a 3.0 tag, or vice versa
Deprecated features: VPAID, AdServingId misuse, old delivery attributes
Duplicate impressions: multiple <Impression> URIs that are identical
SkipOffset: value must be a valid time offset or percentage

Understanding validation results

Every issue has a severity level:

Severity	Meaning	Action
Error	Tag violates a MUST requirement in the spec	Fix before serving. Likely to cause render failure.
Warning	Tag violates a SHOULD requirement or uses deprecated syntax	Fix soon. May cause issues on some players.
Info	Advisory. Tag is valid but could be improved.	Optional improvement

Language-specific validator guides

If you are choosing a production library rather than doing a one-off browser check, start with the runtime-specific guide.

Best for Node.js →Best for Go →Best for Rust →