Your web scraper stops returning prices. One day they're there, the next day they're null. Your monitoring catches it immediately because the data structure broke—a required field went missing. This is the kind of failure teams can work with. It's loud, obvious, fixable.

Rule-based validation exists for these moments. When websites change in ways that break data structure, these tools catch it before bad data flows downstream. They verify types match expectations, required fields exist, formats follow patterns. When something breaks, they stop everything and tell you exactly what's wrong.

Rule-based validation catches structural breaks reliably. But websites don't just break. They drift, and that's a different problem entirely.

What Actually Breaks

Websites change constantly, and those changes interact with web automation in specific ways. A date format shifts from YYYY-MM-DD to MM/DD/YYYY and breaks your parser. A price field moves from one div to another, and your selector starts returning empty strings. A product title that was always present becomes optional on certain pages.

At web agent scale, these structural breaks multiply. The same site serves different HTML to different regions. A/B tests randomly change selectors mid-session. Bot detection serves simplified pages to automated browsers. Your validation needs to catch not just that something broke, but understand which variation of "broken" you're seeing.

Rule-based validation tools like Pydantic and Cerberus catch these structural breaks. They define what valid data looks like—field types, required vs optional, format patterns, value constraints—and reject anything that doesn't match. When a scraper pulls a string where you expect an integer, or returns null for a required field, validation stops the pipeline.

The real value: preventing downstream failures before they happen. Systems don't break three layers deep in your data pipeline. Analysts don't spend hours debugging why their reports look wrong. Product teams don't make decisions based on incomplete data they didn't know was incomplete.

Pydantic vs Cerberus: Different Operational Contexts

Pydantic handles complex data structures where strict typing matters. It uses Python's type hints to define exactly what each field should be, then raises errors immediately when data doesn't match. Teams building data pipelines that feed directly into production systems tend to reach for Pydantic because it's unforgiving. If the data doesn't match the schema, nothing moves forward.

The tool provides automatic type conversion where it makes sense—turning the string "123" into the integer 123—but stays strict about what's acceptable. When you're scraping product catalogs where price accuracy is non-negotiable, or inventory data that feeds real-time availability systems, this strictness prevents silent failures.

Cerberus fits scenarios where validation needs flexibility. It's schema-based but less rigid, better for smaller projects or situations where some data inconsistency is acceptable. The tool validates against defined rules but doesn't halt everything on the first problem. Instead, it collects all validation errors and returns them together.

Teams use Cerberus when they're exploring new data sources, building prototypes, or working with sites where data quality varies naturally. It catches obvious problems without being so strict that minor variations stop everything.

The operational moment that reveals the difference: when you're running agents across 50 e-commerce sites, each with different authentication patterns and regional variations, Pydantic ensures nothing slips through when those patterns shift. Cerberus lets you iterate faster when you're still figuring out what "valid" looks like for a new data source.

What These Tools Catch Well

Both approaches excel at the same categories of structural problems:

• Type mismatches surface when a field that should be a number comes back as text, or a date appears as a string. These look trivial until you're debugging why your pricing analysis crashed at 2 AM.

• Structural breaks happen when expected fields disappear or new fields appear unexpectedly. A website redesign might move product descriptions from one location to another. Your selectors still work, but they're now pulling navigation text instead of descriptions.

• Format violations catch dates, phone numbers, emails, URLs—anything with expected patterns. Using regex and custom validators, these tools flag when formats drift from expectations.

• Value constraints verify that numbers fall within expected ranges, strings aren't too long or too short, and values come from allowed sets. When a product price shows up as $0 or $999,999, validation flags it before it reaches your database.

Where This Approach Works

We run thousands of browser sessions across different sites daily, and scale reveals why validation isn't optional. When you're extracting data from hundreds of sites—each with different authentication flows, regional variations, and bot detection patterns—even a 2% error rate means thousands of bad data points flowing into production systems. At that scale, you can't manually review failures. You need validation that catches problems immediately, before bad data compounds across your infrastructure.

Teams reach for rule-based validation at specific operational moments:

Integration requirements drive adoption when scraped data feeds directly into customer-facing products or business-critical systems. Real-time data quality becomes non-negotiable because downstream failures are visible and expensive.

Scale reveals the need when you're scraping hundreds of thousands of pages daily. A 2% error rate across a million records means 20,000 bad data points flowing into your systems. You can't manually review that volume.

Reliability needs emerge when manual data checking becomes impossible. You need automated validation you can trust, running on every extraction, catching problems immediately.

For teams building web agent infrastructure, rule-based validation is the foundation layer. It catches structural breaks reliably and cheaply. But it's not the complete picture, because websites don't just break. They also drift, and drift is harder to catch.