---
name: input-validation
description: Validate and constrain untrusted input at the boundary. Use on any handler that accepts external data.
when_to_use: request bodies, query params, file uploads, webhook payloads, form data
---
# Input Validation
Validate at the edge, before the data touches logic or storage.
- **Schema** — type, required fields, allowed values. Reject unknown fields rather than ignoring them.
- **Bounds** — string length, number ranges, array size. An unbounded input is a DoS and a memory bomb.
- **Format** — emails, UUIDs, dates parsed and re-validated, not trusted as strings.
- **Files** — size limit, type allowlist (check content, not just extension), no path traversal in names.
- Reject with a clear 4xx and a message that says what's wrong — without leaking internals.
Never trust "it comes from our own frontend". The request can come from anywhere.