Validate Backend Inputs

Purpose

Standardize how backend services validate untrusted inputs at the HTTP boundary and how they communicate validation failures to clients.

When to use

Use this skill when you are:

• •Adding or modifying endpoint request shapes
• •Introducing new query parameters or path params
• •Implementing partial update endpoints (PATCH/PUT)
• •Debugging validation errors or inconsistent error responses

Inputs

• •The endpoint contract (params/query/body)
• •Validation rules (required fields, formats, min/max, allowed enums)
• •Desired error response shape for invalid input

Outputs

• •Validation schemas for request inputs
• •A consistent validation error response mapping (400 / 422 depending on your convention)
• •Examples of valid and invalid payloads

Rules

• •All external inputs MUST be validated (params, query, body).
• •Validation MUST happen before calling the service layer.
• •Validation failures MUST return consistent status codes and error shapes.
• •Validation logic SHOULD be centralized and reusable to avoid divergence.

Steps

1. •Define request DTO(s) and choose a schema library (or built-in validation).
2. •Implement schemas for:
   • •params
   • •query
   • •body
3. •Parse/validate at controller boundary.
4. •Convert schema errors to your standard error response.
5. •Add:
   • •one example valid payload
   • •one example invalid payload
6. •Verify by hitting the endpoint with both payloads.

Common patterns

• •

  Create vs update schemas

  • •Create schemas typically require fields.
  • •Update schemas often use .partial() or optional fields.
• •

  Enums

  • •Prefer explicit enums instead of free-form strings.
• •

  Discriminated unions

  • •Use when request shape varies by a type field.

Verification

• • Valid payloads pass validation and reach the service layer
• • Invalid payloads return consistent 400/422 with structured error details
• • All external inputs (params, query, body) are validated
• • Validation errors include field name and failure reason
• • Schemas match API documentation / OpenAPI spec
• • At least one valid and one invalid payload example exist

Boundaries

• •MUST NOT skip validation for any external input
• •MUST NOT call service layer with unvalidated data
• •MUST NOT expose internal validation library errors directly to clients
• •MUST NOT use free-form strings where enums are appropriate
• •SHOULD NOT duplicate validation logic across endpoints (centralize schemas)
• •SHOULD NOT validate business rules in the validation layer (delegate to services)

Included assets

• •Templates: ./templates/ includes common schema patterns and a validation error formatter.
• •Examples: ./examples/ includes DTO + schema examples with valid/invalid payloads.