---
title: "Parse input once at the boundary · Agent Wranglers"
url: https://agentwranglers.se/playbook/architecture/data-shapes/parse-at-the-boundary
updated: 2026-10-20
licence: "All rights reserved"
---

# Parse input once at the boundary

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

- Level: Required
- Chapter: [Data shapes](https://agentwranglers.se/playbook/architecture/data-shapes)
- Rule: Turn outside input into a typed, valid object as soon as it arrives. Code after that point trusts the type and does not check again.
- How: Use a schema validator, such as zod, pydantic or Symfony Validator, in the controller, queue consumer or file reader. Reject bad input there with a clear error. Pass only the parsed object onward.
- Check: Each request handler calls a parser before any other logic. A test sends a request body with a missing and a wrong-typed field. It expects a 422 answer that names both fields, as in [validation errors](https://agentwranglers.se/playbook/api-design/errors/validation-errors).

Example:

```python
# Bad: every function re-checks the raw dictionary
def book(data: dict):
    if "email" not in data: raise ValueError("email")

# Good: parse once, then trust the type
class BookingRequest(BaseModel):
    email: EmailStr
    start: datetime

def book(request: BookingRequest): ...
```

```ts
import { z } from 'zod';

// Good: parse once, then trust the type
const BookingRequest = z.object({ email: z.email(), start: z.coerce.date() });
type BookingRequest = z.infer<typeof BookingRequest>;

function book(request: BookingRequest) { /* ... */ }
book(BookingRequest.parse(body));
```

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers architecture playbook: https://agentwranglers.se/playbook/architecture/data-shapes/parse-at-the-boundary`.

- Previous: [Convert to arrays and JSON only at the edge](https://agentwranglers.se/playbook/architecture/data-shapes/convert-at-the-edge)
- Next: [Make data objects immutable](https://agentwranglers.se/playbook/architecture/data-shapes/immutable-data)
- [Data shapes](https://agentwranglers.se/playbook/architecture/data-shapes)