bug: POST /api/v1/quote returns 400 for invalid JSON body instead of 422

[ChainsQueen]

Problem

Sending an invalid JSON body to the quote endpoint returns
400 instead of 422. The OpenAPI spec defines 422 as the
correct status code for unparseable JSON bodies.

Impact

Low — does not block core functionality but causes confusion
for SDK clients and integrators who follow the OpenAPI spec
to handle errors correctly.

To reproduce

1. POST https://api.cow.fi/sepolia/api/v1/quote
2. Header: Content-Type: application/json
3. Body: {broken json
4. Observe 400 Bad Request

Expected behaviour

Status: 422
Per OpenAPI spec:
422 → Unable to parse request body as valid JSON

Actual behaviour

Status: 400 Bad Request
Content-Type: text/plain; charset=utf-8
Body: Failed to parse the request body as JSON:
key must be a string at line 1 column 2

Screenshots/logs

See attached Postman screenshot showing the 400 response.

services version/commit hash and environment

Version: v2.362.3-dirty
Environment: production (https://api.cow.fi/sepolia)

Additional context

Reference: https://docs.cow.fi/cow-protocol/reference/apis/orderbook

[4ddx]

Hi @ChainsQueen
This looks like a basic fix of changing return status on that API, do you mind if I work on this?

Thanks,
Ishan