# 8. The client side: decoding problems with `RestClient`

[← 7. Silent 500s](07-silent-500s.md) · [Index](../README.md)

`RestClient`'s default status handler throws `HttpClientErrorException` /
`HttpServerErrorException`; `getResponseBodyAs(ProblemDetail.class)` decodes the body with the
client's converters. [`/diag/decode`](../src/main/java/com/ankurm/problems/diag/DiagController.java)
reports what came back ([`client-decoding.txt`](output/client-decoding.txt)).

## What works

Extension members land in `getProperties()` - `orderId`, the `errors` list, `errorId`. The mixin
works in both directions.

## Two things to code defensively against

**`getType()` is `null` when the server omitted it**, which Spring Framework 7 servers do for every
framework error ([chapter 4](04-i18n-and-types.md)). Treat `null` as `about:blank`.

**Decoding succeeds on a body that is not a problem at all.** Against the `defaults` profile the
server sends Boot's `application/json` error, and it still decodes:

```
    "exception": "org.springframework.web.client.HttpServerErrorException$InternalServerError",
    "status": 500,
    "contentType": "application/json",
    "problemDetail": {
        "type": "null",
        "title": "Internal Server Error",
        "status": 500,
        "detail": null,
        "instance": "null",
        "properties": {
            "timestamp": "2026-09-11T17:05:01.408Z",
            "error": "Internal Server Error",
            "path": "/orders/999"
        }
    }
```

Unknown members go into `properties`, so any JSON object "is" a `ProblemDetail`. Check the
`Content-Type` is `application/problem+json` before believing you have one.