Handle errors

Goal: return the right HTTP status and JSON error body from a handler, without hand-writing impl IntoResponse yourself.

This assumes a working controller — see Add a controller. Every handler that returns loco_rs::Result<T> (i.e. Result<T, Error>) gets its error automatically converted to an HTTP response by impl IntoResponse for Error — you never call .into_response() on an error yourself. For the exhaustive variant list and constructors, see the Error model reference.

1. The three common-case helpers

loco_rs::prelude re-exports three free functions for the HTTP-facing error variants you’ll reach for most:

use loco_rs::prelude::*;

async fn get_one(Path(id): Path<i64>, State(ctx): State<AppContext>) -> Result<Response> {
    let Some(item) = find_item(&ctx, id).await? else {
        return not_found();
    };
    format::json(item)
}

async fn login(State(ctx): State<AppContext>, Json(params): Json<LoginParams>) -> Result<Response> {
    let Ok(user) = find_user(&ctx, &params.email).await else {
        return unauthorized("invalid credentials");
    };
    // ...
    format::json(user)
}

async fn create(Json(params): Json<CreateParams>) -> Result<Response> {
    if params.title.is_empty() {
        return bad_request("title is required");
    }
    // ...
    format::empty()
}

Each returns Result<U> (always the Err arm), so return unauthorized(msg) type-checks against any handler’s Result<Response> return type. All three are also just regular ways to construct an Error and propagate it with ? from a helper function you call from the handler.

Fn	HTTP status	Response body	Notes
`not_found()`	404	`{"error":"not_found","description":"Resource was not found"}`	Takes no message.
`unauthorized(msg)`	401	`{"error":"unauthorized","description":"You do not have permission to access this resource"}`	`msg` is logged (`tracing::warn!`) but not sent to the client.
`bad_request(msg)`	400	`{"error":"Bad Request","description":"<msg>"}`	`msg` is sent to the client.

2. Everything else falls through to 500

Error is #[non_exhaustive] with ~28 more variants (DB, Model, IO, Tera, Message, InternalServerError, …). Only seven variants get a specific status; the framework’s IntoResponse match ends in a wildcard arm — every other variant becomes 500 Internal Server Error with body {"error":"internal_server_error","description":"Internal Server Error"}.

In practice this means: if you ?-propagate a sea_orm::DbErr, an std::io::Error, or anything else that converts into Error via #[from], and you haven’t matched it explicitly, the client gets a generic 500 — which is usually what you want (don’t leak internals), and every response is logged at tracing::error! first regardless of variant, so you still see the real cause server-side.

The full variant → status table, including Validation (→ 400, from the validator crate) and JsonRejection (→ axum’s own rejection status), is in the Error model reference.

3. Return an arbitrary status: `Error::CustomError`

When none of the built-in helpers fit — a 409 Conflict, a 429 Too Many Requests, a body shape the framework doesn’t produce — build one directly:

use loco_rs::prelude::*;
use loco_rs::controller::ErrorDetail;
use axum::http::StatusCode;

async fn create(Json(params): Json<CreateParams>) -> Result<Response> {
    if already_exists(&params).await? {
        return Err(Error::CustomError(
            StatusCode::CONFLICT,
            ErrorDetail::new("conflict", "a resource with this name already exists"),
        ));
    }
    format::empty()
}

Error::CustomError(StatusCode, ErrorDetail) passes both the status and the body through unchanged — it’s the one variant the response mapping doesn’t rewrite. ErrorDetail::new(error, description) sets both fields (an empty description collapses to None); ErrorDetail::with_reason(error) sets only error. The response body is always {error, description, errors} with None fields omitted from the JSON.

4. Convert a foreign error without a dedicated variant

Use Error::wrap/Error::msg at a ?/.map_err(..) call site to fold any std::error::Error into Error::Any/Error::Message (both fall through to the 500 catch-all above):

let parsed: MyType = serde_json::from_str(&raw).map_err(Error::wrap)?;

Reach for Error::string("...") when you have a plain &str/message and no source error to wrap.

5. Content-type-aware error handling

If an endpoint needs to render errors differently for HTML vs. JSON clients, match on both the fallible call’s Result and the negotiated format in one place — see Respond with different formats.

Verify

curl -i localhost:5150/notes/999999   # -> 404 {"error":"not_found",...}
curl -i -X POST localhost:5150/auth/login -d '{"email":"x","password":"y"}' -H 'content-type: application/json'
# -> 401 {"error":"unauthorized",...}

Validate requests — the Validation variant and structured field errors
Respond with different formats
Error model reference — full variant list and constructors