DioxusLabs/dioxus · error · syn::Error

keyword argument repeated: `encoding`

Error message

keyword argument repeated: `encoding`

What it means

`encoding` is the legacy shortcut in `#[server(...)]` that sets both input and output codecs (e.g. `encoding = postcard`). It may be specified only once; the parser rejects a second `encoding = ...` (packages/fullstack-macro/src/lib.rs:1572-1578).

Source

Thrown at packages/fullstack-macro/src/lib.rs:1574

                    if key == "name" {
                        if struct_name.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `name`",
                            ));
                        }
                        struct_name = Some(stream.parse()?);
                    } else if key == "prefix" {
                        if prefix.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `prefix`",
                            ));
                        }
                        prefix = Some(stream.parse()?);
                    } else if key == "encoding" {
                        if encoding.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `encoding`",
                            ));
                        }
                        encoding = Some(stream.parse()?);
                    } else if key == "endpoint" {
                        if fn_path.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `endpoint`",
                            ));
                        }
                        fn_path = Some(stream.parse()?);
                    } else if key == "input" {
                        if encoding.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "`encoding` and `input` should not both be \

View on GitHub (pinned to 393d190a80)

Solutions

1. Delete the duplicate so one `encoding = ...` remains.
2. For mixed codecs, drop `encoding` entirely and use `input = Json, output = Postcard` (one each).
3. Prefer the explicit `input`/`output` form in new code; it composes and errors like this one disappear.

Example fix

// before
#[server(encoding = Json, encoding = Postcard)]
async fn ping() -> i32 { ... }

// after
#[server(input = Json, output = Postcard)]
async fn ping() -> i32 { ... }

# Reuse the duplicate-key checker from index 93; encoding is in its key list.

When it happens

Trigger: `#[server(encoding = Json, encoding = Postcard)]`; adding `encoding` when migrating an old attribute that already had one; trying to use different codecs for request vs response via two `encoding` keys.

Common situations: Upgrading old server functions that used `encoding` while also adding a new one; misunderstanding that per-direction configuration uses `input = ...` / `output = ...`, not repeated `encoding`.

Related errors

• `encoding` and `input` should not both be specified
• keyword argument repeated: `input`
• keyword argument repeated: `name`
• keyword argument repeated: `prefix`
• keyword argument repeated: `endpoint`

AI-assisted analysis of DioxusLabs/dioxus@393d190a80 (2026-08-16). Data as JSON: /api/errors/079a3530810f7689. Report an issue: GitHub.