What causes "`encoding` and `input` should not both be specified" in leptos-rs/leptos?

Remove the `encoding = ...` argument and keep `input = ...` for a custom input type.

`encoding` and `input` should not both be specified

Error message

`encoding` and `input` should not both be specified

What it means

The #[server] macro rejects specifying both `encoding` and `input` because they configure the same thing (how the request body is deserialized) through overlapping mechanisms: `encoding` picks a built-in encoding, while `input` gives a fully custom input type. Allowing both would be ambiguous, so the parser errors at compile time.

Source

Thrown at server_fn_macro/src/lib.rs:1113

                    } 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 \
                                 specified",
                            ));
                        } else if input.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `input`",
                            ));
                        }
                        input = Some(stream.parse()?);
                    } else if key == "input_derive" {
                        if input_derive.is_some() {
                            return Err(syn::Error::new(
                                key.span(),
                                "keyword argument repeated: `input_derive`",
                            ));
                        }

View on GitHub (pinned to 32d20f6c9d)

Solutions

Remove the `encoding = ...` argument and keep `input = ...` for a custom input type.
Alternatively remove `input` and keep `encoding` to use a built-in encoding.
Note the companion check: `output` is likewise mutually exclusive with `encoding`.

Example fix

// before
#[server(encoding = "Json", input = MyInput)]
async fn save(data: MyInput) -> Result<(), ServerFnError> { todo!() }

// after
#[server(input = MyInput)]
async fn save(data: MyInput) -> Result<(), ServerFnError> { todo!() }

Defensive patterns

Strategy: validation

Validate before calling

// Reject attributes mixing `encoding` with `input`/`output`:
// grep -rnE '#\[server\([^)\n]*encoding[^)\n]*(input|output)\s*=' src/ && exit 1 || exit 0

Prevention

Remember the rule: `encoding` XOR (`input` and/or `output`).
When migrating to custom types, delete the `encoding` key in the same change.
Check library upgrade notes before porting old attribute signatures.
Let cargo check fail loudly; never suppress macro errors with #[allow].

When it happens

Trigger: Writing #[server(encoding = "...", input = ...)] in the same attribute, e.g. mixing a legacy `encoding` arg with a newer `input` custom-type arg after a migration.

Common situations: Migrating from the older encoding API to custom input/output types and leaving the old `encoding` key in place; cargo expansions after upgrading server_fn versions; examples from docs that mix both styles.

Related errors

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/694ca3ac3bf1833b. Report an issue: GitHub.