leptos-rs/leptos · error

Encoding not found.

Error message

Encoding not found.

What it means

The `encoding` option (or third positional string) is validated against the set of built-in encodings known to the macro (e.g. Url, GetJson, PostJson, PostCbor, etc., matched case-insensitively). Any other value fails this check with 'Encoding not found.' at the encoding literal's span.

Source

Thrown at server_fn_macro/src/lib.rs:1230

                        ));
                    }
                }
            } else if lookahead.peek(LitStr) {
                if use_key_and_value {
                    return Err(syn::Error::new(
                        stream.span(),
                        "If you use keyword arguments (e.g., `name` = \
                         Something), then you can no longer use arguments \
                         without a keyword.",
                    ));
                }
                match arg_pos {
                    1 => return Err(lookahead.error()),
                    2 => prefix = Some(stream.parse()?),
                    3 => encoding = Some(stream.parse()?),
                    4 => fn_path = Some(stream.parse()?),
                    _ => {
                        return Err(syn::Error::new(
                            stream.span(),
                            "unexpected extra argument",
                        ))
                    }
                }
            } else {
                return Err(lookahead.error());
            }

            if !stream.is_empty() {
                stream.parse::<Token![,]>()?;
            }
        }

        // parse legacy encoding into input/output
        let mut builtin_encoding = false;
        if let Some(encoding) = encoding {
            match encoding.value().to_lowercase().as_str() {

View on GitHub (pinned to 32d20f6c9d)

Solutions

1. Use one of the supported builtin encoding strings (Url, GetText, PostText, GetBinary, PostBinary, GetJson, PostJson, PostCbor, GetCBOR-style idents are matched case-insensitively).
2. For non-builtin serialization, omit `encoding` and specify `input`/`output` (and optionally `custom`) types instead.
3. Check spelling/case and any recent renames of encodings in the crate docs.

Example fix

// before
#[server(encoding = "msgpack")]
// after
#[server(encoding = "PostCbor")]
// or for custom:
// #[server(input = MyInput, output = MyOutput)]

const BUILTIN_ENCODINGS: &[&str] = &["url","gettext","posttext","getbinary","postbinary","getjson","postjson","postcbor"];
let enc = "msgpack";
assert!(BUILTIN_ENCODINGS.contains(&enc.to_lowercase().as_str()), "unsupported encoding: {enc}");

When it happens

Trigger: #[server(encoding = "json")] (lowercase name not in the builtin list); misspelling like 'PostJSon' variants that don't match; passing an encoding name from a different framework; using a custom encoding that requires input/output/custom instead.

Common situations: Copying encodings from other libraries (e.g. 'msgpack' when only Cbor is builtin); typos in the encoding string; switching to custom serialization without switching options.

Related errors

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

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/0164ff05c9907f9c. Report an issue: GitHub.