actix/actix-web · warning · Error

Required field is missing "{_0}"

Error message

Required field is missing "{_0}"

What it means

This is `actix_multipart::form::Error::MissingField(String)` raised by actix-multipart's typed form deserialization (`MultipartForm`). It means a required (non-Option) struct field did not receive a corresponding multipart part in the submitted body. The library cannot construct the target struct, so it fails the request with this validation error.

Source

Thrown at actix-multipart/src/error.rs:91

    #[display("Stream is not consumed")]
    NotConsumed,

    /// Form field handler raised error.
    #[display("An error occurred processing field \"{name}\"")]
    Field {
        name: String,
        source: actix_web::Error,
    },

    /// Duplicate field found (for structure that opted-in to denying duplicate fields).
    #[display("Duplicate field found \"{_0}\"")]
    #[from(ignore)]
    DuplicateField(#[error(not(source))] String),

    /// Required field is missing.
    #[display("Required field is missing \"{_0}\"")]
    #[from(ignore)]
    MissingField(#[error(not(source))] String),

    /// Unknown field (for structure that opted-in to denying unknown fields).
    #[display("Unknown field \"{_0}\"")]
    #[from(ignore)]
    UnknownField(#[error(not(source))] String),
}

impl ResponseError for Error {
    fn status_code(&self) -> StatusCode {
        match &self {
            Error::Field { source, .. } => source.as_response_error().status_code(),
            Error::ContentTypeIncompatible => StatusCode::UNSUPPORTED_MEDIA_TYPE,
            _ => StatusCode::BAD_REQUEST,
        }
    }
}

#[cfg(test)]

View on GitHub (pinned to c215607f4b)

Solutions

1. Make the field optional in the struct (`Option<T>`) if the client is allowed to omit it.
2. Fix the client form so every required field's `name` matches the Rust struct field (or `#[multipart(rename = "...")]`).
3. Verify the request uses `Content-Type: multipart/form-data` with a proper boundary and that the part name matches exactly (case-sensitive).
4. Test with curl `-F name=value` to reproduce and confirm which part the server expects.

Example fix

// before
struct Form { title: String } // client sometimes omits 'title'
// after
struct Form { title: Option<String> } // or send the part: curl -F title=hello ...

// client-side, before submit
const required = ["title", "file"];
const missing = required.filter((n) => !formData.has(n));
if (missing.length) throw new Error(`missing multipart fields: ${missing.join(", ")}`);

// server-side: mark truly-optional fields as Option<T>
fn is_optional<T>(_: &Option<T>) -> bool { true }

match form_result {
    Err(e) if e.to_string().contains("Required field is missing") =>
        ErrHttpResponse::build(StatusCode::BAD_REQUEST).body("missing required form field"),
    other => other,
}

When it happens

Trigger: A `#[derive(MultipartForm)]` struct has a non-Option field, and `MultipartForm::from_request` finishes consuming parts without ever seeing a part with that field's name.

Common situations: Client form omitted an input the server requires, the input's `name` attribute does not match the Rust field name, curl/tests built manually without all parts, or the client sent JSON instead of multipart/form-data.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Duplicate field found "{_0}"
• Unknown field "{_0}"
• `MultipartForm` can only be derived for a struct with named
• Could not parse size limit `{}`: {}
• Multiple fields named: `{}`

AI-assisted analysis of actix/actix-web@c215607f4b (2026-09-09). Data as JSON: /api/errors/6b80b99367f0b01a. Report an issue: GitHub.