seanmonstar/warp · error

illegal Method

Error message

illegal Method

What it means

The `allow_method` builder on the CORS filter converts the given value into `http::Method`; if the conversion fails (e.g. an invalid method string), the library panics with "illegal Method" because the CORS builder API is meant to be configured with statically valid methods at startup.

Solutions

1. Validate the method string before passing it: it must be a valid HTTP token (e.g. "GET", "POST", "PUT", "DELETE", "PATCH", "HEAD", "OPTIONS").
2. Use `http::Method::try_from(s)` yourself first, or pass `http::Method` constants directly (`http::Method::GET`) to guarantee success.
3. If methods come from config, filter/validate the list at load time and fail fast with a clear config error instead of a panic in the builder.
4. Fix typos or stray whitespace/uppercase issues in the configured method names.

Example fix

// before
let cors = warp::cors().allow_method(cfg.method_str); // panics if invalid

// after
let method: http::Method = cfg
    .method_str
    .parse()
    .expect("CORS method must be a valid HTTP method like GET or POST");
let cors = warp::cors().allow_method(method);

fn is_valid_method(s: &str) -> bool {
    http::Method::try_from(s).is_ok()
}
// before calling: assert!(is_valid_method(cfg.method_str));

fn as_method(s: &str) -> Option<http::Method> {
    http::Method::try_from(s).ok()
}

// panic-based API; validate instead of catching:
let m = http::Method::try_from(input)
    .map_err(|e| format!("invalid CORS method '{}': {}", input, e))?;
let cors = warp::cors().allow_method(m);

When it happens

Trigger: Calling `warp::cors().allow_method(m)` where `m` cannot be converted to `http::Method` — most commonly a `&str` that is not a valid HTTP method token (e.g. `allow_method("GET-POST")`, empty string, or a string with illegal characters).

Common situations: Reading allowed methods from config/env where values are typos or contain whitespace, dynamically building method lists from user input, version changes in method parsing rules, passing lowercase or malformed tokens.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• illegal Header
• invalid Origin
• invalid header name
• invalid header value
• Origin is always a valid HeaderValue

AI-assisted analysis of seanmonstar/warp@ff34d7213e (2026-09-09). Data as JSON: /api/errors/e495c1e582a16a1b. Report an issue: GitHub.

Appendix: source

Thrown at src/filters/cors.rs:91

impl Builder {
    /// Sets whether to add the `Access-Control-Allow-Credentials` header.
    pub fn allow_credentials(mut self, allow: bool) -> Self {
        self.credentials = allow;
        self
    }

    /// Adds a method to the existing list of allowed request methods.
    ///
    /// # Panics
    ///
    /// Panics if the provided argument is not a valid `http::Method`.
    pub fn allow_method<M>(mut self, method: M) -> Self
    where
        http::Method: TryFrom<M>,
    {
        let method = match TryFrom::try_from(method) {
            Ok(m) => m,
            Err(_) => panic!("illegal Method"),
        };
        self.methods.insert(method);
        self
    }

    /// Adds multiple methods to the existing list of allowed request methods.
    ///
    /// # Panics
    ///
    /// Panics if the provided argument is not a valid `http::Method`.
    pub fn allow_methods<I>(mut self, methods: I) -> Self
    where
        I: IntoIterator,
        http::Method: TryFrom<I::Item>,
    {
        let iter = methods.into_iter().map(|m| match TryFrom::try_from(m) {
            Ok(m) => m,
            Err(_) => panic!("illegal Method"),

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