leviath-mcp 0.3.6

//! OAuth 2.1 for MCP servers: browser login, token exchange, and refresh.
//!
//! MCP servers advertise themselves as OAuth *public clients* (no secret), so
//! this implements the authorization-code flow with PKCE (RFC 7636) plus
//! dynamic client registration (RFC 7591) and resource indicators (RFC 8707).
//! A standards-correct implementation needs no per-server code.
//!
//! The interactive flow (browser + loopback redirect) lives in
//! `OAuthClient::login`; `OAuthClient::refresh` is non-interactive so a
//! background process can keep a session alive without ever opening a browser.

// `pub(crate)` so the transport can reuse `same_origin` and
// `is_safe_discovery_url` rather than growing a second opinion about what "the
// server's own origin" means. The OAuth chain and the transport have to agree:
// they are guarding the same token against the same server.
pub(crate) mod metadata;
mod pkce;
pub mod store;

use std::collections::HashMap;
use std::time::Duration;

use reqwest::Url;
use serde::Deserialize;
use tokio::net::TcpListener;

use metadata::{AuthServerMetadata, ProtectedResourceMetadata};
use pkce::Pkce;
pub use store::{AuthStore, ServerAuth};

/// How the browser gets opened. Injected so tests never launch one.
///
/// A boxed closure, not a bare `fn` pointer, so a platform binding can capture
/// context: desktop passes `Arc::new(leviath_sys::open_url)`, a future mobile
/// entry point passes an `Arc::new(move |url| ...)` closing over its native
/// handle (an Android `Activity`, an iOS callback), and tests pass a stub that
/// drives the callback directly. `Send + Sync` because it may be invoked from
/// an async task. Returns whether the launcher spawned.
pub type BrowserOpener = std::sync::Arc<dyn Fn(&str) -> bool + Send + Sync>;

/// How long to wait for the user to finish authorizing in the browser.
const CALLBACK_TIMEOUT: Duration = Duration::from_secs(300);

/// The scopes requested when the server advertises none.
const DEFAULT_SCOPES: &str = "openid profile email";

/// The token endpoint's response.
#[derive(Debug, Deserialize)]
struct TokenResponse {
    access_token: String,
    #[serde(default)]
    refresh_token: Option<String>,
    #[serde(default)]
    expires_in: Option<u64>,
    #[serde(default)]
    scope: Option<String>,
}

/// A dynamic client registration response (only the id is needed).
#[derive(Debug, Deserialize)]
struct RegistrationResponse {
    client_id: String,
}

/// What an MCP endpoint said when asked, with the configured headers, whether
/// it wants credentials.
enum Probe {
    /// It asked for credentials: a `401`/`403`, a `WWW-Authenticate`, or both.
    /// The header, when present, names where the metadata lives.
    Challenge(Option<String>),
    /// It answered the request as it stood, so nothing needs logging in. Either
    /// the server is open, or the headers already configured for it are enough.
    Satisfied,
    /// It did not answer at all. Says nothing either way, so the caller carries
    /// on to discovery, where a failure produces a message worth reading.
    Unreachable,
}

#[cfg(test)]
impl Probe {
    /// A one-word name for the variant, so a test can assert on it with
    /// `assert_eq!` rather than a `matches!` whose other arm never runs.
    fn label(&self) -> &'static str {
        match self {
            Self::Challenge(_) => "challenge",
            Self::Satisfied => "satisfied",
            Self::Unreachable => "unreachable",
        }
    }
}

/// The result of [`OAuthClient::login`].
#[derive(Debug)]
pub enum LoginOutcome {
    /// The browser flow ran and produced credentials worth storing.
    ///
    /// Boxed because it is much larger than the other variant, and every caller
    /// stores it rather than reading it in place.
    Authenticated(Box<ServerAuth>),
    /// The server does not want an OAuth login. There is nothing to store, and
    /// nothing went wrong.
    NotRequired,
}

impl LoginOutcome {
    /// The credentials this login produced, or `None` if none were needed.
    ///
    /// For a caller that only wants to store what came back and has nothing to
    /// say about the other case.
    pub fn authenticated(self) -> Option<ServerAuth> {
        match self {
            Self::Authenticated(auth) => Some(*auth),
            Self::NotRequired => None,
        }
    }
}

/// Drives OAuth against one MCP server's authorization server.
pub struct OAuthClient {
    http: reqwest::Client,
}

impl Default for OAuthClient {
    fn default() -> Self {
        Self::new()
    }
}

impl OAuthClient {
    /// Build a client with sensible network timeouts.
    pub fn new() -> Self {
        let http = reqwest::Client::builder()
            .connect_timeout(Duration::from_secs(30))
            .timeout(Duration::from_secs(60))
            .build()
            .expect("failed to build reqwest client");
        Self { http }
    }

    /// Run the full interactive login for `mcp_url`.
    ///
    /// Returns [`LoginOutcome::NotRequired`] when the server answers a probe
    /// carrying `headers` without demanding credentials. A server configured
    /// with its own API token is the ordinary case: there is no OAuth flow to
    /// run, and pushing one anyway is what sent every such server into a
    /// discovery request it does not serve.
    ///
    /// `now` (Unix seconds) is passed in rather than read from the clock so the
    /// computed `expires_at` is deterministic under test. `reuse_client_id`
    /// short-circuits dynamic registration when a previous login already
    /// registered this client with the authorization server.
    pub async fn login(
        &self,
        mcp_url: &str,
        headers: &HashMap<String, String>,
        allow_env: &[String],
        opener: BrowserOpener,
        now: u64,
        reuse_client_id: Option<&str>,
    ) -> anyhow::Result<LoginOutcome> {
        let mcp = Url::parse(mcp_url)
            .map_err(|e| anyhow::anyhow!("Invalid MCP server url '{}': {}", mcp_url, e))?;

        // An unreachable server still goes down the discovery path, which is
        // where the useful error message comes from. Only a live answer that
        // asked for nothing ends the flow here.
        let www_authenticate = match self.probe_challenge(&mcp, headers, allow_env).await {
            Probe::Satisfied => return Ok(LoginOutcome::NotRequired),
            Probe::Challenge(header) => header,
            Probe::Unreachable => None,
        };

        let (resource, server_meta) = self.discover(&mcp, www_authenticate.as_deref()).await?;

        // Bind the loopback listener first, so its port is known before both
        // registration (which needs the redirect URI) and the authorize URL.
        // Binding an OS-assigned loopback port does not fail in practice; a
        // failure here would mean the machine has no working loopback stack.
        let listener = TcpListener::bind("127.0.0.1:0")
            .await
            .expect("binding an ephemeral loopback port cannot fail");
        let port = listener
            .local_addr()
            .expect("a bound listener always has a local address")
            .port();
        let redirect_uri = format!("http://127.0.0.1:{port}/callback");

        let client_id = match reuse_client_id {
            Some(id) => id.to_string(),
            None => self.register(&server_meta, &redirect_uri).await?,
        };

        let pkce = Pkce::generate();
        let scope = if server_meta.scopes_supported.is_empty() {
            DEFAULT_SCOPES.to_string()
        } else {
            server_meta.scopes_supported.join(" ")
        };
        // Already validated in `validate_auth_server_metadata`; `.expect` rather
        // than a second fallible parse, which could not fail.
        let authorize_endpoint = Url::parse(&server_meta.authorization_endpoint)
            .expect("the authorization endpoint was parsed during metadata validation");
        let authorize_url = build_authorize_url(
            &authorize_endpoint,
            &client_id,
            &redirect_uri,
            &pkce,
            &scope,
            &resource,
        );

        // Always print the URL: on a headless or SSH session the browser can't
        // open, and the user needs to paste it themselves.
        println!("Opening your browser to authorize:\n  {authorize_url}");
        if !(*opener)(authorize_url.as_str()) {
            println!("(couldn't open a browser automatically - open the link above)");
        }

        let code = wait_for_callback(listener, &pkce.state, CALLBACK_TIMEOUT).await?;

        let token = self
            .exchange_code(
                &server_meta.token_endpoint,
                &client_id,
                &redirect_uri,
                &code,
                &pkce.verifier,
                &resource,
            )
            .await?;

        Ok(LoginOutcome::Authenticated(Box::new(build_server_auth(
            resource,
            &server_meta,
            client_id,
            token,
            now,
        ))))
    }

    /// Refresh `auth` non-interactively. Never opens a browser.
    pub async fn refresh(&self, auth: &ServerAuth, now: u64) -> anyhow::Result<ServerAuth> {
        let refresh_token = auth
            .refresh_token
            .as_deref()
            .ok_or_else(|| anyhow::anyhow!("no refresh token available"))?;

        let params = [
            ("grant_type", "refresh_token"),
            ("refresh_token", refresh_token),
            ("client_id", auth.client_id.as_str()),
            ("resource", auth.resource.as_str()),
        ];
        let value = self
            .post_form(&auth.token_endpoint, &params)
            .await
            .map_err(|e| anyhow::anyhow!("token refresh failed: {}", e))?;
        let token: TokenResponse = serde_json::from_value(value)
            .map_err(|e| anyhow::anyhow!("could not parse token response: {}", e))?;

        let mut refreshed = auth.clone();
        refreshed.access_token = token.access_token;
        // A refresh may or may not rotate the refresh token; keep the old one
        // if the server did not send a new one.
        if let Some(new_refresh) = token.refresh_token {
            refreshed.refresh_token = Some(new_refresh);
        }
        refreshed.expires_at = expires_at(token.expires_in, now);
        if let Some(scope) = token.scope {
            refreshed.scope = scope;
        }
        Ok(refreshed)
    }

    /// Resolve the `Authorization` header for a stored server, refreshing the
    /// token first if it is at or near expiry.
    ///
    /// Non-interactive: a dead refresh returns an error naming the login
    /// command rather than opening a browser, so the daemon can call this
    /// safely. A refreshed token is written back to `store_path`. Returns
    /// `None` when the server has no stored auth (e.g. an unauthenticated
    /// server, or one using a static header).
    pub async fn authorization_header(
        &self,
        server_name: &str,
        store_path: &std::path::Path,
        now: u64,
    ) -> anyhow::Result<Option<(String, String)>> {
        self.authorization_header_with(server_name, store_path, now, None)
            .await
    }

    /// [`authorization_header`](Self::authorization_header) reading and writing
    /// grants through `credentials` - the OS credential store, when
    /// `[security] credential_store = "keychain"` is set.
    ///
    /// `None` is the file backend. A refreshed token is written back through the
    /// same backend it was read from, so a refresh in keychain mode does not
    /// quietly land the new refresh token on disk.
    pub async fn authorization_header_with(
        &self,
        server_name: &str,
        store_path: &std::path::Path,
        now: u64,
        credentials: Option<&dyn leviath_core::CredentialStore>,
    ) -> anyhow::Result<Option<(String, String)>> {
        let mut store = AuthStore::load_with(store_path, credentials)?;
        let Some(auth) = store.get(server_name) else {
            return Ok(None);
        };

        let token = if auth.is_expired_at(now) {
            let refreshed = self.refresh(auth, now).await.map_err(|e| {
                anyhow::anyhow!(
                    "MCP server '{server_name}' token expired and could not be \
                     refreshed ({e}); re-authenticate with `lev mcp login {server_name}`"
                )
            })?;
            let access = refreshed.access_token.clone();
            store.set(server_name, refreshed);
            store.save_with(store_path, credentials)?;
            access
        } else {
            auth.access_token.clone()
        };

        Ok(Some((
            "Authorization".to_string(),
            format!("Bearer {token}"),
        )))
    }

    /// Discover the resource identifier and authorization-server metadata.
    async fn discover(
        &self,
        mcp: &Url,
        www_authenticate: Option<&str>,
    ) -> anyhow::Result<(String, AuthServerMetadata)> {
        let hinted = metadata::resource_metadata_url(www_authenticate);
        // The hint comes out of a header the *remote server* wrote, and whatever
        // it names is then fetched by us, from inside the user's network. Bind it
        // to the MCP server's own origin: a server may point at its own metadata
        // document, which is the legitimate use, and may not point at anything
        // else. Without this, connecting to a hostile MCP server was enough to
        // make Leviath fetch an arbitrary URL - cloud metadata included.
        let resource_meta_url = match hinted {
            Some(hint) => {
                let parsed = Url::parse(&hint)
                    .map_err(|e| anyhow::anyhow!("invalid resource_metadata URL '{hint}': {e}"))?;
                if !metadata::same_origin(&parsed, mcp) {
                    anyhow::bail!(
                        "MCP server at {mcp} pointed resource_metadata at a different origin \
                         ({parsed}) - refusing to follow it"
                    );
                }
                parsed
            }
            None => metadata::well_known_resource_url(mcp),
        };
        self.require_safe_discovery_url(&resource_meta_url)?;

        let value = self
            .get_json(resource_meta_url.as_str())
            .await
            .map_err(|e| anyhow::anyhow!("failed to fetch resource metadata: {}", e))?;
        let resource_meta: ProtectedResourceMetadata = serde_json::from_value(value)
            .map_err(|e| anyhow::anyhow!("failed to parse resource metadata: {}", e))?;

        let issuer = resource_meta
            .authorization_servers
            .first()
            .ok_or_else(|| anyhow::anyhow!("resource metadata names no authorization server"))?;
        // Fall back to the MCP URL itself as the resource identifier if the
        // document omits it (some servers do).
        let resource = if resource_meta.resource.is_empty() {
            mcp.to_string()
        } else {
            resource_meta.resource.clone()
        };

        let server_meta = self.fetch_auth_server_metadata(issuer).await?;
        Ok((resource, server_meta))
    }

    /// Refuse a discovery URL that would carry a bearer token in cleartext.
    fn require_safe_discovery_url(&self, url: &Url) -> anyhow::Result<()> {
        match metadata::is_safe_discovery_url(url) {
            true => Ok(()),
            false => anyhow::bail!(
                "refusing OAuth discovery over an insecure URL ({url}): the flow carries a \
                 bearer token, so it must use https (http is permitted only on loopback)"
            ),
        }
    }

    /// Fetch AS metadata, trying RFC 8414 then the OpenID fallback.
    ///
    /// The returned document is validated against `issuer` before use. RFC 8414
    /// §3.3 requires the `issuer` in the metadata to match the one that was
    /// requested, and this never checked - so a hostile
    /// `authorization_servers[0]` in the resource document could redirect the
    /// entire flow to an attacker's authorization server and harvest the code.
    async fn fetch_auth_server_metadata(&self, issuer: &str) -> anyhow::Result<AuthServerMetadata> {
        let mut last_err = None;
        // Parsed once here and passed down: `auth_server_metadata_urls` already
        // parses `issuer` and errors on a bad one, so a second parse inside the
        // validator could never fail.
        let issuer_url = Url::parse(issuer)
            .map_err(|e| anyhow::anyhow!("invalid authorization server issuer '{issuer}': {e}"))?;
        for url in metadata::auth_server_metadata_urls(&issuer_url) {
            self.require_safe_discovery_url(&url)?;
            match self.fetch_one_auth_server_metadata(url.as_str()).await {
                Ok(meta) => {
                    self.validate_auth_server_metadata(&issuer_url, &meta)?;
                    return Ok(meta);
                }
                Err(e) => last_err = Some(e),
            }
        }
        Err(anyhow::anyhow!(
            "failed to fetch authorization server metadata: {}",
            last_err.expect("at least one candidate URL is always tried")
        ))
    }

    /// Check a fetched AS metadata document against the issuer it claims to
    /// describe.
    ///
    /// Three things, all of which a hostile document would otherwise get for
    /// free:
    ///
    /// 1. The document's own `issuer` matches the one requested (RFC 8414 §3.3).
    /// 2. The authorization and token endpoints share the issuer's origin, so a
    ///    valid-looking document cannot send the user's browser - and the
    ///    resulting code - somewhere else.
    /// 3. Both endpoints are safe to use at all (https, or loopback).
    fn validate_auth_server_metadata(
        &self,
        issuer_url: &Url,
        meta: &AuthServerMetadata,
    ) -> anyhow::Result<()> {
        let issuer = issuer_url.as_str();

        // RFC 8414 §3.3. Compared as parsed URLs so a trailing slash is not a
        // spurious mismatch.
        if !meta.issuer.is_empty() {
            let claimed = Url::parse(&meta.issuer).map_err(|e| {
                anyhow::anyhow!("invalid issuer '{}' in metadata: {e}", meta.issuer)
            })?;
            if !metadata::same_origin(&claimed, issuer_url) {
                anyhow::bail!(
                    "authorization server metadata claims issuer '{}' but was fetched for \
                     '{issuer}' - refusing (RFC 8414 §3.3)",
                    meta.issuer
                );
            }
        }

        for (label, endpoint) in [
            ("authorization_endpoint", &meta.authorization_endpoint),
            ("token_endpoint", &meta.token_endpoint),
        ] {
            let parsed = Url::parse(endpoint)
                .map_err(|e| anyhow::anyhow!("invalid {label} '{endpoint}': {e}"))?;
            self.require_safe_discovery_url(&parsed)?;
            if !metadata::same_origin(&parsed, issuer_url) {
                anyhow::bail!(
                    "{label} '{endpoint}' is not on the issuer's origin ('{issuer}') - refusing"
                );
            }
        }
        Ok(())
    }

    /// Fetch and parse AS metadata from one candidate URL.
    async fn fetch_one_auth_server_metadata(
        &self,
        url: &str,
    ) -> anyhow::Result<AuthServerMetadata> {
        let value = self.get_json(url).await?;
        Ok(serde_json::from_value(value)?)
    }

    /// Probe the MCP endpoint with the configured headers to see whether it
    /// demands OAuth at all.
    ///
    /// The headers matter: a server configured with an API token of its own
    /// answers this request normally, and asking it to run a browser login
    /// afterwards is asking for a second credential it never wanted.
    async fn probe_challenge(
        &self,
        mcp: &Url,
        headers: &HashMap<String, String>,
        allow_env: &[String],
    ) -> Probe {
        let mut request = self.http.post(mcp.clone()).body("{}");
        for (name, value) in headers {
            // Expand exactly as the transport will. Probing with a literal
            // `${TOKEN}` asks the server a question about a credential nobody
            // will ever send it, and a server that checks the value answers
            // `401` - sending a correctly configured client into an OAuth flow
            // it does not need.
            let value = crate::transport::http::expand_env_allowing(value, allow_env);
            request = request.header(name, value);
        }
        let Ok(response) = request.send().await else {
            return Probe::Unreachable;
        };
        let challenge = response
            .headers()
            .get(reqwest::header::WWW_AUTHENTICATE)
            .and_then(|v| v.to_str().ok())
            .map(str::to_string);
        // A 401 or 403 is the server asking for credentials, whether or not it
        // bothered to describe how. Anything else, with no challenge attached,
        // means the request was accepted as it stood.
        let demands_auth = matches!(
            response.status(),
            reqwest::StatusCode::UNAUTHORIZED | reqwest::StatusCode::FORBIDDEN
        );
        match (challenge, demands_auth) {
            (Some(header), _) => Probe::Challenge(Some(header)),
            (None, true) => Probe::Challenge(None),
            (None, false) => Probe::Satisfied,
        }
    }

    /// Register this client dynamically (RFC 7591), returning its id.
    async fn register(
        &self,
        server_meta: &AuthServerMetadata,
        redirect_uri: &str,
    ) -> anyhow::Result<String> {
        let endpoint = server_meta
            .registration_endpoint
            .as_deref()
            .ok_or_else(|| {
                anyhow::anyhow!(
                    "authorization server does not support dynamic client registration; \
                 a client id must be configured manually"
                )
            })?;

        let body = serde_json::json!({
            "client_name": "Leviath",
            "redirect_uris": [redirect_uri],
            "grant_types": ["authorization_code", "refresh_token"],
            "response_types": ["code"],
            "token_endpoint_auth_method": "none",
        });
        let response = self
            .http
            .post(endpoint)
            .json(&body)
            .send()
            .await
            .map_err(|e| anyhow::anyhow!("client registration request failed: {}", e))?;
        if !response.status().is_success() {
            let status = response.status();
            let text = response.text().await.unwrap_or_default();
            return Err(anyhow::anyhow!(
                "client registration failed with HTTP {}: {}",
                status,
                text.trim()
            ));
        }
        let registration: RegistrationResponse = response
            .json()
            .await
            .map_err(|e| anyhow::anyhow!("failed to parse registration response: {}", e))?;
        Ok(registration.client_id)
    }

    /// Exchange an authorization code for tokens.
    async fn exchange_code(
        &self,
        token_endpoint: &str,
        client_id: &str,
        redirect_uri: &str,
        code: &str,
        verifier: &str,
        resource: &str,
    ) -> anyhow::Result<TokenResponse> {
        let params = [
            ("grant_type", "authorization_code"),
            ("code", code),
            ("redirect_uri", redirect_uri),
            ("client_id", client_id),
            ("code_verifier", verifier),
            ("resource", resource),
        ];
        let value = self
            .post_form(token_endpoint, &params)
            .await
            .map_err(|e| anyhow::anyhow!("token exchange failed: {}", e))?;
        serde_json::from_value(value)
            .map_err(|e| anyhow::anyhow!("could not parse token response: {}", e))
    }

    /// GET a URL and return its JSON body as a value.
    ///
    /// Non-generic on purpose: a `<T>` version generates a separate llvm-cov
    /// instantiation per return type, and the error arms of the unused ones
    /// read as uncovered. Callers deserialize the returned value concretely.
    async fn get_json(&self, url: &str) -> anyhow::Result<serde_json::Value> {
        let response = self.http.get(url).send().await?;
        if !response.status().is_success() {
            anyhow::bail!("HTTP {}", response.status());
        }
        Ok(response.json().await?)
    }

    /// POST a form and return the JSON response as a value, surfacing an OAuth
    /// error body rather than a bare status. Non-generic for the same reason as
    /// [`Self::get_json`].
    async fn post_form(
        &self,
        url: &str,
        params: &[(&str, &str)],
    ) -> anyhow::Result<serde_json::Value> {
        let response = self.http.post(url).form(params).send().await?;
        let status = response.status();
        let body = response.text().await.unwrap_or_default();
        if !status.is_success() {
            anyhow::bail!("HTTP {}: {}", status, body.trim());
        }
        serde_json::from_str(&body)
            .map_err(|e| anyhow::anyhow!("could not parse token response: {}", e))
    }
}

/// A [`crate::transport::BearerRefresher`] backed by the on-disk token store.
///
/// On a mid-session `401` the HTTP transport calls this: it refreshes the
/// stored token non-interactively, persists the rotation, and hands back the
/// new `Authorization` header value.
pub struct StoredTokenRefresher {
    server_name: String,
    store_path: std::path::PathBuf,
    /// Current Unix time; a fn so a long-lived transport stays current.
    clock: fn() -> u64,
}

impl StoredTokenRefresher {
    /// A refresher for `server_name`, reading and writing `store_path`.
    pub fn new(server_name: impl Into<String>, store_path: std::path::PathBuf) -> Self {
        Self {
            server_name: server_name.into(),
            store_path,
            clock: system_now_secs,
        }
    }
}

/// Wall-clock Unix time in seconds.
fn system_now_secs() -> u64 {
    std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_secs())
        .unwrap_or(0)
}

#[async_trait::async_trait]
impl crate::transport::BearerRefresher for StoredTokenRefresher {
    async fn refresh(&self) -> anyhow::Result<String> {
        let mut store = AuthStore::load(&self.store_path)?;
        let auth = store.get(&self.server_name).ok_or_else(|| {
            anyhow::anyhow!(
                "no stored credentials for MCP server '{}'",
                self.server_name
            )
        })?;
        let refreshed = OAuthClient::new().refresh(auth, (self.clock)()).await?;
        let value = format!("Bearer {}", refreshed.access_token);
        store.set(&self.server_name, refreshed);
        store.save(&self.store_path)?;
        Ok(value)
    }
}

/// Compose the browser authorization URL.
/// Takes an already-parsed endpoint rather than a string: by the time login
/// reaches this, `validate_auth_server_metadata` has parsed the endpoint,
/// required it to be https-or-loopback, and required it to share the issuer's
/// origin. Re-parsing here would be a failure branch nothing can reach.
fn build_authorize_url(
    endpoint: &Url,
    client_id: &str,
    redirect_uri: &str,
    pkce: &Pkce,
    scope: &str,
    resource: &str,
) -> Url {
    let mut url = endpoint.clone();
    url.query_pairs_mut()
        .append_pair("response_type", "code")
        .append_pair("client_id", client_id)
        .append_pair("redirect_uri", redirect_uri)
        .append_pair("code_challenge", &pkce.challenge)
        .append_pair("code_challenge_method", "S256")
        .append_pair("state", &pkce.state)
        .append_pair("scope", scope)
        // RFC 8707: bind the issued token to this specific MCP server.
        .append_pair("resource", resource);
    url
}

/// Assemble the stored auth from a token response.
fn build_server_auth(
    resource: String,
    server_meta: &AuthServerMetadata,
    client_id: String,
    token: TokenResponse,
    now: u64,
) -> ServerAuth {
    ServerAuth {
        resource,
        issuer: server_meta.issuer.clone(),
        authorization_endpoint: server_meta.authorization_endpoint.clone(),
        token_endpoint: server_meta.token_endpoint.clone(),
        client_id,
        access_token: token.access_token,
        refresh_token: token.refresh_token,
        expires_at: expires_at(token.expires_in, now),
        scope: token.scope.unwrap_or_default(),
    }
}

/// Absolute expiry from a relative `expires_in`, or `0` (unknown) when the
/// server omits it.
fn expires_at(expires_in: Option<u64>, now: u64) -> u64 {
    match expires_in {
        Some(secs) => now.saturating_add(secs),
        None => 0,
    }
}

/// Accept the browser redirect on the loopback listener and return the code.
///
/// Validates `state` to reject a forged or replayed callback, replies with a
/// human-friendly page, and gives up after [`CALLBACK_TIMEOUT`].
async fn wait_for_callback(
    listener: TcpListener,
    expected_state: &str,
    timeout: Duration,
) -> anyhow::Result<String> {
    let accept = async {
        loop {
            // Accepting on a freshly-bound loopback listener does not fail;
            // connection resets surface later, on read, not here.
            let (stream, _) = listener
                .accept()
                .await
                .expect("accepting on a bound loopback listener cannot fail");
            // A browser may make incidental requests (favicon, etc); only the
            // one carrying our params counts.
            if let Some(result) = handle_callback_connection(stream, expected_state).await? {
                return Ok(result);
            }
        }
    };

    match tokio::time::timeout(timeout, accept).await {
        Ok(result) => result,
        Err(_) => Err(anyhow::anyhow!(
            "timed out waiting for browser authorization"
        )),
    }
}

/// Handle one loopback connection.
///
/// Returns `Ok(Some(code))` for the authorization callback, `Ok(None)` for an
/// unrelated request (so the caller keeps listening), and `Err` for a callback
/// that arrived but was invalid (mismatched state, or an OAuth `error`).
async fn handle_callback_connection(
    mut stream: tokio::net::TcpStream,
    expected_state: &str,
) -> anyhow::Result<Option<String>> {
    use tokio::io::AsyncReadExt;

    let mut buf = vec![0u8; 8192];
    let n = stream.read(&mut buf).await.unwrap_or(0);
    let request = String::from_utf8_lossy(&buf[..n]);
    let Some(target) = request_target(&request) else {
        return Ok(None);
    };
    if !target.starts_with("/callback") {
        write_response(&mut stream, "404 Not Found", "Not found.").await;
        return Ok(None);
    }

    let params = query_params(target);
    if let Some(error) = params.get("error") {
        write_response(&mut stream, "400 Bad Request", "Authorization failed.").await;
        return Err(anyhow::anyhow!("authorization server returned: {}", error));
    }
    match (params.get("code"), params.get("state")) {
        // Constant-time: the state is 128 bits of fresh entropy over loopback, so
        // a timing oracle here is theoretical - but it was the one secret
        // comparison in the codebase still using `==`, and "theoretical" is not
        // a reason for the comparison to differ from every other one.
        (Some(code), Some(state)) if leviath_core::constant_time_eq(state, expected_state) => {
            write_response(
                &mut stream,
                "200 OK",
                "Authorization complete - you can close this tab and return to Leviath.",
            )
            .await;
            Ok(Some(code.clone()))
        }
        (_, Some(_)) => {
            // A state mismatch means a forged or stale callback.
            write_response(
                &mut stream,
                "400 Bad Request",
                "Invalid authorization state.",
            )
            .await;
            Err(anyhow::anyhow!("OAuth state mismatch - rejecting callback"))
        }
        _ => {
            write_response(
                &mut stream,
                "400 Bad Request",
                "Missing authorization code.",
            )
            .await;
            Err(anyhow::anyhow!("callback missing code or state"))
        }
    }
}

/// The request target (`/callback?…`) from an HTTP request line.
fn request_target(request: &str) -> Option<&str> {
    let line = request.lines().next()?;
    let mut parts = line.split_whitespace();
    let _method = parts.next()?;
    parts.next()
}

/// Parse the query string of a request target into a map.
fn query_params(target: &str) -> HashMap<String, String> {
    let query = target.split_once('?').map(|(_, q)| q).unwrap_or("");
    form_urlencoded::parse(query.as_bytes())
        .map(|(k, v)| (k.into_owned(), v.into_owned()))
        .collect()
}

/// Write a minimal HTML response and close the connection.
async fn write_response(stream: &mut tokio::net::TcpStream, status: &str, message: &str) {
    use tokio::io::AsyncWriteExt;
    let body = format!("<!doctype html><meta charset=utf-8><p>{message}</p>");
    let response = format!(
        "HTTP/1.1 {status}\r\nContent-Type: text/html; charset=utf-8\r\n\
         Content-Length: {}\r\nConnection: close\r\n\r\n{body}",
        body.len()
    );
    let _ = stream.write_all(response.as_bytes()).await;
    let _ = stream.flush().await;
}

#[cfg(test)]
mod tests {
    use super::*;

    // ─── build_authorize_url ──────────────────────────────────────────────

    fn fixed_pkce() -> Pkce {
        Pkce {
            verifier: "verifier".to_string(),
            challenge: "challenge".to_string(),
            state: "state123".to_string(),
        }
    }

    #[test]
    fn authorize_url_carries_every_required_parameter() {
        let url = build_authorize_url(
            &Url::parse("https://auth.example.com/authorize").unwrap(),
            "client-1",
            "http://127.0.0.1:5000/callback",
            &fixed_pkce(),
            "openid profile",
            "https://mcp.example.com/mcp",
        );
        let params: HashMap<_, _> = url.query_pairs().into_owned().collect();
        assert_eq!(params["response_type"], "code");
        assert_eq!(params["client_id"], "client-1");
        assert_eq!(params["redirect_uri"], "http://127.0.0.1:5000/callback");
        assert_eq!(params["code_challenge"], "challenge");
        assert_eq!(params["code_challenge_method"], "S256");
        assert_eq!(params["state"], "state123");
        assert_eq!(params["scope"], "openid profile");
        // RFC 8707 resource binding is mandatory since MCP 2025-06-18.
        assert_eq!(params["resource"], "https://mcp.example.com/mcp");
    }

    // `authorize_url_rejects_a_bad_endpoint` is gone with the `&str` parameter:
    // `build_authorize_url` now takes an already-parsed `Url`, because
    // `validate_auth_server_metadata` parses and origin-checks the endpoint
    // before login ever gets here. An unparseable endpoint is covered end to end
    // by `login_fails_when_the_authorize_endpoint_is_malformed`.

    // ─── expires_at ───────────────────────────────────────────────────────

    #[test]
    fn expires_at_adds_the_relative_lifetime() {
        assert_eq!(expires_at(Some(3600), 1_000), 4_600);
    }

    #[test]
    fn expires_at_is_zero_when_unknown() {
        assert_eq!(expires_at(None, 1_000), 0);
    }

    // ─── request parsing ──────────────────────────────────────────────────

    #[test]
    fn request_target_reads_the_path() {
        assert_eq!(
            request_target("GET /callback?code=abc HTTP/1.1\r\nHost: x\r\n\r\n"),
            Some("/callback?code=abc")
        );
    }

    #[test]
    fn request_target_of_garbage_is_none() {
        assert_eq!(request_target(""), None);
        // A method with no target (the `?` on the second token).
        assert_eq!(request_target("GET"), None);
        // A whitespace-only line: a non-empty first line that yields no tokens.
        assert_eq!(request_target("   \r\n"), None);
    }

    #[test]
    fn query_params_parses_pairs() {
        let params = query_params("/callback?code=abc&state=xyz");
        assert_eq!(params["code"], "abc");
        assert_eq!(params["state"], "xyz");
    }

    #[test]
    fn query_params_of_a_bare_path_is_empty() {
        assert!(query_params("/callback").is_empty());
    }

    // ─── build_server_auth ────────────────────────────────────────────────

    fn server_meta() -> AuthServerMetadata {
        serde_json::from_value(serde_json::json!({
            "issuer": "https://auth.example.com",
            "authorization_endpoint": "https://auth.example.com/authorize",
            "token_endpoint": "https://auth.example.com/token",
        }))
        .unwrap()
    }

    #[test]
    fn build_server_auth_populates_every_field() {
        let token = TokenResponse {
            access_token: "at".to_string(),
            refresh_token: Some("rt".to_string()),
            expires_in: Some(3600),
            scope: Some("openid".to_string()),
        };
        let auth = build_server_auth(
            "https://mcp.example.com/mcp".to_string(),
            &server_meta(),
            "client-1".to_string(),
            token,
            1_000,
        );
        assert_eq!(auth.resource, "https://mcp.example.com/mcp");
        assert_eq!(auth.issuer, "https://auth.example.com");
        assert_eq!(auth.client_id, "client-1");
        assert_eq!(auth.access_token, "at");
        assert_eq!(auth.refresh_token.as_deref(), Some("rt"));
        assert_eq!(auth.expires_at, 4_600);
        assert_eq!(auth.scope, "openid");
    }

    #[test]
    fn build_server_auth_defaults_a_missing_scope() {
        let token = TokenResponse {
            access_token: "at".to_string(),
            refresh_token: None,
            expires_in: None,
            scope: None,
        };
        let auth = build_server_auth(
            "https://mcp".to_string(),
            &server_meta(),
            "c".to_string(),
            token,
            0,
        );
        assert_eq!(auth.scope, "");
        assert_eq!(auth.expires_at, 0);
        assert!(auth.refresh_token.is_none());
    }

    // ─── loopback callback handling ───────────────────────────────────────
    //
    // wait_for_callback binds a real listener; these drive it with a real TCP
    // client, exactly as a browser redirect would, so the accept loop, state
    // check, and response writing are all exercised without a browser.

    use tokio::io::{AsyncReadExt, AsyncWriteExt};
    use tokio::net::TcpStream;

    /// Send one raw HTTP request line to `addr` and return the response text.
    async fn hit(addr: std::net::SocketAddr, target: &str) -> String {
        let mut stream = TcpStream::connect(addr).await.unwrap();
        let request = format!("GET {target} HTTP/1.1\r\nHost: localhost\r\n\r\n");
        stream.write_all(request.as_bytes()).await.unwrap();
        stream.flush().await.unwrap();
        let mut buf = Vec::new();
        let _ = stream.read_to_end(&mut buf).await;
        String::from_utf8_lossy(&buf).into_owned()
    }

    #[tokio::test]
    async fn callback_returns_the_code_on_a_matching_state() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let server = tokio::spawn(async move {
            wait_for_callback(listener, "st8", Duration::from_secs(5)).await
        });

        let response = hit(addr, "/callback?code=the-code&state=st8").await;
        assert!(response.contains("200 OK"), "got: {response}");
        assert!(
            response.contains("Authorization complete"),
            "got: {response}"
        );
        assert_eq!(server.await.unwrap().unwrap(), "the-code");
    }

    #[tokio::test]
    async fn callback_skips_unrelated_requests_then_accepts_the_real_one() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let server = tokio::spawn(async move {
            wait_for_callback(listener, "st8", Duration::from_secs(5)).await
        });

        // A browser often fetches /favicon.ico first; it must not end the wait.
        let favicon = hit(addr, "/favicon.ico").await;
        assert!(favicon.contains("404"), "got: {favicon}");
        let ok = hit(addr, "/callback?code=c&state=st8").await;
        assert!(ok.contains("200 OK"));
        assert_eq!(server.await.unwrap().unwrap(), "c");
    }

    #[tokio::test]
    async fn callback_rejects_a_mismatched_state() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let server = tokio::spawn(async move {
            wait_for_callback(listener, "expected", Duration::from_secs(5)).await
        });

        let response = hit(addr, "/callback?code=c&state=forged").await;
        assert!(response.contains("400"), "got: {response}");
        let err = server.await.unwrap().expect_err("mismatch must fail");
        assert!(err.to_string().contains("state mismatch"), "got: {err}");
    }

    #[tokio::test]
    async fn callback_surfaces_an_oauth_error() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let server =
            tokio::spawn(
                async move { wait_for_callback(listener, "s", Duration::from_secs(5)).await },
            );

        let response = hit(addr, "/callback?error=access_denied").await;
        assert!(response.contains("400"), "got: {response}");
        let err = server.await.unwrap().expect_err("error param must fail");
        assert!(err.to_string().contains("access_denied"), "got: {err}");
    }

    #[tokio::test]
    async fn callback_rejects_a_request_missing_code_and_state() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let server =
            tokio::spawn(
                async move { wait_for_callback(listener, "s", Duration::from_secs(5)).await },
            );

        let response = hit(addr, "/callback?nothing=here").await;
        assert!(response.contains("400"), "got: {response}");
        assert!(server.await.unwrap().is_err());
    }

    #[tokio::test]
    async fn handle_callback_ignores_an_empty_connection() {
        // A connection that sends nothing yields no request line, so it is
        // neither the callback nor an error - just skipped.
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        let addr = listener.local_addr().unwrap();
        let accept = tokio::spawn(async move {
            let (stream, _) = listener.accept().await.unwrap();
            handle_callback_connection(stream, "s").await
        });
        // Connect and immediately close without writing.
        let stream = TcpStream::connect(addr).await.unwrap();
        drop(stream);
        let outcome = accept
            .await
            .unwrap()
            .expect("empty connection is not an error");
        assert!(outcome.is_none(), "an empty connection yields no code");
    }

    // ─── full OAuth flows against a mock authorization server ─────────────

    use axum::extract::State;
    use axum::http::StatusCode;
    use axum::response::IntoResponse;
    use axum::routing::{get, post};
    use axum::{Json, Router};
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};

    #[derive(Clone)]
    struct MockAs {
        base: String,
        registrations: Arc<AtomicUsize>,
    }

    /// A standards-correct mock: protected-resource + AS metadata, dynamic
    /// registration, and a token endpoint. `variant` toggles which discovery
    /// quirks to exercise.
    async fn mock_auth_server(variant: &'static str) -> MockAs {
        let registrations = Arc::new(AtomicUsize::new(0));
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let state = MockAs {
            base: base.clone(),
            registrations: registrations.clone(),
        };

        let app_state = (base.clone(), variant, registrations.clone());
        let app = Router::new()
            .route(
                "/mcp",
                post(|State((base, variant, _)): State<(String, &'static str, Arc<AtomicUsize>)>| async move {
                    // Unauthenticated probe → 401 with the resource hint
                    // pointing at this server's own well-known document.
                    let hint = if variant == "bad_hint" {
                        // Not a URL at all: the hint is server-controlled, so a
                        // malformed one must be rejected rather than parsed
                        // leniently.
                        "Bearer resource_metadata=\"not a url\"".to_string()
                    } else {
                        format!(
                            "Bearer resource_metadata=\"{base}/.well-known/oauth-protected-resource\""
                        )
                    };
                    (
                        StatusCode::UNAUTHORIZED,
                        [(reqwest::header::WWW_AUTHENTICATE, hint)],
                    )
                }),
            )
            .route(
                "/.well-known/oauth-protected-resource",
                get(|State((base, variant, _)): State<(String, &'static str, Arc<AtomicUsize>)>| async move {
                    if variant == "discover_fails" {
                        return StatusCode::INTERNAL_SERVER_ERROR.into_response();
                    }
                    if variant == "resource_not_object" {
                        // Valid JSON, but not a ProtectedResourceMetadata object.
                        return Json(serde_json::json!("just a string")).into_response();
                    }
                    let resource = if variant == "no_resource" {
                        serde_json::Value::String(String::new())
                    } else {
                        serde_json::json!(format!("{base}/mcp"))
                    };
                    let servers = match variant {
                        "no_auth_server" => serde_json::json!([]),
                        "bad_issuer" => serde_json::json!(["not a url"]),
                        // A *remote* http issuer: the discovery URLs derived
                        // from it would carry a bearer token in cleartext.
                        "http_issuer" => serde_json::json!(["http://auth.example.com"]),
                        _ => serde_json::json!([base]),
                    };
                    Json(serde_json::json!({
                        "resource": resource,
                        "authorization_servers": servers,
                    }))
                    .into_response()
                }),
            )
            .route(
                "/.well-known/oauth-authorization-server",
                get(|State((base, variant, _)): State<(String, &'static str, Arc<AtomicUsize>)>| async move {
                    if variant == "no_rfc8414" || variant == "no_metadata" {
                        return StatusCode::NOT_FOUND.into_response();
                    }
                    if variant == "as_bad_rfc8414" {
                        // Valid JSON, but missing the required AS metadata
                        // fields, so parsing fails and discovery tries OpenID.
                        return Json(serde_json::json!({ "not": "metadata" })).into_response();
                    }
                    as_metadata(&base, variant).into_response()
                }),
            )
            .route(
                "/.well-known/openid-configuration",
                get(|State((base, variant, _)): State<(String, &'static str, Arc<AtomicUsize>)>| async move {
                    if variant == "no_metadata" {
                        return StatusCode::NOT_FOUND.into_response();
                    }
                    as_metadata(&base, variant).into_response()
                }),
            )
            .route(
                "/register",
                post(|State((base, variant, regs)): State<(String, &'static str, Arc<AtomicUsize>)>, _body: String| async move {
                    regs.fetch_add(1, Ordering::SeqCst);
                    let _ = base;
                    if variant == "register_fails" {
                        return (StatusCode::BAD_REQUEST, "invalid_redirect_uri").into_response();
                    }
                    if variant == "register_bad_json" {
                        return (StatusCode::OK, "not json").into_response();
                    }
                    Json(serde_json::json!({ "client_id": "registered-client" })).into_response()
                }),
            )
            .route(
                "/token",
                post(|State((_, variant, _)): State<(String, &'static str, Arc<AtomicUsize>)>, body: String| async move {
                    // A refresh with a bad token is the one failure we model.
                    if body.contains("refresh_token=bad") {
                        return (StatusCode::BAD_REQUEST, "invalid_grant").into_response();
                    }
                    if variant == "bad_token_json" {
                        return (StatusCode::OK, "not json").into_response();
                    }
                    if variant == "exchange_fails" && body.contains("authorization_code") {
                        return (StatusCode::BAD_REQUEST, "invalid_grant").into_response();
                    }
                    if variant == "minimal_token" {
                        return Json(serde_json::json!({ "access_token": "minimal" }))
                            .into_response();
                    }
                    if variant == "token_no_access" {
                        // Valid JSON, but not a TokenResponse (no access_token).
                        return Json(serde_json::json!({ "wat": true })).into_response();
                    }
                    Json(serde_json::json!({
                        "access_token": "new-access",
                        "refresh_token": "new-refresh",
                        "expires_in": 3600,
                        "scope": "openid",
                    }))
                    .into_response()
                }),
            )
            .with_state(app_state);

        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));
        state
    }

    fn as_metadata(base: &str, variant: &'static str) -> Json<serde_json::Value> {
        let scopes: Vec<&str> = if variant == "no_scopes" {
            vec![]
        } else {
            vec!["openid", "profile"]
        };
        let authorize = match variant {
            "bad_authorize" => "not a url".to_string(),
            // A valid URL on somebody else's origin: the shape a hostile
            // document uses to send the user's browser - and the resulting
            // authorization code - somewhere the issuer does not control.
            "foreign_endpoint" => "https://evil.example.com/authorize".to_string(),
            // Remote *http*: refused for the scheme before the origin check
            // even runs.
            "http_endpoint" => "http://evil.example.com/authorize".to_string(),
            _ => format!("{base}/authorize"),
        };
        let issuer = match variant {
            // Claims to be a different issuer than the one we fetched it for.
            // RFC 8414 §3.3 requires these to match.
            "issuer_mismatch" => "https://someone-else.example.com".to_string(),
            // Distinct from the existing `bad_issuer`, which makes the
            // *resource* document name a non-URL authorization server. This one
            // is the AS document's own `issuer` field.
            "as_unparseable_issuer" => "not a url".to_string(),
            // Omitted entirely. The RFC 8414 §3.3 cross-check is skipped rather
            // than failing, since there is nothing to compare - the endpoint
            // origin check below still applies.
            "no_issuer_field" => String::new(),
            _ => base.to_string(),
        };
        let mut meta = serde_json::json!({
            "issuer": issuer,
            "authorization_endpoint": authorize,
            "token_endpoint": format!("{base}/token"),
            "scopes_supported": scopes,
        });
        if variant != "no_registration" {
            meta["registration_endpoint"] = serde_json::json!(format!("{base}/register"));
        }
        Json(meta)
    }

    /// Drive the loopback redirect exactly as a browser would after consent.
    ///
    /// `state_override` forges the CSRF state (for the mismatch test);
    /// otherwise the real state from the authorize URL is echoed back. One
    /// spawn site shared by both consent stubs.
    fn drive_callback(authorize_url: &str, state_override: Option<&str>) {
        let url = Url::parse(authorize_url).unwrap();
        let params: HashMap<_, _> = url.query_pairs().into_owned().collect();
        let redirect = params["redirect_uri"].clone();
        let state = state_override
            .map(String::from)
            .unwrap_or_else(|| params["state"].clone());
        // Spawned onto the same runtime; login is concurrently awaiting accept.
        tokio::spawn(async move {
            let callback = format!("{redirect}?code=auth-code&state={state}");
            let _ = reqwest::Client::new().get(&callback).send().await;
        });
    }

    /// A fake browser that consents successfully.
    fn auto_consent() -> BrowserOpener {
        Arc::new(|authorize_url: &str| {
            drive_callback(authorize_url, None);
            true
        })
    }

    #[tokio::test]
    async fn full_login_round_trip() {
        let server = mock_auth_server("default").await;
        let auth = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                1_000,
                None,
            )
            .await
            .expect("login should complete")
            .authenticated()
            .expect("the flow should have produced credentials");

        assert_eq!(auth.access_token, "new-access");
        assert_eq!(auth.refresh_token.as_deref(), Some("new-refresh"));
        assert_eq!(auth.expires_at, 4_600);
        assert_eq!(auth.client_id, "registered-client");
        assert_eq!(server.registrations.load(Ordering::SeqCst), 1);
    }

    #[tokio::test]
    async fn login_reuses_a_known_client_id_and_skips_registration() {
        let server = mock_auth_server("default").await;
        OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                Some("existing-client"),
            )
            .await
            .expect("login should complete")
            .authenticated()
            .expect("the flow should have produced credentials");
        assert_eq!(
            server.registrations.load(Ordering::SeqCst),
            0,
            "a known client id must not re-register"
        );
    }

    #[tokio::test]
    async fn login_falls_back_when_rfc8414_metadata_is_malformed() {
        // RFC 8414 returns unparseable metadata; discovery must recover via the
        // OpenID document rather than giving up.
        let server = mock_auth_server("as_bad_rfc8414").await;
        let auth = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("openid recovery should work")
            .authenticated()
            .expect("the flow should have produced credentials");
        assert_eq!(auth.access_token, "new-access");
    }

    #[tokio::test]
    async fn login_falls_back_to_openid_configuration() {
        // RFC 8414 404s, so discovery must try the OpenID document.
        let server = mock_auth_server("no_rfc8414").await;
        let auth = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("openid fallback should work")
            .authenticated()
            .expect("the flow should have produced credentials");
        assert_eq!(auth.access_token, "new-access");
    }

    #[tokio::test]
    async fn login_fails_when_registration_is_unsupported() {
        let server = mock_auth_server("no_registration").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("no registration endpoint and no client id must fail");
        assert!(
            err.to_string().contains("dynamic client registration"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn refresh_rotates_the_tokens() {
        let server = mock_auth_server("default").await;
        let auth = ServerAuth {
            resource: format!("{}/mcp", server.base),
            issuer: server.base.clone(),
            authorization_endpoint: format!("{}/authorize", server.base),
            token_endpoint: format!("{}/token", server.base),
            client_id: "c".to_string(),
            access_token: "old".to_string(),
            refresh_token: Some("good".to_string()),
            expires_at: 500,
            scope: String::new(),
        };
        let refreshed = OAuthClient::new().refresh(&auth, 2_000).await.unwrap();
        assert_eq!(refreshed.access_token, "new-access");
        assert_eq!(refreshed.refresh_token.as_deref(), Some("new-refresh"));
        assert_eq!(refreshed.expires_at, 5_600);
    }

    #[tokio::test]
    async fn refresh_keeps_the_old_token_when_none_is_returned() {
        // A server that returns only an access_token must not wipe the refresh
        // token or scope we already hold.
        let server = mock_auth_server("minimal_token").await;
        let auth = ServerAuth {
            token_endpoint: format!("{}/token", server.base),
            refresh_token: Some("keep-me".to_string()),
            scope: "openid".to_string(),
            ..Default::default()
        };
        let refreshed = OAuthClient::new().refresh(&auth, 0).await.unwrap();
        assert_eq!(refreshed.access_token, "minimal");
        assert_eq!(refreshed.refresh_token.as_deref(), Some("keep-me"));
        assert_eq!(refreshed.scope, "openid");
        assert_eq!(refreshed.expires_at, 0);
    }

    #[tokio::test]
    async fn authorization_header_is_none_without_stored_auth() {
        let dir = tempfile::tempdir().unwrap();
        let store = dir.path().join("mcp-auth.json");
        let header = OAuthClient::new()
            .authorization_header("unknown", &store, 0)
            .await
            .unwrap();
        assert!(header.is_none());
    }

    #[tokio::test]
    async fn authorization_header_returns_a_fresh_token_unchanged() {
        let dir = tempfile::tempdir().unwrap();
        let store_path = dir.path().join("mcp-auth.json");
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                access_token: "still-good".to_string(),
                expires_at: 10_000,
                ..Default::default()
            },
        );
        store.save(&store_path).unwrap();

        let header = OAuthClient::new()
            .authorization_header("srv", &store_path, 1_000)
            .await
            .unwrap()
            .expect("a stored token yields a header");
        assert_eq!(
            header,
            ("Authorization".to_string(), "Bearer still-good".to_string())
        );
    }

    #[tokio::test]
    async fn authorization_header_refreshes_an_expired_token_and_persists_it() {
        let server = mock_auth_server("default").await;
        let dir = tempfile::tempdir().unwrap();
        let store_path = dir.path().join("mcp-auth.json");
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                token_endpoint: format!("{}/token", server.base),
                access_token: "expired".to_string(),
                refresh_token: Some("good".to_string()),
                expires_at: 100,
                ..Default::default()
            },
        );
        store.save(&store_path).unwrap();

        let header = OAuthClient::new()
            .authorization_header("srv", &store_path, 1_000)
            .await
            .unwrap()
            .expect("an expired token is refreshed");
        assert_eq!(header.1, "Bearer new-access");
        // The rotated token is written back for next time.
        let reloaded = AuthStore::load(&store_path).unwrap();
        assert_eq!(reloaded.get("srv").unwrap().access_token, "new-access");
    }

    #[tokio::test]
    async fn authorization_header_names_the_login_command_when_refresh_fails() {
        let dir = tempfile::tempdir().unwrap();
        let store_path = dir.path().join("mcp-auth.json");
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                token_endpoint: "http://127.0.0.1:1/token".to_string(),
                access_token: "expired".to_string(),
                refresh_token: Some("good".to_string()),
                expires_at: 100,
                ..Default::default()
            },
        );
        store.save(&store_path).unwrap();

        let err = OAuthClient::new()
            .authorization_header("srv", &store_path, 1_000)
            .await
            .expect_err("a dead refresh must fail");
        assert!(err.to_string().contains("lev mcp login srv"), "got: {err}");
    }

    // ─── StoredTokenRefresher ─────────────────────────────────────────────

    use crate::transport::BearerRefresher;

    fn refresher_at(dir: &std::path::Path) -> StoredTokenRefresher {
        StoredTokenRefresher {
            server_name: "srv".to_string(),
            store_path: dir.join("mcp-auth.json"),
            clock: || 2_000,
        }
    }

    #[tokio::test]
    async fn stored_refresher_rotates_and_persists_the_token() {
        let server = mock_auth_server("default").await;
        let dir = tempfile::tempdir().unwrap();
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                token_endpoint: format!("{}/token", server.base),
                refresh_token: Some("good".to_string()),
                expires_at: 1,
                ..Default::default()
            },
        );
        let refresher = refresher_at(dir.path());
        store.save(&refresher.store_path).unwrap();

        let value = refresher.refresh().await.expect("refresh should succeed");
        assert_eq!(value, "Bearer new-access");
        // The rotation is persisted.
        let reloaded = AuthStore::load(&refresher.store_path).unwrap();
        assert_eq!(reloaded.get("srv").unwrap().access_token, "new-access");
    }

    #[tokio::test]
    async fn stored_refresher_errors_without_stored_credentials() {
        let dir = tempfile::tempdir().unwrap();
        let refresher = refresher_at(dir.path());
        // Empty store → nothing to refresh.
        let err = refresher.refresh().await.expect_err("no creds must fail");
        assert!(
            err.to_string().contains("no stored credentials"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn stored_refresher_surfaces_a_refresh_failure() {
        let dir = tempfile::tempdir().unwrap();
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                token_endpoint: "http://127.0.0.1:1/token".to_string(),
                refresh_token: Some("good".to_string()),
                expires_at: 1,
                ..Default::default()
            },
        );
        let refresher = refresher_at(dir.path());
        store.save(&refresher.store_path).unwrap();
        assert!(refresher.refresh().await.is_err());
    }

    #[test]
    fn stored_refresher_new_uses_the_system_clock() {
        let r = StoredTokenRefresher::new("s", std::path::PathBuf::from("/tmp/x"));
        assert!((r.clock)() > 1_600_000_000);
    }

    #[test]
    fn system_now_secs_advances_past_the_epoch() {
        assert!(system_now_secs() > 1_600_000_000);
    }

    #[tokio::test]
    async fn authorization_header_surfaces_an_unreadable_store() {
        // The store path is a directory, so loading it fails.
        let dir = tempfile::tempdir().unwrap();
        assert!(
            OAuthClient::new()
                .authorization_header("srv", dir.path(), 0)
                .await
                .is_err()
        );
    }

    #[tokio::test]
    async fn authorization_header_surfaces_an_unwritable_store_after_refresh() {
        // Refresh succeeds, but the read-only store can't persist the rotated
        // token.
        let server = mock_auth_server("default").await;
        let dir = tempfile::tempdir().unwrap();
        let store_path = dir.path().join("mcp-auth.json");
        let mut store = AuthStore::default();
        store.set(
            "srv",
            ServerAuth {
                token_endpoint: format!("{}/token", server.base),
                refresh_token: Some("good".to_string()),
                expires_at: 100,
                ..Default::default()
            },
        );
        store.save(&store_path).unwrap();
        let mut perms = std::fs::metadata(&store_path).unwrap().permissions();
        perms.set_readonly(true);
        std::fs::set_permissions(&store_path, perms).unwrap();

        assert!(
            OAuthClient::new()
                .authorization_header("srv", &store_path, 1_000)
                .await
                .is_err()
        );
    }

    #[tokio::test]
    async fn refresh_without_a_token_is_an_error() {
        let mut auth = ServerAuth {
            token_endpoint: "http://127.0.0.1:1/token".to_string(),
            ..Default::default()
        };
        auth.refresh_token = None;
        let err = OAuthClient::new()
            .refresh(&auth, 0)
            .await
            .expect_err("no refresh token must fail");
        assert!(err.to_string().contains("no refresh token"), "got: {err}");
    }

    #[tokio::test]
    async fn refresh_surfaces_a_rejected_grant() {
        let server = mock_auth_server("default").await;
        let auth = ServerAuth {
            token_endpoint: format!("{}/token", server.base),
            refresh_token: Some("bad".to_string()),
            ..Default::default()
        };
        let err = OAuthClient::new()
            .refresh(&auth, 0)
            .await
            .expect_err("a rejected grant must fail");
        assert!(err.to_string().contains("refresh failed"), "got: {err}");
    }

    /// A server that demands auth but does not say where its metadata lives.
    /// The fallback URL has to be the RFC 9728 one, which keeps the resource's
    /// path after the well-known segment. Serving the document *only* at the
    /// suffixed path is what makes this test fail if the path is dropped, which
    /// is exactly how GitHub's MCP server behaves.
    #[tokio::test]
    async fn a_bare_challenge_falls_back_to_the_path_suffixed_well_known_url() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new()
            .route("/mcp", post(|| async { StatusCode::UNAUTHORIZED }))
            .route(
                "/.well-known/oauth-protected-resource/mcp",
                get({
                    let base = base.clone();
                    move || {
                        let base = base.clone();
                        async move {
                            Json(serde_json::json!({
                                "resource": format!("{base}/mcp"),
                                "authorization_servers": [base],
                            }))
                        }
                    }
                }),
            )
            .route(
                "/.well-known/oauth-authorization-server",
                get({
                    let base = base.clone();
                    move || {
                        let base = base.clone();
                        async move { as_metadata(&base, "default") }
                    }
                }),
            )
            .route(
                "/register",
                post(|| async { Json(serde_json::json!({ "client_id": "c" })) }),
            )
            .route(
                "/token",
                post(|| async {
                    Json(serde_json::json!({"access_token": "at", "expires_in": 60}))
                }),
            );
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));

        let auth = OAuthClient::new()
            .login(
                &format!("{base}/mcp"),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("well-known discovery should work")
            .authenticated()
            .expect("the flow should have produced credentials");
        assert_eq!(auth.access_token, "at");
    }

    #[test]
    fn oauth_client_default_matches_new() {
        // Both build a usable client; `default` just delegates.
        let _ = OAuthClient::default();
    }

    #[tokio::test]
    async fn callback_times_out_when_no_redirect_arrives() {
        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
        // Nobody connects, so the tiny timeout must fire.
        let err = wait_for_callback(listener, "s", Duration::from_millis(100))
            .await
            .expect_err("must time out");
        assert!(err.to_string().contains("timed out"), "got: {err}");
    }

    #[tokio::test]
    async fn login_sends_configured_probe_headers() {
        // A non-empty header map exercises the probe header loop; the server
        // does not require it, so login still completes.
        let server = mock_auth_server("default").await;
        let headers = HashMap::from([("X-Probe".to_string(), "1".to_string())]);
        OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &headers,
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("login with probe headers should complete")
            .authenticated()
            .expect("the flow should have produced credentials");
    }

    #[tokio::test]
    async fn login_still_completes_when_the_browser_cannot_open() {
        // The opener reports failure (headless/SSH), but the user "pastes" the
        // link: the callback is still driven, so login succeeds via the
        // print-the-URL path.
        let failing_opener: BrowserOpener = Arc::new(|authorize_url: &str| {
            drive_callback(authorize_url, None);
            false
        });
        let server = mock_auth_server("default").await;
        OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                failing_opener,
                0,
                None,
            )
            .await
            .expect("login should complete even without a browser")
            .authenticated()
            .expect("the flow should have produced credentials");
    }

    #[tokio::test]
    async fn login_uses_default_scopes_when_the_server_advertises_none() {
        let server = mock_auth_server("no_scopes").await;
        OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("login should complete with default scopes")
            .authenticated()
            .expect("the flow should have produced credentials");
    }

    #[tokio::test]
    async fn login_falls_back_to_the_mcp_url_when_resource_is_omitted() {
        let server = mock_auth_server("no_resource").await;
        let auth = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("login should complete")
            .authenticated()
            .expect("the flow should have produced credentials");
        // The resource identifier defaulted to the MCP URL itself.
        assert_eq!(auth.resource, format!("{}/mcp", server.base));
    }

    #[tokio::test]
    async fn login_fails_when_registration_is_rejected() {
        let server = mock_auth_server("register_fails").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a rejected registration must fail");
        assert!(
            err.to_string().contains("registration failed"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn discovery_fails_when_no_metadata_document_is_reachable() {
        // Both the RFC 8414 and OpenID endpoints 404.
        let server = mock_auth_server("no_metadata").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("no reachable metadata must fail");
        assert!(
            err.to_string().contains("authorization server metadata"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn discovery_fails_when_resource_metadata_is_unavailable() {
        let server = mock_auth_server("discover_fails").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a 500 on resource metadata must fail");
        assert!(err.to_string().contains("resource metadata"), "got: {err}");
    }

    #[tokio::test]
    async fn login_fails_when_registration_returns_bad_json() {
        let server = mock_auth_server("register_bad_json").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("unparseable registration must fail");
        assert!(
            err.to_string().contains("registration response"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_fails_when_the_token_exchange_is_rejected() {
        let server = mock_auth_server("exchange_fails").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a rejected code exchange must fail");
        assert!(
            err.to_string().contains("token exchange failed"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_fails_when_the_token_response_is_not_json() {
        let server = mock_auth_server("bad_token_json").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("an unparseable token response must fail");
        assert!(
            err.to_string().contains("parse token response"),
            "got: {err}"
        );
    }

    /// A hostile MCP server pointing `resource_metadata` at somebody else's
    /// origin. The URL comes out of a `WWW-Authenticate` header the server
    /// controls entirely; fetching it without validation means connecting to
    /// a malicious server is enough to make Leviath issue a request to any
    /// URL from inside the user's network.
    #[tokio::test]
    async fn login_refuses_a_cross_origin_resource_metadata_hint() {
        // A server whose 401 points at a *different* origin. The target does not
        // need to exist: the refusal must happen before the fetch.
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = axum::Router::new().route(
            "/mcp",
            post(|| async {
                (
                    StatusCode::UNAUTHORIZED,
                    [(
                        reqwest::header::WWW_AUTHENTICATE,
                        "Bearer resource_metadata=\"http://169.254.169.254/latest/meta-data/\"",
                    )],
                )
            }),
        );
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));

        let err = OAuthClient::new()
            .login(
                &format!("{base}/mcp"),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a cross-origin resource_metadata hint must be refused");
        let msg = err.to_string();
        assert!(msg.contains("different origin"), "got: {msg}");
        assert!(msg.contains("169.254.169.254"), "got: {msg}");
    }

    /// The `resource_metadata` hint is a string the remote server wrote. A value
    /// that is not a URL must be refused, not parsed leniently.
    #[tokio::test]
    async fn login_refuses_a_malformed_resource_metadata_hint() {
        let server = mock_auth_server("bad_hint").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a malformed hint must be refused");
        assert!(
            err.to_string().contains("invalid resource_metadata URL"),
            "got: {err}"
        );
    }

    /// RFC 8414 §3.3: the document's own `issuer` must match the issuer it was
    /// fetched for. Without this check, a hostile `authorization_servers[0]` in
    /// the resource document redirects the whole flow to an attacker's
    /// authorization server.
    #[tokio::test]
    async fn login_refuses_metadata_claiming_a_different_issuer() {
        let server = mock_auth_server("issuer_mismatch").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("an issuer mismatch must be refused");
        assert!(err.to_string().contains("RFC 8414"), "got: {err}");
    }

    /// A document whose `issuer` is not a URL at all.
    #[tokio::test]
    async fn login_refuses_metadata_with_an_unparseable_issuer() {
        let server = mock_auth_server("as_unparseable_issuer").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("an unparseable issuer must be refused");
        assert!(err.to_string().contains("invalid issuer"), "got: {err}");
    }

    /// An `authorization_endpoint` that parses fine but sits on somebody else's
    /// origin - where the user's browser, and the code it comes back with,
    /// would go.
    #[tokio::test]
    async fn login_refuses_an_endpoint_off_the_issuers_origin() {
        let server = mock_auth_server("foreign_endpoint").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a foreign endpoint must be refused");
        assert!(
            err.to_string().contains("is not on the issuer's origin"),
            "got: {err}"
        );
    }

    /// A document that omits `issuer` skips the §3.3 cross-check rather than
    /// failing it - there is nothing to compare against. The endpoint-origin
    /// check still applies, so this is a narrowing, not a bypass: the login
    /// proceeds normally.
    #[tokio::test]
    async fn metadata_without_an_issuer_field_still_completes() {
        let server = mock_auth_server("no_issuer_field").await;
        let auth = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("an absent issuer is not itself a failure")
            .authenticated()
            .expect("the flow should have produced credentials");
        assert!(!auth.access_token.is_empty());
    }

    /// Every URL in the discovery chain is checked, not just the first. Three
    /// call sites, three ways to reach a remote `http://`:
    ///
    /// 1. the resource-metadata URL itself, when the MCP server is remote;
    /// 2. the authorization-server metadata URLs, derived from the issuer the
    ///    resource document names;
    /// 3. the authorization/token endpoints inside that document.
    #[tokio::test]
    async fn every_step_of_discovery_refuses_remote_http() {
        let insecure = |err: anyhow::Error| {
            let msg = err.to_string();
            assert!(msg.contains("refusing OAuth discovery"), "got: {msg}");
        };

        // (1) A remote http MCP URL. No server needed: the probe simply fails,
        // discovery falls back to the well-known path, and that URL is refused.
        insecure(
            OAuthClient::new()
                .login(
                    "http://mcp.example.invalid/mcp",
                    &HashMap::new(),
                    &[],
                    auto_consent(),
                    0,
                    None,
                )
                .await
                .expect_err("a remote http MCP URL must be refused"),
        );

        // (2) The resource document names a remote http authorization server.
        let server = mock_auth_server("http_issuer").await;
        insecure(
            OAuthClient::new()
                .login(
                    &format!("{}/mcp", server.base),
                    &HashMap::new(),
                    &[],
                    auto_consent(),
                    0,
                    None,
                )
                .await
                .expect_err("a remote http issuer must be refused"),
        );

        // (3) The authorization endpoint inside an otherwise-valid document.
        let server = mock_auth_server("http_endpoint").await;
        insecure(
            OAuthClient::new()
                .login(
                    &format!("{}/mcp", server.base),
                    &HashMap::new(),
                    &[],
                    auto_consent(),
                    0,
                    None,
                )
                .await
                .expect_err("a remote http endpoint must be refused"),
        );
    }

    /// Plain HTTP to a *remote* host is refused: the flow carries a bearer
    /// token, so it would be on the wire in cleartext. The loopback exemption
    /// (which every mock server in this module relies on) is what keeps local
    /// development working.
    #[test]
    fn discovery_over_remote_http_is_refused() {
        let client = OAuthClient::new();
        let err = client
            .require_safe_discovery_url(&Url::parse("http://auth.example.com/x").unwrap())
            .expect_err("remote http must be refused");
        assert!(err.to_string().contains("must use https"), "got: {err}");
        assert!(
            client
                .require_safe_discovery_url(&Url::parse("https://auth.example.com/x").unwrap())
                .is_ok()
        );
    }

    #[tokio::test]
    async fn login_fails_when_the_authorize_endpoint_is_malformed() {
        let server = mock_auth_server("bad_authorize").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a bad authorize endpoint must fail");
        // Caught during metadata validation now, which runs before the URL is
        // built - so the message names the field rather than the later
        // build-the-authorize-URL step. Earlier is better: the endpoint never
        // reaches the browser opener.
        assert!(
            err.to_string().contains("authorization_endpoint"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_fails_when_no_authorization_server_is_named() {
        let server = mock_auth_server("no_auth_server").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("empty authorization_servers must fail");
        assert!(
            err.to_string().contains("no authorization server"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_fails_when_the_issuer_is_malformed() {
        let server = mock_auth_server("bad_issuer").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a bad issuer must fail");
        assert!(err.to_string().contains("issuer"), "got: {err}");
    }

    #[tokio::test]
    async fn login_fails_when_the_callback_is_forged() {
        // The "browser" returns a mismatched state, so wait_for_callback
        // rejects it and login propagates the failure.
        let forge: BrowserOpener = Arc::new(|authorize_url: &str| {
            drive_callback(authorize_url, Some("WRONG"));
            true
        });
        let server = mock_auth_server("default").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                forge,
                0,
                None,
            )
            .await
            .expect_err("a forged callback must fail login");
        assert!(err.to_string().contains("state mismatch"), "got: {err}");
    }

    // ─── private HTTP helpers, driven directly ────────────────────────────
    //
    // Their network-error arms only fire on a failed request, which the
    // happy-path flow never produces. Calling them against a dead port or a
    // bad-body server exercises those arms deterministically.

    #[tokio::test]
    async fn get_json_errors_on_a_dead_connection() {
        let err = OAuthClient::new()
            .get_json("http://127.0.0.1:1/x")
            .await
            .expect_err("a refused connection must fail");
        let _ = err;
    }

    #[tokio::test]
    async fn get_json_errors_on_a_non_success_status() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new().route("/x", get(|| async { StatusCode::NOT_FOUND }));
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));
        let err = OAuthClient::new()
            .get_json(&format!("{base}/x"))
            .await
            .expect_err("404 must fail");
        assert!(err.to_string().contains("404"), "got: {err}");
    }

    #[tokio::test]
    async fn get_json_errors_on_an_unparseable_body() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new().route("/x", get(|| async { "not json" }));
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));
        assert!(
            OAuthClient::new()
                .get_json(&format!("{base}/x"))
                .await
                .is_err()
        );
    }

    #[tokio::test]
    async fn post_form_errors_on_a_dead_connection() {
        assert!(
            OAuthClient::new()
                .post_form("http://127.0.0.1:1/token", &[("a", "b")])
                .await
                .is_err()
        );
    }

    #[tokio::test]
    async fn register_errors_on_a_dead_connection() {
        let meta: AuthServerMetadata = serde_json::from_value(serde_json::json!({
            "issuer": "https://x",
            "authorization_endpoint": "https://x/a",
            "token_endpoint": "https://x/t",
            "registration_endpoint": "http://127.0.0.1:1/register",
        }))
        .unwrap();
        let err = OAuthClient::new()
            .register(&meta, "http://127.0.0.1:5000/callback")
            .await
            .expect_err("a dead registration endpoint must fail");
        assert!(
            err.to_string().contains("registration request failed"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn probe_of_a_dead_server_is_unreachable() {
        let mcp = Url::parse("http://127.0.0.1:1/mcp").unwrap();
        assert_eq!(
            OAuthClient::new()
                .probe_challenge(&mcp, &HashMap::new(), &[])
                .await
                .label(),
            "unreachable"
        );
    }

    /// The case this whole path exists for. A server holding its own API token
    /// answers the probe normally, so there is no OAuth flow to run, and the
    /// old code went looking for a discovery document such a server does not
    /// publish.
    #[tokio::test]
    async fn probe_with_headers_the_server_accepts_is_satisfied() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new().route(
            "/mcp",
            post(|headers: axum::http::HeaderMap| async move {
                match headers.get(reqwest::header::AUTHORIZATION) {
                    Some(_) => StatusCode::OK,
                    None => StatusCode::UNAUTHORIZED,
                }
            }),
        );
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));

        let mcp = Url::parse(&format!("{base}/mcp")).unwrap();
        let client = OAuthClient::new();

        // Same server, same endpoint: the headers are the only difference.
        assert_eq!(
            client
                .probe_challenge(&mcp, &HashMap::new(), &[])
                .await
                .label(),
            "challenge",
            "no credentials, so the server should ask for some"
        );
        let headers = HashMap::from([(
            "Authorization".to_string(),
            "Bearer configured-token".to_string(),
        )]);
        assert_eq!(
            client.probe_challenge(&mcp, &headers, &[]).await.label(),
            "satisfied",
            "the configured header should be enough"
        );
    }

    /// The probe has to send what the transport will send, `${VAR}` expanded
    /// and all. A server that checks the *value* of the credential rejects a
    /// literal `${TOKEN}`, and a correctly configured client was being sent
    /// into an OAuth flow on the strength of that rejection.
    #[tokio::test]
    async fn the_probe_expands_variables_the_way_the_transport_will() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new().route(
            "/mcp",
            post(|headers: axum::http::HeaderMap| async move {
                // Only the expanded value is accepted.
                match headers
                    .get(reqwest::header::AUTHORIZATION)
                    .map(|v| v.as_bytes())
                {
                    Some(b"Bearer the-real-value") => StatusCode::OK,
                    _ => StatusCode::UNAUTHORIZED,
                }
            }),
        );
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));

        let mcp = Url::parse(&format!("{base}/mcp")).unwrap();
        let headers = HashMap::from([(
            "Authorization".to_string(),
            "Bearer ${LEVIATH_PROBE_TEST_TOKEN}".to_string(),
        )]);
        let client = OAuthClient::new();
        let allow = ["LEVIATH_PROBE_TEST_TOKEN".to_string()];

        let (allowed, refused) = temp_env::async_with_vars(
            [("LEVIATH_PROBE_TEST_TOKEN", Some("the-real-value"))],
            async {
                let allowed = client.probe_challenge(&mcp, &headers, &allow).await;
                // The same header with no allowlist: the value is refused, so
                // the server sees a credential it does not recognise and OAuth
                // genuinely is the right answer.
                let refused = client.probe_challenge(&mcp, &headers, &[]).await;
                (allowed, refused)
            },
        )
        .await;

        assert_eq!(
            allowed.label(),
            "satisfied",
            "an allowed variable expands, so the server accepts the request"
        );
        assert_eq!(
            refused.label(),
            "challenge",
            "a refused variable leaves a credential the server rejects"
        );
    }

    /// End to end through `login`: a server the headers already satisfy must
    /// come back as "nothing to do" rather than being dragged into discovery.
    /// The endpoint below serves no metadata at all, so a login that tries to
    /// discover fails loudly here.
    #[tokio::test]
    async fn login_is_not_required_when_the_configured_headers_suffice() {
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
        let base = format!("http://{}", listener.local_addr().unwrap());
        let app = Router::new().route("/mcp", post(|| async { StatusCode::OK }));
        tokio::spawn(std::future::IntoFuture::into_future(axum::serve(
            listener, app,
        )));

        let outcome = OAuthClient::new()
            .login(
                &format!("{base}/mcp"),
                &HashMap::from([("Authorization".to_string(), "Bearer tok".to_string())]),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect("a server that wants nothing is not a login failure");
        assert!(
            outcome.authenticated().is_none(),
            "there are no credentials to store"
        );
    }

    #[tokio::test]
    async fn login_fails_when_resource_metadata_is_not_an_object() {
        let server = mock_auth_server("resource_not_object").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("malformed resource metadata must fail");
        assert!(
            err.to_string().contains("parse resource metadata"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_fails_when_the_token_lacks_an_access_token() {
        let server = mock_auth_server("token_no_access").await;
        let err = OAuthClient::new()
            .login(
                &format!("{}/mcp", server.base),
                &HashMap::new(),
                &[],
                auto_consent(),
                0,
                None,
            )
            .await
            .expect_err("a token without access_token must fail");
        assert!(
            err.to_string().contains("parse token response"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn refresh_fails_when_the_token_lacks_an_access_token() {
        let server = mock_auth_server("token_no_access").await;
        let auth = ServerAuth {
            token_endpoint: format!("{}/token", server.base),
            refresh_token: Some("good".to_string()),
            ..Default::default()
        };
        let err = OAuthClient::new()
            .refresh(&auth, 0)
            .await
            .expect_err("a malformed refresh token response must fail");
        assert!(
            err.to_string().contains("parse token response"),
            "got: {err}"
        );
    }

    #[tokio::test]
    async fn login_rejects_a_bad_mcp_url() {
        let err = OAuthClient::new()
            .login("not a url", &HashMap::new(), &[], auto_consent(), 0, None)
            .await
            .expect_err("bad url must fail");
        assert!(
            err.to_string().contains("Invalid MCP server url"),
            "got: {err}"
        );
    }
}