mcp_proxy/

proxy.rs

//! Core proxy construction and serving.

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

use anyhow::{Context, Result};
use axum::Router;
use tokio::process::Command;
use tower::timeout::TimeoutLayer;
use tower::util::BoxCloneService;
use tower::{Layer, ServiceExt};
use tower_mcp::SessionHandle;
use tower_mcp::auth::{AuthLayer, StaticBearerValidator};
use tower_mcp::client::StdioClientTransport;
use tower_mcp::proxy::McpProxy;
use tower_mcp::{RouterRequest, RouterResponse};
use tower_resilience::retry::RetryLayer;

use crate::admin::BackendMeta;
use crate::alias;
use crate::cache;
use crate::coalesce;
use crate::config::{AuthConfig, ProxyConfig, TransportType};
use crate::filter::CapabilityFilterService;
#[cfg(feature = "oauth")]
use crate::rbac::{RbacConfig, RbacService};
use crate::validation::{ValidationConfig, ValidationService};

/// A fully constructed MCP proxy ready to serve or embed.
pub struct Proxy {
    router: Router,
    session_handle: SessionHandle,
    inner: McpProxy,
    config: ProxyConfig,
    #[cfg(feature = "discovery")]
    discovery_index: Option<crate::discovery::SharedDiscoveryIndex>,
}

impl Proxy {
    /// Build a proxy from a [`ProxyConfig`].
    ///
    /// Connects to all backends, builds the middleware stack, and prepares
    /// the axum router. Call [`serve()`](Self::serve) to run standalone or
    /// [`into_router()`](Self::into_router) to embed in an existing app.
    pub async fn from_config(config: ProxyConfig) -> Result<Self> {
        let (mcp_proxy, cb_handles) = build_mcp_proxy(&config).await?;
        let proxy_for_admin = mcp_proxy.clone();
        // Only the discovery index build mutates this clone.
        #[cfg_attr(not(feature = "discovery"), allow(unused_mut))]
        let mut proxy_for_caller = mcp_proxy.clone();
        let proxy_for_management = mcp_proxy.clone();

        // Install Prometheus metrics recorder (must happen before middleware)
        #[cfg(feature = "metrics")]
        let metrics_handle = if config.observability.metrics.enabled {
            tracing::info!("Prometheus metrics enabled at /admin/metrics");
            let builder = metrics_exporter_prometheus::PrometheusBuilder::new();
            let handle = builder
                .install_recorder()
                .context("installing Prometheus metrics recorder")?;
            Some(handle)
        } else {
            None
        };
        #[cfg(not(feature = "metrics"))]
        let metrics_handle = None;

        let (service, cache_handle) = build_middleware_stack(&config, mcp_proxy)?;

        let (router, session_handle) =
            tower_mcp::transport::http::HttpTransport::from_service(service)
                .into_router_with_handle();

        // Inbound authentication (axum-level middleware)
        let router = apply_auth(&config, router).await?;

        // Collect backend metadata for the health checker
        let backend_meta: std::collections::HashMap<String, BackendMeta> = config
            .backends
            .iter()
            .map(|b| {
                (
                    b.name.clone(),
                    BackendMeta {
                        transport: format!("{:?}", b.transport).to_lowercase(),
                    },
                )
            })
            .collect();

        // Admin API
        let admin_state = crate::admin::spawn_health_checker(
            proxy_for_admin,
            config.proxy.name.clone(),
            config.proxy.version.clone(),
            config.backends.len(),
            backend_meta,
        );
        let router = router.nest(
            "/admin",
            crate::admin::admin_router(
                admin_state.clone(),
                metrics_handle,
                session_handle.clone(),
                cache_handle,
                proxy_for_management,
                &config,
                config.source_path.clone(),
                cb_handles,
            ),
        );
        tracing::info!("Admin API enabled at /admin/backends");

        // Build discovery index if enabled (search mode implies discovery)
        #[cfg(feature = "discovery")]
        let discovery_enabled = config.proxy.tool_discovery
            || config.proxy.tool_exposure == crate::config::ToolExposure::Search;
        #[cfg(feature = "discovery")]
        let (discovery_index, discovery_tools) = if discovery_enabled {
            let index =
                crate::discovery::build_index(&mut proxy_for_caller, &config.proxy.separator).await;
            let tools = crate::discovery::build_discovery_tools(index.clone());
            (Some(index), Some(tools))
        } else {
            (None, None)
        };
        #[cfg(not(feature = "discovery"))]
        let discovery_tools: Option<Vec<tower_mcp::Tool>> = None;

        // MCP admin tools (proxy/ namespace)
        if let Err(e) = crate::admin_tools::register_admin_tools(
            &proxy_for_caller,
            admin_state,
            session_handle.clone(),
            &config,
            discovery_tools,
        )
        .await
        {
            tracing::warn!("Failed to register admin tools: {e}");
        } else {
            tracing::info!("MCP admin tools registered under proxy/ namespace");
        }

        Ok(Self {
            router,
            session_handle,
            inner: proxy_for_caller,
            config,
            #[cfg(feature = "discovery")]
            discovery_index,
        })
    }

    /// Get a reference to the session handle for monitoring active sessions.
    pub fn session_handle(&self) -> &SessionHandle {
        &self.session_handle
    }

    /// Get a reference to the underlying [`McpProxy`] for dynamic operations.
    ///
    /// Use this to add backends dynamically via [`McpProxy::add_backend()`].
    pub fn mcp_proxy(&self) -> &McpProxy {
        &self.inner
    }

    /// Enable hot reload by watching the given config file path.
    ///
    /// New backends added to the config file will be connected dynamically
    /// without restarting the proxy.
    pub fn enable_hot_reload(&self, config_path: std::path::PathBuf) {
        tracing::info!("Hot reload enabled, watching config file for changes");
        crate::reload::spawn_config_watcher(
            config_path,
            self.inner.clone(),
            #[cfg(feature = "discovery")]
            self.discovery_index
                .as_ref()
                .map(|idx| (idx.clone(), self.config.proxy.separator.clone())),
        );
    }

    /// Consume the proxy and return the axum Router and SessionHandle.
    ///
    /// Use this to embed the proxy in an existing axum application:
    ///
    /// ```rust,ignore
    /// let (proxy_router, session_handle) = proxy.into_router();
    ///
    /// let app = Router::new()
    ///     .nest("/mcp", proxy_router)
    ///     .route("/health", get(|| async { "ok" }));
    /// ```
    pub fn into_router(self) -> (Router, SessionHandle) {
        (self.router, self.session_handle)
    }

    /// Serve the proxy on the configured listen address.
    ///
    /// Blocks until a shutdown signal (SIGTERM/SIGINT) is received,
    /// then drains connections for the configured timeout period.
    pub async fn serve(self) -> Result<()> {
        let addr = format!(
            "{}:{}",
            self.config.proxy.listen.host, self.config.proxy.listen.port
        );

        tracing::info!(listen = %addr, "Proxy ready");

        let listener = tokio::net::TcpListener::bind(&addr)
            .await
            .with_context(|| format!("binding to {}", addr))?;

        let shutdown_timeout = Duration::from_secs(self.config.proxy.shutdown_timeout_seconds);
        axum::serve(listener, self.router)
            .with_graceful_shutdown(shutdown_signal(shutdown_timeout))
            .await
            .context("server error")?;

        tracing::info!("Proxy shut down");
        Ok(())
    }
}

/// Circuit breaker handle type alias.
pub type CbHandle = tower_resilience::circuitbreaker::CircuitBreakerHandle;

/// Build the circuit breaker layer for one backend from its config.
///
/// The count-based breaker only evaluates once its sliding window is full,
/// and the window defaults to 100 calls. Aligning the window with
/// `minimum_calls` makes the failure rate cover the most recent
/// `minimum_calls` calls and evaluation begin exactly at the documented
/// threshold (#220).
fn build_breaker_layer(
    cb: &crate::config::CircuitBreakerConfig,
    backend_name: &str,
) -> (
    tower_resilience::circuitbreaker::CircuitBreakerLayer,
    CbHandle,
) {
    tower_resilience::circuitbreaker::CircuitBreakerLayer::builder()
        .failure_rate_threshold(cb.failure_rate_threshold)
        .minimum_number_of_calls(cb.minimum_calls)
        .sliding_window_size(cb.minimum_calls)
        .wait_duration_in_open(Duration::from_secs(cb.wait_duration_seconds))
        .permitted_calls_in_half_open(cb.permitted_calls_in_half_open)
        .name(format!("{backend_name}-cb"))
        .build_with_handle()
}

type InfallibleService = BoxCloneService<RouterRequest, RouterResponse, Infallible>;
type MwService = BoxCloneService<RouterRequest, RouterResponse, tower::BoxError>;
type MwStage = Box<dyn Fn(MwService) -> MwService + Send + Sync>;

/// Every configured per-backend middleware composed into ONE tower-mcp
/// backend layer. tower-mcp's `backend_layer` replaces the previously applied
/// layer rather than stacking (joshrotenberg/tower-mcp#1173), so handing it
/// the middlewares one call at a time silently keeps only the last.
///
/// Composition happens in three zones matching the middlewares' type
/// contracts: retry and hedging operate on the raw `Error = Infallible`
/// backend service (both classify failures from the response, and hedging
/// requires a `Clone` error type); concurrency, rate limit, timeout, and
/// circuit breaker operate in a widened `BoxError` plane where they can
/// produce service-level errors; a `CatchError` fold then converts those
/// errors into JSON-RPC error responses, and outlier detection sits on top,
/// observing the response plane (its `Service` impl requires
/// `Error = Infallible`).
#[derive(Default)]
struct BackendMiddlewareLayer {
    /// Innermost: retries against the raw backend service.
    retry: Option<RetryLayer<RouterRequest, RouterResponse, Infallible>>,
    /// Outside retry, still in the Infallible zone.
    hedge: Option<tower_resilience::hedge::HedgeLayer>,
    /// BoxError zone, applied outward in push order: concurrency, rate
    /// limit, timeout, circuit breaker.
    stages: Vec<MwStage>,
    /// Outermost, after the error fold: observes response-plane errors.
    outlier: Option<crate::outlier::OutlierDetectionLayer>,
}

impl BackendMiddlewareLayer {
    fn is_empty(&self) -> bool {
        self.retry.is_none()
            && self.hedge.is_none()
            && self.stages.is_empty()
            && self.outlier.is_none()
    }

    fn stage<L>(&mut self, layer: L)
    where
        L: Layer<MwService> + Send + Sync + 'static,
        L::Service:
            tower::Service<RouterRequest, Response = RouterResponse> + Clone + Send + 'static,
        <L::Service as tower::Service<RouterRequest>>::Error: std::fmt::Display,
        <L::Service as tower::Service<RouterRequest>>::Future: Send + 'static,
    {
        self.stages.push(Box::new(move |inner| {
            BoxCloneService::new(
                layer
                    .layer(inner)
                    .map_err(|e| -> tower::BoxError { e.to_string().into() }),
            )
        }));
    }
}

impl<S> Layer<S> for BackendMiddlewareLayer
where
    S: tower::Service<RouterRequest, Response = RouterResponse, Error = Infallible>
        + Clone
        + Send
        + 'static,
    S::Future: Send + 'static,
{
    type Service = InfallibleService;

    fn layer(&self, base: S) -> InfallibleService {
        fn widen(e: Infallible) -> tower::BoxError {
            match e {}
        }

        // Zone A: retry classifies failures from responses over the
        // Infallible base (hedging also needs a Clone inner error, which
        // Infallible satisfies).
        let after_retry: InfallibleService = match &self.retry {
            Some(retry) => BoxCloneService::new(retry.layer(base)),
            None => BoxCloneService::new(base),
        };

        // Zone B: error-producing middlewares in the BoxError plane. Hedging
        // wraps the inner error type (`HedgeError<Infallible>`), so it forms
        // the floor of this zone.
        let mut svc: MwService = match &self.hedge {
            Some(hedge) => BoxCloneService::new(
                hedge
                    .layer(after_retry)
                    .map_err(|e| -> tower::BoxError { e.to_string().into() }),
            ),
            None => BoxCloneService::new(after_retry.map_err(widen)),
        };
        for stage in &self.stages {
            svc = stage(svc);
        }

        // Fold middleware errors into JSON-RPC error responses.
        let folded: InfallibleService =
            BoxCloneService::new(tower_mcp::transport::CatchError::new(svc));

        // Zone C: outlier detection observes the response plane.
        match &self.outlier {
            Some(outlier) => BoxCloneService::new(outlier.layer(folded)),
            None => folded,
        }
    }
}

/// Build the McpProxy with all backends and per-backend middleware.
/// Returns the proxy and a map of backend name -> circuit breaker handle.
async fn build_mcp_proxy(config: &ProxyConfig) -> Result<(McpProxy, HashMap<String, CbHandle>)> {
    let mut builder = McpProxy::builder(&config.proxy.name, &config.proxy.version)
        .separator(&config.proxy.separator);
    let mut cb_handles: HashMap<String, CbHandle> = HashMap::new();

    if let Some(instructions) = &config.proxy.instructions {
        builder = builder.instructions(instructions);
    }

    // Create shared outlier detector if any backend has outlier_detection configured.
    // Use the max of all max_ejection_percent values.
    let outlier_detector = {
        let max_pct = config
            .backends
            .iter()
            .filter_map(|b| b.outlier_detection.as_ref())
            .map(|od| od.max_ejection_percent)
            .max();
        max_pct.map(crate::outlier::OutlierDetector::new)
    };

    for backend in &config.backends {
        tracing::info!(name = %backend.name, transport = ?backend.transport, "Adding backend");

        match backend.transport {
            TransportType::Stdio => {
                let command = backend.command.as_deref().unwrap();
                let args: Vec<&str> = backend.args.iter().map(|s| s.as_str()).collect();

                let mut cmd = Command::new(command);
                cmd.args(&args);

                for (key, value) in &backend.env {
                    cmd.env(key, value);
                }

                let transport = StdioClientTransport::spawn_command(&mut cmd)
                    .await
                    .with_context(|| format!("spawning backend '{}'", backend.name))?;

                builder = builder.backend(&backend.name, transport).await;
            }
            TransportType::Http => {
                let url = backend.url.as_deref().unwrap();
                let mut transport = tower_mcp::client::HttpClientTransport::new(url);
                if let Some(token) = &backend.bearer_token {
                    transport = transport.bearer_token(token);
                }

                builder = builder.backend(&backend.name, transport).await;
            }
            #[cfg(feature = "websocket")]
            TransportType::Websocket => {
                let url = backend.url.as_deref().unwrap();
                tracing::info!(url = %url, "Connecting to WebSocket backend");
                let transport = if let Some(token) = &backend.bearer_token {
                    crate::ws_transport::WebSocketClientTransport::connect_with_bearer_token(
                        url, token,
                    )
                    .await
                    .with_context(|| {
                        format!("connecting to WebSocket backend '{}'", backend.name)
                    })?
                } else {
                    crate::ws_transport::WebSocketClientTransport::connect(url)
                        .await
                        .with_context(|| {
                            format!("connecting to WebSocket backend '{}'", backend.name)
                        })?
                };

                builder = builder.backend(&backend.name, transport).await;
            }
            #[cfg(not(feature = "websocket"))]
            TransportType::Websocket => {
                anyhow::bail!(
                    "WebSocket transport requires the 'websocket' feature. \
                     Rebuild with: cargo install mcp-proxy --features websocket"
                );
            }
        }

        // Per-backend middleware stack (applied in order: inner -> outer).
        // Collected into one BackendMiddlewareLayer and handed to tower-mcp
        // as a single backend_layer call; see the type's doc comment.
        let mut mw = BackendMiddlewareLayer::default();

        // Retry (innermost -- retries happen before other middleware)
        if let Some(retry_cfg) = &backend.retry {
            tracing::info!(
                backend = %backend.name,
                max_retries = retry_cfg.max_retries,
                initial_backoff_ms = retry_cfg.initial_backoff_ms,
                max_backoff_ms = retry_cfg.max_backoff_ms,
                "Applying retry policy"
            );
            mw.retry = Some(crate::retry::build_retry_layer(retry_cfg, &backend.name));
        }

        // Hedging (after retry, before concurrency -- hedges are separate requests)
        if let Some(hedge_cfg) = &backend.hedging {
            let delay = Duration::from_millis(hedge_cfg.delay_ms);
            let max_attempts = hedge_cfg.max_hedges + 1; // +1 for the primary request
            tracing::info!(
                backend = %backend.name,
                delay_ms = hedge_cfg.delay_ms,
                max_hedges = hedge_cfg.max_hedges,
                "Applying request hedging"
            );
            let layer = if delay.is_zero() {
                tower_resilience::hedge::HedgeLayer::builder()
                    .no_delay()
                    .max_hedged_attempts(max_attempts)
                    .name(format!("{}-hedge", backend.name))
                    .build()
            } else {
                tower_resilience::hedge::HedgeLayer::builder()
                    .delay(delay)
                    .max_hedged_attempts(max_attempts)
                    .name(format!("{}-hedge", backend.name))
                    .build()
            };
            mw.hedge = Some(layer);
        }

        // Concurrency limit
        if let Some(cc) = &backend.concurrency {
            tracing::info!(
                backend = %backend.name,
                max = cc.max_concurrent,
                "Applying concurrency limit"
            );
            mw.stage(tower::limit::ConcurrencyLimitLayer::new(cc.max_concurrent));
        }

        // Rate limit
        if let Some(rl) = &backend.rate_limit {
            tracing::info!(
                backend = %backend.name,
                requests = rl.requests,
                period_seconds = rl.period_seconds,
                "Applying rate limit"
            );
            let layer = tower_resilience::ratelimiter::RateLimiterLayer::builder()
                .limit_for_period(rl.requests)
                .refresh_period(Duration::from_secs(rl.period_seconds))
                .name(format!("{}-ratelimit", backend.name))
                .build();
            mw.stage(layer);
        }

        // Timeout
        if let Some(timeout) = &backend.timeout {
            tracing::info!(
                backend = %backend.name,
                seconds = timeout.seconds,
                "Applying timeout"
            );
            mw.stage(TimeoutLayer::new(Duration::from_secs(timeout.seconds)));
        }

        // Circuit breaker
        if let Some(cb) = &backend.circuit_breaker {
            tracing::info!(
                backend = %backend.name,
                failure_rate = cb.failure_rate_threshold,
                wait_seconds = cb.wait_duration_seconds,
                "Applying circuit breaker"
            );
            let (layer, handle) = build_breaker_layer(cb, &backend.name);
            cb_handles.insert(backend.name.clone(), handle);
            mw.stage(layer);
        }

        // Outlier detection (outermost -- observes errors after all other middleware)
        if let Some(od) = &backend.outlier_detection
            && let Some(ref detector) = outlier_detector
        {
            tracing::info!(
                backend = %backend.name,
                consecutive_errors = od.consecutive_errors,
                base_ejection_seconds = od.base_ejection_seconds,
                max_ejection_percent = od.max_ejection_percent,
                "Applying outlier detection"
            );
            let layer = crate::outlier::OutlierDetectionLayer::new(
                backend.name.clone(),
                od.clone(),
                detector.clone(),
            );
            mw.outlier = Some(layer);
        }

        if !mw.is_empty() {
            builder = builder.backend_layer(mw);
        }
    }

    let result = builder.build().await?;

    if !result.skipped.is_empty() {
        for s in &result.skipped {
            tracing::warn!("Skipped backend: {s}");
        }
    }

    Ok((result.proxy, cb_handles))
}

/// Build a scope-enforcement layer from configured OAuth `required_scopes`.
///
/// Returns `None` when no scopes are required (the layer would be a no-op).
/// Otherwise returns a [`ScopeEnforcementLayer`](tower_mcp::oauth::ScopeEnforcementLayer)
/// whose default policy requires *all* of `required_scopes` to be present in the
/// token (AND semantics) for every request.
#[cfg(feature = "oauth")]
fn oauth_scope_layer(
    required_scopes: &[String],
) -> Option<tower_mcp::oauth::ScopeEnforcementLayer> {
    if required_scopes.is_empty() {
        return None;
    }
    let policy = tower_mcp::oauth::ScopePolicy::new().default_scopes(
        tower_mcp::oauth::ScopeRequirement::all(required_scopes.iter().cloned()),
    );
    Some(tower_mcp::oauth::ScopeEnforcementLayer::new(policy))
}

/// Build the MCP-level middleware stack around the proxy.
fn build_middleware_stack(
    config: &ProxyConfig,
    proxy: McpProxy,
) -> Result<(
    BoxCloneService<RouterRequest, RouterResponse, Infallible>,
    Option<cache::CacheHandle>,
)> {
    let mut service: BoxCloneService<RouterRequest, RouterResponse, Infallible> =
        BoxCloneService::new(proxy);
    let mut cache_handle: Option<cache::CacheHandle> = None;

    // Argument injection (innermost -- merges default/per-tool args into CallTool requests)
    let injection_rules: Vec<_> = config
        .backends
        .iter()
        .filter(|b| !b.default_args.is_empty() || !b.inject_args.is_empty())
        .map(|b| {
            let namespace = format!("{}{}", b.name, config.proxy.separator);
            tracing::info!(
                backend = %b.name,
                default_args = b.default_args.len(),
                tool_rules = b.inject_args.len(),
                "Applying argument injection"
            );
            crate::inject::InjectionRules::new(
                namespace,
                b.default_args.clone(),
                b.inject_args.clone(),
            )
        })
        .collect();

    if !injection_rules.is_empty() {
        service = BoxCloneService::new(crate::inject::InjectArgsService::new(
            service,
            injection_rules,
        ));
    }

    // Parameter overrides (after inject, before filter -- hides/renames tool params)
    let param_overrides: Vec<_> = config
        .backends
        .iter()
        .filter(|b| !b.param_overrides.is_empty())
        .flat_map(|b| {
            let namespace = format!("{}{}", b.name, config.proxy.separator);
            tracing::info!(
                backend = %b.name,
                overrides = b.param_overrides.len(),
                "Applying parameter overrides"
            );
            b.param_overrides
                .iter()
                .map(move |c| crate::param_override::ToolOverride::new(&namespace, c))
        })
        .collect();

    if !param_overrides.is_empty() {
        service = BoxCloneService::new(crate::param_override::ParamOverrideService::new(
            service,
            param_overrides,
        ));
    }

    // Canary routing (rewrites requests from primary to canary namespace based on weight)
    let canary_mappings: std::collections::HashMap<String, (String, u32, u32)> = config
        .backends
        .iter()
        .filter_map(|b| {
            b.canary_of.as_ref().map(|primary_name| {
                // Find the primary backend's weight
                let primary_weight = config
                    .backends
                    .iter()
                    .find(|p| p.name == *primary_name)
                    .map(|p| p.weight)
                    .unwrap_or(100);
                (
                    primary_name.clone(),
                    (b.name.clone(), primary_weight, b.weight),
                )
            })
        })
        .collect();

    if !canary_mappings.is_empty() {
        for (primary, (canary, pw, cw)) in &canary_mappings {
            tracing::info!(
                primary = %primary,
                canary = %canary,
                primary_weight = pw,
                canary_weight = cw,
                "Enabling canary routing"
            );
        }
        service = BoxCloneService::new(crate::canary::CanaryService::new(
            service,
            canary_mappings,
            &config.proxy.separator,
        ));
    }

    // Failover routing (deterministic fallback on primary error)
    // Collect failover backends grouped by primary, sorted by priority (ascending).
    let mut failover_groups: std::collections::HashMap<String, Vec<(u32, String)>> =
        std::collections::HashMap::new();
    for b in &config.backends {
        if let Some(ref primary) = b.failover_for {
            failover_groups
                .entry(primary.clone())
                .or_default()
                .push((b.priority, b.name.clone()));
        }
    }
    // Sort each group by priority (lower = preferred)
    let failover_mappings: std::collections::HashMap<String, Vec<String>> = failover_groups
        .into_iter()
        .map(|(primary, mut backends)| {
            backends.sort_by_key(|(priority, _)| *priority);
            let names: Vec<String> = backends.into_iter().map(|(_, name)| name).collect();
            (primary, names)
        })
        .collect();

    if !failover_mappings.is_empty() {
        for (primary, failovers) in &failover_mappings {
            tracing::info!(
                primary = %primary,
                failovers = ?failovers,
                "Enabling failover routing"
            );
        }
        service = BoxCloneService::new(crate::failover::FailoverService::new(
            service,
            failover_mappings,
            &config.proxy.separator,
        ));
    }

    // Traffic mirroring (sends cloned requests through the proxy)
    let mirror_mappings: std::collections::HashMap<String, (String, u32)> = config
        .backends
        .iter()
        .filter_map(|b| {
            b.mirror_of
                .as_ref()
                .map(|source| (source.clone(), (b.name.clone(), b.mirror_percent)))
        })
        .collect();

    if !mirror_mappings.is_empty() {
        for (source, (mirror, pct)) in &mirror_mappings {
            tracing::info!(
                source = %source,
                mirror = %mirror,
                percent = pct,
                "Enabling traffic mirroring"
            );
        }
        service = BoxCloneService::new(crate::mirror::MirrorService::new(
            service,
            mirror_mappings,
            &config.proxy.separator,
        ));
    }

    // Response caching
    let cache_configs: Vec<_> = config
        .backends
        .iter()
        .filter_map(|b| {
            b.cache
                .as_ref()
                .map(|c| (format!("{}{}", b.name, config.proxy.separator), c))
        })
        .collect();

    if !cache_configs.is_empty() {
        for (ns, cfg) in &cache_configs {
            tracing::info!(
                backend = %ns.trim_end_matches(&config.proxy.separator),
                resource_ttl = cfg.resource_ttl_seconds,
                tool_ttl = cfg.tool_ttl_seconds,
                max_entries = cfg.max_entries,
                "Applying response cache"
            );
        }
        let (cache_svc, handle) = cache::CacheService::new(service, cache_configs, &config.cache);
        service = BoxCloneService::new(cache_svc);
        cache_handle = Some(handle);
    }

    // Request coalescing
    if config.performance.coalesce_requests {
        tracing::info!("Request coalescing enabled");
        service = BoxCloneService::new(coalesce::CoalesceService::new(service));
    }

    // Request validation
    if config.security.max_argument_size.is_some() {
        let validation = ValidationConfig {
            max_argument_size: config.security.max_argument_size,
        };
        if let Some(max) = validation.max_argument_size {
            tracing::info!(max_argument_size = max, "Applying request validation");
        }
        service = BoxCloneService::new(ValidationService::new(service, validation));
    }

    // Static capability filtering
    let filters: Vec<_> = config
        .backends
        .iter()
        .filter_map(|b| b.build_filter(&config.proxy.separator).transpose())
        .collect::<anyhow::Result<Vec<_>>>()?;

    if !filters.is_empty() {
        for f in &filters {
            tracing::info!(
                backend = %f.namespace.trim_end_matches(&config.proxy.separator),
                tool_filter = ?f.tool_filter,
                resource_filter = ?f.resource_filter,
                prompt_filter = ?f.prompt_filter,
                "Applying capability filter"
            );
        }
        service = BoxCloneService::new(CapabilityFilterService::new(service, filters));
    }

    // Search-mode filtering: hide all tools except proxy/ namespace
    if config.proxy.tool_exposure == crate::config::ToolExposure::Search {
        let prefix = format!("proxy{}", config.proxy.separator);
        tracing::info!(
            prefix = %prefix,
            "Search mode: ListTools will only show proxy/ namespace tools"
        );
        service =
            BoxCloneService::new(crate::filter::SearchModeFilterService::new(service, prefix));
    }

    // Tool aliasing
    let alias_mappings: Vec<_> = config
        .backends
        .iter()
        .flat_map(|b| {
            let ns = format!("{}{}", b.name, config.proxy.separator);
            b.aliases
                .iter()
                .map(move |a| (ns.clone(), a.from.clone(), a.to.clone()))
        })
        .collect();

    if let Some(alias_map) = alias::AliasMap::new(alias_mappings) {
        let count = alias_map.forward.len();
        tracing::info!(aliases = count, "Applying tool aliases");
        service = BoxCloneService::new(alias::AliasService::new(service, alias_map));
    }

    // Composite tools (fan-out to multiple backend tools)
    if !config.composite_tools.is_empty() {
        let count = config.composite_tools.len();
        tracing::info!(composite_tools = count, "Applying composite tool fan-out");
        service = BoxCloneService::new(crate::composite::CompositeService::new(
            service,
            config.composite_tools.clone(),
        ));
    }

    // Bearer token scoping (per-token allow/deny lists)
    #[cfg(feature = "oauth")]
    if matches!(
        &config.auth,
        Some(AuthConfig::Bearer {
            scoped_tokens,
            ..
        }) if !scoped_tokens.is_empty()
    ) {
        tracing::info!("Enabling bearer token scoping middleware");
        service = BoxCloneService::new(crate::bearer_scope::BearerScopingService::new(service));
    }

    // RBAC (JWT auth only)
    #[cfg(feature = "oauth")]
    {
        let rbac_config = match &config.auth {
            Some(
                AuthConfig::Jwt {
                    roles,
                    role_mapping: Some(mapping),
                    ..
                }
                | AuthConfig::OAuth {
                    roles,
                    role_mapping: Some(mapping),
                    ..
                },
            ) if !roles.is_empty() => {
                tracing::info!(
                    roles = roles.len(),
                    claim = %mapping.claim,
                    "Enabling RBAC"
                );
                Some(RbacConfig::new(roles, mapping))
            }
            _ => None,
        };

        if let Some(rbac) = rbac_config {
            service = BoxCloneService::new(RbacService::new(service, rbac));
        }

        // OAuth `required_scopes` enforcement: a coarse global gate that rejects
        // any token missing one of the configured scopes (AND semantics). Runs
        // outside RBAC so a token lacking the required scopes is denied for every
        // operation, including `tools/list`. Reads TokenClaims injected by the
        // OAuth auth layer; requests without claims pass through (already rejected
        // upstream by the HTTP auth layer when auth is enabled).
        let required_scopes: &[String] = match &config.auth {
            Some(AuthConfig::OAuth {
                required_scopes, ..
            }) => required_scopes,
            _ => &[],
        };
        if let Some(layer) = oauth_scope_layer(required_scopes) {
            tracing::info!(
                scopes = ?required_scopes,
                "Enabling OAuth required_scopes enforcement"
            );
            service = BoxCloneService::new(tower::Layer::layer(&layer, service));
        }

        // Token passthrough (inject ClientToken for forward_auth backends)
        let forward_namespaces: std::collections::HashSet<String> = config
            .backends
            .iter()
            .filter(|b| b.forward_auth)
            .map(|b| format!("{}{}", b.name, config.proxy.separator))
            .collect();

        if !forward_namespaces.is_empty() {
            tracing::info!(
                backends = ?forward_namespaces,
                "Enabling token passthrough for forward_auth backends"
            );
            service = BoxCloneService::new(crate::token::TokenPassthroughService::new(
                service,
                forward_namespaces,
            ));
        }
    }

    // Metrics
    #[cfg(feature = "metrics")]
    if config.observability.metrics.enabled {
        service = BoxCloneService::new(crate::metrics::MetricsService::new(service));
    }

    // Structured access logging
    if config.observability.access_log.enabled {
        tracing::info!("Access logging enabled (target: mcp::access)");
        service = BoxCloneService::new(crate::access_log::AccessLogService::new(
            service,
            &config.proxy.separator,
        ));
    }

    // Audit logging
    if config.observability.audit {
        tracing::info!("Audit logging enabled (target: mcp::audit)");
        let audited = tower::Layer::layer(&tower_mcp::AuditLayer::new(), service);
        service = BoxCloneService::new(tower_mcp::CatchError::new(audited));
    }

    // Global rate limit (outermost -- protects entire proxy)
    if let Some(ref rl) = config.proxy.rate_limit {
        tracing::info!(
            requests = rl.requests,
            period_seconds = rl.period_seconds,
            "Applying global rate limit"
        );
        let layer = tower_resilience::ratelimiter::RateLimiterLayer::builder()
            .limit_for_period(rl.requests)
            .refresh_period(Duration::from_secs(rl.period_seconds))
            .name("global-ratelimit")
            .build();
        let limited = tower::Layer::layer(&layer, service);
        service = BoxCloneService::new(tower_mcp::CatchError::new(limited));
    }

    Ok((service, cache_handle))
}

/// Apply inbound authentication middleware to the router.
async fn apply_auth(config: &ProxyConfig, router: Router) -> Result<Router> {
    let router = if let Some(auth) = &config.auth {
        match auth {
            AuthConfig::Bearer {
                tokens,
                scoped_tokens,
            } => {
                let total = tokens.len() + scoped_tokens.len();
                if scoped_tokens.is_empty() {
                    // Simple bearer auth: use StaticBearerValidator
                    tracing::info!(token_count = total, "Enabling bearer token auth");
                    let validator = StaticBearerValidator::new(tokens.iter().cloned());
                    let layer = AuthLayer::new(validator);
                    router.layer(layer)
                } else {
                    // Scoped bearer auth: use custom layer that injects TokenClaims
                    #[cfg(feature = "oauth")]
                    {
                        tracing::info!(
                            token_count = total,
                            scoped = scoped_tokens.len(),
                            "Enabling bearer token auth with per-token scoping"
                        );
                        let layer =
                            crate::bearer_scope::ScopedBearerAuthLayer::new(tokens, scoped_tokens);
                        router.layer(layer)
                    }
                    #[cfg(not(feature = "oauth"))]
                    {
                        anyhow::bail!(
                            "Per-token tool scoping requires the 'oauth' feature. \
                             Rebuild with: cargo install mcp-proxy --features oauth"
                        );
                    }
                }
            }
            #[cfg(feature = "oauth")]
            AuthConfig::Jwt {
                issuer,
                audience,
                jwks_uri,
                ..
            } => {
                tracing::info!(
                    issuer = %issuer,
                    audience = %audience,
                    jwks_uri = %jwks_uri,
                    "Enabling JWT auth (JWKS)"
                );
                let validator = tower_mcp::oauth::JwksValidator::builder(jwks_uri)
                    .expected_audience(audience)
                    .expected_issuer(issuer)
                    .build()
                    .await
                    .context("building JWKS validator")?;

                let addr = format!(
                    "http://{}:{}",
                    config.proxy.listen.host, config.proxy.listen.port
                );
                let metadata = tower_mcp::oauth::ProtectedResourceMetadata::new(&addr)
                    .authorization_server(issuer);

                let layer = tower_mcp::oauth::OAuthLayer::new(validator, metadata);
                router.layer(layer)
            }
            #[cfg(not(feature = "oauth"))]
            AuthConfig::Jwt { .. } => {
                anyhow::bail!(
                    "JWT auth requires the 'oauth' feature. Rebuild with: cargo install mcp-proxy --features oauth"
                );
            }
            #[cfg(feature = "oauth")]
            AuthConfig::OAuth {
                issuer,
                audience,
                token_validation,
                jwks_uri,
                introspection_endpoint,
                client_id,
                client_secret,
                ..
            } => {
                use crate::config::TokenValidationStrategy;

                tracing::info!(
                    issuer = %issuer,
                    audience = %audience,
                    strategy = ?token_validation,
                    "Enabling OAuth 2.1 auth"
                );

                // Auto-discover endpoints from issuer if not overridden
                let discovered = crate::introspection::discover_auth_server(issuer)
                    .await
                    .context("discovering OAuth authorization server")?;

                let effective_jwks_uri = jwks_uri
                    .as_deref()
                    .or(discovered.jwks_uri.as_deref())
                    .ok_or_else(|| {
                        anyhow::anyhow!(
                            "JWKS URI not found via discovery and not configured manually"
                        )
                    })?;

                let effective_introspection = introspection_endpoint
                    .as_deref()
                    .or(discovered.introspection_endpoint.as_deref());

                let addr = format!(
                    "http://{}:{}",
                    config.proxy.listen.host, config.proxy.listen.port
                );
                let metadata = tower_mcp::oauth::ProtectedResourceMetadata::new(&addr)
                    .authorization_server(issuer);

                match token_validation {
                    TokenValidationStrategy::Jwt => {
                        let validator =
                            tower_mcp::oauth::JwksValidator::builder(effective_jwks_uri)
                                .expected_audience(audience)
                                .expected_issuer(issuer)
                                .build()
                                .await
                                .context("building JWKS validator")?;
                        let layer = tower_mcp::oauth::OAuthLayer::new(validator, metadata);
                        router.layer(layer)
                    }
                    TokenValidationStrategy::Introspection => {
                        let endpoint = effective_introspection.ok_or_else(|| {
                            anyhow::anyhow!(
                                "introspection endpoint not found via discovery and not configured"
                            )
                        })?;
                        let validator = crate::introspection::IntrospectionValidator::new(
                            endpoint,
                            client_id.as_deref().unwrap(),
                            client_secret.as_deref().unwrap(),
                        )
                        .expected_audience(audience);
                        let layer = tower_mcp::oauth::OAuthLayer::new(validator, metadata);
                        router.layer(layer)
                    }
                    TokenValidationStrategy::Both => {
                        let endpoint = effective_introspection.ok_or_else(|| {
                            anyhow::anyhow!(
                                "introspection endpoint not found via discovery and not configured"
                            )
                        })?;
                        let jwt_validator =
                            tower_mcp::oauth::JwksValidator::builder(effective_jwks_uri)
                                .expected_audience(audience)
                                .expected_issuer(issuer)
                                .build()
                                .await
                                .context("building JWKS validator")?;
                        let introspection_validator =
                            crate::introspection::IntrospectionValidator::new(
                                endpoint,
                                client_id.as_deref().unwrap(),
                                client_secret.as_deref().unwrap(),
                            )
                            .expected_audience(audience);
                        let fallback = crate::introspection::FallbackValidator::new(
                            jwt_validator,
                            introspection_validator,
                        );
                        let layer = tower_mcp::oauth::OAuthLayer::new(fallback, metadata);
                        router.layer(layer)
                    }
                }
            }
            #[cfg(not(feature = "oauth"))]
            AuthConfig::OAuth { .. } => {
                anyhow::bail!(
                    "OAuth auth requires the 'oauth' feature. Rebuild with: cargo install mcp-proxy --features oauth"
                );
            }
        }
    } else {
        router
    };
    Ok(router)
}

/// Wait for SIGTERM or SIGINT, then log and return.
pub async fn shutdown_signal(timeout: Duration) {
    let ctrl_c = tokio::signal::ctrl_c();
    #[cfg(unix)]
    {
        let mut sigterm = tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate())
            .expect("SIGTERM handler");
        tokio::select! {
            _ = ctrl_c => {},
            _ = sigterm.recv() => {},
        }
    }
    #[cfg(not(unix))]
    {
        ctrl_c.await.ok();
    }
    tracing::info!(
        timeout_seconds = timeout.as_secs(),
        "Shutdown signal received, draining connections"
    );
}

#[cfg(all(test, feature = "oauth"))]
mod scope_enforcement_tests {
    use std::collections::HashMap;

    use tower::{Layer, Service};
    use tower_mcp::oauth::token::TokenClaims;
    use tower_mcp::protocol::{CallToolParams, McpRequest, RequestId};
    use tower_mcp::router::Extensions;

    use super::oauth_scope_layer;
    use crate::test_util::MockService;

    /// Build a `tools/call` request, optionally carrying a token with `scope`.
    fn call_with_scope(scope: Option<&str>) -> tower_mcp::RouterRequest {
        let mut extensions = Extensions::new();
        if let Some(scope) = scope {
            extensions.insert(TokenClaims {
                sub: Some("user".into()),
                iss: None,
                aud: None,
                exp: None,
                scope: Some(scope.to_string()),
                client_id: None,
                extra: HashMap::new(),
            });
        }
        tower_mcp::RouterRequest {
            id: RequestId::Number(1),
            inner: McpRequest::CallTool(CallToolParams {
                name: "fs/read".into(),
                arguments: serde_json::json!({}),
                input_responses: None,
                request_state: None,
                meta: None,
                task: None,
            }),
            extensions,
        }
    }

    #[test]
    fn no_required_scopes_yields_no_layer() {
        assert!(oauth_scope_layer(&[]).is_none());
    }

    #[tokio::test]
    async fn token_missing_required_scope_is_rejected() {
        let required = vec!["mcp:access".to_string()];
        let layer = oauth_scope_layer(&required).expect("layer for non-empty scopes");
        let mut svc = layer.layer(MockService::with_tools(&["fs/read"]));

        // Token carries a different scope -> missing the required one.
        let resp = svc
            .call(call_with_scope(Some("other:scope")))
            .await
            .unwrap();
        let err = resp.inner.unwrap_err();
        assert!(
            err.message.to_lowercase().contains("scope"),
            "expected insufficient-scope error, got: {}",
            err.message
        );
    }

    #[tokio::test]
    async fn token_with_all_required_scopes_is_allowed() {
        let required = vec!["mcp:access".to_string(), "mcp:read".to_string()];
        let layer = oauth_scope_layer(&required).expect("layer for non-empty scopes");
        let mut svc = layer.layer(MockService::with_tools(&["fs/read"]));

        let resp = svc
            .call(call_with_scope(Some("mcp:access mcp:read mcp:extra")))
            .await
            .unwrap();
        assert!(
            resp.inner.is_ok(),
            "token carrying all required scopes should be allowed"
        );
    }
}

#[cfg(test)]
mod middleware_stack_tests {
    //! Regression tests for #218: the per-backend middleware chain must
    //! compose as one stack. Under tower-mcp's last-wins `backend_layer`
    //! semantics (joshrotenberg/tower-mcp#1173), only the final middleware
    //! survived and the first test here fails at "call 1 should time out".

    use std::pin::Pin;
    use std::sync::Arc;
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
    use std::task::{Context, Poll};

    use tower::Service;
    use tower_mcp::CallToolResult;
    use tower_mcp::protocol::{CallToolParams, McpRequest, McpResponse, RequestId};
    use tower_mcp::router::Extensions;
    use tower_mcp_types::JsonRpcError;

    use super::*;

    /// Infallible backend with a hit counter, switchable slowness, and a
    /// countdown of response-plane failures to emit before succeeding.
    #[derive(Clone)]
    struct FlakyBackend {
        hits: Arc<AtomicUsize>,
        slow: Arc<AtomicBool>,
        fail_responses: Arc<AtomicUsize>,
    }

    impl FlakyBackend {
        fn new() -> (Self, Arc<AtomicUsize>, Arc<AtomicBool>, Arc<AtomicUsize>) {
            let hits = Arc::new(AtomicUsize::new(0));
            let slow = Arc::new(AtomicBool::new(false));
            let fail_responses = Arc::new(AtomicUsize::new(0));
            (
                Self {
                    hits: hits.clone(),
                    slow: slow.clone(),
                    fail_responses: fail_responses.clone(),
                },
                hits,
                slow,
                fail_responses,
            )
        }
    }

    impl tower::Service<RouterRequest> for FlakyBackend {
        type Response = RouterResponse;
        type Error = Infallible;
        type Future = Pin<Box<dyn Future<Output = Result<RouterResponse, Infallible>> + Send>>;

        fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Infallible>> {
            Poll::Ready(Ok(()))
        }

        fn call(&mut self, req: RouterRequest) -> Self::Future {
            let hits = self.hits.clone();
            let slow = self.slow.clone();
            let fail_responses = self.fail_responses.clone();
            Box::pin(async move {
                hits.fetch_add(1, Ordering::SeqCst);
                if slow.load(Ordering::SeqCst) {
                    tokio::time::sleep(Duration::from_millis(100)).await;
                }
                let inner = if fail_responses
                    .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |n| n.checked_sub(1))
                    .is_ok()
                {
                    Err(JsonRpcError::internal_error("transient backend failure"))
                } else {
                    Ok(McpResponse::CallTool(CallToolResult::text("pong")))
                };
                Ok(RouterResponse { id: req.id, inner })
            })
        }
    }

    fn req() -> RouterRequest {
        RouterRequest {
            id: RequestId::Number(1),
            inner: McpRequest::CallTool(CallToolParams {
                name: "ping".to_string(),
                arguments: serde_json::json!({}),
                input_responses: None,
                request_state: None,
                meta: None,
                task: None,
            }),
            extensions: Extensions::new(),
        }
    }

    async fn drive(svc: &mut InfallibleService) -> RouterResponse {
        svc.ready()
            .await
            .expect("infallible")
            .call(req())
            .await
            .expect("infallible")
    }

    fn test_breaker(
        minimum_calls: usize,
        wait_in_open: Duration,
        permitted_in_half_open: usize,
    ) -> tower_resilience::circuitbreaker::CircuitBreakerLayer {
        let (layer, _handle) = tower_resilience::circuitbreaker::CircuitBreakerLayer::builder()
            .failure_rate_threshold(0.5)
            .minimum_number_of_calls(minimum_calls)
            // The count-based window must be full before the breaker
            // evaluates; align it with minimum_calls (see #220 for the
            // production-side mapping fix).
            .sliding_window_size(minimum_calls)
            .wait_duration_in_open(wait_in_open)
            .permitted_calls_in_half_open(permitted_in_half_open)
            .name("test-cb")
            .build_with_handle();
        layer
    }

    /// Timeout and circuit breaker both apply; timeouts count as failures,
    /// the breaker opens, rejects without reaching the backend, and closes
    /// again through half-open probes once the fault clears.
    #[tokio::test]
    async fn timeout_and_breaker_compose_and_breaker_trips() {
        let (backend, hits, slow, _fail) = FlakyBackend::new();
        slow.store(true, Ordering::SeqCst);

        let mut mw = BackendMiddlewareLayer::default();
        mw.stage(TimeoutLayer::new(Duration::from_millis(10)));
        mw.stage(test_breaker(4, Duration::from_millis(200), 2));

        let mut svc = mw.layer(backend);

        for i in 1..=4 {
            let resp = drive(&mut svc).await;
            assert!(resp.inner.is_err(), "call {i} should time out");
        }
        assert_eq!(
            hits.load(Ordering::SeqCst),
            4,
            "all four calls reach the backend"
        );

        let resp = drive(&mut svc).await;
        assert!(resp.inner.is_err(), "open breaker rejects");
        assert_eq!(
            hits.load(Ordering::SeqCst),
            4,
            "rejected call must not reach the backend"
        );

        tokio::time::sleep(Duration::from_millis(300)).await;
        slow.store(false, Ordering::SeqCst);
        for i in 1..=3 {
            let resp = drive(&mut svc).await;
            assert!(resp.inner.is_ok(), "recovered call {i} succeeds");
        }
        assert_eq!(hits.load(Ordering::SeqCst), 7);
    }

    /// Retry (Infallible zone) and timeout (BoxError zone) coexist: the
    /// retry policy absorbs transient response-plane failures while the
    /// timeout stays armed above it.
    #[tokio::test]
    async fn retry_and_timeout_compose() {
        let (backend, hits, _slow, fail) = FlakyBackend::new();
        fail.store(2, Ordering::SeqCst);

        let cfg = crate::config::RetryConfig {
            max_retries: 3,
            initial_backoff_ms: 1,
            max_backoff_ms: 5,
            budget_percent: None,
            min_retries_per_sec: 10,
        };
        let mut mw = BackendMiddlewareLayer {
            retry: Some(crate::retry::build_retry_layer(&cfg, "test")),
            ..Default::default()
        };
        mw.stage(TimeoutLayer::new(Duration::from_millis(500)));

        let mut svc = mw.layer(backend);

        let resp = drive(&mut svc).await;
        assert!(resp.inner.is_ok(), "retries absorb the transient failures");
        assert_eq!(
            hits.load(Ordering::SeqCst),
            3,
            "two failures plus the successful attempt"
        );
    }

    /// The empty layer is the identity.
    #[tokio::test]
    async fn empty_middleware_layer_is_identity() {
        let (backend, hits, _slow, _fail) = FlakyBackend::new();
        let mw = BackendMiddlewareLayer::default();
        assert!(mw.is_empty());

        let mut svc = mw.layer(backend);
        let resp = drive(&mut svc).await;
        assert!(resp.inner.is_ok());
        assert_eq!(hits.load(Ordering::SeqCst), 1);
    }

    /// Regression for #220: a breaker built through the production config
    /// mapping opens after exactly `minimum_calls` failing calls, not after
    /// the tower-resilience default 100-call window fills.
    #[tokio::test]
    async fn config_mapped_breaker_opens_at_minimum_calls() {
        let (backend, hits, slow, _fail) = FlakyBackend::new();
        slow.store(true, Ordering::SeqCst);

        let cfg = crate::config::CircuitBreakerConfig {
            failure_rate_threshold: 0.5,
            minimum_calls: 4,
            wait_duration_seconds: 60,
            permitted_calls_in_half_open: 1,
        };
        let (breaker, _handle) = build_breaker_layer(&cfg, "test");

        let mut mw = BackendMiddlewareLayer::default();
        mw.stage(TimeoutLayer::new(Duration::from_millis(10)));
        mw.stage(breaker);

        let mut svc = mw.layer(backend);

        for i in 1..=4 {
            let resp = drive(&mut svc).await;
            assert!(resp.inner.is_err(), "call {i} should time out");
        }
        assert_eq!(hits.load(Ordering::SeqCst), 4);

        let resp = drive(&mut svc).await;
        assert!(resp.inner.is_err(), "open breaker rejects");
        assert_eq!(
            hits.load(Ordering::SeqCst),
            4,
            "rejected call must not reach the backend"
        );
    }
}