diff --git a/sgl-model-gateway/src/app_context.rs b/sgl-model-gateway/src/app_context.rs
index 02c56f8ab..7c1b81cfa 100644
--- a/sgl-model-gateway/src/app_context.rs
+++ b/sgl-model-gateway/src/app_context.rs
@@ -14,6 +14,7 @@ use crate::{
     },
     mcp::McpManager,
     middleware::TokenBucket,
+    observability::inflight_tracker::InFlightRequestTracker,
     policies::PolicyRegistry,
     reasoning_parser::ParserFactory as ReasoningParserFactory,
     routers::router_manager::RouterManager,
@@ -62,6 +63,7 @@ pub struct AppContext {
     pub mcp_manager: Arc<OnceLock<Arc<McpManager>>>,
     pub wasm_manager: Option<Arc<WasmModuleManager>>,
     pub worker_service: Arc<WorkerService>,
+    pub inflight_tracker: Arc<InFlightRequestTracker>,
 }
 
 pub struct AppContextBuilder {
@@ -275,6 +277,7 @@ impl AppContextBuilder {
                 .ok_or(AppContextBuildError("mcp_manager"))?,
             wasm_manager: self.wasm_manager,
             worker_service,
+            inflight_tracker: InFlightRequestTracker::new(),
         })
     }
 
diff --git a/sgl-model-gateway/src/middleware.rs b/sgl-model-gateway/src/middleware.rs
index 5ad2c1875..84d51b321 100644
--- a/sgl-model-gateway/src/middleware.rs
+++ b/sgl-model-gateway/src/middleware.rs
@@ -26,7 +26,10 @@ use tracing::{debug, error, field::Empty, info, info_span, warn, Span};
 
 pub use crate::core::token_bucket::TokenBucket;
 use crate::{
-    observability::metrics::{method_to_static_str, metrics_labels, Metrics},
+    observability::{
+        inflight_tracker::InFlightRequestTracker,
+        metrics::{method_to_static_str, metrics_labels, Metrics},
+    },
     routers::error::extract_error_code_from_response,
     server::AppState,
     wasm::{
@@ -607,12 +610,14 @@ pub async fn concurrency_limit_middleware(
 static ACTIVE_HTTP_CONNECTIONS: AtomicU64 = AtomicU64::new(0);
 
 /// Tower Layer for HTTP metrics collection (SMG Layer 1 metrics)
-#[derive(Clone, Copy, Default)]
-pub struct HttpMetricsLayer;
+#[derive(Clone)]
+pub struct HttpMetricsLayer {
+    tracker: Arc<InFlightRequestTracker>,
+}
 
 impl HttpMetricsLayer {
-    pub fn new() -> Self {
-        Self
+    pub fn new(tracker: Arc<InFlightRequestTracker>) -> Self {
+        Self { tracker }
     }
 }
 
@@ -620,7 +625,10 @@ impl<S> Layer<S> for HttpMetricsLayer {
     type Service = HttpMetricsMiddleware<S>;
 
     fn layer(&self, inner: S) -> Self::Service {
-        HttpMetricsMiddleware { inner }
+        HttpMetricsMiddleware {
+            inner,
+            in_flight_request_tracker: self.tracker.clone(),
+        }
     }
 }
 
@@ -628,6 +636,7 @@ impl<S> Layer<S> for HttpMetricsLayer {
 #[derive(Clone)]
 pub struct HttpMetricsMiddleware<S> {
     inner: S,
+    in_flight_request_tracker: Arc<InFlightRequestTracker>,
 }
 
 impl<S> Service<Request> for HttpMetricsMiddleware<S>
@@ -651,15 +660,20 @@ where
         let start = Instant::now();
 
         let mut inner = self.inner.clone();
+        let in_flight_request_tracker = self.in_flight_request_tracker.clone();
 
         Box::pin(async move {
             // Increment inside async block - ensures no leak if future is dropped before polling
             let active = ACTIVE_HTTP_CONNECTIONS.fetch_add(1, Ordering::Relaxed) + 1;
             Metrics::set_http_connections_active(active as usize);
 
+            let guard = in_flight_request_tracker.track();
+
             // Capture result before decrementing to ensure decrement happens on error too
             let result = inner.call(req).await;
 
+            drop(guard);
+
             // Always decrement, regardless of success or failure
             let active = ACTIVE_HTTP_CONNECTIONS.fetch_sub(1, Ordering::Relaxed) - 1;
             Metrics::set_http_connections_active(active as usize);
diff --git a/sgl-model-gateway/src/observability/inflight_tracker.rs b/sgl-model-gateway/src/observability/inflight_tracker.rs
new file mode 100644
index 000000000..19a0639f5
--- /dev/null
+++ b/sgl-model-gateway/src/observability/inflight_tracker.rs
@@ -0,0 +1,225 @@
+use std::{
+    sync::{
+        atomic::{AtomicU64, Ordering},
+        Arc, OnceLock,
+    },
+    time::Instant,
+};
+
+use dashmap::DashMap;
+
+use super::metrics::Metrics;
+use crate::policies::utils::PeriodicTask;
+
+const AGE_BUCKET_BOUNDS: &[u64] = &[30, 60, 180, 300, 600, 1200, 3600, 7200, 14400, 28800, 86400];
+const AGE_BUCKET_LABELS: &[&str] = &[
+    "30", "60", "180", "300", "600", "1200", "3600", "7200", "14400", "28800", "86400", "+Inf",
+];
+
+pub struct InFlightRequestTracker {
+    requests: DashMap<u64, Instant>,
+    next_id: AtomicU64,
+    sampler: OnceLock<PeriodicTask>,
+}
+
+impl InFlightRequestTracker {
+    pub fn new() -> Arc<Self> {
+        Arc::new(Self {
+            requests: DashMap::new(),
+            next_id: AtomicU64::new(0),
+            sampler: OnceLock::new(),
+        })
+    }
+
+    pub fn start_sampler(self: &Arc<Self>, interval_secs: u64) {
+        let tracker = self.clone();
+        let task = PeriodicTask::spawn(interval_secs, "InFlightRequestSampler", move || {
+            tracker.sample_and_record();
+        });
+        self.sampler.set(task).unwrap();
+    }
+
+    pub fn track(self: &Arc<Self>) -> InFlightGuard {
+        let request_id = self.next_id.fetch_add(1, Ordering::Relaxed);
+        self.requests.insert(request_id, Instant::now());
+        InFlightGuard {
+            tracker: self.clone(),
+            request_id,
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.requests.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.requests.is_empty()
+    }
+
+    pub fn compute_bucket_counts(&self) -> [usize; AGE_BUCKET_LABELS.len()] {
+        let now = Instant::now();
+        let inf_idx = AGE_BUCKET_LABELS.len() - 1;
+
+        let instants: Vec<Instant> = self.requests.iter().map(|entry| *entry.value()).collect();
+
+        let mut non_cumulative_counts = [0usize; AGE_BUCKET_LABELS.len()];
+        for inst in instants {
+            let age_secs = now.duration_since(inst).as_secs();
+            let bucket_idx = AGE_BUCKET_BOUNDS
+                .iter()
+                .position(|&bound| age_secs <= bound)
+                .unwrap_or(inf_idx);
+            non_cumulative_counts[bucket_idx] += 1;
+        }
+
+        let mut counts = [0usize; AGE_BUCKET_LABELS.len()];
+        let mut cumulative = 0;
+        for i in 0..counts.len() {
+            cumulative += non_cumulative_counts[i];
+            counts[i] = cumulative;
+        }
+
+        counts
+    }
+
+    fn sample_and_record(&self) {
+        let counts = self.compute_bucket_counts();
+        for (i, &label) in AGE_BUCKET_LABELS.iter().enumerate() {
+            Metrics::set_inflight_request_age_count(label, counts[i]);
+        }
+    }
+}
+
+pub struct InFlightGuard {
+    tracker: Arc<InFlightRequestTracker>,
+    request_id: u64,
+}
+
+impl Drop for InFlightGuard {
+    fn drop(&mut self) {
+        self.tracker.requests.remove(&self.request_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use super::*;
+
+    impl InFlightRequestTracker {
+        fn insert_with_time(&self, request_id: u64, start_time: Instant) {
+            self.requests.insert(request_id, start_time);
+        }
+    }
+
+    #[test]
+    fn test_track_and_drop() {
+        let tracker = InFlightRequestTracker::new();
+        {
+            let _guard1 = tracker.track();
+            let _guard2 = tracker.track();
+            assert_eq!(tracker.len(), 2);
+        }
+        assert_eq!(tracker.len(), 0);
+    }
+
+    #[test]
+    fn test_guard_auto_deregister() {
+        let tracker = InFlightRequestTracker::new();
+        let guard = tracker.track();
+        assert_eq!(tracker.len(), 1);
+        drop(guard);
+        assert_eq!(tracker.len(), 0);
+    }
+
+    #[test]
+    fn test_request_age_tracking() {
+        let tracker = InFlightRequestTracker::new();
+        let _guard = tracker.track();
+        std::thread::sleep(Duration::from_millis(100));
+
+        let entry = tracker.requests.iter().next().unwrap();
+        let age = entry.value().elapsed();
+        assert!(age >= Duration::from_millis(100));
+    }
+
+    #[test]
+    fn test_empty_tracker_buckets() {
+        let tracker = InFlightRequestTracker::new();
+        let counts = tracker.compute_bucket_counts();
+        assert!(counts.iter().all(|&c| c == 0));
+    }
+
+    #[test]
+    fn test_cumulative_bucket_counts() {
+        let tracker = InFlightRequestTracker::new();
+        let now = Instant::now();
+
+        tracker.insert_with_time(1, now);
+        tracker.insert_with_time(2, now - Duration::from_secs(45));
+        tracker.insert_with_time(3, now - Duration::from_secs(100));
+        tracker.insert_with_time(4, now - Duration::from_secs(250));
+        tracker.insert_with_time(5, now - Duration::from_secs(500));
+        tracker.insert_with_time(6, now - Duration::from_secs(700));
+
+        let counts = tracker.compute_bucket_counts();
+        assert_eq!(counts[0], 1, "bucket 0");
+        assert_eq!(counts[1], 2, "bucket 1");
+        assert_eq!(counts[2], 3, "bucket 2");
+        assert_eq!(counts[3], 4, "bucket 3");
+        assert_eq!(counts[4], 5, "bucket 4");
+        assert_eq!(counts[5], 6, "bucket 5");
+        assert_eq!(*counts.last().unwrap(), 6, "bucket +Inf");
+    }
+
+    #[test]
+    fn test_bucket_boundary_values() {
+        let tracker = InFlightRequestTracker::new();
+        let now = Instant::now();
+
+        tracker.insert_with_time(1, now - Duration::from_secs(30));
+        tracker.insert_with_time(2, now - Duration::from_secs(31));
+
+        let counts = tracker.compute_bucket_counts();
+        assert_eq!(counts[0], 1, "bucket 0 includes exact boundary");
+        assert_eq!(counts[1], 2, "bucket 1 includes both");
+        assert_eq!(*counts.last().unwrap(), 2, "bucket +Inf includes all");
+    }
+
+    #[test]
+    fn test_concurrent_tracking() {
+        use std::thread;
+
+        let tracker = InFlightRequestTracker::new();
+        let mut handles = vec![];
+
+        for _ in 0..10 {
+            let t = tracker.clone();
+            handles.push(thread::spawn(move || {
+                (0..100).map(|_| t.track()).collect::<Vec<_>>()
+            }));
+        }
+
+        let all_guards: Vec<_> = handles
+            .into_iter()
+            .flat_map(|h| h.join().unwrap())
+            .collect();
+
+        assert_eq!(tracker.len(), 1000);
+        drop(all_guards);
+        assert_eq!(tracker.len(), 0);
+    }
+
+    #[test]
+    fn test_unique_ids() {
+        let tracker = InFlightRequestTracker::new();
+        let g1 = tracker.track();
+        let g2 = tracker.track();
+        let g3 = tracker.track();
+
+        assert_ne!(g1.request_id, g2.request_id);
+        assert_ne!(g2.request_id, g3.request_id);
+        assert_eq!(tracker.len(), 3);
+    }
+}
diff --git a/sgl-model-gateway/src/observability/metrics.rs b/sgl-model-gateway/src/observability/metrics.rs
index c4ef2ffe0..3cabf199c 100644
--- a/sgl-model-gateway/src/observability/metrics.rs
+++ b/sgl-model-gateway/src/observability/metrics.rs
@@ -153,6 +153,10 @@ pub fn init_metrics() {
         "smg_http_request_duration_seconds",
         "HTTP request duration by method and path"
     );
+    describe_gauge!(
+        "smg_http_inflight_request_age_count",
+        "Count of currently in-flight HTTP requests by age"
+    );
     describe_counter!(
         "smg_http_responses_total",
         "Total HTTP responses by status_code and error_code"
@@ -491,7 +495,17 @@ impl Metrics {
         .record(duration.as_secs_f64());
     }
 
-    /// Set active HTTP connections count.
+    /// Set the cumulative count of in-flight requests for a given age bucket.
+    /// Uses `le` label to match Prometheus histogram convention.
+    pub fn set_inflight_request_age_count(le: &'static str, count: usize) {
+        gauge!(
+            "smg_http_inflight_request_age_count",
+            "le" => le
+        )
+        .set(count as f64);
+    }
+
+    /// Set active HTTP connections count
     pub fn set_http_connections_active(count: usize) {
         gauge!("smg_http_connections_active").set(count as f64);
     }
diff --git a/sgl-model-gateway/src/observability/mod.rs b/sgl-model-gateway/src/observability/mod.rs
index 8a8f436d5..cd4677430 100644
--- a/sgl-model-gateway/src/observability/mod.rs
+++ b/sgl-model-gateway/src/observability/mod.rs
@@ -1,6 +1,7 @@
 //! Observability utilities for logging, metrics, and tracing.
 
 pub mod events;
+pub mod inflight_tracker;
 pub mod logging;
 pub mod metrics;
 pub mod otel_trace;
diff --git a/sgl-model-gateway/src/server.rs b/sgl-model-gateway/src/server.rs
index f3bfe6032..1b0a7e556 100644
--- a/sgl-model-gateway/src/server.rs
+++ b/sgl-model-gateway/src/server.rs
@@ -654,7 +654,9 @@ pub fn build_app(
             max_payload_size,
         ))
         .layer(middleware::create_logging_layer())
-        .layer(middleware::HttpMetricsLayer::new())
+        .layer(middleware::HttpMetricsLayer::new(
+            app_state.context.inflight_tracker.clone(),
+        ))
         .layer(middleware::RequestIdLayer::new(request_id_headers))
         .layer(create_cors_layer(cors_allowed_origins))
         .fallback(sink_handler)
@@ -714,6 +716,10 @@ pub async fn startup(config: ServerConfig) -> Result<(), Box<dyn std::error::Err
         AppContext::from_config(config.router_config.clone(), config.request_timeout_secs).await?,
     );
 
+    if config.prometheus_config.is_some() {
+        app_context.inflight_tracker.start_sampler(20);
+    }
+
     let weak_context = Arc::downgrade(&app_context);
     let worker_job_queue = JobQueue::new(JobQueueConfig::default(), weak_context);
     app_context
diff --git a/sgl-model-gateway/src/service_discovery.rs b/sgl-model-gateway/src/service_discovery.rs
index c89e15462..e9344e50e 100644
--- a/sgl-model-gateway/src/service_discovery.rs
+++ b/sgl-model-gateway/src/service_discovery.rs
@@ -609,7 +609,10 @@ mod tests {
     }
 
     async fn create_test_app_context() -> Arc<AppContext> {
-        use crate::{config::RouterConfig, core::WorkerService, middleware::TokenBucket};
+        use crate::{
+            config::RouterConfig, core::WorkerService, middleware::TokenBucket,
+            observability::inflight_tracker::InFlightRequestTracker,
+        };
 
         let router_config = RouterConfig::builder()
             .worker_startup_timeout_secs(1)
@@ -649,6 +652,7 @@ mod tests {
                 worker_job_queue,
                 router_config,
             )),
+            inflight_tracker: InFlightRequestTracker::new(),
         })
     }
 
diff --git a/sgl-model-gateway/tests/inflight_tracker_test.rs b/sgl-model-gateway/tests/inflight_tracker_test.rs
new file mode 100644
index 000000000..ee39da32d
--- /dev/null
+++ b/sgl-model-gateway/tests/inflight_tracker_test.rs
@@ -0,0 +1,152 @@
+mod common;
+
+use std::time::Duration;
+
+use axum::{
+    body::Body,
+    extract::Request,
+    http::{header::CONTENT_TYPE, StatusCode},
+};
+use common::{
+    mock_worker::{HealthStatus, MockWorkerConfig, WorkerType},
+    AppTestContext,
+};
+use serde_json::json;
+use tower::ServiceExt;
+
+#[tokio::test]
+async fn test_multiple_concurrent_requests_tracking() {
+    let ctx = AppTestContext::new(vec![MockWorkerConfig {
+        port: 19002,
+        worker_type: WorkerType::Regular,
+        health_status: HealthStatus::Healthy,
+        response_delay_ms: 50,
+        fail_rate: 0.0,
+    }])
+    .await;
+
+    let tracker = ctx.app_context.inflight_tracker.clone();
+
+    let mut handles = vec![];
+    for i in 0..5 {
+        let app = ctx.create_app().await;
+        handles.push(tokio::spawn(async move {
+            let payload = json!({
+                "text": format!("Request {}", i),
+                "stream": false
+            });
+
+            let req = Request::builder()
+                .method("POST")
+                .uri("/generate")
+                .header(CONTENT_TYPE, "application/json")
+                .body(Body::from(serde_json::to_string(&payload).unwrap()))
+                .unwrap();
+
+            app.oneshot(req).await.unwrap()
+        }));
+    }
+
+    for handle in handles {
+        let resp = handle.await.unwrap();
+        assert_eq!(resp.status(), StatusCode::OK);
+    }
+
+    assert!(tracker.is_empty());
+
+    ctx.shutdown().await;
+}
+
+#[tokio::test]
+async fn test_inflight_request_appears_in_bucket() {
+    let ctx = AppTestContext::new(vec![MockWorkerConfig {
+        port: 19004,
+        worker_type: WorkerType::Regular,
+        health_status: HealthStatus::Healthy,
+        response_delay_ms: 2000,
+        fail_rate: 0.0,
+    }])
+    .await;
+
+    let tracker = &ctx.app_context.inflight_tracker;
+    assert!(tracker.is_empty(), "Tracker should start empty");
+
+    let app = ctx.create_app().await;
+    let payload = json!({
+        "text": "Long running request",
+        "stream": false
+    });
+
+    let req = Request::builder()
+        .method("POST")
+        .uri("/generate")
+        .header(CONTENT_TYPE, "application/json")
+        .body(Body::from(serde_json::to_string(&payload).unwrap()))
+        .unwrap();
+
+    let tracker_clone = ctx.app_context.inflight_tracker.clone();
+    let response_future = tokio::spawn(async move { app.oneshot(req).await });
+
+    tokio::time::sleep(Duration::from_millis(500)).await;
+
+    let inflight_count = tracker_clone.len();
+    assert!(
+        inflight_count > 0,
+        "Should have at least one in-flight request, got {}",
+        inflight_count
+    );
+
+    let buckets = tracker_clone.compute_bucket_counts();
+    assert!(buckets[0] > 0, "first bucket should have requests");
+    assert!(
+        *buckets.last().unwrap() > 0,
+        "+Inf bucket should have requests"
+    );
+
+    let resp = response_future.await.unwrap().unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+
+    tokio::time::sleep(Duration::from_millis(50)).await;
+    assert!(
+        tracker_clone.is_empty(),
+        "Request should be deregistered after completion"
+    );
+
+    ctx.shutdown().await;
+}
+
+#[tokio::test]
+async fn test_failed_request_still_deregisters() {
+    let ctx = AppTestContext::new(vec![MockWorkerConfig {
+        port: 19003,
+        worker_type: WorkerType::Regular,
+        health_status: HealthStatus::Healthy,
+        response_delay_ms: 0,
+        fail_rate: 1.0,
+    }])
+    .await;
+
+    let tracker = &ctx.app_context.inflight_tracker;
+    assert!(tracker.is_empty());
+
+    let app = ctx.create_app().await;
+
+    let payload = json!({
+        "text": "This should fail",
+        "stream": false
+    });
+
+    let req = Request::builder()
+        .method("POST")
+        .uri("/generate")
+        .header(CONTENT_TYPE, "application/json")
+        .body(Body::from(serde_json::to_string(&payload).unwrap()))
+        .unwrap();
+
+    let resp = app.oneshot(req).await.unwrap();
+    assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
+
+    assert!(tracker.is_empty());
+
+    ctx.shutdown().await;
+}