feat: add worker observation websocket proxy

crates/worker-runtime/src/http_server.rs

@@ -14,15 +14,21 @@ use crate::fs_store::FsRuntimeStoreOptions;
 use crate::identity::{RuntimeId, WorkerId, WorkerRef};
 use crate::interaction::{WorkerInput, WorkerInteractionAck};
 use crate::management::{RuntimeLimits, RuntimeOptions, RuntimeSummary};
+#[cfg(feature = "ws-server")]
+use crate::observation::WorkerObservationCursor;
 use crate::observation::{TranscriptProjection, TranscriptQuery};
 use axum::body::{Body, Bytes};
 use axum::extract::rejection::{JsonRejection, QueryRejection};
+#[cfg(feature = "ws-server")]
+use axum::extract::ws::{Message as WsMessage, WebSocket, WebSocketUpgrade};
 use axum::extract::{Path, Query, State};
 use axum::http::{Request, StatusCode, header};
 use axum::middleware::{self, Next};
 use axum::response::{IntoResponse, Response};
 use axum::routing::{get, post};
 use axum::{Json, Router};
+#[cfg(feature = "ws-server")]
+use futures::StreamExt;
 use serde::{Deserialize, Serialize};
 use std::fmt;
 use std::net::SocketAddr;
@@ -157,7 +163,7 @@ pub fn runtime_http_router(runtime: Runtime, local_token: Option<String>) -> Rou
         local_token: local_token.map(Arc::<str>::from),
     };
 
-    Router::new()
+    let router = Router::new()
         .route("/v1/runtime", get(get_runtime))
         .route("/v1/workers", get(list_workers).post(create_worker))
         .route("/v1/workers/{worker_id}", get(get_worker))
@@ -167,7 +173,12 @@ pub fn runtime_http_router(runtime: Runtime, local_token: Option<String>) -> Rou
         .route(
             "/v1/workers/{worker_id}/transcript",
             get(get_worker_transcript),
-        )
+        );
+
+    #[cfg(feature = "ws-server")]
+    let router = router.route("/v1/workers/{worker_id}/events/ws", get(worker_events_ws));
+
+    router
         .with_state(state.clone())
         .layer(middleware::from_fn_with_state(state, require_local_token))
 }
@@ -255,6 +266,43 @@ pub struct RuntimeHttpErrorDetail {
     pub message: String,
 }
 
+/// Runtime-owned WebSocket frame for worker-scoped observation.
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(tag = "kind", rename_all = "snake_case")]
+pub enum RuntimeWorkerEventWsFrame {
+    Event {
+        envelope: RuntimeWorkerEventWsEnvelope,
+    },
+    Diagnostic {
+        diagnostic: RuntimeWorkerEventWsDiagnostic,
+    },
+}
+
+/// Runtime-local protocol event envelope.
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct RuntimeWorkerEventWsEnvelope {
+    pub cursor: String,
+    pub event_id: String,
+    pub worker_id: WorkerId,
+    pub payload: protocol::Event,
+}
+
+/// Runtime-local observation diagnostic.
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct RuntimeWorkerEventWsDiagnostic {
+    pub code: String,
+    pub message: String,
+}
+
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Debug, Default, Deserialize)]
+struct RuntimeWorkerEventsWsQuery {
+    cursor: Option<String>,
+}
+
 #[derive(Clone, Debug, Deserialize)]
 struct RuntimeHttpTranscriptQuery {
     #[serde(default)]
@@ -267,6 +315,51 @@ fn default_transcript_limit() -> usize {
     256
 }
 
+#[cfg(feature = "ws-server")]
+impl RuntimeWorkerEventWsFrame {
+    fn event(
+        cursor: String,
+        event_id: String,
+        worker_id: WorkerId,
+        payload: protocol::Event,
+    ) -> Self {
+        Self::Event {
+            envelope: RuntimeWorkerEventWsEnvelope {
+                cursor,
+                event_id,
+                worker_id,
+                payload,
+            },
+        }
+    }
+
+    fn diagnostic(code: impl Into<String>, message: impl Into<String>) -> Self {
+        Self::Diagnostic {
+            diagnostic: RuntimeWorkerEventWsDiagnostic {
+                code: code.into(),
+                message: message.into(),
+            },
+        }
+    }
+}
+
+#[cfg(feature = "ws-server")]
+async fn send_ws_frame(socket: &mut WebSocket, frame: &RuntimeWorkerEventWsFrame) -> bool {
+    match serde_json::to_string(frame) {
+        Ok(text) => socket.send(WsMessage::Text(text.into())).await.is_ok(),
+        Err(error) => {
+            let fallback = RuntimeWorkerEventWsFrame::diagnostic(
+                "runtime.serialize_failed",
+                format!("failed to serialize observation frame: {error}"),
+            );
+            let Ok(text) = serde_json::to_string(&fallback) else {
+                return false;
+            };
+            socket.send(WsMessage::Text(text.into())).await.is_ok()
+        }
+    }
+}
+
 type RestResult<T> = Result<Json<T>, RuntimeHttpRestError>;
 
 async fn get_runtime(
@@ -313,6 +406,182 @@ async fn create_worker(
     Ok(Json(RuntimeHttpWorkerResponse { worker }))
 }
 
+#[cfg(feature = "ws-server")]
+async fn worker_events_ws(
+    State(state): State<RuntimeHttpState>,
+    Path(worker_id): Path<String>,
+    Query(query): Query<RuntimeWorkerEventsWsQuery>,
+    ws: WebSocketUpgrade,
+) -> Result<Response, RuntimeHttpRestError> {
+    let worker_ref = worker_ref_for(&state.runtime, worker_id)?;
+    state
+        .runtime
+        .worker_detail(&worker_ref)
+        .map_err(RuntimeHttpRestError::runtime)?;
+    Ok(ws
+        .on_upgrade(move |socket| {
+            worker_events_ws_session(state.runtime, worker_ref, query, socket)
+        })
+        .into_response())
+}
+
+#[cfg(feature = "ws-server")]
+async fn worker_events_ws_session(
+    runtime: Runtime,
+    worker_ref: WorkerRef,
+    query: RuntimeWorkerEventsWsQuery,
+    mut socket: WebSocket,
+) {
+    let mut cursor = match query.cursor.as_deref() {
+        Some(raw) => match WorkerObservationCursor::decode(raw) {
+            Some(cursor) => cursor,
+            None => {
+                let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                    "runtime.cursor_malformed",
+                    format!("malformed worker observation cursor: {raw}"),
+                );
+                let _ = send_ws_frame(&mut socket, &frame).await;
+                return;
+            }
+        },
+        None => match runtime.worker_observation_cursor_now(&worker_ref) {
+            Ok(cursor) => cursor,
+            Err(error) => {
+                let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                    "runtime.worker_not_found",
+                    error.to_string(),
+                );
+                let _ = send_ws_frame(&mut socket, &frame).await;
+                return;
+            }
+        },
+    };
+
+    let mut receiver = match runtime.subscribe_worker_observation() {
+        Ok(receiver) => receiver,
+        Err(error) => {
+            let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                "runtime.unavailable",
+                format!("runtime observation bus unavailable: {error}"),
+            );
+            let _ = send_ws_frame(&mut socket, &frame).await;
+            return;
+        }
+    };
+
+    let snapshot = match runtime.worker_observation_snapshot(&worker_ref) {
+        Ok(snapshot) => snapshot,
+        Err(error) => {
+            let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                "runtime.worker_not_found",
+                error.to_string(),
+            );
+            let _ = send_ws_frame(&mut socket, &frame).await;
+            return;
+        }
+    };
+    let snapshot_cursor = cursor.encode();
+    let snapshot_frame = RuntimeWorkerEventWsFrame::event(
+        snapshot_cursor.clone(),
+        format!("snapshot:{snapshot_cursor}"),
+        worker_ref.worker_id.clone(),
+        snapshot,
+    );
+    if !send_ws_frame(&mut socket, &snapshot_frame).await {
+        return;
+    }
+
+    match runtime.read_worker_observation_events(&worker_ref, cursor) {
+        Ok(backlog) => {
+            for event in backlog {
+                cursor = WorkerObservationCursor::new(event.sequence);
+                let frame = RuntimeWorkerEventWsFrame::event(
+                    event.cursor,
+                    event.event_id,
+                    event.worker_ref.worker_id,
+                    event.payload,
+                );
+                if !send_ws_frame(&mut socket, &frame).await {
+                    return;
+                }
+            }
+        }
+        Err(error) => {
+            let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                "runtime.cursor_unknown_or_expired",
+                error.to_string(),
+            );
+            let _ = send_ws_frame(&mut socket, &frame).await;
+            return;
+        }
+    }
+
+    loop {
+        tokio::select! {
+            inbound = socket.next() => {
+                match inbound {
+                    Some(Ok(WsMessage::Close(_))) | None => return,
+                    Some(Ok(WsMessage::Ping(payload))) => {
+                        if socket.send(WsMessage::Pong(payload)).await.is_err() {
+                            return;
+                        }
+                    }
+                    Some(Ok(WsMessage::Pong(_))) => {}
+                    Some(Ok(_)) => {
+                        let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                            "runtime.observation_only",
+                            "runtime worker event WebSocket is observation-only",
+                        );
+                        let _ = send_ws_frame(&mut socket, &frame).await;
+                        return;
+                    }
+                    Some(Err(error)) => {
+                        let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                            "runtime.websocket_error",
+                            format!("runtime WebSocket receive error: {error}"),
+                        );
+                        let _ = send_ws_frame(&mut socket, &frame).await;
+                        return;
+                    }
+                }
+            }
+            event = receiver.recv() => {
+                match event {
+                    Ok(event) if event.worker_ref == worker_ref && event.sequence > cursor.sequence => {
+                        cursor = WorkerObservationCursor::new(event.sequence);
+                        let frame = RuntimeWorkerEventWsFrame::event(
+                            event.cursor,
+                            event.event_id,
+                            event.worker_ref.worker_id,
+                            event.payload,
+                        );
+                        if !send_ws_frame(&mut socket, &frame).await {
+                            return;
+                        }
+                    }
+                    Ok(_) => {}
+                    Err(tokio::sync::broadcast::error::RecvError::Lagged(_)) => {
+                        let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                            "runtime.cursor_expired",
+                            "runtime observation backlog was overrun",
+                        );
+                        let _ = send_ws_frame(&mut socket, &frame).await;
+                        return;
+                    }
+                    Err(tokio::sync::broadcast::error::RecvError::Closed) => {
+                        let frame = RuntimeWorkerEventWsFrame::diagnostic(
+                            "runtime.upstream_closed",
+                            "runtime observation bus closed",
+                        );
+                        let _ = send_ws_frame(&mut socket, &frame).await;
+                        return;
+                    }
+                }
+            }
+        }
+    }
+}
+
 async fn send_worker_input(
     State(state): State<RuntimeHttpState>,
     Path(worker_id): Path<String>,
@@ -688,3 +957,159 @@ mod tests {
         assert!(error.error.message.contains("worker-missing"));
     }
 }
+
+#[cfg(all(test, feature = "ws-server"))]
+mod ws_tests {
+    use super::*;
+    use futures::{SinkExt, StreamExt};
+    use tokio_tungstenite::connect_async;
+    use tokio_tungstenite::tungstenite::Message;
+
+    async fn spawn_runtime_server() -> (Runtime, WorkerRef, String) {
+        let runtime = Runtime::new_memory();
+        let worker = runtime
+            .create_worker(CreateWorkerRequest::default())
+            .unwrap();
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn({
+            let runtime = runtime.clone();
+            async move { serve_runtime_http(runtime, listener, None).await.unwrap() }
+        });
+        (
+            runtime,
+            worker.worker_ref.clone(),
+            format!(
+                "ws://{addr}/v1/workers/{}/events/ws",
+                worker.worker_ref.worker_id
+            ),
+        )
+    }
+
+    async fn next_frame(
+        stream: &mut tokio_tungstenite::WebSocketStream<
+            tokio_tungstenite::MaybeTlsStream<tokio::net::TcpStream>,
+        >,
+    ) -> RuntimeWorkerEventWsFrame {
+        let message = stream.next().await.unwrap().unwrap();
+        let Message::Text(text) = message else {
+            panic!("expected text frame");
+        };
+        serde_json::from_str(&text).unwrap()
+    }
+
+    #[tokio::test]
+    async fn runtime_ws_connect_sends_snapshot_and_live_worker_events() {
+        let (runtime, worker_ref, url) = spawn_runtime_server().await;
+        let (mut stream, _) = connect_async(&url).await.unwrap();
+
+        match next_frame(&mut stream).await {
+            RuntimeWorkerEventWsFrame::Event { envelope } => {
+                assert_eq!(envelope.worker_id, worker_ref.worker_id);
+                assert!(matches!(envelope.payload, protocol::Event::Snapshot { .. }));
+            }
+            RuntimeWorkerEventWsFrame::Diagnostic { diagnostic } => {
+                panic!("unexpected diagnostic: {diagnostic:?}");
+            }
+        }
+
+        let stored = runtime
+            .observe_worker_event(
+                &worker_ref,
+                protocol::Event::TextDelta {
+                    text: "started".into(),
+                },
+            )
+            .unwrap();
+        match next_frame(&mut stream).await {
+            RuntimeWorkerEventWsFrame::Event { envelope } => {
+                assert_eq!(envelope.worker_id, worker_ref.worker_id);
+                assert_eq!(envelope.cursor, stored.cursor);
+                assert!(matches!(
+                    envelope.payload,
+                    protocol::Event::TextDelta { .. }
+                ));
+            }
+            RuntimeWorkerEventWsFrame::Diagnostic { diagnostic } => {
+                panic!("unexpected diagnostic: {diagnostic:?}");
+            }
+        }
+    }
+
+    #[tokio::test]
+    async fn runtime_ws_cursor_resume_is_duplicate_safe_and_filters_workers() {
+        let (runtime, worker_ref, url) = spawn_runtime_server().await;
+        let other = runtime
+            .create_worker(CreateWorkerRequest::default())
+            .unwrap();
+        let first = runtime
+            .observe_worker_event(
+                &worker_ref,
+                protocol::Event::TextDelta {
+                    text: "started".into(),
+                },
+            )
+            .unwrap();
+        runtime
+            .observe_worker_event(
+                &other.worker_ref,
+                protocol::Event::TextDelta {
+                    text: "started".into(),
+                },
+            )
+            .unwrap();
+
+        let (mut stream, _) = connect_async(format!("{url}?cursor={}", first.cursor))
+            .await
+            .unwrap();
+        assert!(matches!(
+            next_frame(&mut stream).await,
+            RuntimeWorkerEventWsFrame::Event { envelope } if matches!(envelope.payload, protocol::Event::Snapshot { .. })
+        ));
+
+        let second = runtime
+            .observe_worker_event(
+                &worker_ref,
+                protocol::Event::TextDone {
+                    text: "done".into(),
+                },
+            )
+            .unwrap();
+        match next_frame(&mut stream).await {
+            RuntimeWorkerEventWsFrame::Event { envelope } => {
+                assert_eq!(envelope.cursor, second.cursor);
+                assert_ne!(envelope.cursor, first.cursor);
+                assert!(matches!(envelope.payload, protocol::Event::TextDone { .. }));
+            }
+            RuntimeWorkerEventWsFrame::Diagnostic { diagnostic } => {
+                panic!("unexpected diagnostic: {diagnostic:?}");
+            }
+        }
+    }
+
+    #[tokio::test]
+    async fn runtime_ws_reports_malformed_cursor_and_observation_only_input() {
+        let (_runtime, _worker_ref, url) = spawn_runtime_server().await;
+        let (mut malformed, _) = connect_async(format!("{url}?cursor=bad")).await.unwrap();
+        match next_frame(&mut malformed).await {
+            RuntimeWorkerEventWsFrame::Diagnostic { diagnostic } => {
+                assert_eq!(diagnostic.code, "runtime.cursor_malformed");
+            }
+            RuntimeWorkerEventWsFrame::Event { envelope } => {
+                panic!("unexpected event: {envelope:?}");
+            }
+        }
+
+        let (mut stream, _) = connect_async(&url).await.unwrap();
+        let _ = next_frame(&mut stream).await;
+        stream.send(Message::Text("{}".into())).await.unwrap();
+        match next_frame(&mut stream).await {
+            RuntimeWorkerEventWsFrame::Diagnostic { diagnostic } => {
+                assert_eq!(diagnostic.code, "runtime.observation_only");
+            }
+            RuntimeWorkerEventWsFrame::Event { envelope } => {
+                panic!("unexpected event: {envelope:?}");
+            }
+        }
+    }
+}

crates/worker-runtime/src/observation.rs

@@ -93,3 +93,62 @@ pub struct RuntimeEventBatch {
     pub events: Vec<RuntimeEvent>,
     pub has_more: bool,
 }
+
+/// Runtime-local cursor for worker-scoped WebSocket observation.
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct WorkerObservationCursor {
+    pub sequence: u64,
+}
+
+#[cfg(feature = "ws-server")]
+impl WorkerObservationCursor {
+    pub const PREFIX: &'static str = "wo";
+
+    pub fn new(sequence: u64) -> Self {
+        Self { sequence }
+    }
+
+    pub fn zero() -> Self {
+        Self { sequence: 0 }
+    }
+
+    pub fn encode(self) -> String {
+        format!("{}_{:016x}", Self::PREFIX, self.sequence)
+    }
+
+    pub fn decode(value: &str) -> Option<Self> {
+        let encoded = value.strip_prefix("wo_")?;
+        if encoded.len() != 16 {
+            return None;
+        }
+        u64::from_str_radix(encoded, 16)
+            .ok()
+            .map(|sequence| Self { sequence })
+    }
+}
+
+/// One protocol event observed from a runtime Worker.
+#[cfg(feature = "ws-server")]
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct WorkerObservationEvent {
+    pub cursor: String,
+    pub event_id: String,
+    pub sequence: u64,
+    pub worker_ref: WorkerRef,
+    pub payload: protocol::Event,
+}
+
+#[cfg(feature = "ws-server")]
+impl WorkerObservationEvent {
+    pub fn new(sequence: u64, worker_ref: WorkerRef, payload: protocol::Event) -> Self {
+        let cursor = WorkerObservationCursor::new(sequence).encode();
+        Self {
+            event_id: cursor.clone(),
+            cursor,
+            sequence,
+            worker_ref,
+            payload,
+        }
+    }
+}

crates/worker-runtime/src/runtime.rs

@@ -16,9 +16,15 @@ use crate::observation::{
     EventCursor, EventSubscription, EventSubscriptionMode, RuntimeEvent, RuntimeEventBatch,
     RuntimeEventKind, TranscriptEntry, TranscriptProjection, TranscriptQuery, TranscriptRole,
 };
+#[cfg(feature = "ws-server")]
+use crate::observation::{WorkerObservationCursor, WorkerObservationEvent};
 use std::collections::BTreeMap;
+#[cfg(feature = "ws-server")]
+use std::collections::VecDeque;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex, MutexGuard};
+#[cfg(feature = "ws-server")]
+use tokio::sync::broadcast;
 
 static NEXT_RUNTIME_SEQUENCE: AtomicU64 = AtomicU64::new(1);
 
@@ -395,6 +401,88 @@ impl Runtime {
         })
     }
 
+    /// Cursor pointing after the current worker-scoped protocol observation event.
+    #[cfg(feature = "ws-server")]
+    pub fn worker_observation_cursor_now(
+        &self,
+        worker_ref: &WorkerRef,
+    ) -> Result<WorkerObservationCursor, RuntimeError> {
+        let state = self.lock()?;
+        state.ensure_worker_ref(worker_ref)?;
+        let sequence = state
+            .observation_events
+            .iter()
+            .rev()
+            .find(|event| &event.worker_ref == worker_ref)
+            .map(|event| event.sequence)
+            .unwrap_or(0);
+        Ok(WorkerObservationCursor::new(sequence))
+    }
+
+    /// Build the current Worker Snapshot event used as the first observation frame.
+    #[cfg(feature = "ws-server")]
+    pub fn worker_observation_snapshot(
+        &self,
+        worker_ref: &WorkerRef,
+    ) -> Result<protocol::Event, RuntimeError> {
+        let state = self.lock()?;
+        let _worker = state.worker(worker_ref)?;
+        Ok(protocol::Event::Snapshot {
+            entries: Vec::new(),
+            greeting: protocol::Greeting {
+                worker_name: worker_ref.worker_id.to_string(),
+                cwd: String::new(),
+                provider: "worker-runtime".to_string(),
+                model: "worker-runtime".to_string(),
+                scope_summary: "runtime worker observation".to_string(),
+                tools: Vec::new(),
+                context_window: 0,
+                context_tokens: 0,
+            },
+            status: protocol::WorkerStatus::Idle,
+            in_flight: protocol::InFlightSnapshot { blocks: Vec::new() },
+        })
+    }
+
+    /// Replay retained worker-scoped protocol observation events after a cursor.
+    #[cfg(feature = "ws-server")]
+    pub fn read_worker_observation_events(
+        &self,
+        worker_ref: &WorkerRef,
+        cursor: WorkerObservationCursor,
+    ) -> Result<Vec<WorkerObservationEvent>, RuntimeError> {
+        let state = self.lock()?;
+        state.ensure_worker_ref(worker_ref)?;
+        state.validate_worker_observation_cursor(worker_ref, cursor)?;
+        Ok(state
+            .observation_events
+            .iter()
+            .filter(|event| &event.worker_ref == worker_ref && event.sequence > cursor.sequence)
+            .cloned()
+            .collect())
+    }
+
+    /// Subscribe to live protocol observation events.
+    #[cfg(feature = "ws-server")]
+    pub fn subscribe_worker_observation(
+        &self,
+    ) -> Result<broadcast::Receiver<WorkerObservationEvent>, RuntimeError> {
+        Ok(self.lock()?.observation_tx.subscribe())
+    }
+
+    /// Append a Worker protocol event to the observation bus.
+    #[cfg(feature = "ws-server")]
+    pub fn observe_worker_event(
+        &self,
+        worker_ref: &WorkerRef,
+        payload: protocol::Event,
+    ) -> Result<WorkerObservationEvent, RuntimeError> {
+        let mut state = self.lock()?;
+        state.ensure_worker_ref(worker_ref)?;
+        let event = state.push_worker_observation_event(worker_ref.clone(), payload);
+        Ok(event)
+    }
+
     /// Snapshot current diagnostics.
     pub fn diagnostics(&self) -> Result<Vec<RuntimeDiagnostic>, RuntimeError> {
         Ok(self.lock()?.diagnostics.clone())
@@ -465,6 +553,12 @@ struct RuntimeState {
     workers: BTreeMap<WorkerId, WorkerRecord>,
     events: Vec<RuntimeEvent>,
     diagnostics: Vec<RuntimeDiagnostic>,
+    #[cfg(feature = "ws-server")]
+    next_observation_sequence: u64,
+    #[cfg(feature = "ws-server")]
+    observation_events: VecDeque<WorkerObservationEvent>,
+    #[cfg(feature = "ws-server")]
+    observation_tx: broadcast::Sender<WorkerObservationEvent>,
 }
 
 impl RuntimeState {
@@ -482,6 +576,12 @@ impl RuntimeState {
             workers: BTreeMap::new(),
             events: Vec::new(),
             diagnostics: Vec::new(),
+            #[cfg(feature = "ws-server")]
+            next_observation_sequence: 1,
+            #[cfg(feature = "ws-server")]
+            observation_events: VecDeque::new(),
+            #[cfg(feature = "ws-server")]
+            observation_tx: broadcast::channel(256).0,
         }
     }
 
@@ -505,6 +605,12 @@ impl RuntimeState {
             workers: BTreeMap::new(),
             events: Vec::new(),
             diagnostics: Vec::new(),
+            #[cfg(feature = "ws-server")]
+            next_observation_sequence: 1,
+            #[cfg(feature = "ws-server")]
+            observation_events: VecDeque::new(),
+            #[cfg(feature = "ws-server")]
+            observation_tx: broadcast::channel(256).0,
         }
     }
 
@@ -762,6 +868,54 @@ impl RuntimeState {
         self.next_event_id.saturating_sub(1)
     }
 
+    #[cfg(feature = "ws-server")]
+    fn validate_worker_observation_cursor(
+        &self,
+        worker_ref: &WorkerRef,
+        cursor: WorkerObservationCursor,
+    ) -> Result<(), RuntimeError> {
+        if let Some(first) = self
+            .observation_events
+            .iter()
+            .find(|event| &event.worker_ref == worker_ref)
+        {
+            if cursor.sequence != 0 && cursor.sequence < first.sequence {
+                return Err(RuntimeError::InvalidRequest(format!(
+                    "worker observation cursor {} is expired for worker {}",
+                    cursor.encode(),
+                    worker_ref.worker_id
+                )));
+            }
+        }
+        if cursor.sequence >= self.next_observation_sequence {
+            return Err(RuntimeError::InvalidRequest(format!(
+                "worker observation cursor {} is unknown for worker {}",
+                cursor.encode(),
+                worker_ref.worker_id
+            )));
+        }
+        Ok(())
+    }
+
+    #[cfg(feature = "ws-server")]
+    fn push_worker_observation_event(
+        &mut self,
+        worker_ref: WorkerRef,
+        payload: protocol::Event,
+    ) -> WorkerObservationEvent {
+        const MAX_OBSERVATION_BACKLOG: usize = 1024;
+
+        let sequence = self.next_observation_sequence;
+        self.next_observation_sequence += 1;
+        let event = WorkerObservationEvent::new(sequence, worker_ref, payload);
+        self.observation_events.push_back(event.clone());
+        while self.observation_events.len() > MAX_OBSERVATION_BACKLOG {
+            self.observation_events.pop_front();
+        }
+        let _ = self.observation_tx.send(event.clone());
+        event
+    }
+
     fn push_diagnostic(
         &mut self,
         severity: DiagnosticSeverity,