fix: surface disconnect reason, switch health probe to /global/health (#978)

- event-pipeline: onDisconnect now carries a reason tag (ws_closed:code=N, ws_error_frame, ws_closed_before_ready, sse_error)
- useConfigStore: store lastDisconnectReason, clear on reconnect
- send guards: embed reason in Connection lost toast so we can tell which path tripped
- lifecycle: isOpenCodeProcessHealthy hits /global/health (healthy flag) with 5s timeout instead of /session with 2s, avoids false restarts under stream load

packages/ui/src/sync/event-pipeline.ts

@@ -34,7 +34,7 @@ export type EventPipelineInput = {
   /** Called after stream reconnects (visibility restore or heartbeat timeout). */
   onReconnect?: () => void
   /** Called when the stream disconnects (heartbeat timeout, network error, or transport failure). */
-  onDisconnect?: () => void
+  onDisconnect?: (reason: string) => void
   transport?: "auto" | "ws" | "sse"
 }
 
@@ -429,6 +429,7 @@ export function createEventPipeline(input: EventPipelineInput) {
 
         if (frame.type === "error") {
           const error = new Error(frame.message || "Message stream WebSocket error")
+          ;(error as Error & { reason?: string }).reason = `ws_error_frame:${frame.message || "unknown"}`
           setFallbackCode(error)
           settleReject(error)
           try {
@@ -463,13 +464,16 @@ export function createEventPipeline(input: EventPipelineInput) {
         void 0
       }
 
-      socket.onclose = () => {
+      socket.onclose = (event) => {
         if (signal.aborted) {
           settleResolve()
           return
         }
 
         const error = new Error("Global message stream WebSocket closed")
+        ;(error as Error & { reason?: string }).reason = opened
+          ? `ws_closed:code=${event?.code ?? "?"}`
+          : "ws_closed_before_ready"
         setFallbackCode(error)
         settleReject(error)
       }
@@ -521,7 +525,18 @@ export function createEventPipeline(input: EventPipelineInput) {
           // setState calls on every failed retry attempt.
           if (!disconnected) {
             disconnected = true
-            onDisconnect?.()
+            const taggedReason = typeof error === "object" && error !== null
+              ? (error as { reason?: unknown }).reason
+              : undefined
+            const message = typeof error === "object" && error !== null
+              ? (error as { message?: unknown }).message
+              : undefined
+            const reason = typeof taggedReason === "string" && taggedReason.length > 0
+              ? taggedReason
+              : typeof message === "string" && message.length > 0
+                ? `${currentTransport}_error:${message.slice(0, 80)}`
+                : `${currentTransport}_error:unknown`
+            onDisconnect?.(reason)
           }
         }
       } finally {

packages/ui/src/sync/session-actions.ts

@@ -54,6 +54,12 @@ function dir() {
   return _getDirectory() || undefined
 }
 
+function connectionLostError(): Error {
+  const reason = useConfigStore.getState().lastDisconnectReason
+  const suffix = reason ? ` (${reason})` : " (never connected)"
+  return new Error(`Connection lost${suffix}. Please wait for reconnection.`)
+}
+
 function getSessionDirectory(sessionId: string): string | undefined {
   return useSessionUIStore.getState().getDirectoryForSession(sessionId) || dir()
 }
@@ -327,7 +333,7 @@ export async function optimisticSend(input: {
   }
 
   if (!useConfigStore.getState().isConnected) {
-    throw new Error("Connection lost. Please wait for reconnection.")
+    throw connectionLostError()
   }
 
   const store = dirStore()
@@ -414,7 +420,7 @@ export async function respondToPermission(
   response: "once" | "always" | "reject",
 ): Promise<void> {
   if (!useConfigStore.getState().isConnected) {
-    throw new Error("Connection lost. Please wait for reconnection.")
+    throw connectionLostError()
   }
   const result = await getRequestReplyClient("permission", sessionId, requestId).permission.reply({
     requestID: requestId,
@@ -430,7 +436,7 @@ export async function dismissPermission(
   requestId: string,
 ): Promise<void> {
   if (!useConfigStore.getState().isConnected) {
-    throw new Error("Connection lost. Please wait for reconnection.")
+    throw connectionLostError()
   }
   const result = await getRequestReplyClient("permission", sessionId, requestId).permission.reply({
     requestID: requestId,
@@ -451,7 +457,7 @@ export async function respondToQuestion(
   answers: string[] | string[][],
 ): Promise<void> {
   if (!useConfigStore.getState().isConnected) {
-    throw new Error("Connection lost. Please wait for reconnection.")
+    throw connectionLostError()
   }
   const result = await getRequestReplyClient("question", sessionId, requestId).question.reply({
     requestID: requestId,
@@ -467,7 +473,7 @@ export async function rejectQuestion(
   requestId: string,
 ): Promise<void> {
   if (!useConfigStore.getState().isConnected) {
-    throw new Error("Connection lost. Please wait for reconnection.")
+    throw connectionLostError()
   }
   const result = await getRequestReplyClient("question", sessionId, requestId).question.reject({
     requestID: requestId,

packages/ui/src/sync/sync-context.tsx

@@ -1347,7 +1347,7 @@ export function SyncProvider(props: {
         handleEvent(directory, payload, childStores, routingIndex)
       },
       onReconnect: () => {
-        useConfigStore.setState({ isConnected: true })
+        useConfigStore.setState({ isConnected: true, lastDisconnectReason: null })
         for (const [dir, store] of childStores.children) {
           if (reconnectResyncing.has(dir)) continue
           if (getReconnectCandidateSessionIds(store.getState()).length === 0) continue
@@ -1362,8 +1362,8 @@ export function SyncProvider(props: {
             })
         }
       },
-      onDisconnect: () => {
-        useConfigStore.setState({ isConnected: false })
+      onDisconnect: (reason) => {
+        useConfigStore.setState({ isConnected: false, lastDisconnectReason: reason })
       },
     })
     return cleanup