diff --git a/packages/ui/src/components/chat/ChatInput.tsx b/packages/ui/src/components/chat/ChatInput.tsx
index 8ad7a789..a81bef1e 100644
--- a/packages/ui/src/components/chat/ChatInput.tsx
+++ b/packages/ui/src/components/chat/ChatInput.tsx
@@ -1520,7 +1520,9 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({ onOpenSettings, scrollTo
             else if (commandName === 'compact' && currentSessionId) {
                 try {
                     if (!useConfigStore.getState().isConnected) {
-                        throw new Error("Connection lost. Please wait for reconnection.");
+                        const reason = useConfigStore.getState().lastDisconnectReason;
+                        const suffix = reason ? ` (${reason})` : " (never connected)";
+                        throw new Error(`Connection lost${suffix}. Please wait for reconnection.`);
                     }
                     const { opencodeClient } = await import('@/lib/opencode/client');
                     const sdk = opencodeClient.getSdkClient();
diff --git a/packages/ui/src/stores/useConfigStore.ts b/packages/ui/src/stores/useConfigStore.ts
index 9226e888..d8903c35 100644
--- a/packages/ui/src/stores/useConfigStore.ts
+++ b/packages/ui/src/stores/useConfigStore.ts
@@ -468,6 +468,7 @@ interface ConfigStore {
     agentModelSelections: { [agentName: string]: { providerId: string; modelId: string } };
     defaultProviders: { [key: string]: string };
     isConnected: boolean;
+    lastDisconnectReason: string | null;
     isInitialized: boolean;
     modelsMetadata: Map<string, ModelMetadata>;
     // OpenChamber settings-based defaults (take precedence over agent preferences)
@@ -588,6 +589,7 @@ export const useConfigStore = create<ConfigStore>()(
                 agentModelSelections: {},
                 defaultProviders: {},
                 isConnected: false,
+                lastDisconnectReason: null,
                 isInitialized: false,
                 modelsMetadata: new Map<string, ModelMetadata>(),
                 settingsDefaultModel: undefined,
diff --git a/packages/ui/src/sync/event-pipeline.ts b/packages/ui/src/sync/event-pipeline.ts
index 49220065..2ce3d99b 100644
--- a/packages/ui/src/sync/event-pipeline.ts
+++ b/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 {
diff --git a/packages/ui/src/sync/session-actions.ts b/packages/ui/src/sync/session-actions.ts
index bfbbaca8..25b576c2 100644
--- a/packages/ui/src/sync/session-actions.ts
+++ b/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,
diff --git a/packages/ui/src/sync/sync-context.tsx b/packages/ui/src/sync/sync-context.tsx
index cbac4822..ef15d374 100644
--- a/packages/ui/src/sync/sync-context.tsx
+++ b/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
diff --git a/packages/web/server/lib/opencode/lifecycle.js b/packages/web/server/lib/opencode/lifecycle.js
index a4be1884..bf8781c5 100644
--- a/packages/web/server/lib/opencode/lifecycle.js
+++ b/packages/web/server/lib/opencode/lifecycle.js
@@ -319,12 +319,17 @@ export const createOpenCodeLifecycleRuntime = (deps) => {
     }
 
     try {
-      const response = await fetch(buildOpenCodeUrl('/session', ''), {
+      const response = await fetch(buildOpenCodeUrl('/global/health', ''), {
         method: 'GET',
-        headers: getOpenCodeAuthHeaders(),
-        signal: AbortSignal.timeout(2000),
+        headers: {
+          Accept: 'application/json',
+          ...getOpenCodeAuthHeaders(),
+        },
+        signal: AbortSignal.timeout(5000),
       });
-      return response.ok;
+      if (!response.ok) return false;
+      const body = await response.json().catch(() => null);
+      return body?.healthy === true;
     } catch {
       return false;
     }