diff --git a/web/lib/gsd-workspace-store.tsx b/web/lib/gsd-workspace-store.tsx
index d22c16dc0..a34d91cf1 100644
--- a/web/lib/gsd-workspace-store.tsx
+++ b/web/lib/gsd-workspace-store.tsx
@@ -470,6 +470,10 @@ export type WorkspaceEvent =
   | TurnEndEvent
   | ({ type: Exclude<string, "bridge_status" | "live_state_invalidation" | "extension_ui_request" | "extension_error" | "message_update" | "tool_execution_start" | "tool_execution_end" | "agent_end" | "turn_end">; [key: string]: unknown } & Record<string, unknown>)
 
+export function isWorkspaceEvent(value: unknown): value is WorkspaceEvent {
+  return value !== null && typeof value === "object" && typeof (value as Record<string, unknown>).type === "string"
+}
+
 export interface WorkspaceCommandResponse {
   type: "response"
   command: string
@@ -4866,8 +4870,15 @@ export class GSDWorkspaceStore {
 
     stream.onmessage = (message) => {
       try {
-        const payload = JSON.parse(message.data) as WorkspaceEvent
-        this.handleEvent(payload)
+        const parsed: unknown = JSON.parse(message.data)
+        if (!isWorkspaceEvent(parsed)) {
+          this.patchState({
+            lastClientError: "Malformed event received from stream",
+            terminalLines: withTerminalLine(this.state.terminalLines, createTerminalLine("error", "Malformed event received from stream")),
+          })
+          return
+        }
+        this.handleEvent(parsed)
       } catch (error) {
         const text = normalizeClientError(error)
         this.patchState({
@@ -4945,6 +4956,15 @@ export class GSDWorkspaceStore {
       case "tool_execution_end":
         this.handleToolExecutionEnd(event as ToolExecutionEndEvent)
         break
+      case "bridge_status":
+        // Handled upstream in handleEvent with early return — never reaches here
+        break
+      case "live_state_invalidation":
+        // Handled upstream in handleEvent via handleLiveStateInvalidation — no live interaction state update needed
+        break
+      case "extension_error":
+        // Terminal line produced by summarizeEvent — no live interaction state update needed
+        break
     }
   }