fix(clients): allow portable provider handoffs after detach

packages/client-runtime/src/state/threadWorkflows.test.ts

@@ -62,7 +62,8 @@ describe("thread workflows", () => {
         ...projection,
         providerThreads: [{ ...projection.providerThreads[0]!, nativeThreadRef: null }],
       }),
-    ).toBe(false);
+    ).toBe(true);
+    expect(threadSupportsProviderHandoff({ ...projection, providerThreads: [] })).toBe(false);
   });
 
   it("sorts queued messages and gates promotion from capabilities", () => {

packages/client-runtime/src/state/threadWorkflows.ts

@@ -109,14 +109,14 @@ export function threadSupportsProviderHandoff(projection: Projection | null | un
   if (resolveActiveThreadRun(projection) !== null) return false;
   if (projection.thread.historyOrigin === "v1_import" || projection.runs.length === 0) return true;
 
-  // Detaching a stopped session removes it from the projection, but its native
-  // provider thread remains available for the next turn's portable handoff.
+  // Detaching a stopped session removes it from the projection. The server still
+  // hands off from saved app history, which needs the source provider thread but
+  // not its native ref.
   return projection.providerThreads.some(
     (thread) =>
       thread.id === projection.thread.activeProviderThreadId &&
       thread.appThreadId === projection.thread.id &&
-      thread.providerInstanceId === projection.thread.modelSelection.instanceId &&
-      thread.nativeThreadRef !== null,
+      thread.providerInstanceId === projection.thread.modelSelection.instanceId,
   );
 }