fix(server): steers keep the newest next-turn selection (#14725)

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: juliusmarminge

apps/server/src/orchestration-v2/EffectWorker.ts

@@ -244,7 +244,12 @@ export const executorLayer: Layer.Layer<
                       text: message.text,
                       ...(message.context ? { context: message.context } : {}),
                       attachments: message.attachments,
-                      modelSelection: run.modelSelection,
+                      // A user's follow-up starts on the thread's saved selection,
+                      // which already holds the steer's choice. A delegated
+                      // completion stays pinned to the run it reports to.
+                      ...(message.delegatedCompletion === undefined
+                        ? {}
+                        : { modelSelection: run.modelSelection }),
                       dispatchMode: {
                         type:
                           message.delegatedCompletion === undefined

apps/server/src/orchestration-v2/Orchestrator.ts

@@ -3631,17 +3631,25 @@ const makeOrchestrator = Effect.fn("orchestrationV2.Orchestrator.layer")(functio
       );
 
       if (steeringPolicy === "active_steering") {
+        // The steer's selection becomes the saved next-turn choice, even when it
+        // matches the running run again. A delegated completion carries the
+        // run's selection, not a user choice, so it never replaces the saved one.
+        // The saved choice may name another instance, so it moves with the steer.
+        const instanceChanged =
+          input.projection.thread.providerInstanceId !== input.modelSelection.instanceId;
         if (
-          selectionChanged &&
-          !modelSelectionsEqual(input.projection.thread.modelSelection, input.modelSelection)
+          input.delegatedCompletion === undefined &&
+          (instanceChanged ||
+            !modelSelectionsEqual(input.projection.thread.modelSelection, input.modelSelection))
         ) {
           yield* emitEvent({
-            type: "thread.model-selection-updated",
+            type: instanceChanged ? "thread.provider-switched" : "thread.model-selection-updated",
             threadId: input.command.threadId,
             providerInstanceId: input.modelSelection.instanceId,
             occurredAt: now,
             payload: {
               ...input.projection.thread,
+              providerInstanceId: input.modelSelection.instanceId,
               modelSelection: input.modelSelection,
               updatedAt: now,
             },