fix(sessions): guard the remaining in-flight-turn invalidation paths

app/api/sessions/[id]/route.ts

@@ -343,10 +343,12 @@ export async function DELETE(
     for (const deletedId of [...deletedSessionIds].reverse()) {
       if (deletedId === id) continue;
       try { await abortSubagent(deletedId); } catch { /* idle or completed */ }
-      await getRpcSession(deletedId)?.shutdown();
+      const child = getRpcSession(deletedId);
+      if (!child?.isRunning()) await child?.shutdown();
     }
     try { await abortSubagent(id); } catch { /* ordinary session */ }
-    await getRpcSession(id)?.shutdown();
+    const target = getRpcSession(id);
+    if (!target?.isRunning()) await target?.shutdown();
     for (const [deletedId, deletedPath] of deletedPaths) {
       try {
         unlinkSync(deletedPath);

lib/rpc-manager.test.mjs

@@ -229,7 +229,8 @@ test("normal session teardown paths use graceful extension shutdown", async () =
   assert.match(replacementShutdownSource, /await this\.shutdown\(\)/);
   assert.match(forkSource, /shutdownAfterSessionReplacement\("fork"\)/);
   assert.match(cloneSource, /shutdownAfterSessionReplacement\("clone"\)/);
-  assert.match(deleteRouteSource, /await getRpcSession\(id\)\?\.shutdown\(\)/);
+  assert.match(deleteRouteSource, /const target = getRpcSession\(id\);\s*if \(!target\?\.isRunning\(\)\) await target\?\.shutdown\(\)/);
+  assert.match(deleteRouteSource, /const child = getRpcSession\(deletedId\);\s*if \(!child\?\.isRunning\(\)\) await child\?\.shutdown\(\)/);
   assert.match(trustRouteSource, /await destroyRpcSessionsForCwd\(result\.cwd\)/);
 });
 

lib/rpc-manager.ts

@@ -1210,6 +1210,9 @@ export class AgentSessionWrapper {
       }
 
       case "reload": {
+        if (this.isSessionRunningForReplacement()) {
+          throw new Error("Cannot reload while the session is running");
+        }
         if (this.extensionUiAbortController.signal.aborted) {
           this.extensionUiAbortController = new AbortController();
         }
@@ -1959,6 +1962,9 @@ export class AgentSessionWrapper {
         this.navigateTreeKeepingToolSelection(targetId, { summarize: options?.summarize }),
       switchSession: async () => ({ cancelled: true }),
       reload: async () => {
+        if (this.isSessionRunningForReplacement()) {
+          throw new Error("Cannot reload (extension ctx) while the session is running");
+        }
         this.extensionStatuses.clear();
         this.resetExtensionWidgetsForReload();
         this.syncProjectTrust();
@@ -2295,7 +2301,7 @@ export function hasBusyRpcSessionForCwd(cwd: string): boolean {
 export async function destroyRpcSessionsForCwd(cwd: string): Promise<number> {
   const targetCwd = normalizeRpcCwd(cwd);
   const sessions = Array.from(getRegistry().values()).filter(
-    (session) => normalizeRpcCwd(session.cwd) === targetCwd,
+    (session) => normalizeRpcCwd(session.cwd) === targetCwd && !session.isRunning(),
   );
   await Promise.all(sessions.map((session) => session.shutdown()));
   return sessions.length;

lib/subagent-runtime.ts

@@ -1,3 +1,4 @@
+import { registerSessionLivenessProvider } from "./session-liveness";
 import type { ThinkingLevel } from "@earendil-works/pi-agent-core";
 import {
   createAgentSessionFromServices,
@@ -540,9 +541,17 @@ export function createSubagentController(
         finishQueuedAbort,
       );
       stored.cancelQueued = queued.cancel;
+      // A background subagent outlives its parent's turn; keep the parent session from
+      // being automatically idle-evicted while the run is still active. Registered here,
+      // after the last call that can throw, so the release below cannot be skipped.
+      const releaseParentLiveness = registerSessionLivenessProvider({
+        name: "pi-web-subagent-run",
+        sessionId: parentSessionId,
+        isActive: () => getSubagentRuns().get(initialRun.sessionId) === stored,
+      });
       void queued.promise.then(resolveCompletion, (error) => {
         resolveCompletion({ ...initialRun, status: "failed", completedAt: new Date().toISOString(), error: error instanceof Error ? error.message : String(error) });
-      });
+      }).finally(releaseParentLiveness);
 
       return { run: stored.run, completion: stored.completion };
     } catch (error) {
@@ -663,7 +672,13 @@ export function createSubagentController(
       dependencies.invalidateSessionList();
     }, finishQueuedAbort);
     stored.cancelQueued = queued.cancel;
-    void queued.promise.then(resolveCompletion, (error) => resolveCompletion({ ...initialRun, status: "failed", completedAt: new Date().toISOString(), error: error instanceof Error ? error.message : String(error) }));
+    // Keep the resumed run's parent alive past its own turn (see start()).
+    const releaseParentLiveness = registerSessionLivenessProvider({
+      name: "pi-web-subagent-run",
+      sessionId: parentSessionId,
+      isActive: () => getSubagentRuns().get(request.sessionId) === stored,
+    });
+    void queued.promise.then(resolveCompletion, (error) => resolveCompletion({ ...initialRun, status: "failed", completedAt: new Date().toISOString(), error: error instanceof Error ? error.message : String(error) })).finally(releaseParentLiveness);
     return { run: stored.run, completion };
   }