fix(webapp): improve dead-run friendlyId fallback and swap-race error path

Author: ericallam

apps/webapp/app/services/realtime/sessionRunManager.server.ts

@@ -123,10 +123,19 @@ export async function ensureRunForSession(
     // agent as `previousRunId` so its boot gate flips
     // `couldHavePriorState` and replays the persisted state instead of
     // treating this as a fresh chat. See `chat.agent`'s boot orchestration
-    // in `packages/trigger-sdk/src/v3/ai.ts`. Falls back to the cuid on
-    // probe miss (rare — replica miss on a row we just read) so the
-    // continuation flag still propagates with degraded id fidelity.
-    priorDeadRunFriendlyId = probe?.friendlyId ?? session.currentRunId;
+    // in `packages/trigger-sdk/src/v3/ai.ts`.
+    if (probe?.friendlyId) {
+      priorDeadRunFriendlyId = probe.friendlyId;
+    } else {
+      // Replica miss on a row we just observed via `currentRunId`. Retry
+      // on the writer so the customer's `runs.retrieve(previousRunId)`
+      // gets the public `run_*` form rather than the internal cuid.
+      const writerProbe = await prisma.taskRun.findFirst({
+        where: { id: session.currentRunId },
+        select: { friendlyId: true },
+      });
+      priorDeadRunFriendlyId = writerProbe?.friendlyId ?? session.currentRunId;
+    }
   }
 
   // 2. Validate config + trigger upfront. Continuation overrides
@@ -457,8 +466,19 @@ export async function swapSessionRun(
     select: { currentRunId: true },
   });
 
+  // Mirror `ensureRunForSession`'s "session vanished" branch: if we
+  // can't find the row (or it has no current run) on the writer right
+  // after losing the race, surface as an error rather than handing back
+  // `callingRunId` with `swapped: false` — that would tell the caller
+  // it's still the canonical run when in fact we don't know who is.
+  if (!fresh?.currentRunId) {
+    throw new SessionRunManagerError(
+      `Session ${session.id} has no currentRunId after preempted swap`
+    );
+  }
+
   return {
-    runId: fresh?.currentRunId ?? callingRunId,
+    runId: fresh.currentRunId,
     swapped: false,
   };
 }