diff --git a/clients/shared/Network/EventStreamClient.swift b/clients/shared/Network/EventStreamClient.swift
index 875722f73b4..e42ead28e90 100644
--- a/clients/shared/Network/EventStreamClient.swift
+++ b/clients/shared/Network/EventStreamClient.swift
@@ -277,6 +277,15 @@ public final class EventStreamClient {
         sseTask = Task { @MainActor [weak self] in
             guard let self else { return }
 
+            // A back-to-back call to `startSSEStream()` on the MainActor can
+            // cancel this task and invalidate `session` before it runs its
+            // first instruction. Calling `session.bytes(for:)` on an already
+            // invalidated session throws an ObjC NSGenericException from
+            // `-[__NSURLSessionLocal taskForClassInfo:]`, which is
+            // uncatchable in Swift and crashes the process with SIGABRT.
+            // Bail out before touching the session if we've been superseded.
+            guard !Task.isCancelled, self.sseSession === session else { return }
+
             do {
                 let (bytes, response) = try await GatewayHTTPClient.stream(
                     path: "assistants/{assistantId}/events",