@@ -3778,6 +3778,36 @@ it.layer(McpProviderSessions.layer)("OpenCode2 adapter", (it) => {
     }).pipe(Effect.scoped),
   );
 
+  it.effect("forks after a Stop before the interrupted execution's end event arrives", () =>
+    Effect.gen(function* () {
+      const FORK = "ses_f1484db83ffeLGtrRCFimo1H0e";
+      const { runtime, thread } = yield* resumed([
+        out("session.prompt", { sessionID: SESSION, text: "<any>" }),
+        promptAccepted,
+        event("session.execution.started", { sessionID: SESSION }),
+        out("session.interrupt", { sessionID: SESSION }),
+        reply("session.interrupt", { interrupted: true }),
+        // OpenCode has ended the run; its event has not reached T3 yet.
+        out("session.active"),
+        replyData("session.active", {}),
+        out("session.fork", { sessionID: SESSION }),
+        replyData("session.fork", sessionInfo({ id: FORK })),
+        out("session.update", { sessionID: FORK, permissions: "<any>" }),
+        reply("session.update", null),
+      ]);
+      yield* runtime.startTurn(turnInput(thread));
+      yield* runtime.interruptTurn({
+        providerThread: thread,
+        providerTurnId: yield* providerTurnId,
+      });
+      const forked = yield* runtime.forkThread({
+        sourceProviderThread: thread,
+        targetThreadId: ThreadId.make("thread:opencode2-adapter:fork-after-stop"),
+      });
+      assert.equal(forked.nativeThreadRef?.nativeId, FORK);
+    }).pipe(Effect.scoped),
+  );
+
   it.effect("moves a fork into its target thread's worktree before the first prompt", () =>
     Effect.gen(function* () {
       const FORK = "ses_f1484db83ffeLGtrRCFimo1H0e";
 
@@ -2899,16 +2899,19 @@ export const make = Effect.fn("OpenCode2Adapter.make")(function* (instanceId: Pr
       verb: "roll back" | "fork",
     ) {
       const state = threads.get(sessionId);
+      const background = state !== undefined && verb === "roll back" && hasBackground(state);
       const busyHere =
-        state !== undefined &&
-        (state.active !== undefined ||
-          busy.has(sessionId) ||
-          (verb === "roll back" && hasBackground(state)));
-      const busyThere =
-        !busyHere &&
-        (state === undefined || state.unsettled) &&
-        sessionId in (yield* client.session.active().pipe(Effect.timeout(ACTIVE_CHECK_TIMEOUT)));
-      if (busyHere || busyThere) {
+        state !== undefined && (state.active !== undefined || busy.has(sessionId) || background);
+      // A Stop replies before OpenCode's execution-end event reaches T3. The
+      // run is already over there, so ask the server instead of refusing.
+      const stopping = state?.active?.interrupted === true;
+      const active =
+        (busyHere && stopping) || state === undefined || state.unsettled
+          ? yield* client.session.active().pipe(Effect.timeout(ACTIVE_CHECK_TIMEOUT))
+          : undefined;
+      const busyThere = active !== undefined && sessionId in active;
+      const stoppedBeforeEvent = stopping && !background && active !== undefined && !busyThere;
+      if ((busyHere && !stoppedBeforeEvent) || busyThere) {
         return yield* new ProviderAdapter.ProviderAdapterProtocolError({
           driver: OPENCODE_PROVIDER,
           detail: `Cannot ${verb} OpenCode session ${sessionId} while it is still working`,