From a0ca1de51e21aad39acdeec2271571db644073d1 Mon Sep 17 00:00:00 2001
From: Federico Jaramillo Martinez <federico@klingit.com>
Date: Tue, 12 May 2026 15:55:19 +0200
Subject: [PATCH] Disable forking active sessions

---
 .../sessions/sessionCommandService.test.ts    | 30 +++++++++++++++++++
 src/server/sessions/sessionCommandService.ts  | 11 +++++++
 2 files changed, 41 insertions(+)

diff --git a/src/server/sessions/sessionCommandService.test.ts b/src/server/sessions/sessionCommandService.test.ts
index 53a19ef..1bff313 100644
--- a/src/server/sessions/sessionCommandService.test.ts
+++ b/src/server/sessions/sessionCommandService.test.ts
@@ -114,4 +114,34 @@ describe("SessionCommandService", () => {
     expect(active.runtime.fork).toHaveBeenCalledWith("newest");
     await expect(service.respond("s1", result.requestId, "newest")).resolves.toEqual({ type: "unsupported", message: "Command request expired" });
   });
+
+  it("rejects fork and clone while the session has active work", async () => {
+    const active = activeSession({ isStreaming: true });
+    const service = new SessionCommandService(() => getActive(active), vi.fn(), { publish: vi.fn() } as never);
+
+    await expect(service.run("s1", "/fork")).resolves.toEqual({
+      type: "unsupported",
+      message: "Cannot fork while the session is active. Stop current activity before forking.",
+    });
+    await expect(service.run("s1", "/clone")).resolves.toEqual({
+      type: "unsupported",
+      message: "Cannot clone while the session is active. Stop current activity before cloning.",
+    });
+    expect(active.runtime.fork).not.toHaveBeenCalled();
+  });
+
+  it("rejects fork responses if the session becomes active after choosing fork", async () => {
+    const active = activeSession();
+    const service = new SessionCommandService(() => getActive(active), vi.fn(), { publish: vi.fn() } as never);
+
+    const result = await service.run("s1", "/fork");
+    if (result.type !== "select") throw new Error("Expected select result");
+    (active.runtime.session as Record<string, unknown>)["isStreaming"] = true;
+
+    await expect(service.respond("s1", result.requestId, "m1")).resolves.toEqual({
+      type: "unsupported",
+      message: "Cannot fork while the session is active. Stop current activity before forking.",
+    });
+    expect(active.runtime.fork).not.toHaveBeenCalled();
+  });
 });
diff --git a/src/server/sessions/sessionCommandService.ts b/src/server/sessions/sessionCommandService.ts
index 5ca1d9c..73e85a7 100644
--- a/src/server/sessions/sessionCommandService.ts
+++ b/src/server/sessions/sessionCommandService.ts
@@ -52,6 +52,7 @@ export class SessionCommandService {
     this.pendingSelects.delete(requestId);
 
     const active = await this.getActive(sessionId);
+    if (sessionHasActiveWork(active.runtime.session)) return forkActiveUnsupported("fork");
     const result = await active.runtime.fork(value);
     if (result.cancelled) return { type: "done", message: "Fork cancelled" };
     return { type: "done", message: "Session forked", session: clientSessionFromRuntime(active.runtime) };
@@ -84,6 +85,7 @@ export class SessionCommandService {
   }
 
   private async clone(active: ActiveSession): Promise<ClientCommandResult> {
+    if (sessionHasActiveWork(active.runtime.session)) return forkActiveUnsupported("clone");
     const leafId = active.runtime.session.sessionManager.getLeafId();
     if (leafId === null || leafId === "") return { type: "unsupported", message: "Cannot clone: no current session entry" };
     const result = await active.runtime.fork(leafId, { position: "at" });
@@ -92,6 +94,7 @@ export class SessionCommandService {
   }
 
   private fork(active: ActiveSession): ClientCommandResult {
+    if (sessionHasActiveWork(active.runtime.session)) return forkActiveUnsupported("fork");
     const messages = active.runtime.session.getUserMessagesForForking();
     if (!messages.length) return { type: "unsupported", message: "No user messages to fork from" };
     const requestId = crypto.randomUUID();
@@ -127,6 +130,14 @@ function clientSessionFromRuntime(runtime: AgentSessionRuntime): ClientSession {
   };
 }
 
+function sessionHasActiveWork(session: AgentSession): boolean {
+  return session.isStreaming || session.isBashRunning || session.isCompacting || session.pendingMessageCount > 0;
+}
+
+function forkActiveUnsupported(command: "fork" | "clone"): ClientCommandResult {
+  return { type: "unsupported", message: `Cannot ${command} while the session is active. Stop current activity before ${command === "fork" ? "forking" : "cloning"}.` };
+}
+
 function formatSessionStats(session: AgentSession): string {
   const stats = session.getSessionStats();
   return [