diff --git a/ui/src/ui/chat/slash-command-executor.node.test.ts b/ui/src/ui/chat/slash-command-executor.node.test.ts
index b915ea403ea..2e47b79aa43 100644
--- a/ui/src/ui/chat/slash-command-executor.node.test.ts
+++ b/ui/src/ui/chat/slash-command-executor.node.test.ts
@@ -141,4 +141,39 @@ describe("executeSlashCommand /kill", () => {
       sessionKey: "agent:main:subagent:two",
     });
   });
+
+  it("treats the legacy main session key as the default agent scope", async () => {
+    const request = vi.fn(async (method: string, _payload?: unknown) => {
+      if (method === "sessions.list") {
+        return {
+          sessions: [
+            row("main"),
+            row("agent:main:subagent:one"),
+            row("agent:main:subagent:two"),
+            row("agent:other:subagent:three"),
+          ],
+        };
+      }
+      if (method === "chat.abort") {
+        return { ok: true, aborted: true };
+      }
+      throw new Error(`unexpected method: ${method}`);
+    });
+
+    const result = await executeSlashCommand(
+      { request } as unknown as GatewayBrowserClient,
+      "main",
+      "kill",
+      "all",
+    );
+
+    expect(result.content).toBe("Aborted 2 sub-agent sessions.");
+    expect(request).toHaveBeenNthCalledWith(1, "sessions.list", {});
+    expect(request).toHaveBeenNthCalledWith(2, "chat.abort", {
+      sessionKey: "agent:main:subagent:one",
+    });
+    expect(request).toHaveBeenNthCalledWith(3, "chat.abort", {
+      sessionKey: "agent:main:subagent:two",
+    });
+  });
 });
diff --git a/ui/src/ui/chat/slash-command-executor.ts b/ui/src/ui/chat/slash-command-executor.ts
index d0c4b01e3cf..c7ddced286d 100644
--- a/ui/src/ui/chat/slash-command-executor.ts
+++ b/ui/src/ui/chat/slash-command-executor.ts
@@ -3,7 +3,12 @@
  * Calls gateway RPC methods and returns formatted results.
  */
 
-import { isSubagentSessionKey, parseAgentSessionKey } from "../../../../src/routing/session-key.js";
+import {
+  DEFAULT_AGENT_ID,
+  DEFAULT_MAIN_KEY,
+  isSubagentSessionKey,
+  parseAgentSessionKey,
+} from "../../../../src/routing/session-key.js";
 import type { GatewayBrowserClient } from "../gateway.ts";
 import type {
   AgentsListResult,
@@ -350,6 +355,9 @@ function resolveKillTargets(
   const keys = new Set<string>();
   const normalizedCurrentSessionKey = currentSessionKey.trim().toLowerCase();
   const currentParsed = parseAgentSessionKey(normalizedCurrentSessionKey);
+  const currentAgentId =
+    currentParsed?.agentId ??
+    (normalizedCurrentSessionKey === DEFAULT_MAIN_KEY ? DEFAULT_AGENT_ID : undefined);
   for (const session of sessions) {
     const key = session?.key?.trim();
     if (!key || !isSubagentSessionKey(key)) {
@@ -360,7 +368,7 @@ function resolveKillTargets(
     const belongsToCurrentSession = isWithinCurrentSessionSubtree(
       normalizedKey,
       normalizedCurrentSessionKey,
-      currentParsed?.agentId,
+      currentAgentId,
       parsed?.agentId,
     );
     const isMatch =