diff --git a/lib/chat/tools/__tests__/createPromptSandboxStreamingTool.test.ts b/lib/chat/tools/__tests__/createPromptSandboxStreamingTool.test.ts
index 0609f0a5..66f08e28 100644
--- a/lib/chat/tools/__tests__/createPromptSandboxStreamingTool.test.ts
+++ b/lib/chat/tools/__tests__/createPromptSandboxStreamingTool.test.ts
@@ -239,6 +239,28 @@ describe("createPromptSandboxStreamingTool", () => {
     });
   });
 
+  describe("description explains runId background task behavior", () => {
+    it("tells LLM not to interpret empty output when runId is present", () => {
+      const tool = createPromptSandboxStreamingTool("acc_1", "key_1");
+
+      expect(tool.description).toContain("runId");
+      expect(tool.description).toContain("background task");
+      expect(tool.description).toContain("do NOT");
+    });
+
+    it("tells LLM not to poll task status", () => {
+      const tool = createPromptSandboxStreamingTool("acc_1", "key_1");
+
+      expect(tool.description).toContain("Do NOT automatically poll");
+    });
+
+    it("tells LLM to let user know they can ask for status", () => {
+      const tool = createPromptSandboxStreamingTool("acc_1", "key_1");
+
+      expect(tool.description).toContain("ask you to check");
+    });
+  });
+
   describe("description mentions release management", () => {
     it("includes release management as a primary use case", () => {
       const tool = createPromptSandboxStreamingTool("acc_1", "key_1");
diff --git a/lib/chat/tools/createPromptSandboxStreamingTool.ts b/lib/chat/tools/createPromptSandboxStreamingTool.ts
index e7a1d7e4..8cddbb18 100644
--- a/lib/chat/tools/createPromptSandboxStreamingTool.ts
+++ b/lib/chat/tools/createPromptSandboxStreamingTool.ts
@@ -52,7 +52,12 @@ export function createPromptSandboxStreamingTool(
       "file operations, data analysis, content generation, and any multi-step task. " +
       "The sandbox has skills for managing RELEASE.md documents, generating deliverables, and more. " +
       "Reuses the account's existing running sandbox or creates one from the latest snapshot. " +
-      "Streams output in real-time.",
+      "Streams output in real-time. " +
+      "IMPORTANT: When the result contains a `runId`, it means the sandbox is being set up for the first time " +
+      "and the command was dispatched to a background task. The output will be empty because the task is still running. " +
+      "The UI automatically shows a live progress view for background tasks — do NOT summarize or interpret the empty output. " +
+      "Simply tell the user their request is being processed in the sandbox and the results will appear in the task progress view above. " +
+      "Do NOT automatically poll or check the task status — instead, let the user know they can ask you to check on it whenever they want.",
     inputSchema: promptSandboxSchema,
     execute: async function* ({ prompt }, { abortSignal }) {
       yield { status: "booting" as const, output: "" };