diff --git a/actions/setup/js/copilot_sdk_driver.test.cjs b/actions/setup/js/copilot_sdk_driver.test.cjs
index fd88ddcecd1..ab2dc541bfa 100644
--- a/actions/setup/js/copilot_sdk_driver.test.cjs
+++ b/actions/setup/js/copilot_sdk_driver.test.cjs
@@ -1,10 +1,26 @@
-import { describe, it, expect, vi } from "vitest";
+import { describe, it, expect, vi, beforeAll, afterAll } from "vitest";
 import { createRequire } from "module";
+import * as fs from "fs";
+import * as os from "os";
+import * as path from "path";
 
 const require = createRequire(import.meta.url);
 const { runWithCopilotSDK, parsePermissionConfigFromServerArgs } = require("./copilot_sdk_driver.cjs");
 
 describe("copilot_sdk_driver.cjs", () => {
+  let testSessionStateDir;
+  let prevSessionStateDir;
+  beforeAll(() => {
+    prevSessionStateDir = process.env.GH_AW_SESSION_STATE_BASE_DIR;
+    testSessionStateDir = fs.mkdtempSync(path.join(os.tmpdir(), "gh-aw-test-session-state-"));
+    process.env.GH_AW_SESSION_STATE_BASE_DIR = testSessionStateDir;
+  });
+  afterAll(() => {
+    if (prevSessionStateDir === undefined) delete process.env.GH_AW_SESSION_STATE_BASE_DIR;
+    else process.env.GH_AW_SESSION_STATE_BASE_DIR = prevSessionStateDir;
+    if (testSessionStateDir) fs.rmSync(testSessionStateDir, { recursive: true, force: true });
+  });
+
   describe("runWithCopilotSDK", () => {
     it("disconnects session and stops client on success", async () => {
       const disconnect = vi.fn().mockResolvedValue(undefined);
diff --git a/actions/setup/js/copilot_sdk_session.cjs b/actions/setup/js/copilot_sdk_session.cjs
index c8510abb884..82b04c323cc 100644
--- a/actions/setup/js/copilot_sdk_session.cjs
+++ b/actions/setup/js/copilot_sdk_session.cjs
@@ -82,10 +82,11 @@ function extractPromptFromArgs(args) {
  *     RuntimeConnection: typeof import("@github/copilot-sdk").RuntimeConnection,
  *     approveAll: typeof import("@github/copilot-sdk").approveAll
  *   },
+ *   sessionStateBaseDir?: string,
  * }} options
  * @returns {Promise<{exitCode: number, output: string, hasOutput: boolean, durationMs: number}>}
  */
-async function runWithCopilotSDK({ sdkUri, prompt, logger, attempt = 0, model, connectionToken, provider, maxToolDenials, permissionConfig, coreLogger, sdkModule }) {
+async function runWithCopilotSDK({ sdkUri, prompt, logger, attempt = 0, model, connectionToken, provider, maxToolDenials, permissionConfig, coreLogger, sdkModule, sessionStateBaseDir }) {
   // Lazy-require to avoid loading the SDK when it is not needed.
   // The SDK is large and has side-effects on import (worker threads, etc.).
   const { CopilotClient, RuntimeConnection, approveAll } = sdkModule ?? require("@github/copilot-sdk");
@@ -106,7 +107,9 @@ async function runWithCopilotSDK({ sdkUri, prompt, logger, attempt = 0, model, c
 
   // Session state directory — mirrors the target path used by unified_timeline.cjs.
   // /tmp/gh-aw/sandbox/agent/logs/copilot-session-state/{sessionId}/events.jsonl
-  const sessionStateBase = path.join(os.tmpdir(), "gh-aw", "sandbox", "agent", "logs", "copilot-session-state");
+  // GH_AW_SESSION_STATE_BASE_DIR may be set in tests to redirect writes to an isolated directory.
+  const defaultSessionStateBase = path.join(os.tmpdir(), "gh-aw", "sandbox", "agent", "logs", "copilot-session-state");
+  const sessionStateBase = sessionStateBaseDir ?? process.env.GH_AW_SESSION_STATE_BASE_DIR ?? defaultSessionStateBase;
 
   /** @type {ReadonlyArray<NonNullable<import("@github/copilot-sdk").CopilotClientOptions["logLevel"]>>} */
   const VALID_LOG_LEVELS = ["none", "error", "warning", "info", "debug", "all"];