diff --git a/packages/adapter-utils/src/acpx-engine/execute.test.ts b/packages/adapter-utils/src/acpx-engine/execute.test.ts
index 56e41dc6feb..8918aec7c25 100644
--- a/packages/adapter-utils/src/acpx-engine/execute.test.ts
+++ b/packages/adapter-utils/src/acpx-engine/execute.test.ts
@@ -41,6 +41,13 @@ async function onlyChildDir(parent: string): Promise<string> {
   return path.join(parent, entries[0]!);
 }
 
+async function readSingleWrapperEnv(stateDir: string): Promise<string> {
+  const wrappers = await fs.readdir(path.join(stateDir, "wrappers"));
+  const envFileName = wrappers.find((name) => name.endsWith(".env"));
+  expect(envFileName).toBeTypeOf("string");
+  return fs.readFile(path.join(stateDir, "wrappers", envFileName!), "utf8");
+}
+
 async function createSkill(root: string, name: string, body = `---\nrequired: false\n---\n# ${name}\n`) {
   const skillDir = path.join(root, name);
   await fs.mkdir(skillDir, { recursive: true });
@@ -696,6 +703,46 @@ describe("shared ACPX engine runtime behavior", () => {
     expect(env).not.toContain("attacker-issue");
   });
 
+  it("injects CLAUDE_CODE_DISABLE_BACKGROUND_TASKS for Claude ACP runs", async () => {
+    const root = await makeTempRoot();
+    const stateDir = path.join(root, "state");
+    const cwd = path.join(root, "worktree");
+    await fs.mkdir(cwd, { recursive: true });
+
+    await runExecutor({ agent: "claude", stateDir, cwd });
+
+    const env = await readSingleWrapperEnv(stateDir);
+    expect(env).toContain("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS='1'");
+  });
+
+  it("preserves an explicit CLAUDE_CODE_DISABLE_BACKGROUND_TASKS value for Claude ACP runs", async () => {
+    const root = await makeTempRoot();
+    const stateDir = path.join(root, "state");
+    const cwd = path.join(root, "worktree");
+    await fs.mkdir(cwd, { recursive: true });
+
+    await runExecutor({
+      agent: "claude",
+      stateDir,
+      cwd,
+      env: { CLAUDE_CODE_DISABLE_BACKGROUND_TASKS: "0" },
+    });
+
+    const env = await readSingleWrapperEnv(stateDir);
+    expect(env).toContain("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS='0'");
+    expect(env).not.toContain("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS='1'");
+  });
+
+  it("does not inject CLAUDE_CODE_DISABLE_BACKGROUND_TASKS for non-Claude ACP runs", async () => {
+    const root = await makeTempRoot();
+    const stateDir = path.join(root, "state");
+
+    await runExecutor({ agent: "codex", agentCommand: "node ./fake-acp.js", stateDir });
+
+    const env = await readSingleWrapperEnv(stateDir);
+    expect(env).not.toContain("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS=");
+  });
+
   it("busts the session fingerprint when resolved adapter env changes but not across wakes", async () => {
     const root = await makeTempRoot();
     const stateDir = path.join(root, "state");
diff --git a/packages/adapter-utils/src/acpx-engine/execute.ts b/packages/adapter-utils/src/acpx-engine/execute.ts
index d23841e35f9..df7406d9fce 100644
--- a/packages/adapter-utils/src/acpx-engine/execute.ts
+++ b/packages/adapter-utils/src/acpx-engine/execute.ts
@@ -1117,6 +1117,9 @@ async function buildRuntime(input: {
   if (requestedModel && acpxAgent === "claude" && !env.ANTHROPIC_MODEL) {
     env.ANTHROPIC_MODEL = requestedModel;
   }
+  if (acpxAgent === "claude" && !env.CLAUDE_CODE_DISABLE_BACKGROUND_TASKS) {
+    env.CLAUDE_CODE_DISABLE_BACKGROUND_TASKS = "1";
+  }
   if (acpxAgent === "codex") {
     const codexStartupConfig = buildCodexStartupConfig({
       existingConfig: env.CODEX_CONFIG,