diff --git a/packages/adapter-utils/src/acpx-engine/execute.test.ts b/packages/adapter-utils/src/acpx-engine/execute.test.ts
index 8918aec7c25..c4f1a34d00d 100644
--- a/packages/adapter-utils/src/acpx-engine/execute.test.ts
+++ b/packages/adapter-utils/src/acpx-engine/execute.test.ts
@@ -733,6 +733,24 @@ describe("shared ACPX engine runtime behavior", () => {
     expect(env).not.toContain("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS='1'");
   });
 
+  it("preserves an explicit empty 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: "" },
+    });
+
+    const env = await readSingleWrapperEnv(stateDir);
+    expect(env).toMatch(/CLAUDE_CODE_DISABLE_BACKGROUND_TASKS=\x27\x27/);
+    expect(env).not.toMatch(/CLAUDE_CODE_DISABLE_BACKGROUND_TASKS=\x271\x27/);
+  });
+
   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");
diff --git a/packages/adapter-utils/src/acpx-engine/execute.ts b/packages/adapter-utils/src/acpx-engine/execute.ts
index df7406d9fce..f7de00fd382 100644
--- a/packages/adapter-utils/src/acpx-engine/execute.ts
+++ b/packages/adapter-utils/src/acpx-engine/execute.ts
@@ -1117,7 +1117,7 @@ async function buildRuntime(input: {
   if (requestedModel && acpxAgent === "claude" && !env.ANTHROPIC_MODEL) {
     env.ANTHROPIC_MODEL = requestedModel;
   }
-  if (acpxAgent === "claude" && !env.CLAUDE_CODE_DISABLE_BACKGROUND_TASKS) {
+  if (acpxAgent === "claude" && !("CLAUDE_CODE_DISABLE_BACKGROUND_TASKS" in env)) {
     env.CLAUDE_CODE_DISABLE_BACKGROUND_TASKS = "1";
   }
   if (acpxAgent === "codex") {