diff --git a/src/__tests__/subagentAutoBackground.test.ts b/src/__tests__/subagentAutoBackground.test.ts
new file mode 100644
index 0000000..4a2814a
--- /dev/null
+++ b/src/__tests__/subagentAutoBackground.test.ts
@@ -0,0 +1,51 @@
+/**
+ * item 12: 子代理自动后台化阈值解析单测 (CC 2.1.198/232)
+ */
+import { describe, expect, it } from "bun:test";
+import { getSubagentAutoBackgroundMs } from "../tools/AgentTool/autoBackground.js";
+
+describe("getSubagentAutoBackgroundMs", () => {
+	const orig = process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS;
+
+	it("未设 → 0 (default off, byte-identical 旧行为)", () => {
+		delete process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS;
+		expect(getSubagentAutoBackgroundMs()).toBe(0);
+	});
+
+	it("空串 → 0", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "";
+		expect(getSubagentAutoBackgroundMs()).toBe(0);
+	});
+
+	it("正值 60000 → 60000", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "60000";
+		expect(getSubagentAutoBackgroundMs()).toBe(60000);
+	});
+
+	it("120000 (对齐 CC 默认) → 120000", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "120000";
+		expect(getSubagentAutoBackgroundMs()).toBe(120000);
+	});
+
+	it("0 → 0 (显式 off)", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "0";
+		expect(getSubagentAutoBackgroundMs()).toBe(0);
+	});
+
+	it("负数 → 0 (fail-off)", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "-5";
+		expect(getSubagentAutoBackgroundMs()).toBe(0);
+	});
+
+	it("非数 → 0 (fail-off)", () => {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = "abc";
+		expect(getSubagentAutoBackgroundMs()).toBe(0);
+	});
+
+	// 还原
+	if (orig === undefined) {
+		delete process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS;
+	} else {
+		process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS = orig;
+	}
+});
diff --git a/src/tools/AgentTool/AgentTool.tsx b/src/tools/AgentTool/AgentTool.tsx
index c965a4b..c40272a 100644
--- a/src/tools/AgentTool/AgentTool.tsx
+++ b/src/tools/AgentTool/AgentTool.tsx
@@ -121,6 +121,7 @@ import {
 	isForkSubagentEnabled,
 	isInForkChild,
 } from "./forkSubagent.js";
+import { getSubagentAutoBackgroundMs } from "./autoBackground.js";
 import type { AgentDefinition } from "./loadAgentsDir.js";
 import {
 	filterAgentsByMcpRequirements,
@@ -159,6 +160,14 @@ const isBackgroundTasksDisabled =
 // Auto-background agent tasks after this many ms (0 = disabled)
 // Enabled by env var OR GrowthBook gate (checked lazily since GB may not be ready at module load)
 function getAutoBackgroundMs(): number {
+	// item 12: FUSION_SUBAGENT_AUTO_BACKGROUND_MS 显式阈值 (并行 item 4
+	// FUSION_MCP_AUTO_BACKGROUND_MS)。default 0=off, byte-identical 旧行为。
+	// 设正值 → subagent 运行该毫秒后转后台, 复用 registerAgentForeground
+	// 定时器→backgroundSignal→mid-run 后台化解阻塞 turn。
+	const fusionMs = getSubagentAutoBackgroundMs();
+	if (fusionMs > 0) {
+		return fusionMs;
+	}
 	if (
 		isEnvTruthy(process.env.CLAUDE_AUTO_BACKGROUND_TASKS) ||
 		getFeatureValue_CACHED_MAY_BE_STALE("tengu_auto_background_agents", false)
diff --git a/src/tools/AgentTool/autoBackground.ts b/src/tools/AgentTool/autoBackground.ts
new file mode 100644
index 0000000..0ed7f82
--- /dev/null
+++ b/src/tools/AgentTool/autoBackground.ts
@@ -0,0 +1,16 @@
+// item 12 (CC 2.1.198/232): 子代理自动后台化阈值解析。
+// 并行 item 4 FUSION_MCP_AUTO_BACKGROUND_MS (src/services/mcp/client.ts)。
+// FUSION_SUBAGENT_AUTO_BACKGROUND_MS 设正值 → subagent 运行该毫秒后转后台,
+// 复用 registerAgentForeground 定时器→backgroundSignal→mid-run 后台化解阻塞 turn。
+// default 0=off, byte-identical 旧行为。
+export function getSubagentAutoBackgroundMs(): number {
+	const raw = process.env.FUSION_SUBAGENT_AUTO_BACKGROUND_MS;
+	if (raw === undefined || raw === "") {
+		return 0;
+	}
+	const parsed = parseInt(raw, 10);
+	if (!Number.isFinite(parsed) || parsed <= 0) {
+		return 0;
+	}
+	return parsed;
+}