diff --git a/src/auto-reply/envelope.test.ts b/src/auto-reply/envelope.test.ts
index 179bd69abbe..69571636282 100644
--- a/src/auto-reply/envelope.test.ts
+++ b/src/auto-reply/envelope.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it } from "vitest";
+import { withEnv } from "../test-utils/env.js";
 import {
   formatAgentEnvelope,
   formatInboundEnvelope,
@@ -7,56 +8,47 @@ import {
 
 describe("formatAgentEnvelope", () => {
   it("includes channel, from, ip, host, and timestamp", () => {
-    const originalTz = process.env.TZ;
-    process.env.TZ = "UTC";
+    withEnv({ TZ: "UTC" }, () => {
+      const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z
+      const body = formatAgentEnvelope({
+        channel: "WebChat",
+        from: "user1",
+        host: "mac-mini",
+        ip: "10.0.0.5",
+        timestamp: ts,
+        envelope: { timezone: "utc" },
+        body: "hello",
+      });
 
-    const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z
-    const body = formatAgentEnvelope({
-      channel: "WebChat",
-      from: "user1",
-      host: "mac-mini",
-      ip: "10.0.0.5",
-      timestamp: ts,
-      envelope: { timezone: "utc" },
-      body: "hello",
+      expect(body).toBe("[WebChat user1 mac-mini 10.0.0.5 Thu 2025-01-02T03:04Z] hello");
     });
-
-    process.env.TZ = originalTz;
-
-    expect(body).toBe("[WebChat user1 mac-mini 10.0.0.5 Thu 2025-01-02T03:04Z] hello");
   });
 
   it("formats timestamps in local timezone by default", () => {
-    const originalTz = process.env.TZ;
-    process.env.TZ = "America/Los_Angeles";
+    withEnv({ TZ: "America/Los_Angeles" }, () => {
+      const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z
+      const body = formatAgentEnvelope({
+        channel: "WebChat",
+        timestamp: ts,
+        body: "hello",
+      });
 
-    const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z
-    const body = formatAgentEnvelope({
-      channel: "WebChat",
-      timestamp: ts,
-      body: "hello",
+      expect(body).toMatch(/\[WebChat Wed 2025-01-01 19:04 [^\]]+\] hello/);
     });
-
-    process.env.TZ = originalTz;
-
-    expect(body).toMatch(/\[WebChat Wed 2025-01-01 19:04 [^\]]+\] hello/);
   });
 
   it("formats timestamps in UTC when configured", () => {
-    const originalTz = process.env.TZ;
-    process.env.TZ = "America/Los_Angeles";
+    withEnv({ TZ: "America/Los_Angeles" }, () => {
+      const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z (19:04 PST)
+      const body = formatAgentEnvelope({
+        channel: "WebChat",
+        timestamp: ts,
+        envelope: { timezone: "utc" },
+        body: "hello",
+      });
 
-    const ts = Date.UTC(2025, 0, 2, 3, 4); // 2025-01-02T03:04:00Z (19:04 PST)
-    const body = formatAgentEnvelope({
-      channel: "WebChat",
-      timestamp: ts,
-      envelope: { timezone: "utc" },
-      body: "hello",
+      expect(body).toBe("[WebChat Thu 2025-01-02T03:04Z] hello");
     });
-
-    process.env.TZ = originalTz;
-
-    expect(body).toBe("[WebChat Thu 2025-01-02T03:04Z] hello");
   });
 
   it("formats timestamps in user timezone when configured", () => {
diff --git a/src/config/config.pruning-defaults.test.ts b/src/config/config.pruning-defaults.test.ts
index b6a0c4563d3..c37b9ba8f45 100644
--- a/src/config/config.pruning-defaults.test.ts
+++ b/src/config/config.pruning-defaults.test.ts
@@ -1,6 +1,7 @@
 import fs from "node:fs/promises";
 import path from "node:path";
 import { describe, expect, it } from "vitest";
+import { withEnvAsync } from "../test-utils/env.js";
 import { loadConfig } from "./config.js";
 import { withTempHome } from "./test-helpers.js";
 
@@ -16,27 +17,15 @@ async function writeConfigForTest(home: string, config: unknown): Promise<void>
 
 describe("config pruning defaults", () => {
   it("does not enable contextPruning by default", async () => {
-    const prevApiKey = process.env.ANTHROPIC_API_KEY;
-    const prevOauthToken = process.env.ANTHROPIC_OAUTH_TOKEN;
-    process.env.ANTHROPIC_API_KEY = "";
-    process.env.ANTHROPIC_OAUTH_TOKEN = "";
-    await withTempHome(async (home) => {
-      await writeConfigForTest(home, { agents: { defaults: {} } });
+    await withEnvAsync({ ANTHROPIC_API_KEY: "", ANTHROPIC_OAUTH_TOKEN: "" }, async () => {
+      await withTempHome(async (home) => {
+        await writeConfigForTest(home, { agents: { defaults: {} } });
 
-      const cfg = loadConfig();
+        const cfg = loadConfig();
 
-      expect(cfg.agents?.defaults?.contextPruning?.mode).toBeUndefined();
+        expect(cfg.agents?.defaults?.contextPruning?.mode).toBeUndefined();
+      });
     });
-    if (prevApiKey === undefined) {
-      delete process.env.ANTHROPIC_API_KEY;
-    } else {
-      process.env.ANTHROPIC_API_KEY = prevApiKey;
-    }
-    if (prevOauthToken === undefined) {
-      delete process.env.ANTHROPIC_OAUTH_TOKEN;
-    } else {
-      process.env.ANTHROPIC_OAUTH_TOKEN = prevOauthToken;
-    }
   });
 
   it("enables cache-ttl pruning + 1h heartbeat for Anthropic OAuth", async () => {
diff --git a/src/infra/env.test.ts b/src/infra/env.test.ts
index ce968a6e47f..42eb0b921cf 100644
--- a/src/infra/env.test.ts
+++ b/src/infra/env.test.ts
@@ -1,52 +1,31 @@
 import { describe, expect, it } from "vitest";
+import { withEnv } from "../test-utils/env.js";
 import { isTruthyEnvValue, normalizeZaiEnv } from "./env.js";
 
 describe("normalizeZaiEnv", () => {
-  function withZaiEnv(env: { zaiApiKey?: string; legacyZaiApiKey?: string }, run: () => void) {
-    const prevZai = process.env.ZAI_API_KEY;
-    const prevLegacy = process.env.Z_AI_API_KEY;
-    if (env.zaiApiKey === undefined) {
-      delete process.env.ZAI_API_KEY;
-    } else {
-      process.env.ZAI_API_KEY = env.zaiApiKey;
-    }
-    if (env.legacyZaiApiKey === undefined) {
-      delete process.env.Z_AI_API_KEY;
-    } else {
-      process.env.Z_AI_API_KEY = env.legacyZaiApiKey;
-    }
-    try {
-      run();
-    } finally {
-      if (prevZai === undefined) {
-        delete process.env.ZAI_API_KEY;
-      } else {
-        process.env.ZAI_API_KEY = prevZai;
-      }
-      if (prevLegacy === undefined) {
-        delete process.env.Z_AI_API_KEY;
-      } else {
-        process.env.Z_AI_API_KEY = prevLegacy;
-      }
-    }
-  }
-
   it("copies Z_AI_API_KEY to ZAI_API_KEY when missing", () => {
-    withZaiEnv({ zaiApiKey: "", legacyZaiApiKey: "zai-legacy" }, () => {
+    withEnv({ ZAI_API_KEY: "", Z_AI_API_KEY: "zai-legacy" }, () => {
       normalizeZaiEnv();
       expect(process.env.ZAI_API_KEY).toBe("zai-legacy");
     });
   });
 
   it("does not override existing ZAI_API_KEY", () => {
-    withZaiEnv({ zaiApiKey: "zai-current", legacyZaiApiKey: "zai-legacy" }, () => {
+    withEnv({ ZAI_API_KEY: "zai-current", Z_AI_API_KEY: "zai-legacy" }, () => {
       normalizeZaiEnv();
       expect(process.env.ZAI_API_KEY).toBe("zai-current");
     });
   });
 
   it("ignores blank legacy Z_AI_API_KEY values", () => {
-    withZaiEnv({ zaiApiKey: "", legacyZaiApiKey: "   " }, () => {
+    withEnv({ ZAI_API_KEY: "", Z_AI_API_KEY: "   " }, () => {
+      normalizeZaiEnv();
+      expect(process.env.ZAI_API_KEY).toBe("");
+    });
+  });
+
+  it("does not copy when legacy Z_AI_API_KEY is unset", () => {
+    withEnv({ ZAI_API_KEY: "", Z_AI_API_KEY: undefined }, () => {
       normalizeZaiEnv();
       expect(process.env.ZAI_API_KEY).toBe("");
     });