diff --git a/src/auth/codex/instructions.test.ts b/src/auth/codex/instructions.test.ts
index 63a798b79..95fcdc0d8 100644
--- a/src/auth/codex/instructions.test.ts
+++ b/src/auth/codex/instructions.test.ts
@@ -1,119 +1,133 @@
-import { afterEach, beforeEach, describe, expect, mock, test } from "bun:test";
+import { afterAll, afterEach, beforeEach, describe, expect, mock, test } from "bun:test"
 
 // Reused by both the TUI and exec boot paths (src/tui/runner.ts,
 // src/exec/runner.ts) to refresh the pinned Codex instructions before first
 // Codex inference. This exercises the shared refresh/fallback logic directly,
 // with disk I/O faked so tests never touch the real ~/.corbits cache.
+//
+// Shallow-copy + afterAll restore is required: Bun mutates the live module
+// namespace when mock.module runs, and CI loads ./src before ./tests — a leaked
+// node:fs mock turns later suites into ENOENT / missing-settings failures.
 
-let fakeDisk = new Map<string, string>();
+const realFs = { ...(await import("node:fs")) }
+
+let fakeDisk = new Map<string, string>()
 
 mock.module("node:fs", () => ({
+  ...realFs,
   readFileSync: (path: string) => {
-    const contents = fakeDisk.get(path);
+    const contents = fakeDisk.get(path)
     if (contents === undefined) {
-      const err = new Error("ENOENT") as NodeJS.ErrnoException;
-      err.code = "ENOENT";
-      throw err;
+      const err = new Error("ENOENT") as NodeJS.ErrnoException
+      err.code = "ENOENT"
+      throw err
     }
-    return contents;
+    return contents
   },
   writeFileSync: (path: string, contents: string) => {
-    fakeDisk.set(path, contents);
+    fakeDisk.set(path, contents)
   },
   mkdirSync: () => undefined,
-}));
+}))
+
+afterAll(() => {
+  mock.module("node:fs", () => realFs)
+})
 
 const { refreshCodexInstructions, codexInstructions, codexInstructionsHash } = await import(
-  "./instructions.js"
-);
-const { GPT_5_CODEX_PROMPT } = await import("./prompts/gpt-5-codex.js");
+  "./instructions.js",
+)
+const { GPT_5_CODEX_PROMPT } = await import("./prompts/gpt-5-codex.js")
 
-const VALID_PROMPT = `You are Codex${"x".repeat(1200)}`;
+const VALID_PROMPT = `You are Codex${"x".repeat(1200)}`
 
 function mockFetchSequence(tag: string, promptResponse: () => Response): typeof fetch {
   return (async (input: RequestInfo | URL) => {
-    const url = String(input);
+    const url = String(input)
     if (url.includes("releases/latest")) {
-      return new Response(JSON.stringify({ tag_name: tag }), { status: 200 });
+      return new Response(JSON.stringify({ tag_name: tag }), { status: 200 })
     }
-    return promptResponse();
-  }) as typeof fetch;
+    return promptResponse()
+  }) as typeof fetch
 }
 
 describe("refreshCodexInstructions", () => {
-  const originalFetch = global.fetch;
+  const originalFetch = global.fetch
 
   beforeEach(() => {
-    fakeDisk = new Map();
-  });
+    fakeDisk = new Map()
+  })
 
   afterEach(() => {
-    global.fetch = originalFetch;
-  });
+    global.fetch = originalFetch
+  })
 
   test("bundled copy is used before any refresh", () => {
-    expect(codexInstructions()).toBe(GPT_5_CODEX_PROMPT);
-  });
+    expect(codexInstructions()).toBe(GPT_5_CODEX_PROMPT)
+  })
 
   test("updates in-memory instructions on a successful fetch", async () => {
-    global.fetch = mockFetchSequence("rust-v1.2.3", () => new Response(VALID_PROMPT, { status: 200 }));
+    global.fetch = mockFetchSequence("rust-v1.2.3", () => new Response(VALID_PROMPT, { status: 200 }))
 
-    await refreshCodexInstructions();
-    expect(codexInstructions()).toBe(VALID_PROMPT);
-  });
+    await refreshCodexInstructions()
+    expect(codexInstructions()).toBe(VALID_PROMPT)
+  })
 
   test("falls back without throwing the run when the release lookup network call fails", async () => {
-    const before = codexInstructions();
-    global.fetch = (() => Promise.reject(new Error("network down"))) as unknown as typeof fetch;
+    const before = codexInstructions()
+    global.fetch = (() => Promise.reject(new Error("network down"))) as unknown as typeof fetch
 
     // The function itself rejects; callers (TUI/exec boot) catch this and
     // keep running on cache/bundled instructions — see src/exec/runner.ts and
     // src/tui/runner.ts refresh call sites.
-    await expect(refreshCodexInstructions()).rejects.toThrow("network down");
-    expect(codexInstructions()).toBe(before);
-  });
+    await expect(refreshCodexInstructions()).rejects.toThrow("network down")
+    expect(codexInstructions()).toBe(before)
+  })
 
   test("falls back without throwing when the prompt fetch returns a non-200", async () => {
-    const before = codexInstructions();
-    global.fetch = mockFetchSequence("rust-v1.2.3", () => new Response("not found", { status: 404 }));
+    const before = codexInstructions()
+    global.fetch = mockFetchSequence("rust-v1.2.3", () => new Response("not found", { status: 404 }))
 
-    await expect(refreshCodexInstructions()).rejects.toThrow(/HTTP 404/);
-    expect(codexInstructions()).toBe(before);
-  });
+    await expect(refreshCodexInstructions()).rejects.toThrow(/HTTP 404/)
+    expect(codexInstructions()).toBe(before)
+  })
 
   test("rejects a 200 response whose body is not a valid Codex prompt (CDN error page)", async () => {
-    const before = codexInstructions();
-    global.fetch = mockFetchSequence("rust-v1.2.3", () => new Response("<html>oops</html>", { status: 200 }));
+    const before = codexInstructions()
+    global.fetch = mockFetchSequence(
+      "rust-v1.2.3",
+      () => new Response("<html>oops</html>", { status: 200 }),
+    )
 
-    await expect(refreshCodexInstructions()).rejects.toThrow(/unexpected body/);
-    expect(codexInstructions()).toBe(before);
-  });
+    await expect(refreshCodexInstructions()).rejects.toThrow(/unexpected body/)
+    expect(codexInstructions()).toBe(before)
+  })
 
   test("rejects within the timeout when a fetch never resolves (hung connection)", async () => {
-    const before = codexInstructions();
+    const before = codexInstructions()
     global.fetch = ((_input: RequestInfo | URL, init?: RequestInit) => {
       return new Promise((_resolve, reject) => {
-        const signal = init?.signal;
+        const signal = init?.signal
         if (signal) {
-          signal.addEventListener("abort", () => reject(signal.reason as Error));
+          signal.addEventListener("abort", () => reject(signal.reason as Error))
         }
-      });
-    }) as unknown as typeof fetch;
+      })
+    }) as unknown as typeof fetch
 
-    const started = Date.now();
-    await expect(refreshCodexInstructions()).rejects.toBeTruthy();
-    expect(Date.now() - started).toBeLessThan(15_000);
-    expect(codexInstructions()).toBe(before);
-  }, 20_000);
+    const started = Date.now()
+    await expect(refreshCodexInstructions()).rejects.toBeTruthy()
+    expect(Date.now() - started).toBeLessThan(15_000)
+    expect(codexInstructions()).toBe(before)
+  }, 20_000)
 
   test("codexInstructionsHash reflects the currently resolved instructions text", async () => {
-    const hashBefore = codexInstructionsHash();
-    expect(hashBefore).toMatch(/^[0-9a-f]{12}$/);
+    const hashBefore = codexInstructionsHash()
+    expect(hashBefore).toMatch(/^[0-9a-f]{12}$/)
 
-    const otherPrompt = `You are Codex${"y".repeat(1200)}`;
-    global.fetch = mockFetchSequence("rust-v1.2.4", () => new Response(otherPrompt, { status: 200 }));
-    await refreshCodexInstructions();
+    const otherPrompt = `You are Codex${"y".repeat(1200)}`
+    global.fetch = mockFetchSequence("rust-v1.2.4", () => new Response(otherPrompt, { status: 200 }))
+    await refreshCodexInstructions()
 
-    expect(codexInstructionsHash()).not.toBe(hashBefore);
-  });
-});
+    expect(codexInstructionsHash()).not.toBe(hashBefore)
+  })
+})