diff --git a/packages/adapters/codex-local/src/server/auth-precedence.test.ts b/packages/adapters/codex-local/src/server/auth-precedence.test.ts
new file mode 100644
index 00000000000..375068ced74
--- /dev/null
+++ b/packages/adapters/codex-local/src/server/auth-precedence.test.ts
@@ -0,0 +1,124 @@
+import { describe, expect, it } from "vitest";
+import {
+  CODEX_SANDBOX_AUTH_EXISTS_COMMAND,
+  CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING,
+  CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING_LOG_LINE,
+  resolveCodexAuthPrecedence,
+} from "./auth-precedence.js";
+
+describe("resolveCodexAuthPrecedence", () => {
+  describe("precedence order", () => {
+    it("configured_api_key wins over all other sources", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: true,
+        hostAuthJson: true,
+        sandboxAuthJson: true,
+      });
+      expect(result.winner).toBe("configured_api_key");
+    });
+
+    it("host_auth_json wins when no configured api key", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: true,
+        sandboxAuthJson: true,
+      });
+      expect(result.winner).toBe("host_auth_json");
+    });
+
+    it("sandbox_auth_json wins when no host credentials", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: false,
+        sandboxAuthJson: true,
+      });
+      expect(result.winner).toBe("sandbox_auth_json");
+    });
+
+    it("none when no credentials present", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: false,
+        sandboxAuthJson: false,
+      });
+      expect(result.winner).toBe("none");
+    });
+  });
+
+  describe("sandbox login shadowing and warning", () => {
+    it("warns when configured_api_key shadows sandbox login", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: true,
+        hostAuthJson: false,
+        sandboxAuthJson: true,
+      });
+      expect(result.sandboxLoginShadowed).toBe(true);
+      expect(result.shouldWarn).toBe(true);
+    });
+
+    it("warns when host_auth_json shadows sandbox login", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: true,
+        sandboxAuthJson: true,
+      });
+      expect(result.sandboxLoginShadowed).toBe(true);
+      expect(result.shouldWarn).toBe(true);
+    });
+
+    it("does not warn when sandbox login wins (no host credentials)", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: false,
+        sandboxAuthJson: true,
+      });
+      expect(result.sandboxLoginShadowed).toBe(false);
+      expect(result.shouldWarn).toBe(false);
+    });
+
+    it("does not warn when no sandbox login exists (sandbox-only gate)", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: true,
+        hostAuthJson: true,
+        sandboxAuthJson: false,
+      });
+      expect(result.sandboxLoginShadowed).toBe(false);
+      expect(result.shouldWarn).toBe(false);
+    });
+
+    it("does not warn when no credentials exist at all (fail-open)", () => {
+      const result = resolveCodexAuthPrecedence({
+        configuredApiKey: false,
+        hostAuthJson: false,
+        sandboxAuthJson: false,
+      });
+      expect(result.sandboxLoginShadowed).toBe(false);
+      expect(result.shouldWarn).toBe(false);
+    });
+  });
+
+  describe("constants", () => {
+    it("warning message is stable and human-readable", () => {
+      expect(CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING).toBe(
+        "snapshot login present but host credentials take precedence",
+      );
+    });
+
+    it("log line wraps the warning message", () => {
+      expect(CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING_LOG_LINE).toContain(
+        CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING,
+      );
+      expect(CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING_LOG_LINE).toMatch(
+        /^\[paperclip\] Warning:/,
+      );
+    });
+
+    it("sandbox auth exists command is a static shell test", () => {
+      expect(CODEX_SANDBOX_AUTH_EXISTS_COMMAND).toBe(
+        'test -f "$HOME/.codex/auth.json"',
+      );
+      expect(CODEX_SANDBOX_AUTH_EXISTS_COMMAND).not.toContain("cat");
+      expect(CODEX_SANDBOX_AUTH_EXISTS_COMMAND).not.toContain("readFile");
+    });
+  });
+});