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..80a4705f5b8
--- /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 configured or 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");
+    });
+  });
+});
\ No newline at end of file
diff --git a/packages/adapters/codex-local/src/server/auth-precedence.ts b/packages/adapters/codex-local/src/server/auth-precedence.ts
new file mode 100644
index 00000000000..734a25617e9
--- /dev/null
+++ b/packages/adapters/codex-local/src/server/auth-precedence.ts
@@ -0,0 +1,46 @@
+export const CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING =
+  "snapshot login present but configured or host credentials take precedence";
+export const CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING_LOG_LINE =
+  `[paperclip] Warning: ${CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING}.\n`;
+export const CODEX_SANDBOX_AUTH_EXISTS_COMMAND =
+  'test -f "$HOME/.codex/auth.json"';
+
+export type CodexAuthPrecedenceWinner =
+  | "configured_api_key"
+  | "host_auth_json"
+  | "sandbox_auth_json"
+  | "none";
+
+export interface CodexAuthPrecedenceInput {
+  configuredApiKey: boolean;
+  hostAuthJson: boolean;
+  sandboxAuthJson: boolean;
+}
+
+export interface CodexAuthPrecedenceResolution {
+  winner: CodexAuthPrecedenceWinner;
+  sandboxLoginShadowed: boolean;
+  shouldWarn: boolean;
+}
+
+export function resolveCodexAuthPrecedence(
+  input: CodexAuthPrecedenceInput,
+): CodexAuthPrecedenceResolution {
+  const winner: CodexAuthPrecedenceWinner =
+    input.configuredApiKey
+      ? "configured_api_key"
+      : input.hostAuthJson
+        ? "host_auth_json"
+        : input.sandboxAuthJson
+          ? "sandbox_auth_json"
+          : "none";
+  const sandboxLoginShadowed =
+    input.sandboxAuthJson &&
+    (winner === "configured_api_key" || winner === "host_auth_json");
+
+  return {
+    winner,
+    sandboxLoginShadowed,
+    shouldWarn: sandboxLoginShadowed,
+  };
+}
\ No newline at end of file
diff --git a/packages/adapters/codex-local/src/server/index.ts b/packages/adapters/codex-local/src/server/index.ts
index 02643e2c477..f1c39ef4ada 100644
--- a/packages/adapters/codex-local/src/server/index.ts
+++ b/packages/adapters/codex-local/src/server/index.ts
@@ -1,4 +1,13 @@
 export { execute, ensureCodexSkillsInjected } from "./execute.js";
+export {
+  resolveCodexAuthPrecedence,
+  CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING,
+  CODEX_SANDBOX_AUTH_PRECEDENCE_WARNING_LOG_LINE,
+  CODEX_SANDBOX_AUTH_EXISTS_COMMAND,
+  type CodexAuthPrecedenceInput,
+  type CodexAuthPrecedenceResolution,
+  type CodexAuthPrecedenceWinner,
+} from "./auth-precedence.js";
 export * from "./acp.js";
 export { getConfigSchema } from "./config-schema.js";
 export {
@@ -82,4 +91,4 @@ export const sessionCodec: AdapterSessionCodec = {
       null
     );
   },
-};
+};
\ No newline at end of file