diff --git a/open-sse/config/codexInstructions.ts b/open-sse/config/codexInstructions.ts
index 7a5b4a5aa2..9a736211e1 100644
--- a/open-sse/config/codexInstructions.ts
+++ b/open-sse/config/codexInstructions.ts
@@ -1,6 +1,8 @@
 // Default instructions for Codex models
 // Source: CLIProxyAPI internal/misc/codex_instructions/
 
+export const CODEX_CHAT_DEFAULT_INSTRUCTIONS = "You are a ChatGPT agent.";
+
 export const CODEX_DEFAULT_INSTRUCTIONS = `You are Codex, based on GPT-5. You are running as a coding agent in the Codex CLI on a user's computer.
 
 ## General
diff --git a/open-sse/executors/codex.ts b/open-sse/executors/codex.ts
index 7930888432..720930e4d1 100644
--- a/open-sse/executors/codex.ts
+++ b/open-sse/executors/codex.ts
@@ -5,7 +5,10 @@ import {
   setUserAgentHeader,
   type ExecuteInput,
 } from "./base.ts";
-import { CODEX_DEFAULT_INSTRUCTIONS } from "../config/codexInstructions.ts";
+import {
+  CODEX_CHAT_DEFAULT_INSTRUCTIONS,
+  CODEX_DEFAULT_INSTRUCTIONS,
+} from "../config/codexInstructions.ts";
 import { PROVIDERS } from "../config/constants.ts";
 import { getCodexClientVersion, getCodexUserAgent } from "../config/codexClient.ts";
 import { getAccessToken } from "../services/tokenRefresh.ts";
@@ -1037,9 +1040,9 @@ export class CodexExecutor extends BaseExecutor {
         body.instructions = "Follow the developer instructions in the conversation.";
       }
     } else {
-      // Translated: use CODEX_DEFAULT_INSTRUCTIONS as fallback when no system
-      // prompt was provided by the client, BUT only if tools are requested.
-      // Injecting tool instructions on bare requests causes Harmony leaks (#1686).
+      // Translated: keep the full Codex tool instructions only for tool-capable
+      // requests. Bare chat requests still need a neutral instructions value
+      // because the Codex Responses backend rejects requests without it.
       const hasTools = Array.isArray(body.tools) && body.tools.length > 0;
       if (
         !body.instructions ||
@@ -1048,7 +1051,7 @@ export class CodexExecutor extends BaseExecutor {
         if (hasTools) {
           body.instructions = CODEX_DEFAULT_INSTRUCTIONS;
         } else {
-          delete body.instructions;
+          body.instructions = CODEX_CHAT_DEFAULT_INSTRUCTIONS;
         }
       }
     }
diff --git a/tests/unit/executor-codex.test.ts b/tests/unit/executor-codex.test.ts
index 68b5ed6997..05c67da79b 100644
--- a/tests/unit/executor-codex.test.ts
+++ b/tests/unit/executor-codex.test.ts
@@ -17,6 +17,7 @@ import {
   setThinkingBudgetConfig,
   ThinkingMode,
 } from "../../open-sse/services/thinkingBudget.ts";
+import { CODEX_CHAT_DEFAULT_INSTRUCTIONS } from "../../open-sse/config/codexInstructions.ts";
 
 test.afterEach(() => {
   setThinkingBudgetConfig(DEFAULT_THINKING_CONFIG);
@@ -200,6 +201,37 @@ test("CodexExecutor.transformRequest injects default instructions, clamps reason
   assert.equal(result.stream_options, undefined);
 });
 
+test("CodexExecutor.transformRequest sends neutral instructions for bare chat requests", () => {
+  const executor = new CodexExecutor();
+  const body = {
+    model: "gpt-5.5-medium",
+    input: [
+      {
+        type: "message",
+        role: "user",
+        content: [
+          {
+            type: "input_text",
+            text: "Calculate 79530+41475, and reply with the result only.",
+          },
+        ],
+      },
+    ],
+    instructions: "",
+    stream: false,
+  };
+
+  const result = executor.transformRequest("gpt-5.5-medium", body, false, {
+    requestEndpointPath: "/responses",
+  });
+
+  assert.equal(result.instructions, CODEX_CHAT_DEFAULT_INSTRUCTIONS);
+  assert.equal(result.stream, true);
+  assert.equal(result.model, "gpt-5.5");
+  assert.equal(result.input.length, 1);
+  assert.equal(result.tools, undefined);
+});
+
 test("CodexExecutor.transformRequest preserves compact requests and native passthrough semantics", () => {
   const executor = new CodexExecutor();
   const body = {