diff --git a/src/providers/proxies/codex-proxy.ts b/src/providers/proxies/codex-proxy.ts
index 6a854a7..2951bc6 100644
--- a/src/providers/proxies/codex-proxy.ts
+++ b/src/providers/proxies/codex-proxy.ts
@@ -89,6 +89,15 @@ export const mergeHeaderValue = (
   headers.set(name, values.join(","));
 };
 
+const hasInstructionContext = (body: JsonObject): boolean =>
+  Boolean(trimString(body.instructions)) ||
+  (Array.isArray(body.input) &&
+    body.input.some(
+      (item) =>
+        isObjectRecord(item) &&
+        (item.role === "system" || item.role === "developer")
+    ));
+
 export const transformCodexBodyJson = (
   bodyJson: unknown,
   promptCacheKey?: string | null
@@ -97,7 +106,7 @@ export const transformCodexBodyJson = (
     return null;
   }
 
-  // ChatGPT rejects output limits and requires instructions and stateless storage.
+  // ChatGPT rejects output limits and requires stateless storage.
   // https://github.com/anomalyco/opencode/blob/d9987ef9c896383eb47b9103f6c93fa772a44b83/packages/core/src/plugin/provider/openai.ts#L311-L317
   const {
     max_completion_tokens: _maxCompletionTokens,
@@ -105,22 +114,12 @@ export const transformCodexBodyJson = (
     ...body
   } = bodyJson;
 
-  const incomingInstructions = trimString(body.instructions);
-  const input = Array.isArray(body.input) ? body.input : [];
-  const firstInput = input[0];
-  const promotedInstructions =
-    !incomingInstructions &&
-    isObjectRecord(firstInput) &&
-    (firstInput.role === "system" || firstInput.role === "developer")
-      ? trimString(firstInput.content)
-      : "";
-  const instructions =
-    incomingInstructions || promotedInstructions || CODEX_DEFAULT_INSTRUCTIONS;
-
   return {
     ...body,
-    ...(promotedInstructions ? { input: input.slice(1) } : {}),
-    instructions,
+    // Requests without any instruction context get the bundled defaults.
+    ...(hasInstructionContext(body)
+      ? {}
+      : { instructions: CODEX_DEFAULT_INSTRUCTIONS }),
     store: false,
     ...(promptCacheKey ? { prompt_cache_key: promptCacheKey } : {}),
   };
diff --git a/src/usage/prompt-cache.ts b/src/usage/prompt-cache.ts
index 3fd409e..22b1a34 100644
--- a/src/usage/prompt-cache.ts
+++ b/src/usage/prompt-cache.ts
@@ -130,11 +130,10 @@ const firstItem = (
   include: (item: unknown) => boolean = () => true
 ): unknown => (Array.isArray(items) ? items.find(include) : items);
 
-const isCodexTask = (item: unknown): boolean =>
-  !(
-    isObjectRecord(item) &&
-    (item.role === "system" || item.role === "developer")
-  );
+const isCodexInstruction = (item: unknown): boolean =>
+  isObjectRecord(item) && (item.role === "system" || item.role === "developer");
+
+const isCodexTask = (item: unknown): boolean => !isCodexInstruction(item);
 
 // A conversation is one session's turns with the same model, system prompt,
 // tools and first task item, so subagents and side requests in that session
@@ -166,7 +165,16 @@ export const readPromptCacheRequest = (
   }
 
   const sessionHash = hashJson([input.apiKeyId, provider, session]);
-  const system = hashJson(claude ? body.system : body.instructions);
+  const system = hashJson(
+    claude
+      ? body.system
+      : [
+          body.instructions,
+          Array.isArray(body.input)
+            ? body.input.filter(isCodexInstruction)
+            : null,
+        ]
+  );
   const tools = hashJson(body.tools);
   const requestClass = input.headers.get(CLAUDE_CODE_REQUEST_CLASS_HEADER);
   return {
diff --git a/tests/http/proxy-usage.scenarios.ts b/tests/http/proxy-usage.scenarios.ts
index b72d4ce..ca9b6f0 100644
--- a/tests/http/proxy-usage.scenarios.ts
+++ b/tests/http/proxy-usage.scenarios.ts
@@ -9,6 +9,7 @@ import {
 import { Hono } from "hono";
 import { migrate } from "drizzle-orm/libsql/migrator";
 import { installErrorHandler } from "../../src/http/error-handler";
+import CODEX_DEFAULT_INSTRUCTIONS from "../../src/providers/codex-default-instructions.txt";
 import {
   CLAUDE_EXTENDED_CACHE_TTL_BETA_HEADER,
   CLAUDE_SYSTEM_IDENTITY,
@@ -520,6 +521,40 @@ test("Codex keeps client request, thread, user-agent and beta headers", async ()
   }
 });
 
+test("Codex instruction context passes through unchanged and only missing context gets the defaults", async () => {
+  const original = globalThis.fetch;
+  let forwarded: unknown = null;
+  globalThis.fetch = (async (_input, init) => {
+    forwarded = JSON.parse(String(init?.body));
+    return Response.json({ output: [] });
+  }) as typeof fetch;
+  const user = { role: "user", content: "hi" };
+  try {
+    for (const [body, instructions] of [
+      [{ input: [{ role: "developer", content: " Base " }, user] }, undefined],
+      [{ input: [user, { role: "system", content: "sys" }] }, undefined],
+      [{ instructions: "  Keep it short  ", input: [user] }, undefined],
+      [{ input: [user] }, CODEX_DEFAULT_INSTRUCTIONS],
+    ] as const) {
+      const response = await app.request("/openai/v1/responses", {
+        method: "POST",
+        body: JSON.stringify({ model: "gpt-fake", ...body }),
+      });
+      expect(response.status).toBe(200);
+      await response.text();
+      expect(forwarded).toEqual({
+        model: "gpt-fake",
+        ...body,
+        ...(instructions ? { instructions } : {}),
+        store: false,
+      });
+    }
+    await flushProxyUsage();
+  } finally {
+    globalThis.fetch = original;
+  }
+});
+
 test("route records one successful SSE request including latest tokens", async () => {
   const original = globalThis.fetch;
   globalThis.fetch = (async () =>
diff --git a/tests/providers/proxy-contract.test.ts b/tests/providers/proxy-contract.test.ts
index 16f4076..14fa463 100644
--- a/tests/providers/proxy-contract.test.ts
+++ b/tests/providers/proxy-contract.test.ts
@@ -346,39 +346,6 @@ describe("proxy contract: codex", () => {
     expect(headers.get(CODEX_ACCOUNT_ID_HEADER)).toBeNull();
   });
 
-  test("promotes OpenCode developer input instead of adding fallback instructions", () => {
-    const headers = new Headers();
-    const bodyJson = {
-      model: "gpt-5.6-sol",
-      input: [
-        {
-          role: "developer",
-          content: "Current OpenCode GPT instructions",
-        },
-        {
-          role: "user",
-          content: [{ type: "input_text", text: "hello" }],
-        },
-      ],
-    };
-
-    const result = prepareCodexProxyRequest({
-      headers,
-      accessToken: "codex-access",
-      accountId: "acct-fallback",
-      metadata: null,
-      bodyText: JSON.stringify(bodyJson),
-      bodyJson,
-    });
-
-    expect(JSON.parse(result.bodyText)).toEqual({
-      model: "gpt-5.6-sol",
-      input: [bodyJson.input[1]],
-      instructions: "Current OpenCode GPT instructions",
-      store: false,
-    });
-  });
-
   test("removes output and completion limits the ChatGPT backend rejects", () => {
     const bodyJson = {
       model: "gpt-5-codex",
diff --git a/tests/usage/prompt-cache.test.ts b/tests/usage/prompt-cache.test.ts
index 29d993b..aa65872 100644
--- a/tests/usage/prompt-cache.test.ts
+++ b/tests/usage/prompt-cache.test.ts
@@ -348,7 +348,7 @@ test("a turn that reads far less than the previous turn cached is reported only
       null,
     ],
     [
-      "a sibling Codex agent whose instructions are promoted",
+      "a sibling Codex agent with different developer instructions",
       [
         codex(codexUsage(20_000), promoted("Agent A")),
         codex(codexUsage(0), promoted("Agent B"), { headers: codexSubagent }),