diff --git a/src/codex/auth-context.ts b/src/codex/auth-context.ts
index 32d124b7fd..353ab1a2e1 100644
--- a/src/codex/auth-context.ts
+++ b/src/codex/auth-context.ts
@@ -1160,6 +1160,7 @@ export function materializeCodexUpstreamAuth(
     if (!stored?.accessToken || !isMainAccountTokenLive()) {
       throw new CodexMainSubstitutionUnavailableError();
     }
+    selected.delete("chatgpt-account-id");
     selected.set("authorization", `Bearer ${stored.accessToken}`);
     if (stored.chatgptAccountId) selected.set("chatgpt-account-id", stored.chatgptAccountId);
     observeSelectedMainCredential(stored, writer);
@@ -1237,6 +1238,7 @@ export async function materializeCodexUpstreamAuthAsync(
     ...(options.nativeMainRefreshDependencies ?? {}),
   });
   if (!stored?.accessToken) throw new CodexMainSubstitutionUnavailableError();
+  selected.delete("chatgpt-account-id");
   selected.set("authorization", `Bearer ${stored.accessToken}`);
   if (stored.chatgptAccountId) selected.set("chatgpt-account-id", stored.chatgptAccountId);
   observeSelectedMainCredential(stored, writer);
diff --git a/structure/data-planes/inbound-compat.md b/structure/data-planes/inbound-compat.md
index e6e6ecd582..2cc844d6c3 100644
--- a/structure/data-planes/inbound-compat.md
+++ b/structure/data-planes/inbound-compat.md
@@ -8,7 +8,8 @@ is scoped to canonical ChatGPT Responses forwarding; other source-area behavior
 `src/server/audio-transcriptions.ts` owns `POST /v1/audio/transcriptions`, independently of
 Responses and Chat conversion. `src/server/audio-upstream.ts` resolves explicit data-plane keys
 on both listeners and substitutes stored OpenAI credentials. Direct stored-main access claims
-the enclosing admission lease; Pool uses the existing sidecar account resolver. A selected
+the enclosing admission lease and derives its account header only from that stored credential;
+caller-supplied account selection is never retained. Pool uses the existing sidecar account resolver. A selected
 ChatGPT authentication failure never falls through to the paid OpenAI provider.
 
 The bounded multipart input accepts one nonempty file up to 25,000,000 bytes within a 32 MiB
diff --git a/structure/providers/openai-tiers.md b/structure/providers/openai-tiers.md
index 527aacba6b..22fc41dc30 100644
--- a/structure/providers/openai-tiers.md
+++ b/structure/providers/openai-tiers.md
@@ -415,7 +415,8 @@ sidecar candidate and cannot hide a failed Codex credential with separately bill
 
 `src/server/audio-upstream.ts` uses the same selection for standalone transcription. Explicit
 native Direct auth remains caller-owned; proxy-key-only Direct claims stored main before
-materialization. `src/providers/openai-sidecar.ts` releases quota-probe ownership on every
+materialization, replacing both bearer and account identity exclusively from that credential.
+`src/providers/openai-sidecar.ts` releases quota-probe ownership on every
 materialization or usability failure before transferring a resolved context to its caller.
 Audio reports one terminal upstream outcome after validating the response body; redirects remain
 neutral and client/shutdown cancellation does not manufacture an account failure.
diff --git a/tests/codex-integration/codex-auth-context.test.ts b/tests/codex-integration/codex-auth-context.test.ts
index c23a1cda2c..9084e1e477 100644
--- a/tests/codex-integration/codex-auth-context.test.ts
+++ b/tests/codex-integration/codex-auth-context.test.ts
@@ -19,6 +19,7 @@ import {
   cooldownErrorResponse,
   headersForCodexAuthContext,
   materializeCodexUpstreamAuth,
+  materializeCodexUpstreamAuthAsync,
   CodexMainSubstitutionUnavailableError,
   isCodexAuthContextUsable,
   resolveCodexAuthContext,
@@ -1664,6 +1665,32 @@ describe("Codex auth context", () => {
     expect(headers.get("openai-beta")).toBe("responses=experimental");
   });
 
+  test.each([
+    ["absent", undefined, null],
+    ["present", "stored_main_acc", "stored_main_acc"],
+  ])("async stored Direct substitution owns account identity when %s", async (_label, accountId, expectedAccountId) => {
+    const storedCredential = liveJwt();
+    writeFileSync(join(testDir, "auth.json"), JSON.stringify({
+      tokens: { access_token: storedCredential, account_id: accountId },
+    }));
+    const inbound = new Headers({
+      authorization: "Bearer ocx_data_localsecret",
+      "chatgpt-account-id": "caller-account",
+      "openai-beta": "responses=experimental",
+    });
+
+    const headers = await materializeCodexUpstreamAuthAsync(
+      inbound,
+      { kind: "main", accountId: null },
+      { substituteMainCredential: true },
+    );
+
+    expect(headers.get("authorization")).toBe(`Bearer ${storedCredential}`);
+    expect(headers.get("chatgpt-account-id")).toBe(expectedAccountId);
+    expect(headers.get("openai-beta")).toBe("responses=experimental");
+    expect(inbound.get("chatgpt-account-id")).toBe("caller-account");
+  });
+
   test("substitution fails closed when no usable main credential exists (#1686)", () => {
     // Falling through here would forward the admission secret upstream, which is exactly
     // the leak the forward guard exists to prevent. Throw before any I/O instead.
diff --git a/tests/server/audio-transcriptions.test.ts b/tests/server/audio-transcriptions.test.ts
index eb6a9cfe75..7141cc1fe0 100644
--- a/tests/server/audio-transcriptions.test.ts
+++ b/tests/server/audio-transcriptions.test.ts
@@ -284,6 +284,27 @@ describe("standalone transcription API", () => {
     expect((await captured[0]!.formData()).get("model")).toBeNull();
   });
 
+  test("stored Direct credentials never inherit a caller account ID", async () => {
+    writeFileSync(join(codex.path, "auth.json"), JSON.stringify({ tokens: { access_token: "fixture-main-access" } }));
+    clearMainAccountInfoCache();
+    const cfg = config();
+    cfg.defaultProvider = "openai";
+    cfg.providers = { openai: { adapter: "openai-responses", baseUrl: "https://chatgpt.com/backend-api/codex", authMode: "forward", codexAccountMode: "direct" } };
+    saveConfig(cfg);
+
+    for (const headers of [
+      { authorization: "", "x-opencodex-api-key": KEY, "chatgpt-account-id": "caller-workspace" },
+      { authorization: "", "x-api-key": KEY, "chatgpt-account-id": "caller-workspace" },
+    ]) {
+      expect((await request(form(), headers)).status).toBe(200);
+    }
+    expect(captured).toHaveLength(2);
+    for (const upstream of captured) {
+      expect(upstream.headers.get("authorization")).toBe("Bearer fixture-main-access");
+      expect(upstream.headers.get("chatgpt-account-id")).toBeNull();
+    }
+  });
+
   test("a missing stored Direct credential fails without paid-provider fallback", async () => {
     const cfg = config();
     cfg.providers.openai = { adapter: "openai-responses", baseUrl: "https://chatgpt.com/backend-api/codex", authMode: "forward", codexAccountMode: "direct" };