diff --git a/backend/crates/providers/openai/src/compat/messages.rs b/backend/crates/providers/openai/src/compat/messages.rs
index d8e2745e1..5d462616e 100644
--- a/backend/crates/providers/openai/src/compat/messages.rs
+++ b/backend/crates/providers/openai/src/compat/messages.rs
@@ -1,8 +1,19 @@
-//! 跨协议生成的 Messages fast 请求补充协议头，不改写原生 Messages 请求
+//! Messages 跨协议转换的目标档位与 fast 协议头适配
 
 use bytes::Bytes;
 use gateway_core::engine::middleware::MiddlewareHeader;
 use reqwest::header::HeaderMap;
+use serde_json::{Map, Value};
+
+pub(crate) fn normalize_translated_codex_service_tier(body: &mut Map<String, Value>) {
+    // 转换生成的默认档位沿用 Codex 客户端的省略表达，原生 Responses 保留调用方声明
+    if matches!(
+        body.get("service_tier").and_then(Value::as_str),
+        Some("auto" | "default")
+    ) {
+        body.shift_remove("service_tier");
+    }
+}
 
 pub(crate) fn add_translated_fast_beta(headers: &mut Vec<MiddlewareHeader>, forwarded: &HeaderMap) {
     const BETA: &[u8] = b"fast-mode-2026-02-01";
diff --git a/backend/crates/providers/openai/src/compat/mod.rs b/backend/crates/providers/openai/src/compat/mod.rs
index ba32061d1..820112513 100644
--- a/backend/crates/providers/openai/src/compat/mod.rs
+++ b/backend/crates/providers/openai/src/compat/mod.rs
@@ -11,7 +11,7 @@ use gateway_core::engine::middleware::MiddlewareHeader;
 use serde_json::{Map, Value};
 
 pub(crate) use headers::is_non_codex_request_header;
-pub(crate) use messages::add_translated_fast_beta;
+pub(crate) use messages::{add_translated_fast_beta, normalize_translated_codex_service_tier};
 pub(crate) use request::normalize_codex_request_body;
 pub(crate) use search_tools::declares_hosted_search;
 
diff --git a/backend/crates/providers/openai/src/compat/request.rs b/backend/crates/providers/openai/src/compat/request.rs
index 0a71105a7..48f87f45f 100644
--- a/backend/crates/providers/openai/src/compat/request.rs
+++ b/backend/crates/providers/openai/src/compat/request.rs
@@ -35,13 +35,6 @@ pub(super) fn normalize_non_codex_request_body(body: &mut Map<String, Value>) {
 }
 
 fn normalize_codex_oauth_options(body: &mut Map<String, Value>) {
-    // Codex Core 省略默认档位，OAuth 不接受公开 API 的 auto/default 字面值
-    if matches!(
-        body.get("service_tier").and_then(Value::as_str),
-        Some("auto" | "default")
-    ) {
-        body.shift_remove("service_tier");
-    }
     // Codex OAuth 拒绝这些客户端选项，即使采样参数取默认值
     for field in [
         "top_p",
diff --git a/backend/crates/providers/openai/src/provider/messages/mod.rs b/backend/crates/providers/openai/src/provider/messages/mod.rs
index aa1c7def7..af13984a4 100644
--- a/backend/crates/providers/openai/src/provider/messages/mod.rs
+++ b/backend/crates/providers/openai/src/provider/messages/mod.rs
@@ -87,8 +87,14 @@ impl CodexProvider {
             let mut source_context = generate.protocol_payload().context().clone();
             retain_target_headers(source, &mut source_context, &mut headers);
             let translator = translator::ResponsesResponseTranslator::new(original);
-            let translated =
+            let mut translated =
                 messages_to_responses_request(original).map_err(|_| invalid_request())?;
+            if matches!(
+                selected.lease.authentication(),
+                CodexRuntimeAuthentication::OAuth(_)
+            ) {
+                crate::compat::normalize_translated_codex_service_tier(&mut translated);
+            }
             let body = serde_json::to_vec(&translated).map_err(|_| invalid_request())?;
             let operation = operation
                 .replace_protocol_wire(None, source_context)
diff --git a/backend/crates/providers/openai/tests/provider/contract/messages.rs b/backend/crates/providers/openai/tests/provider/contract/messages.rs
index 301b30a09..0e196db44 100644
--- a/backend/crates/providers/openai/tests/provider/contract/messages.rs
+++ b/backend/crates/providers/openai/tests/provider/contract/messages.rs
@@ -302,6 +302,71 @@ async fn messages_and_responses_translate_with_target_header_boundaries() {
     }
 }
 
+#[tokio::test]
+async fn translated_messages_service_tiers_follow_the_selected_responses_upstream() {
+    for api_key in [false, true] {
+        for (options, oauth_tier, api_tier) in [
+            (json!({"service_tier":"auto"}), None, Some("auto")),
+            (
+                json!({"service_tier":"standard_only"}),
+                None,
+                Some("default"),
+            ),
+            (json!({"speed":"fast"}), Some("priority"), Some("priority")),
+        ] {
+            let server = MockServer::start().await;
+            let store = Arc::new(MemoryAccountStore::default());
+            if api_key {
+                seed(&store, &server, ApiKeyProtocol::Responses).await;
+            } else {
+                create_account(&store, "acct_provider_contract").await;
+            }
+            Mock::given(method("POST"))
+                .and(path(if api_key {
+                    "/responses"
+                } else {
+                    "/codex/responses"
+                }))
+                .respond_with(
+                    ResponseTemplate::new(200)
+                        .set_body_raw(CAPTURE_COMPLETED_SSE, "text/event-stream"),
+                )
+                .expect(1)
+                .mount(&server)
+                .await;
+            let mut body = json!({"model":"gpt-5.4","max_tokens":128,"stream":true,"messages":[{"role":"user","content":"hello"}]});
+            body.as_object_mut()
+                .unwrap()
+                .extend(options.as_object().unwrap().clone());
+            let operation = Operation::Generate(GenerateRequest::from_protocol_payload(
+                ProtocolPayload::json_object("anthropic", body.as_object().unwrap().clone())
+                    .unwrap()
+                    .with_context(Map::from_iter([("use_websocket".to_owned(), json!(false))])),
+            ));
+            let mut stream = provider_with_base_url(&store, server.uri())
+                .execute(
+                    planned_request("openai", operation),
+                    context("req_translated_tier", CancellationToken::new()),
+                )
+                .await
+                .unwrap();
+            while let Some(event) = stream.next().await {
+                event.unwrap();
+            }
+            let requests = server.received_requests().await.unwrap();
+            let sent = captured_request_body(&requests[0]);
+            let expected = if api_key { api_tier } else { oauth_tier };
+            assert_eq!(sent.get("service_tier").and_then(Value::as_str), expected);
+            let hint = expected.map_or_else(
+                || "model=gpt-5.4".to_owned(),
+                |tier| format!("model=gpt-5.4;tier={tier}"),
+            );
+            assert_eq!(requests[0].headers["x-codex-routing-hint"], hint);
+            server.verify().await;
+        }
+    }
+}
+
 #[tokio::test]
 async fn custom_messages_and_responses_count_locally_without_billable_usage() {
     for (target, source) in [
diff --git a/backend/crates/providers/openai/tests/provider/contract/mod.rs b/backend/crates/providers/openai/tests/provider/contract/mod.rs
index cd34950b0..f26385a57 100644
--- a/backend/crates/providers/openai/tests/provider/contract/mod.rs
+++ b/backend/crates/providers/openai/tests/provider/contract/mod.rs
@@ -8326,7 +8326,7 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 false,
                 Some(json!("default")),
                 Some("priority"),
-                None,
+                Some("default"),
                 Some(3_437_500),
             ),
             (
@@ -8342,7 +8342,7 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 false,
                 Some(json!("priority")),
                 Some("priority"),
-                None,
+                Some("default"),
                 Some(3_437_500),
             ),
             (
@@ -8350,7 +8350,7 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 false,
                 Some(json!("fast")),
                 None,
-                None,
+                Some("default"),
                 Some(3_437_500),
             ),
             (
@@ -8358,7 +8358,7 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 false,
                 Some(json!("default")),
                 None,
-                None,
+                Some("default"),
                 Some(3_437_500),
             ),
             (FastMode::Disabled, false, None, None, None, Some(3_437_500)),
@@ -8431,8 +8431,8 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 true,
                 Some(json!("auto")),
                 None,
+                Some("auto"),
                 None,
-                Some(3_437_500),
             ),
             (FastMode::Enabled, false, None, None, None, Some(3_437_500)),
             (
@@ -8440,7 +8440,7 @@ async fn responses_should_observe_and_bill_the_outbound_service_tier_on_both_tra
                 false,
                 Some(json!("default")),
                 None,
-                None,
+                Some("default"),
                 Some(3_437_500),
             ),
         ] {
@@ -12622,18 +12622,27 @@ async fn fast_policy_changes_preserve_the_websocket_continuation_and_meter_each_
         let mut websocket =
             crate::transport::accept_codex_test_websocket_with(socket, |request, response| {
                 // 提示头只在首次握手发送；后续档位由各自 response.create 正文指定
-                assert_eq!(request.headers()["x-codex-routing-hint"], "model=gpt-5.4");
+                assert_eq!(
+                    request.headers()["x-codex-routing-hint"],
+                    "model=gpt-5.4;tier=default"
+                );
                 response.headers_mut().insert(
                     "sec-websocket-extensions",
                     "permessage-deflate".parse().unwrap(),
                 );
             })
             .await;
-        for (index, tier) in [None, Some("priority"), None, None].into_iter().enumerate() {
+        for (index, tier) in ["default", "priority", "default", "default"]
+            .into_iter()
+            .enumerate()
+        {
             let message = websocket.next().await.unwrap().unwrap();
             let frame: Value = serde_json::from_str(message.to_text().unwrap()).unwrap();
             assert_eq!(frame["type"], "response.create");
-            assert_eq!(frame.get("service_tier").and_then(Value::as_str), tier);
+            assert_eq!(
+                frame.get("service_tier").and_then(Value::as_str),
+                Some(tier)
+            );
             assert_eq!(
                 frame.get("previous_response_id").cloned(),
                 index
@@ -12790,7 +12799,11 @@ async fn fast_policy_changes_preserve_the_websocket_continuation_and_meter_each_
         );
         assert_eq!(
             observed_tier.as_deref(),
-            (fast_mode == FastMode::Enabled).then_some("priority")
+            Some(if fast_mode == FastMode::Enabled {
+                "priority"
+            } else {
+                "default"
+            })
         );
         assert_eq!(
             costs,
diff --git a/backend/crates/providers/openai/tests/provider/contract/passthrough.rs b/backend/crates/providers/openai/tests/provider/contract/passthrough.rs
index 03f0ac97b..74cf2655b 100644
--- a/backend/crates/providers/openai/tests/provider/contract/passthrough.rs
+++ b/backend/crates/providers/openai/tests/provider/contract/passthrough.rs
@@ -302,13 +302,28 @@ async fn websocket_messages_preserve_text_and_metadata_despite_unrecognized_json
 }
 
 #[tokio::test]
-async fn default_service_tiers_are_omitted_only_for_codex_oauth() {
-    for tier in ["auto", "default", "priority", "flex"] {
+async fn responses_service_tiers_reach_each_upstream_on_both_transports() {
+    for tier in [
+        None,
+        Some(Value::Null),
+        Some(json!("auto")),
+        Some(json!("default")),
+        Some(json!("priority")),
+        Some(json!("flex")),
+    ] {
         for api_key in [false, true] {
-            let body = json!({"model":"gpt-5.4","input":"hello","service_tier":tier});
-            let sent = capture_downstream_request(body, api_key, false, false).await;
-            let expected = (api_key || !matches!(tier, "auto" | "default")).then_some(tier);
-            assert_eq!(sent.get("service_tier").and_then(Value::as_str), expected);
+            for websocket in [false, true] {
+                let mut body = json!({"model":"gpt-5.4","input":"hello"});
+                if let Some(tier) = &tier {
+                    body["service_tier"] = tier.clone();
+                }
+                let sent = capture_downstream_request(body, api_key, websocket, false).await;
+                assert_eq!(
+                    sent.get("service_tier"),
+                    tier.as_ref(),
+                    "api_key={api_key}, websocket={websocket}"
+                );
+            }
         }
     }
 }
diff --git a/docs/api.md b/docs/api.md
index 77d240a3c..5c64c51b3 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -395,7 +395,7 @@ adaptive 路径将普通强制选择降为 `auto`，保留串行约束。
 服务档位按目标协议映射：Responses `default` 对应 Messages `standard_only`，`auto` 保留，无法对应的档位省略。
 按 CPA 将 Responses `priority` 与 Messages `speed:fast` 双向映射；这两个字段不代表相同的服务承诺。
 跨协议生成 fast 请求时补充 `fast-mode-2026-02-01` beta，并保留已有 beta；目标是否支持 fast 由上游校验。
-Codex OAuth 出站省略 `auto/default`，Responses API Key 保留这些公开 API 参数。
+Messages 转向 Codex OAuth 时省略转换生成的 `auto/default`，转向 Responses API Key 时保留。
 会话内配置、工具增删和指令生效期限等扩展需要完整保留时使用同协议上游
 
 Messages 错误使用 `{"type":"error","error":{"type":"...","message":"..."}}`；SSE 开始后使用 `event: error`。
@@ -1946,7 +1946,7 @@ GET 和 POST 均返回 `enabled`、`leadMinutes`、`weeklyTriggerRemainingPercen
 - 关闭：将 `priority`（含 `fast` 别名）改为显式 `default`，继续处理请求
 
 策略只作用于 OpenAI Responses 的顶层 `service_tier`，不改变 `flex`、`ultrafast` 等其他显式档位、嵌套字段或其他 Provider。
-选定 Codex OAuth 后，出站兼容层按其合同省略 `auto/default`，默认档位仍按标准价格计费；Responses API Key 保留这两个值。
+OAuth 和 Responses API Key 共用上述档位规则，出站兼容层不额外删除原生 Responses 的 `auto/default`。
 宿主策略在 request 中间件之后应用，形成 attempt 输入基线；attempt 中间件继续改写档位时，以改写后的实际发送值为准。
 HTTP 和每个 WebSocket `response.create` 均使用请求开始时的分组策略，同一请求重试保持该策略；
 HTTP 请求头及新建 WS 的握手提示按当时的最终出站档位构造；复用 WS 时不重发握手头，