diff --git a/backend/internal/service/openai_codex_identity.go b/backend/internal/service/openai_codex_identity.go
index 68d4105d84..47bc19ce0c 100644
--- a/backend/internal/service/openai_codex_identity.go
+++ b/backend/internal/service/openai_codex_identity.go
@@ -11,12 +11,32 @@ import (
 // 若请求携带 version 且低于该值，上游直接 404（issue #3901，2026-07 实测）。
 const codexUpstreamMinVersion = "0.144.0"
 
+// ensureCodexIdentityHeaders 补齐 OAuth（ChatGPT 内部接口）出站请求所需的 Codex 身份头。
+// 已有 User-Agent 与 version 保持不变，交给紧随其后的 enforceCodexIdentityHeaders
+// 做官方身份配对与最低版本校正。
+func ensureCodexIdentityHeaders(h http.Header) {
+	if h == nil {
+		return
+	}
+	if strings.TrimSpace(h.Get("user-agent")) == "" {
+		h.Set("user-agent", codexCLIUserAgent)
+	}
+	if strings.TrimSpace(h.Get("originator")) == "" {
+		h.Set("originator", "codex_cli_rs")
+	}
+	if strings.TrimSpace(h.Get("version")) == "" {
+		h.Set("version", codexCLIVersion)
+	}
+	h.Set("OpenAI-Beta", "responses=experimental")
+}
+
 // enforceCodexIdentityHeaders 收口 OAuth（ChatGPT 内部接口）出站请求的客户端身份头。
 // 上游要求 originator 与 User-Agent 首段配套且为官方客户端标识，version 头（若携带）
 // 不低于 0.144.0，任一不满足即 404（issue #3901）。以最终 User-Agent 为准推导配套
 // originator；推导不出官方身份（第三方 UA / UA 缺失）时整体回退为默认 Codex CLI 身份。
 //
-// 仅对携带 originator 的请求生效——compat messages bridge 故意不带 originator，保持原样。
+// 仅对携带 originator 的请求生效；需要从缺失身份头恢复的调用方应先调用
+// ensureCodexIdentityHeaders。
 // 必须在所有 User-Agent 改写（自定义 UA / ForceCodexCLI / 浏览器 UA 兜底）之后调用。
 func enforceCodexIdentityHeaders(h http.Header) {
 	if h == nil || h.Get("originator") == "" {
diff --git a/backend/internal/service/openai_codex_identity_test.go b/backend/internal/service/openai_codex_identity_test.go
index 7d2c6d8520..ecb6eb7f6f 100644
--- a/backend/internal/service/openai_codex_identity_test.go
+++ b/backend/internal/service/openai_codex_identity_test.go
@@ -7,6 +7,36 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestEnsureCodexIdentityHeaders(t *testing.T) {
+	t.Run("补齐缺失身份头", func(t *testing.T) {
+		h := make(http.Header)
+
+		ensureCodexIdentityHeaders(h)
+		enforceCodexIdentityHeaders(h)
+
+		require.Equal(t, "codex_cli_rs", h.Get("originator"))
+		require.Equal(t, codexCLIUserAgent, h.Get("user-agent"))
+		require.Equal(t, codexCLIVersion, h.Get("version"))
+		require.Equal(t, "responses=experimental", h.Get("OpenAI-Beta"))
+	})
+
+	t.Run("保留已有官方UA和合法version并重新配对", func(t *testing.T) {
+		const tuiUA = "codex-tui/9.9.9 (Mac OS X 14.0; arm64) iTerm (codex-tui; 9.9.9)"
+		h := make(http.Header)
+		h.Set("user-agent", tuiUA)
+		h.Set("version", "9.9.9")
+		h.Set("OpenAI-Beta", "assistants=v2")
+
+		ensureCodexIdentityHeaders(h)
+		enforceCodexIdentityHeaders(h)
+
+		require.Equal(t, "codex-tui", h.Get("originator"))
+		require.Equal(t, tuiUA, h.Get("user-agent"))
+		require.Equal(t, "9.9.9", h.Get("version"))
+		require.Equal(t, "responses=experimental", h.Get("OpenAI-Beta"))
+	})
+}
+
 func TestEnforceCodexIdentityHeaders(t *testing.T) {
 	const tuiUA = "codex-tui/0.140.2 (Mac OS X 14.0; arm64) iTerm (codex-tui; 0.140.2)"
 
@@ -102,13 +132,14 @@ func TestEnforceCodexIdentityHeaders(t *testing.T) {
 	}
 }
 
-// compat messages bridge 故意不带 originator：收口必须保持 no-op，不得注入身份头。
+// enforce 本身仍只负责收口：缺少 originator 时必须保持 no-op，由需要恢复身份的
+// 调用方先显式调用 ensureCodexIdentityHeaders。
 func TestEnforceCodexIdentityHeaders_NoOriginatorIsNoop(t *testing.T) {
 	h := make(http.Header)
-	h.Set("user-agent", "luna/1.0.0")
+	h.Set("user-agent", "third-party-client/1.0.0")
 
 	enforceCodexIdentityHeaders(h)
 
 	require.Empty(t, h.Get("originator"))
-	require.Equal(t, "luna/1.0.0", h.Get("user-agent"))
+	require.Equal(t, "third-party-client/1.0.0", h.Get("user-agent"))
 }
diff --git a/backend/internal/service/openai_compat_model_test.go b/backend/internal/service/openai_compat_model_test.go
index 7c7ac1b94f..69b6ddbca2 100644
--- a/backend/internal/service/openai_compat_model_test.go
+++ b/backend/internal/service/openai_compat_model_test.go
@@ -837,8 +837,7 @@ func TestForwardAsAnthropic_ReusesOAuthCodexTurnState(t *testing.T) {
 	require.NoError(t, err)
 	require.NotNil(t, firstResult)
 	require.Empty(t, upstream.requests[0].Header.Get("x-codex-turn-state"))
-	require.Empty(t, upstream.requests[0].Header.Get("OpenAI-Beta"))
-	require.Empty(t, upstream.requests[0].Header.Get("originator"))
+	requireOpenAIMessagesCodexIdentity(t, upstream.requests[0], codexCLIUserAgent, "codex_cli_rs")
 
 	secondBody := []byte(`{"model":"claude-sonnet-4-5","max_tokens":16,"messages":[{"role":"user","content":"first"},{"role":"assistant","content":"ok"},{"role":"user","content":"second"}],"stream":false}`)
 	secondRec := httptest.NewRecorder()
@@ -852,12 +851,73 @@ func TestForwardAsAnthropic_ReusesOAuthCodexTurnState(t *testing.T) {
 	require.Equal(t, "turn_state_first", upstream.requests[1].Header.Get("x-codex-turn-state"))
 	require.Equal(t, generateSessionUUID(isolateOpenAISessionID(0, "stable-cache-key")), upstream.requests[1].Header.Get("session_id"))
 	require.Empty(t, upstream.requests[1].Header.Get("conversation_id"))
-	require.Empty(t, upstream.requests[1].Header.Get("OpenAI-Beta"))
-	require.Empty(t, upstream.requests[1].Header.Get("originator"))
+	requireOpenAIMessagesCodexIdentity(t, upstream.requests[1], codexCLIUserAgent, "codex_cli_rs")
 	require.False(t, gjson.GetBytes(upstream.bodies[1], "prompt_cache_key").Exists())
 	require.False(t, gjson.GetBytes(upstream.bodies[1], "previous_response_id").Exists())
 }
 
+func TestForwardAsAnthropic_OAuthRestoresCodexIdentityHeaders(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	const tuiUA = "codex-tui/9.9.9 (Mac OS X 14.0; arm64) iTerm (codex-tui; 9.9.9)"
+	tests := []struct {
+		name           string
+		userAgent      string
+		originator     string
+		wantUserAgent  string
+		wantOriginator string
+	}{
+		{
+			name:           "官方UA逐字保留并重新配对",
+			userAgent:      tuiUA,
+			originator:     "opencode",
+			wantUserAgent:  tuiUA,
+			wantOriginator: "codex-tui",
+		},
+		{
+			name:           "第三方UA回退为默认Codex身份",
+			userAgent:      "third-party-client/1.0.0",
+			originator:     "opencode",
+			wantUserAgent:  codexCLIUserAgent,
+			wantOriginator: "codex_cli_rs",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			body := []byte(`{"model":"claude-sonnet-4-5","max_tokens":16,"messages":[{"role":"user","content":"hello"}],"stream":false}`)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
+			c.Request.Header.Set("Content-Type", "application/json")
+			c.Request.Header.Set("User-Agent", tt.userAgent)
+			c.Request.Header.Set("originator", tt.originator)
+
+			upstream := &httpUpstreamRecorder{resp: openAICompatSSECompletedResponse("resp_identity", "gpt-5.4")}
+			svc := &OpenAIGatewayService{
+				httpUpstream: upstream,
+				cfg:          &config.Config{Security: config.SecurityConfig{URLAllowlist: config.URLAllowlistConfig{Enabled: false}}},
+			}
+			account := &Account{
+				ID:          1,
+				Name:        "openai-oauth",
+				Platform:    PlatformOpenAI,
+				Type:        AccountTypeOAuth,
+				Concurrency: 1,
+				Credentials: map[string]any{
+					"access_token":       "oauth-token",
+					"chatgpt_account_id": "chatgpt-acc",
+				},
+			}
+
+			result, err := svc.ForwardAsAnthropic(context.Background(), c, account, body, "", "gpt-5.4")
+			require.NoError(t, err)
+			require.NotNil(t, result)
+			requireOpenAIMessagesCodexIdentity(t, upstream.lastReq, tt.wantUserAgent, tt.wantOriginator)
+		})
+	}
+}
+
 func TestForwardAsAnthropic_OAuthDigestFallbackReusesTurnStateWithoutExplicitKey(t *testing.T) {
 	t.Parallel()
 	gin.SetMode(gin.TestMode)
@@ -896,6 +956,7 @@ func TestForwardAsAnthropic_OAuthDigestFallbackReusesTurnStateWithoutExplicitKey
 	firstSessionID := upstream.requests[0].Header.Get("session_id")
 	require.NotEmpty(t, firstSessionID)
 	require.Empty(t, upstream.requests[0].Header.Get("x-codex-turn-state"))
+	requireOpenAIMessagesCodexIdentity(t, upstream.requests[0], codexCLIUserAgent, "codex_cli_rs")
 	require.False(t, gjson.GetBytes(upstream.bodies[0], "prompt_cache_key").Exists())
 
 	secondBody := []byte(`{"model":"claude-sonnet-4-5","max_tokens":16,"messages":[{"role":"user","content":"first"},{"role":"assistant","content":"ok"},{"role":"user","content":"second"}],"stream":false}`)
@@ -910,6 +971,7 @@ func TestForwardAsAnthropic_OAuthDigestFallbackReusesTurnStateWithoutExplicitKey
 	require.Equal(t, firstSessionID, upstream.requests[1].Header.Get("session_id"))
 	require.Equal(t, "turn_state_digest_first", upstream.requests[1].Header.Get("x-codex-turn-state"))
 	require.Empty(t, upstream.requests[1].Header.Get("conversation_id"))
+	requireOpenAIMessagesCodexIdentity(t, upstream.requests[1], codexCLIUserAgent, "codex_cli_rs")
 	require.False(t, gjson.GetBytes(upstream.bodies[1], "prompt_cache_key").Exists())
 	require.False(t, gjson.GetBytes(upstream.bodies[1], "previous_response_id").Exists())
 }
@@ -1064,8 +1126,7 @@ func TestForwardAsAnthropic_OAuthKeepsSystemAsDeveloperInput(t *testing.T) {
 	instructions := gjson.GetBytes(upstream.lastBody, "instructions")
 	require.True(t, instructions.Exists())
 	require.Empty(t, instructions.String())
-	require.Empty(t, upstream.requests[0].Header.Get("OpenAI-Beta"))
-	require.Empty(t, upstream.requests[0].Header.Get("originator"))
+	requireOpenAIMessagesCodexIdentity(t, upstream.requests[0], codexCLIUserAgent, "codex_cli_rs")
 }
 
 func TestForwardAsAnthropic_OAuthAddsClaudeCodeTodoGuardForCompatModel(t *testing.T) {
@@ -1202,6 +1263,15 @@ func openAICompatSSECompletedResponse(responseID, model string) *http.Response {
 	}
 }
 
+func requireOpenAIMessagesCodexIdentity(t *testing.T, req *http.Request, wantUserAgent, wantOriginator string) {
+	t.Helper()
+	require.NotNil(t, req)
+	require.Equal(t, wantUserAgent, req.Header.Get("User-Agent"))
+	require.Equal(t, wantOriginator, req.Header.Get("originator"))
+	require.Equal(t, codexCLIVersion, req.Header.Get("version"))
+	require.Equal(t, "responses=experimental", req.Header.Get("OpenAI-Beta"))
+}
+
 func openAICompatSSEResponseWithoutUsage(responseID, model string) *http.Response {
 	body := strings.Join([]string{
 		`data: {"type":"response.completed","response":{"id":"` + responseID + `","object":"response","model":"` + model + `","status":"completed","output":[{"type":"message","id":"msg_1","role":"assistant","status":"completed","content":[{"type":"output_text","text":"ok"}]}]}}`,
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index 8dbd9ddad0..5952469409 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -878,6 +878,8 @@ func TestForwardAsAnthropicForGrokUsesXAIResponses(t *testing.T) {
 	c, _ := gin.CreateTestContext(recorder)
 	body := []byte(`{"model":"grok","max_tokens":32,"stream":false,"messages":[{"role":"user","content":"hi"}]}`)
 	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
+	c.Request.Header.Set("OpenAI-Beta", "grok-experimental")
+	c.Request.Header.Set("originator", "opencode")
 
 	account := &Account{
 		ID:          54,
@@ -908,6 +910,9 @@ func TestForwardAsAnthropicForGrokUsesXAIResponses(t *testing.T) {
 	require.Equal(t, xai.DefaultCLIBaseURL+"/responses", upstream.lastReq.URL.String())
 	require.Equal(t, "Bearer access-token", upstream.lastReq.Header.Get("Authorization"))
 	require.Equal(t, "sub2api-grok/1.0", upstream.lastReq.Header.Get("User-Agent"))
+	require.Equal(t, "grok-experimental", upstream.lastReq.Header.Get("OpenAI-Beta"))
+	require.Empty(t, upstream.lastReq.Header.Get("originator"))
+	require.Empty(t, upstream.lastReq.Header.Get("version"))
 	require.Equal(t, "grok-4.5", gjson.GetBytes(upstream.lastBody, "model").String())
 	require.True(t, gjson.GetBytes(upstream.lastBody, "stream").Bool())
 	require.NotContains(t, string(upstream.lastBody), "chatgpt.com")
diff --git a/backend/internal/service/openai_gateway_messages.go b/backend/internal/service/openai_gateway_messages.go
index 219b5e4be4..de332f1404 100644
--- a/backend/internal/service/openai_gateway_messages.go
+++ b/backend/internal/service/openai_gateway_messages.go
@@ -261,9 +261,8 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 	// 6. Build upstream request
 	if account.Type == AccountTypeOAuth && account.Platform != PlatformGrok {
 		// Messages 兼容桥即使 body 未带 todo-guard/prompt_cache_key 标记（如映射到非
-		// gpt-5/codex 模型），也必须让 buildUpstreamRequest 走 bridge 分支：不带
-		// originator、User-Agent 逐字透传，避免身份收口（issue #3901）误改本路径
-		// 刻意最小化的请求形态（下方的 Del(OpenAI-Beta/originator) 兜底保持不变）。
+		// gpt-5/codex 模型），也必须让 buildUpstreamRequest 走 bridge 分支，以保留
+		// 既有 body/session/conversation 行为。身份头在 post-build 阶段统一恢复。
 		setOpenAICompatMessagesBridgeContext(c, true)
 	}
 	upstreamCtx, releaseUpstreamCtx := detachUpstreamContext(ctx)
@@ -288,12 +287,16 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 		}
 	}
 	if account.Type == AccountTypeOAuth && account.Platform != PlatformGrok {
-		// Anthropic Messages compatibility uses the ChatGPT Codex SSE endpoint.
-		// Match airgate-openai's request shape: the SSE endpoint does not need
-		// the Responses experimental beta header, and forcing originator can make
-		// ChatGPT select a different internal continuation path.
-		upstreamReq.Header.Del("OpenAI-Beta")
-		upstreamReq.Header.Del("originator")
+		// buildUpstreamRequest 保留 Messages bridge 的 body/session 兼容行为，并会先
+		// 清除身份头。真正发送前恢复完整 Codex 身份，避免 ChatGPT Codex 上游因缺失
+		// originator/OpenAI-Beta 返回 404（issue #3901）。
+		ensureCodexIdentityHeaders(upstreamReq.Header)
+		enforceCodexIdentityHeaders(upstreamReq.Header)
+		logger.L().Debug("openai messages: upstream identity restored",
+			zap.Int64("account_id", account.ID),
+			zap.String("upstream_model", upstreamModel),
+			zap.Bool("compat_identity_restored", true),
+		)
 	}
 	if account.Type == AccountTypeOAuth && promptCacheKey != "" && strings.TrimSpace(c.GetHeader("conversation_id")) == "" {
 		upstreamReq.Header.Del("conversation_id")
diff --git a/backend/internal/service/openai_gateway_messages_chat_fallback_test.go b/backend/internal/service/openai_gateway_messages_chat_fallback_test.go
index 8bb15c81aa..b36b76ffa8 100644
--- a/backend/internal/service/openai_gateway_messages_chat_fallback_test.go
+++ b/backend/internal/service/openai_gateway_messages_chat_fallback_test.go
@@ -356,6 +356,8 @@ func TestForwardAsAnthropic_ResponsesSupportedAccountStillUsesResponsesEndpoint(
 	c, _ := gin.CreateTestContext(rec)
 	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
 	c.Request.Header.Set("Content-Type", "application/json")
+	c.Request.Header.Set("User-Agent", "third-party-client/1.0.0")
+	c.Request.Header.Set("originator", "opencode")
 
 	upstreamBody := strings.Join([]string{
 		`data: {"type":"response.completed","response":{"id":"resp_native","object":"response","model":"gpt-5.4","status":"completed","output":[{"type":"message","id":"msg_1","role":"assistant","status":"completed","content":[{"type":"output_text","text":"ok"}]}],"usage":{"input_tokens":5,"output_tokens":2,"total_tokens":7}}}`,
@@ -385,5 +387,9 @@ func TestForwardAsAnthropic_ResponsesSupportedAccountStillUsesResponsesEndpoint(
 		"responses-capable account must stay on /v1/responses, got %s", upstream.lastReq.URL.String())
 	require.True(t, gjson.GetBytes(upstream.lastBody, "input").Exists())
 	require.False(t, gjson.GetBytes(upstream.lastBody, "messages").Exists())
+	require.Equal(t, "third-party-client/1.0.0", upstream.lastReq.Header.Get("User-Agent"))
+	require.Equal(t, "opencode", upstream.lastReq.Header.Get("originator"))
+	require.Empty(t, upstream.lastReq.Header.Get("version"))
+	require.Empty(t, upstream.lastReq.Header.Get("OpenAI-Beta"))
 	require.Equal(t, "ok", gjson.Get(rec.Body.String(), "content.0.text").String())
 }