diff --git a/backend/internal/pkg/apicompat/responses_namespace_test.go b/backend/internal/pkg/apicompat/responses_namespace_test.go
index ae686d38bd..4c8e313c5d 100644
--- a/backend/internal/pkg/apicompat/responses_namespace_test.go
+++ b/backend/internal/pkg/apicompat/responses_namespace_test.go
@@ -31,20 +31,31 @@ func TestFlattenResponsesNamespaces_RewritesDeclarationHistoryAndChoice(t *testi
 	require.True(t, changed)
 	require.Equal(t, ResponsesNamespaceName{Namespace: "collaboration", Name: "spawn_agent"}, names["collaboration__spawn_agent"])
 
-	tools := req["tools"].([]any)
+	tools, ok := req["tools"].([]any)
+	require.True(t, ok)
 	require.Len(t, tools, 2)
-	require.Equal(t, "plain", tools[0].(map[string]any)["name"])
-	require.Equal(t, "collaboration__spawn_agent", tools[1].(map[string]any)["name"])
-	require.Equal(t, "spawn", tools[1].(map[string]any)["description"])
+	plainTool, ok := tools[0].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "plain", plainTool["name"])
+	flatTool, ok := tools[1].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "collaboration__spawn_agent", flatTool["name"])
+	require.Equal(t, "spawn", flatTool["description"])
 
-	choice := req["tool_choice"].(map[string]any)
+	choice, ok := req["tool_choice"].(map[string]any)
+	require.True(t, ok)
 	require.Equal(t, "collaboration__spawn_agent", choice["name"])
 	require.NotContains(t, choice, "namespace")
 
-	call := req["input"].([]any)[0].(map[string]any)
+	input, ok := req["input"].([]any)
+	require.True(t, ok)
+	require.Len(t, input, 2)
+	call, ok := input[0].(map[string]any)
+	require.True(t, ok)
 	require.Equal(t, "collaboration__spawn_agent", call["name"])
 	require.NotContains(t, call, "namespace")
-	message := req["input"].([]any)[1].(map[string]any)
+	message, ok := input[1].(map[string]any)
+	require.True(t, ok)
 	require.Equal(t, "spawn_agent", message["name"])
 	require.Equal(t, "collaboration", message["namespace"])
 	require.Equal(t, "gpt-5.5", req["model"])
@@ -96,11 +107,17 @@ func TestFlattenResponsesNamespacesExcept_PreservesBuiltInNamespaceAndChoice(t *
 	require.NoError(t, err)
 	require.True(t, changed)
 	require.Contains(t, names, "collaboration__spawn_agent")
-	tools := req["tools"].([]any)
-	require.Equal(t, "namespace", tools[0].(map[string]any)["type"])
-	require.Equal(t, "image_gen", tools[0].(map[string]any)["name"])
-	require.Equal(t, "function", tools[1].(map[string]any)["type"])
-	require.Equal(t, "collaboration__spawn_agent", tools[1].(map[string]any)["name"])
+	tools, ok := req["tools"].([]any)
+	require.True(t, ok)
+	require.Len(t, tools, 2)
+	preservedTool, ok := tools[0].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "namespace", preservedTool["type"])
+	require.Equal(t, "image_gen", preservedTool["name"])
+	flatTool, ok := tools[1].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "function", flatTool["type"])
+	require.Equal(t, "collaboration__spawn_agent", flatTool["name"])
 	require.Equal(t, map[string]any{"type": "namespace", "name": "image_gen"}, req["tool_choice"])
 }
 
diff --git a/backend/internal/service/openai_gateway_forward.go b/backend/internal/service/openai_gateway_forward.go
index 980d2ffe33..6c7d45946a 100644
--- a/backend/internal/service/openai_gateway_forward.go
+++ b/backend/internal/service/openai_gateway_forward.go
@@ -42,7 +42,11 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	if normalized {
 		body = normalizedBody
 	}
-	if account.Type == AccountTypeOAuth {
+	wsDecision := s.getOpenAIWSProtocolResolver().Resolve(account)
+	// 仅允许 WS 入站请求走 WS 上游，避免出现 HTTP -> WS 协议混用。
+	wsDecision = resolveOpenAIWSDecisionByClientTransport(wsDecision, GetOpenAIClientTransport(c))
+	passthroughEnabled := account.IsOpenAIPassthroughEnabled()
+	if shouldFlattenOpenAIResponsesNamespaces(account, wsDecision.Transport, passthroughEnabled) {
 		body, err = flattenOpenAIResponsesNamespaces(c, body)
 		if err != nil {
 			setOpsUpstreamError(c, http.StatusBadRequest, err.Error(), "")
@@ -74,10 +78,6 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	if isCodexCLI {
 		codexImageGenerationExplicitToolPolicy = account.CodexImageGenerationExplicitToolPolicy()
 	}
-	wsDecision := s.getOpenAIWSProtocolResolver().Resolve(account)
-	clientTransport := GetOpenAIClientTransport(c)
-	// 仅允许 WS 入站请求走 WS 上游，避免出现 HTTP -> WS 协议混用。
-	wsDecision = resolveOpenAIWSDecisionByClientTransport(wsDecision, clientTransport)
 	if c != nil {
 		c.Set("openai_ws_transport_decision", string(wsDecision.Transport))
 		c.Set("openai_ws_transport_reason", wsDecision.Reason)
@@ -106,7 +106,6 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		}
 		return nil, errors.New("openai ws v1 is temporarily unsupported; use ws v2")
 	}
-	passthroughEnabled := account.IsOpenAIPassthroughEnabled()
 	if passthroughEnabled {
 		if isCodexCLI && codexImageGenerationExplicitToolPolicy == codexImageGenerationExplicitToolPolicyStrip {
 			strippedBody, changed, stripErr := stripOpenAIImageGenerationToolsFromRawPayload(body)
diff --git a/backend/internal/service/openai_responses_namespace.go b/backend/internal/service/openai_responses_namespace.go
index 279eb2afeb..7d71b64814 100644
--- a/backend/internal/service/openai_responses_namespace.go
+++ b/backend/internal/service/openai_responses_namespace.go
@@ -11,6 +11,21 @@ import (
 
 const openAIResponsesNamespaceNamesContextKey = "openai_responses_namespace_names"
 
+// shouldFlattenOpenAIResponsesNamespaces 判定原生 Responses 转发前是否摊平
+// Codex namespace 工具。WSv2 上游原生支持 namespace，且 WS 出口
+// （openai_ws_forwarder_v2）原样转发上游事件、不经 HTTP 回程还原，摊平后的
+// 平名无法还原会破坏客户端工具匹配，因此实际走 WSv2 分支的请求保持 namespace
+// 原样。透传账号先于 WSv2 分支经 HTTP 转发返回，仍需摊平。
+func shouldFlattenOpenAIResponsesNamespaces(account *Account, transport OpenAIUpstreamTransport, passthroughEnabled bool) bool {
+	if account == nil || account.Type != AccountTypeOAuth {
+		return false
+	}
+	if transport == OpenAIUpstreamTransportResponsesWebsocketV2 && !passthroughEnabled {
+		return false
+	}
+	return true
+}
+
 func flattenOpenAIResponsesNamespaces(c *gin.Context, body []byte) ([]byte, error) {
 	if !bytes.Contains(body, []byte(`"namespace"`)) {
 		return body, nil
diff --git a/backend/internal/service/openai_responses_namespace_test.go b/backend/internal/service/openai_responses_namespace_test.go
new file mode 100644
index 0000000000..7aa1392260
--- /dev/null
+++ b/backend/internal/service/openai_responses_namespace_test.go
@@ -0,0 +1,34 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestShouldFlattenOpenAIResponsesNamespaces(t *testing.T) {
+	oauth := &Account{Type: AccountTypeOAuth}
+	apiKey := &Account{Type: AccountTypeAPIKey}
+
+	tests := []struct {
+		name               string
+		account            *Account
+		transport          OpenAIUpstreamTransport
+		passthroughEnabled bool
+		want               bool
+	}{
+		{name: "oauth_http", account: oauth, transport: OpenAIUpstreamTransportHTTPSSE, want: true},
+		{name: "oauth_http_passthrough", account: oauth, transport: OpenAIUpstreamTransportHTTPSSE, passthroughEnabled: true, want: true},
+		// WSv2 出口原样转发上游事件、不做回程还原，摊平会让客户端收到无法匹配的平名。
+		{name: "oauth_wsv2", account: oauth, transport: OpenAIUpstreamTransportResponsesWebsocketV2, want: false},
+		// 透传账号先于 WSv2 分支经 HTTP 转发返回，仍需摊平。
+		{name: "oauth_wsv2_passthrough", account: oauth, transport: OpenAIUpstreamTransportResponsesWebsocketV2, passthroughEnabled: true, want: true},
+		{name: "apikey_http", account: apiKey, transport: OpenAIUpstreamTransportHTTPSSE, want: false},
+		{name: "nil_account", account: nil, transport: OpenAIUpstreamTransportHTTPSSE, want: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, shouldFlattenOpenAIResponsesNamespaces(tt.account, tt.transport, tt.passthroughEnabled))
+		})
+	}
+}