diff --git a/backend/internal/service/claude_code_validator.go b/backend/internal/service/claude_code_validator.go
index 923f30b7cc..52f351f1b3 100644
--- a/backend/internal/service/claude_code_validator.go
+++ b/backend/internal/service/claude_code_validator.go
@@ -48,9 +48,14 @@ var claudeCodeSystemPrompts = []string{
 }
 
 const (
+	// These markers identify Claude Code's official security-monitor classifier
+	// request without coupling validation to every wording change in the prompt.
+	claudeCodeSecurityMonitorPromptPrefix = "You are a security monitor for autonomous AI coding agents."
+	claudeCodeSecurityMonitorPromptMinLen = 10_000
+
 	// claudeCodeBillingHeaderPrefix 是 Claude Code 在 system 数组首块注入的计费归因块前缀。
-	// 该块存在于所有真实 Claude Code CLI 请求中（含安全监视器等无身份 prose 的子请求），
-	// 格式固定、不随提示词改版漂移，是比身份 prose 更稳定的客户端标识。
+	// 大多数真实 CLI 请求（含部分无身份 prose 的子请求）会携带该块；不携带该块的
+	// 固定官方辅助请求由独立规则识别。该格式比身份 prose 更稳定。
 	// 生成见 gateway_billing_block.go；同类识别见 pkg/apicompat/anthropic_to_responses.go。
 	claudeCodeBillingHeaderPrefix = "x-anthropic-billing-header"
 	// claudeCodeEntrypointMarker 标识计费块携带入口归因字段。不绑定具体入口值
@@ -167,6 +172,10 @@ func (v *ClaudeCodeValidator) hasClaudeCodeSystemPrompt(body map[string]any) boo
 		return false
 	}
 
+	if isClaudeCodeSecurityMonitorPrompt(systemEntries) {
+		return true
+	}
+
 	// 检查每个 system entry
 	for _, entry := range systemEntries {
 		entryMap, ok := entry.(map[string]any)
@@ -196,6 +205,46 @@ func (v *ClaudeCodeValidator) hasClaudeCodeSystemPrompt(body map[string]any) boo
 	return false
 }
 
+func isClaudeCodeSecurityMonitorPrompt(systemEntries []any) bool {
+	if len(systemEntries) != 1 {
+		return false
+	}
+
+	entry, ok := systemEntries[0].(map[string]any)
+	if !ok {
+		return false
+	}
+
+	entryType, ok := entry["type"].(string)
+	if !ok || entryType != "text" {
+		return false
+	}
+
+	text, ok := entry["text"].(string)
+	if !ok || len(text) < claudeCodeSecurityMonitorPromptMinLen ||
+		!strings.HasPrefix(text, claudeCodeSecurityMonitorPromptPrefix) {
+		return false
+	}
+
+	markers := []string{
+		"## Threat Model",
+		"- `<transcript>`:",
+		"## HARD BLOCK",
+		"## SOFT BLOCK",
+		"## Classification Process",
+		"## Output Format",
+		"<block>yes</block>",
+		"<block>no</block>",
+	}
+	for _, marker := range markers {
+		if !strings.Contains(text, marker) {
+			return false
+		}
+	}
+
+	return true
+}
+
 // bestSimilarityScore 计算文本与所有 Claude Code 模板的最佳相似度
 func (v *ClaudeCodeValidator) bestSimilarityScore(text string) float64 {
 	normalizedText := normalizePrompt(text)
diff --git a/backend/internal/service/claude_code_validator_test.go b/backend/internal/service/claude_code_validator_test.go
index d54177d132..8fd0f2126c 100644
--- a/backend/internal/service/claude_code_validator_test.go
+++ b/backend/internal/service/claude_code_validator_test.go
@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
@@ -135,6 +136,152 @@ func TestClaudeCodeValidator_BillingBlockRecognizedWithoutIdentityPrompt(t *test
 	require.True(t, ok)
 }
 
+func TestClaudeCodeValidator_SecurityMonitorWithoutBillingBlock(t *testing.T) {
+	monitorPrompt, err := os.ReadFile("testdata/security_monitor_system_prompt.txt")
+	require.NoError(t, err)
+
+	validHeaders := map[string]string{
+		"User-Agent":        "claude-cli/2.1.220 (external, cli)",
+		"X-App":             "cli",
+		"anthropic-beta":    "claude-code-20250219",
+		"anthropic-version": "2023-06-01",
+	}
+	validBody := func(prompt string) map[string]any {
+		return map[string]any{
+			"model": "claude-haiku-4-5-20251001",
+			"system": []any{
+				map[string]any{"type": "text", "text": prompt},
+			},
+			"metadata": map[string]any{"user_id": claudeCodeMetadataUserIDJSON},
+		}
+	}
+
+	tests := []struct {
+		name       string
+		headers    map[string]string
+		body       map[string]any
+		wantAccept bool
+	}{
+		{
+			name:       "official classifier request",
+			headers:    validHeaders,
+			body:       validBody(string(monitorPrompt)),
+			wantAccept: true,
+		},
+		{
+			name:    "classifier output with category element",
+			headers: validHeaders,
+			body: validBody(strings.Replace(
+				string(monitorPrompt),
+				"<block>yes</block><reason>",
+				"<block>yes</block><category>Exact BLOCK Rule Name</category><reason>",
+				1,
+			)),
+			wantAccept: true,
+		},
+		{
+			name: "non-Claude user agent",
+			headers: map[string]string{
+				"User-Agent":        "curl/8.0.0",
+				"X-App":             "cli",
+				"anthropic-beta":    "claude-code-20250219",
+				"anthropic-version": "2023-06-01",
+			},
+			body: validBody(string(monitorPrompt)),
+		},
+		{
+			name: "missing X-App",
+			headers: map[string]string{
+				"User-Agent":        validHeaders["User-Agent"],
+				"anthropic-beta":    validHeaders["anthropic-beta"],
+				"anthropic-version": validHeaders["anthropic-version"],
+			},
+			body: validBody(string(monitorPrompt)),
+		},
+		{
+			name: "missing anthropic-beta",
+			headers: map[string]string{
+				"User-Agent":        validHeaders["User-Agent"],
+				"X-App":             validHeaders["X-App"],
+				"anthropic-version": validHeaders["anthropic-version"],
+			},
+			body: validBody(string(monitorPrompt)),
+		},
+		{
+			name: "missing anthropic-version",
+			headers: map[string]string{
+				"User-Agent":     validHeaders["User-Agent"],
+				"X-App":          validHeaders["X-App"],
+				"anthropic-beta": validHeaders["anthropic-beta"],
+			},
+			body: validBody(string(monitorPrompt)),
+		},
+		{
+			name:    "missing metadata",
+			headers: validHeaders,
+			body: map[string]any{
+				"model":  "claude-haiku-4-5-20251001",
+				"system": []any{map[string]any{"type": "text", "text": string(monitorPrompt)}},
+			},
+		},
+		{
+			name:    "invalid metadata user ID",
+			headers: validHeaders,
+			body: func() map[string]any {
+				body := validBody(string(monitorPrompt))
+				body["metadata"] = map[string]any{"user_id": "invalid"}
+				return body
+			}(),
+		},
+		{
+			name:       "unrelated prompt",
+			headers:    validHeaders,
+			body:       validBody("You are a different security classifier for coding agents."),
+			wantAccept: false,
+		},
+		{
+			name:       "opening sentence alone",
+			headers:    validHeaders,
+			body:       validBody(claudeCodeSecurityMonitorPromptPrefix),
+			wantAccept: false,
+		},
+		{
+			name:    "opening sentence plus arbitrary altered suffix",
+			headers: validHeaders,
+			body: validBody(claudeCodeSecurityMonitorPromptPrefix + "\n\n" +
+				strings.Repeat("This is arbitrary altered classifier content. ", 300)),
+			wantAccept: false,
+		},
+		{
+			name:    "multiple system entries without billing block",
+			headers: validHeaders,
+			body: func() map[string]any {
+				body := validBody(string(monitorPrompt))
+				system, ok := body["system"].([]any)
+				require.True(t, ok)
+				body["system"] = append(system, map[string]any{
+					"type": "text",
+					"text": "Additional unrelated system content.",
+				})
+				return body
+			}(),
+			wantAccept: false,
+		},
+	}
+
+	validator := NewClaudeCodeValidator()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodPost, "http://example.com/v1/messages", nil)
+			for name, value := range tt.headers {
+				req.Header.Set(name, value)
+			}
+
+			require.Equal(t, tt.wantAccept, validator.Validate(req, tt.body))
+		})
+	}
+}
+
 func TestClaudeCodeValidator_BillingBlockVSCodeEntrypointRecognized(t *testing.T) {
 	// 回归：Claude Code 在 VSCode 扩展内运行时，计费块入口为 cc_entrypoint=claude-vscode
 	// 而非 cli。其安全监视器子请求同样不携带身份 prose，此前写死 cc_entrypoint=cli 的