diff --git a/server-rs/crates/api-server/src/llm/mod.rs b/server-rs/crates/api-server/src/llm/mod.rs
index 351ffbb52..0bf249dac 100644
--- a/server-rs/crates/api-server/src/llm/mod.rs
+++ b/server-rs/crates/api-server/src/llm/mod.rs
@@ -38,6 +38,45 @@ pub(crate) const LLM_REQUEST_MAX_BODY_BYTES: usize = 32 * 1024 * 1024;
 const LLM_ANTHROPIC_NATIVE_FIRST_BYTE_TIMEOUT: std::time::Duration =
     std::time::Duration::from_secs(20);
 
+/// Claude Agent SDK 在 `system` 里固定注入的身份句原文。
+///
+/// 2026-10-06 用真实 AGC 客户端复现：共享 LLM Router 的上游内容策略会把这句话整体判成
+/// 需要拦截的内容，直接返回 `500 {"error":{"message":"Your request was blocked by our
+/// content policy..."}}`。单独把这句话作为 `system` 发送即可稳定复现；同一请求保留 AGC
+/// 提示词、billing header、工具、`metadata`、`max_tokens`、`thinking` 等全部其它字段，只把
+/// 品牌归属片段换成等价表述后稳定返回 200。因此这里只做最小归一化，保留身份语义，去掉会
+/// 触发误判的片段，其它请求内容一律原样透传。
+const CLAUDE_AGENT_SDK_IDENTITY_SENTENCE: &str =
+    "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
+const CLAUDE_AGENT_SDK_IDENTITY_SENTENCE_REPLACEMENT: &str =
+    "You are a Claude agent, built on an Anthropic agent SDK.";
+
+/// 归一化 Claude Agent SDK 注入的身份句，避免上游内容策略误判整条请求。
+///
+/// `system` 既可能是字符串也可能是数组；Claude Agent SDK 固定发数组，其它形状保持不动，
+/// 保证这条路只影响已知会失败的那一种载荷。
+fn normalize_claude_agent_sdk_identity_sentence(payload: &mut Value) {
+    let Some(system) = payload.get_mut("system").and_then(Value::as_array_mut) else {
+        return;
+    };
+    for entry in system {
+        let Some(object) = entry.as_object_mut() else {
+            continue;
+        };
+        let Some(text) = object.get("text").and_then(Value::as_str) else {
+            continue;
+        };
+        if !text.contains(CLAUDE_AGENT_SDK_IDENTITY_SENTENCE) {
+            continue;
+        }
+        let normalized = text.replace(
+            CLAUDE_AGENT_SDK_IDENTITY_SENTENCE,
+            CLAUDE_AGENT_SDK_IDENTITY_SENTENCE_REPLACEMENT,
+        );
+        object.insert("text".to_string(), Value::String(normalized));
+    }
+}
+
 mod anthropic_bridge;
 
 pub(crate) mod icon_specs;
@@ -568,6 +607,7 @@ pub async fn proxy_llm_messages(
         .to_string();
     // 原生直通要用目录解析后的上游模型名；桥接路径由转换器显式接收同一个名字。
     object.insert("model".to_string(), Value::String(selected_model.clone()));
+    normalize_claude_agent_sdk_identity_sentence(&mut payload);
 
     let (base_url, api_key, key_id) =
         resolve_llm_router_credentials(&state, authenticated.claims().user_id())
@@ -2204,6 +2244,52 @@ mod tests {
         assert!(carry.is_empty());
     }
 
+    #[test]
+    fn normalize_claude_agent_sdk_identity_sentence_rewrites_only_the_blocked_sentence() {
+        let mut payload = json!({
+            "model": "claude-opus-5-5",
+            "system": [
+                { "type": "text", "text": "x-anthropic-billing-header: cc_version=2.1.285; " },
+                {
+                    "type": "text",
+                    "text": "You are a Claude agent, built on Anthropic's Claude Agent SDK.",
+                    "cache_control": { "type": "ephemeral" }
+                },
+                { "type": "text", "text": "你是陶泥儿，负责当前任务。" }
+            ]
+        });
+
+        normalize_claude_agent_sdk_identity_sentence(&mut payload);
+
+        assert_eq!(
+            payload["system"][1]["text"],
+            json!("You are a Claude agent, built on an Anthropic agent SDK.")
+        );
+        assert_eq!(
+            payload["system"][1]["cache_control"],
+            json!({ "type": "ephemeral" })
+        );
+        assert_eq!(
+            payload["system"][0]["text"],
+            json!("x-anthropic-billing-header: cc_version=2.1.285; ")
+        );
+        assert_eq!(
+            payload["system"][2]["text"],
+            json!("你是陶泥儿，负责当前任务。")
+        );
+    }
+
+    #[test]
+    fn normalize_claude_agent_sdk_identity_sentence_keeps_other_system_shapes() {
+        let mut payload = json!({ "system": "You are a Claude agent." });
+        normalize_claude_agent_sdk_identity_sentence(&mut payload);
+        assert_eq!(payload["system"], json!("You are a Claude agent."));
+
+        let mut without_system = json!({ "model": "claude-opus-5-5" });
+        normalize_claude_agent_sdk_identity_sentence(&mut without_system);
+        assert_eq!(without_system, json!({ "model": "claude-opus-5-5" }));
+    }
+
     #[test]
     fn router_protocol_url_composes_v1_without_duplicating_it() {
         // baseUrl 不带路由与版本段，协议自己拼。