diff --git a/changelog.d/fixes/9142-fix.plan.md b/changelog.d/fixes/9142-fix.plan.md
new file mode 100644
index 0000000000..aecf251f9e
--- /dev/null
+++ b/changelog.d/fixes/9142-fix.plan.md
@@ -0,0 +1 @@
+- fix(background): detect Anthropic top-level system prompts for background task detection (#9142)
diff --git a/open-sse/services/backgroundTaskDetector.ts b/open-sse/services/backgroundTaskDetector.ts
index 20ac799c5d..8cbcbd3e9e 100644
--- a/open-sse/services/backgroundTaskDetector.ts
+++ b/open-sse/services/backgroundTaskDetector.ts
@@ -190,15 +190,31 @@ export function getBackgroundTaskReason(
   const messages = toMessageArray(typedBody.messages ?? typedBody.input ?? []);
   if (!Array.isArray(messages) || messages.length === 0) return null;
 
-  // Find system message
+  // Derive system content from messages array (OpenAI format) or top-level
+  // system field (Anthropic format).
   const systemMsg = messages.find(
     (message: BackgroundMessage) => message.role === "system" || message.role === "developer"
   );
-  if (!systemMsg) return null;
-
-  const systemContent =
-    typeof systemMsg.content === "string" ? systemMsg.content.toLowerCase() : "";
-
+  let systemContent = "";
+  if (systemMsg && typeof systemMsg.content === "string") {
+    systemContent = systemMsg.content.toLowerCase();
+  } else if (!systemMsg) {
+    // Anthropic top-level system field: string or array of text blocks
+    const raw = (typedBody as Record<string, unknown>).system;
+    if (typeof raw === "string") {
+      systemContent = raw.toLowerCase();
+    } else if (Array.isArray(raw)) {
+      systemContent = raw
+        .map((part) =>
+          part && typeof (part as { text?: unknown }).text === "string"
+            ? (part as { text: string }).text
+            : ""
+        )
+        .filter(Boolean)
+        .join(" ")
+        .toLowerCase();
+    }
+  }
   if (!systemContent) return null;
 
   // Check against detection patterns
diff --git a/tests/unit/triage-bugs-2026-08-02.test.ts b/tests/unit/triage-bugs-2026-08-02.test.ts
index 88a7cbb300..d27536dba9 100644
--- a/tests/unit/triage-bugs-2026-08-02.test.ts
+++ b/tests/unit/triage-bugs-2026-08-02.test.ts
@@ -42,6 +42,26 @@ test("non-GPT-5.6 models still get max downgraded to xhigh", () => {
     )
   );
   assert.equal(translated.reasoning_effort, "xhigh");
+<<<<<<< HEAD
+});
+
+// ─────────────────────────────────────────────────────────────────────
+// PR #9142 — Anthropic top-level `system` prompts must trigger background detection
+// ─────────────────────────────────────────────────────────────────────
+const { getBackgroundTaskReason, setBackgroundDegradationConfig } =
+  await import("../../open-sse/services/backgroundTaskDetector.ts");
+
+test("#9142 Anthropic top-level system prompts must trigger background detection", () => {
+  setBackgroundDegradationConfig({ enabled: true });
+  assert.equal(
+    getBackgroundTaskReason({
+      system: "Generate a title for this conversation",
+      messages: [{ role: "user", content: "hello" }],
+    }),
+    "system_prompt_pattern"
+  );
+});
+=======
 
 // #9140 — VS Code routes filter out built-in auto models
 const { isUsableChatModel } = await import(
@@ -59,3 +79,4 @@ test("#9140 VS Code listing must accept built-in auto routing entries", () => {
     false,
     "operator-created combo should still be rejected"
   );
+>>>>>>> origin/release/v3.8.50