diff --git a/src/shared/validation/settingsSchemas.ts b/src/shared/validation/settingsSchemas.ts
index 83d87893f9..7bfc8c031f 100644
--- a/src/shared/validation/settingsSchemas.ts
+++ b/src/shared/validation/settingsSchemas.ts
@@ -174,6 +174,8 @@ export const updateSettingsSchema = z.object({
     )
     .optional(),
   customBannedSignals: z.array(z.string().max(200)).optional(),
+  customSystemPromptEnabled: z.boolean().optional(),
+  customSystemPrompt: z.string().max(10000).optional(),
   // #9817: opt-in (default off) — lets a probe-origin (model test-all)
   // failure deactivate a connection like real traffic. Off by default:
   // probe failures are recorded but never mutate routing state.
diff --git a/tests/unit/custom-system-prompt-settings-persistence.test.ts b/tests/unit/custom-system-prompt-settings-persistence.test.ts
new file mode 100644
index 0000000000..2947c5af90
--- /dev/null
+++ b/tests/unit/custom-system-prompt-settings-persistence.test.ts
@@ -0,0 +1,15 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { updateSettingsSchema } from "@/shared/validation/settingsSchemas";
+
+test("updateSettingsSchema preserves customSystemPromptEnabled and customSystemPrompt fields", () => {
+  const input = {
+    customSystemPromptEnabled: true,
+    customSystemPrompt: "Always respond politely",
+  };
+
+  const parsed = updateSettingsSchema.parse(input);
+
+  assert.equal(parsed.customSystemPromptEnabled, true);
+  assert.equal(parsed.customSystemPrompt, "Always respond politely");
+});