diff --git a/apps/Backend/src/ai/internal-chat-workflow.ts b/apps/Backend/src/ai/internal-chat-workflow.ts
index 323bd2a3..b1940cdb 100644
--- a/apps/Backend/src/ai/internal-chat-workflow.ts
+++ b/apps/Backend/src/ai/internal-chat-workflow.ts
@@ -55,15 +55,29 @@ export function deriveSiteKey(provider: string): string {
   return "MH";
 }
 
-// siteKey → autoCheck value used by the insurance-status page prefill
-export function siteKeyToAutoCheck(siteKey: string): string {
+// siteKey → autoCheck value used by the insurance-status page prefill.
+// For MH, pass the patient's DOB so under-21 patients route to CMSP.
+export function siteKeyToAutoCheck(siteKey: string, dob?: string | Date | null): string {
   switch (siteKey) {
     case "CCA":        return "cca";
     case "DDMA":       return "ddma";
     case "DELTA_INS":  return "delta-ins";
     case "TUFTS_SCO":  return "tufts-sco";
     case "UNITED_SCO": return "united-sco";
-    default:           return "mh"; // MH (caller may downgrade to "cmsp" by age)
+    default: {
+      // MassHealth: patients under 21 use the CMSP button
+      if (dob) {
+        try {
+          const birth = dob instanceof Date ? dob : new Date(dob);
+          const today = new Date();
+          let age = today.getFullYear() - birth.getFullYear();
+          const m = today.getMonth() - birth.getMonth();
+          if (m < 0 || (m === 0 && today.getDate() < birth.getDate())) age--;
+          if (age < 21) return "cmsp";
+        } catch {}
+      }
+      return "mh";
+    }
   }
 }
 
@@ -231,7 +245,7 @@ export async function runInternalChatWorkflow(
           memberId: patient.insuranceId,
           dob: resolvedDob,
           siteKey,
-          autoCheck: siteKeyToAutoCheck(siteKey),
+          autoCheck: siteKeyToAutoCheck(siteKey, resolvedDob),
         },
       };
     }