diff --git a/src/auto-reply/reply.triggers.test.ts b/src/auto-reply/reply.triggers.test.ts
index 0e129c928..d8afe0a33 100644
--- a/src/auto-reply/reply.triggers.test.ts
+++ b/src/auto-reply/reply.triggers.test.ts
@@ -1517,7 +1517,7 @@ describe("group intro prompts", () => {
         vi.mocked(runEmbeddedPiAgent).mock.calls.at(-1)?.[0]
           ?.extraSystemPrompt ?? "";
       expect(extraSystemPrompt).toBe(
-        `You are replying inside the WhatsApp group "Ops". Activation: trigger-only (you are invoked only when explicitly mentioned; recent context may be included). ${groupParticipationNote} Address the specific sender noted in the message context.`,
+        `You are replying inside the WhatsApp group "Ops". Activation: trigger-only (you are invoked only when explicitly mentioned; recent context may be included). WhatsApp IDs: SenderId is the participant JID; [message_id: ...] is the message id for reactions (use SenderId as participant). ${groupParticipationNote} Address the specific sender noted in the message context.`,
       );
     });
   });
diff --git a/src/auto-reply/reply/groups.ts b/src/auto-reply/reply/groups.ts
index f3445001a..2cf4dcd0c 100644
--- a/src/auto-reply/reply/groups.ts
+++ b/src/auto-reply/reply/groups.ts
@@ -213,6 +213,10 @@ export function buildGroupIntro(params: {
     activation === "always"
       ? "Activation: always-on (you receive every group message)."
       : "Activation: trigger-only (you are invoked only when explicitly mentioned; recent context may be included).";
+  const whatsappIdsLine =
+    provider === "whatsapp"
+      ? "WhatsApp IDs: SenderId is the participant JID; [message_id: ...] is the message id for reactions (use SenderId as participant)."
+      : undefined;
   const silenceLine =
     activation === "always"
       ? `If no response is needed, reply with exactly "${params.silentToken}" (and nothing else) so Clawdbot stays silent. Do not add any other words, punctuation, tags, markdown/code blocks, or explanations.`
@@ -227,6 +231,7 @@ export function buildGroupIntro(params: {
     subjectLine,
     membersLine,
     activationLine,
+    whatsappIdsLine,
     silenceLine,
     cautionLine,
     lurkLine,
diff --git a/src/web/auto-reply.ts b/src/web/auto-reply.ts
index 5da3376c1..ee9786680 100644
--- a/src/web/auto-reply.ts
+++ b/src/web/auto-reply.ts
@@ -1261,6 +1261,7 @@ export async function monitorWebProvider(
             msg.senderE164,
           ),
           SenderName: msg.senderName,
+          SenderId: msg.senderJid ?? msg.senderE164,
           SenderE164: msg.senderE164,
           WasMentioned: msg.wasMentioned,
           ...(msg.location ? toLocationContext(msg.location) : {}),