diff --git a/src/auto-reply/reply/get-reply-run.media-only.test.ts b/src/auto-reply/reply/get-reply-run.media-only.test.ts
index e8c1c69cc9ac..8bcaa85cbf5f 100644
--- a/src/auto-reply/reply/get-reply-run.media-only.test.ts
+++ b/src/auto-reply/reply/get-reply-run.media-only.test.ts
@@ -371,6 +371,15 @@ describe("runPreparedReply media-only handling", () => {
     expect(directContextParams?.sessionCtx?.Provider).toBe("telegram");
     expect(directContextParams?.sessionCtx?.ChatType).toBe("direct");
     expect(directContextParams?.sourceReplyDeliveryMode).toBe("message_tool_only");
+    expect(buildInboundUserContextPrefix).toHaveBeenCalledWith(
+      expect.objectContaining({
+        ChatType: "direct",
+        OriginatingChannel: "telegram",
+        OriginatingTo: "telegram-direct-test-id",
+      }),
+      expect.anything(),
+      { sourceReplyDeliveryMode: "message_tool_only" },
+    );
   });
 
   it.each(["direct", "dm"] as const)(
diff --git a/src/auto-reply/reply/get-reply-run.ts b/src/auto-reply/reply/get-reply-run.ts
index 49b88005b61c..8732ea830293 100644
--- a/src/auto-reply/reply/get-reply-run.ts
+++ b/src/auto-reply/reply/get-reply-run.ts
@@ -618,6 +618,7 @@ export async function runPreparedReply(
         }
       : { ...sessionCtx, ThreadStarterBody: undefined },
     envelopeOptions,
+    { sourceReplyDeliveryMode: opts?.sourceReplyDeliveryMode },
   );
   const inboundUserContextPromptJoiner = resolveInboundUserContextPromptJoiner(sessionCtx);
   const hasUserBody =
diff --git a/src/auto-reply/reply/inbound-meta.test.ts b/src/auto-reply/reply/inbound-meta.test.ts
index 0f00e60a5127..a50daf11a4f1 100644
--- a/src/auto-reply/reply/inbound-meta.test.ts
+++ b/src/auto-reply/reply/inbound-meta.test.ts
@@ -287,6 +287,40 @@ describe("buildInboundUserContextPrefix", () => {
     expect(conversationInfo["conversation_label"]).toBeUndefined();
   });
 
+  it("adds delivery guidance beside inbound source context for message-tool-only turns", () => {
+    const text = buildInboundUserContextPrefix(
+      {
+        ChatType: "direct",
+        OriginatingChannel: "telegram",
+        OriginatingTo: "telegram:849985193",
+        MessageSid: "776",
+        SenderName: "Nik",
+      } as TemplateContext,
+      undefined,
+      { sourceReplyDeliveryMode: "message_tool_only" },
+    );
+
+    expect(text).toContain("Delivery: to send a message, use the `message` tool.");
+    expect(text.indexOf("Delivery:")).toBeLessThan(text.indexOf("Conversation info"));
+    expect(text).toContain("Conversation info (untrusted metadata):");
+  });
+
+  it("does not add delivery guidance for automatic source delivery", () => {
+    const text = buildInboundUserContextPrefix(
+      {
+        ChatType: "direct",
+        OriginatingChannel: "telegram",
+        OriginatingTo: "telegram:849985193",
+        MessageSid: "776",
+      } as TemplateContext,
+      undefined,
+      { sourceReplyDeliveryMode: "automatic" },
+    );
+
+    expect(text).not.toContain("Delivery: to send a message");
+    expect(text).toContain("Conversation info (untrusted metadata):");
+  });
+
   it("includes message identifiers for direct chats when channel is inferred from Provider", () => {
     const text = buildInboundUserContextPrefix({
       ChatType: "direct",
diff --git a/src/auto-reply/reply/inbound-meta.ts b/src/auto-reply/reply/inbound-meta.ts
index 2b63c5b28411..4eda19c10f71 100644
--- a/src/auto-reply/reply/inbound-meta.ts
+++ b/src/auto-reply/reply/inbound-meta.ts
@@ -7,11 +7,17 @@ import { normalizeOptionalString } from "../../shared/string-coerce.js";
 import { truncateUtf16Safe } from "../../utils.js";
 import type { EnvelopeFormatOptions } from "../envelope.js";
 import { formatEnvelopeTimestamp } from "../envelope.js";
+import type { SourceReplyDeliveryMode } from "../get-reply-options.types.js";
 import type { TemplateContext } from "../templating.js";
 
 const MAX_UNTRUSTED_JSON_STRING_CHARS = 2_000;
 const MAX_UNTRUSTED_HISTORY_ENTRIES = 20;
 const MAX_UNTRUSTED_TRANSCRIPT_FIELD_CHARS = 500;
+const MESSAGE_TOOL_DELIVERY_HINT = "Delivery: to send a message, use the `message` tool.";
+
+type InboundUserContextPrefixOptions = {
+  sourceReplyDeliveryMode?: SourceReplyDeliveryMode;
+};
 
 function stripNullBytes(value: string): string {
   return value.replaceAll("\u0000", "");
@@ -411,8 +417,12 @@ export function buildInboundMetaSystemPrompt(
 export function buildInboundUserContextPrefix(
   ctx: TemplateContext,
   envelope?: EnvelopeFormatOptions,
+  options?: InboundUserContextPrefixOptions,
 ): string {
   const blocks: string[] = [];
+  if (options?.sourceReplyDeliveryMode === "message_tool_only") {
+    blocks.push(MESSAGE_TOOL_DELIVERY_HINT);
+  }
   const chatType = normalizeChatType(ctx.ChatType);
   const isDirect = !chatType || chatType === "direct";
   const directChannelValue = resolveInboundChannel(ctx);