diff --git a/agents/myra/src/system-prompt.ts b/agents/myra/src/system-prompt.ts
index c142ae07b..43b5296f4 100644
--- a/agents/myra/src/system-prompt.ts
+++ b/agents/myra/src/system-prompt.ts
@@ -41,4 +41,4 @@ export const ASSISTANT_SYSTEM_PROMPT =
   "\n" +
   "When you hand a task to another participant, copy the person's " +
   'address from that same "Participants:" block in `to` so they can ' +
-  "follow the conversation.";
+  "follow the conversation, and give every mail you send a short subject.";
diff --git a/apps/web/src/chat/room-roster.test.ts b/apps/web/src/chat/room-roster.test.ts
index c396f90cf..f58b2f0c4 100644
--- a/apps/web/src/chat/room-roster.test.ts
+++ b/apps/web/src/chat/room-roster.test.ts
@@ -29,7 +29,7 @@ describe("appendRoster / stripRoster", () => {
     expect(withRoster).toBe(
       "Please pass this to the scribe.\n\n" +
         "Participants:\nSawyer <usr_sawyer@room.example>\nScribe <run_abc@room.example>\n\n" +
-        "Copy usr_sawyer@room.example in `to` on any mail you send another participant, so they can follow along.",
+        "Copy usr_sawyer@room.example in `to` on any mail you send another participant, so they can follow along, and give every mail a short subject.",
     );
     expect(stripRoster(withRoster)).toBe(body);
   });
@@ -41,7 +41,7 @@ describe("appendRoster / stripRoster", () => {
       { name: "Scribe", address: "run_abc@room.example", kind: "agent" },
     ]);
     expect(withRoster).toContain(
-      "Copy usr_sawyer@room.example, usr_alex@room.example in `to` on any mail you send another participant, so they can follow along.",
+      "Copy usr_sawyer@room.example, usr_alex@room.example in `to` on any mail you send another participant, so they can follow along, and give every mail a short subject.",
     );
   });
 });
diff --git a/apps/web/src/chat/room-roster.ts b/apps/web/src/chat/room-roster.ts
index 04d1cbc69..fbd0c7081 100644
--- a/apps/web/src/chat/room-roster.ts
+++ b/apps/web/src/chat/room-roster.ts
@@ -31,7 +31,9 @@ function ccInstruction(entries: readonly RosterEntry[]): string | undefined {
   const people = entries.filter((entry) => entry.kind === "person");
   if (people.length === 0) return undefined;
   const addresses = people.map((entry) => entry.address).join(", ");
-  return `Copy ${addresses} in \`to\` on any mail you send another participant, so they can follow along.`;
+  // The subject clause is load-bearing: a mail with an empty Subject ends
+  // the receiving agent's run today.
+  return `Copy ${addresses} in \`to\` on any mail you send another participant, so they can follow along, and give every mail a short subject.`;
 }
 
 /** Appends a `Participants:` block listing every entry's name and address,