diff --git a/agent/conversation_loop.py b/agent/conversation_loop.py
index 9efa25ad0a..95c061b3bb 100644
--- a/agent/conversation_loop.py
+++ b/agent/conversation_loop.py
@@ -790,7 +790,11 @@ def _restore_or_build_system_prompt(agent, system_message, conversation_history)
         # fails closed to "reuse" so a probe failure can't burn cache.
         _bot_stale = False
         try:
-            from tools.bot_mode_probe import stored_prompt_capability_stale
+            from tools.bot_mode_probe import (
+                BOT_CHAT_TITLE,
+                stored_bot_chat_prompt_needs_upgrade,
+                stored_prompt_capability_stale,
+            )
 
             _home_for_epoch = None
             try:
@@ -800,6 +804,21 @@ def _restore_or_build_system_prompt(agent, system_message, conversation_history)
             except Exception:
                 pass
             _bot_stale = stored_prompt_capability_stale(stored_prompt, _home_for_epoch)
+            if not _bot_stale and getattr(agent, "_bot_mode_protocol", True):
+                # Legacy upgrade: a Bot Chat whose prompt predates the epoch
+                # mechanism (no stamp, no protocol) gets ONE migration
+                # rebuild — otherwise pre-existing bots would never learn
+                # the messaging protocol. Title-gated so ordinary unstamped
+                # sessions (i.e. all of them) never take this path; the
+                # rebuilt prompt carries the stamp, so it cannot re-fire.
+                _t = str(getattr(agent, "_session_title_hint", "") or "").strip()
+                if not _t and agent._session_db and agent.session_id:
+                    try:
+                        _t = str(agent._session_db.get_session_title(agent.session_id) or "").strip()
+                    except Exception:
+                        _t = ""
+                if _t == BOT_CHAT_TITLE:
+                    _bot_stale = stored_bot_chat_prompt_needs_upgrade(stored_prompt, _home_for_epoch)
         except Exception:
             _bot_stale = False
         if _bot_stale:
diff --git a/tests/tools/test_bot_mode_probe.py b/tests/tools/test_bot_mode_probe.py
index abc2d5d672..a1e9177328 100644
--- a/tests/tools/test_bot_mode_probe.py
+++ b/tests/tools/test_bot_mode_probe.py
@@ -169,3 +169,34 @@ def test_stored_prompt_staleness(tmp_path):
     # prompts without a stamp (every non-Bot-Chat session) are never stale
     assert not bot_mode_probe.stored_prompt_capability_stale("ordinary prompt", home)
     assert not bot_mode_probe.stored_prompt_capability_stale("", home)
+
+
+def test_legacy_bot_chat_upgrade(tmp_path):
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    _make_bot_profile(home, "researcher", managed=True)
+
+    legacy = "old prompt with no protocol and no stamp"
+    # legacy Bot Chat on a managed install → upgrade once
+    assert bot_mode_probe.stored_bot_chat_prompt_needs_upgrade(legacy, home)
+
+    # a rebuilt prompt (stamped) never re-fires
+    upgraded = legacy + "\n\n" + bot_mode_probe.get_bot_mode_protocol_section(home) + "\n\n" + bot_mode_probe.epoch_line(home)
+    assert not bot_mode_probe.stored_bot_chat_prompt_needs_upgrade(upgraded, home)
+
+    # SOUL already carries the legacy plugin-side append → probe silent →
+    # no upgrade (rebuilding would loop: the new prompt would be unstamped too)
+    bot_mode_probe._reset_cache_for_tests()
+    (home / "SOUL.md").write_text("# Me\n\n## Messaging other agents\nlegacy\n", encoding="utf-8")
+    assert not bot_mode_probe.stored_bot_chat_prompt_needs_upgrade(legacy, home)
+
+    # prompt whose SOUL section rode into it → protocol heading present → no upgrade
+    assert not bot_mode_probe.stored_bot_chat_prompt_needs_upgrade(
+        "prompt containing\n## Messaging other agents\nfrom SOUL", home
+    )
+
+    # unmanaged install → probe silent → never upgrades
+    bot_mode_probe._reset_cache_for_tests()
+    home2 = tmp_path / ".hermes2"
+    home2.mkdir()
+    assert not bot_mode_probe.stored_bot_chat_prompt_needs_upgrade(legacy, home2)
diff --git a/tools/bot_mode_probe.py b/tools/bot_mode_probe.py
index 3e7a70988c..7f59b986fe 100644
--- a/tools/bot_mode_probe.py
+++ b/tools/bot_mode_probe.py
@@ -260,6 +260,33 @@ def stored_prompt_capability_stale(stored_prompt: str, home: str | os.PathLike |
         return False
 
 
+def stored_bot_chat_prompt_needs_upgrade(stored_prompt: str, home: str | os.PathLike | None = None) -> bool:
+    """True when a Bot Chat session's stored prompt PREDATES this feature.
+
+    Legacy Bot Chats (created before bundling / this epoch mechanism)
+    persisted prompts with no protocol section and no epoch stamp; without
+    an explicit upgrade they would be stranded forever — the staleness check
+    above only fires on stamped prompts. This is a one-time migration per
+    legacy session: the caller must only invoke it for sessions titled
+    "Bot Chat", and only rebuilds when the probe would actually emit a
+    section (a profile whose SOUL.md already carries the legacy plugin-side
+    append keeps its protocol-free prompt — rebuilding those would loop,
+    since the probe stays silent and the rebuilt prompt would be unstamped
+    again). Fails closed to "no upgrade".
+    """
+    try:
+        if _EPOCH_PREFIX in (stored_prompt or ""):
+            return False
+        if _PROTOCOL_HEADING in (stored_prompt or ""):
+            return False
+        # Only upgrade when the rebuild would actually add the section —
+        # this is what guarantees the rebuilt prompt carries a stamp and
+        # the upgrade can never re-fire.
+        return bool(get_bot_mode_protocol_section(home))
+    except Exception:
+        return False
+
+
 def _reset_cache_for_tests() -> None:
     with _lock:
         _cached.clear()