diff --git a/agent/prompt_builder.py b/agent/prompt_builder.py
index 8494a70eef..2f00020cc1 100644
--- a/agent/prompt_builder.py
+++ b/agent/prompt_builder.py
@@ -513,6 +513,12 @@ PLATFORM_HINTS = {
         "image and is the WRONG path. Bare Unicode emoji in text is also not a substitute "
         "— when a sticker is the right response, use yb_send_sticker."
     ),
+    "api_server": (
+        "You're responding through an API server. The rendering layer is unknown — "
+        "assume plain text. No markdown formatting (no asterisks, bullets, headers, "
+        "code fences). Treat this like a conversation, not a document. Keep responses "
+        "brief and natural."
+    ),
 }
 
 # ---------------------------------------------------------------------------
diff --git a/tests/agent/test_prompt_builder.py b/tests/agent/test_prompt_builder.py
index 88de5186b8..d99e6944ff 100644
--- a/tests/agent/test_prompt_builder.py
+++ b/tests/agent/test_prompt_builder.py
@@ -788,6 +788,7 @@ class TestPromptBuilderConstants:
         assert "discord" in PLATFORM_HINTS
         assert "cron" in PLATFORM_HINTS
         assert "cli" in PLATFORM_HINTS
+        assert "api_server" in PLATFORM_HINTS
 
     def test_cli_hint_does_not_suggest_media_tags(self):
         # Regression: MEDIA:/path tags are intercepted only by messaging