diff --git a/agent/prompt_builder.py b/agent/prompt_builder.py
index 4ae71cbb3a37..0c997ff42843 100644
--- a/agent/prompt_builder.py
+++ b/agent/prompt_builder.py
@@ -884,7 +884,13 @@ PLATFORM_HINTS = {
         "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."
+        "brief and natural. "
+        "File/media delivery: do NOT emit MEDIA:/path tags in your response — "
+        "that convention is only intercepted on messaging platforms (Telegram, "
+        "WebUI, etc.); here it renders as literal, unusable text exposing a raw "
+        "host filesystem path to the caller. If a registered file-delivery tool "
+        "is available in your toolset, use it; otherwise there is no channel to "
+        "deliver a file on this platform."
     ),
     "webui": (
         "You are in the Hermes WebUI, a browser-based chat interface. "
diff --git a/tests/agent/test_prompt_builder.py b/tests/agent/test_prompt_builder.py
index 633fde7c02d9..e1ef1dd170d6 100644
--- a/tests/agent/test_prompt_builder.py
+++ b/tests/agent/test_prompt_builder.py
@@ -1103,6 +1103,19 @@ class TestPromptBuilderConstants:
         hint = PLATFORM_HINTS["whatsapp_cloud"]
         assert "MEDIA:" in hint
 
+    def test_api_server_hint_forbids_media_tag(self):
+        """api_server has no MEDIA: interception (unlike telegram/webui/etc.,
+        which resolve it to an attachment or a validated/inlined data URL) —
+        _handle_runs never calls the resolver other api_server endpoints use.
+        Without an explicit prohibition, the model's general cross-platform
+        habit of using MEDIA:/path for file delivery (correct and taught for
+        several other platforms in this same dict) can surface here too, with
+        no interception to catch it: the tag renders as literal text in the
+        response, exposing a raw host filesystem path to the API caller."""
+        hint = PLATFORM_HINTS["api_server"]
+        assert "MEDIA:" in hint
+        assert "not" in hint.lower()
+
     def test_markdown_converting_platform_hints_do_not_forbid_markdown(self):
         """#12224 — WhatsApp (Baileys) and Signal adapters actively convert
         markdown to native formatting (gateway/platforms/whatsapp_common.py