diff --git a/agent/tools/save_plan.py b/agent/tools/save_plan.py
index ac7f4cbb..3f97061f 100644
--- a/agent/tools/save_plan.py
+++ b/agent/tools/save_plan.py
@@ -43,7 +43,10 @@ async def save_plan(
     shared content.
 
     Write the content in standard Markdown — headings, bullet/numbered lists, and
-    fenced code blocks all render.
+    fenced code blocks all render. Shared responses persist Markdown text only;
+    they do not upload or serve sandbox-local or relative image files. If images
+    or screenshots are needed for a Slack response, post them directly in Slack
+    instead of relying on the shared response page.
 
     Args:
         plan_file_path: Path to the Markdown plan file in the sandbox.
diff --git a/tests/agent/test_plan_review.py b/tests/agent/test_plan_review.py
index d6c321d4..7cc49219 100644
--- a/tests/agent/test_plan_review.py
+++ b/tests/agent/test_plan_review.py
@@ -214,6 +214,14 @@ def test_save_plan_exported_and_wired() -> None:
     assert callable(save_plan)
 
 
+def test_save_plan_description_warns_about_slack_images() -> None:
+    from agent.tools import save_plan
+
+    description = save_plan.__doc__ or ""
+    assert "persist Markdown text only" in description
+    assert "post them directly in Slack" in description
+
+
 def test_plan_status_constants() -> None:
     from agent.dashboard import plan_store