diff --git a/libs/langchain_v1/langchain/agents/middleware/summarization.py b/libs/langchain_v1/langchain/agents/middleware/summarization.py
index 81c47f48622..9d2e4ffac28 100644
--- a/libs/langchain_v1/langchain/agents/middleware/summarization.py
+++ b/libs/langchain_v1/langchain/agents/middleware/summarization.py
@@ -76,6 +76,17 @@ Respond ONLY with the extracted context. Do not include any additional informati
 Messages to summarize:
 {messages}
 </messages>"""  # noqa: E501
+"""Default prompt used to summarize conversation history.
+
+The `<messages>` marker (on its own line) and the `{messages}` placeholder are
+part of this constant's public contract, not just cosmetic formatting.
+Downstream consumers depend on them: for example, deep agents'
+`SummarizationMiddleware` splices an extra instruction block in immediately
+before the `<messages>` marker via `str.replace`. Removing, renaming, or
+reformatting the marker (or the `{messages}` placeholder) is a breaking change
+for those consumers even though it does not alter any function signature, so
+treat edits to it accordingly.
+"""
 
 _DEFAULT_MESSAGES_TO_KEEP = 20
 _DEFAULT_TRIM_TOKEN_LIMIT = 4000