diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index 69917ec6a8a..e4448bacd25 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -415,6 +415,13 @@ def _is_method_not_found_error(exc: BaseException) -> bool:
     an empty result. Structurally inspect ``McpError.error.code`` first, then
     fall back to a substring match so detection survives SDK version drift and
     servers that surface the condition as a plain message.
+
+    The substring fallback matters when a server reports method-not-found
+    without a structural ``-32601`` code (e.g. surfaced as a plain exception
+    string). Besides the canonical "method not found", many JSON-RPC
+    implementations phrase it as "Unknown method: <name>" — agentmemory's MCP
+    server is one such case (#50028). Without matching that phrasing the
+    ping→list_tools fallback never latches and the keepalive reconnect-loops.
     """
     # Structural: mcp.shared.exceptions.McpError carries ErrorData.code.
     err = getattr(exc, "error", None)
@@ -427,6 +434,7 @@ def _is_method_not_found_error(exc: BaseException) -> bool:
     return (
         str(_JSONRPC_METHOD_NOT_FOUND) in msg
         or "method not found" in msg
+        or "unknown method" in msg
         or "not found: ping" in msg
     )