diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 3dd4ca5e2d..a87e5ce29b 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -1266,6 +1266,12 @@ func checkSystemInstructionsWithMode(payload []byte, strictMode bool) []byte {
 			}
 			return true
 		})
+	} else if system.Type == gjson.String && system.String() != "" {
+		// The Anthropic API allows "system" to be a plain string as well as an array
+		// of content blocks. Convert the string into a content block so it is preserved.
+partJSON := `{"type":"text","cache_control":{"type":"ephemeral"}}`
+partJSON, _ = sjson.Set(partJSON, "text", system.String())
+		result += "," + partJSON
 	}
 	result += "]"