diff --git a/R/chat.R b/R/chat.R
index 10ab5d9..1e531b3 100644
--- a/R/chat.R
+++ b/R/chat.R
@@ -251,6 +251,12 @@ Chat <- R6::R6Class("Chat",
     #' @description Register a tool (an R function) that the chatbot can use.
     #'   If the chatbot decides to use the function,  elmer will automatically
     #'   call it and submit the results back.
+    #' 
+    #'   The return value of the function. Generally, this should either be a
+    #'   string, or a JSON-serializable value. If you must have more direct
+    #'   control of the structure of the JSON that's returned, you can return a
+    #'   JSON-serializable value wrapped in [base::I()], which elmer will leave
+    #'   alone until the entire request is JSON-serialized.
     #' @param tool_def Tool definition created by [tool()].
     register_tool = function(tool_def) {
       if (!S7_inherits(tool_def, ToolDef)) {
diff --git a/R/content.R b/R/content.R
index bd83092..0e180bb 100644
--- a/R/content.R
+++ b/R/content.R
@@ -215,8 +215,14 @@ tool_errored <- function(x) !is.null(x@error)
 tool_string <- function(x) {
   if (tool_errored(x)) {
     paste0("Tool calling failed with error ", x@error)
+  } else if (inherits(x@value, "AsIs")) {
+    x@value
+  } else if (inherits(x@value, "json")) {
+    x@value
+  } else if (is.character(x@value)) {
+    paste(x@value, collapse = "\n")
   } else {
-    toString(x@value)
+    jsonlite::toJSON(x@value, auto_unbox = TRUE)
   }
 }