Use ellmer:::tool_string() to format tool results

NEWS.md

@@ -1,5 +1,6 @@
 # mcptools (development version)
 
+* `mcp_server()` now formats tool results in the same way as ellmer (#78 by @gadenbuie).
 * `mcp_tools()` now sends and receives a `"notifications/initialized"` (#77 by @galachad).
 * `mcp_session()` now returns invisibly the nanonext socket used for communicating with the server.
 * `mcp_server()` gains logical argument `session_tools`, allowing users to opt-out of presenting R sessions tools to clients.

R/session.R

@@ -55,18 +55,20 @@ handle_message_from_server <- function(data) {
 
 as_tool_call_result <- function(data, result) {
   is_error <- FALSE
+  format_result <- function(x) paste(x, collapse = "\n")
+
   if (inherits(result, "ellmer::ContentToolResult")) {
     is_error <- !is.null(result@error)
-    result <- result@value %||% result@error
+    format_result <- asNamespace("ellmer")[["tool_string"]] %||% format_result
   }
-
+  
   jsonrpc_response(
     data$id,
     list(
       content = list(
         list(
           type = "text",
-          text = paste(result, collapse = "\n")
+          text = format_result(result)
         )
       ),
       isError = is_error

tests/testthat/test-session.R

@@ -41,7 +41,7 @@ test_that("as_tool_call_result handles ContentToolResult with error", {
 
   output <- as_tool_call_result(data, tool_result)
 
-  expect_equal(output$result$content[[1]]$text, "error message")
+  expect_match(output$result$content[[1]]$text, "error message")
   expect_true(output$result$isError)
 })