fix: handle JSON contents of create_new_file

Handle case where JSON content was parsed into an object by the tool call parser.  If the arguments to the tool call are valid JSON (e.g. the model attempts to create a .json file) the earlier call to JSON.parse() will have deeply parsed the returned contents. If that has happened, convert back to string.

Fixes: #8972

Author: mcowger

core/tools/parseArgs.ts

@@ -18,15 +18,41 @@ export function getStringArg(
   argName: string,
   allowEmpty = false,
 ): string {
-  if (!args || !(argName in args) || typeof args[argName] !== "string") {
+  if (!args || !(argName in args)) {
     throw new Error(
       `\`${argName}\` argument is required${allowEmpty ? "" : " and must not be empty or whitespace-only"}. (type string)`,
     );
   }
-  if (!allowEmpty && !args[argName].trim()) {
+
+  let value = args[argName];
+
+  // Handle case where JSON content was parsed into an object by the tool call parser.
+  // If the arguments to the tool call are valid JSON (e.g. the model attempts to create a .json file)
+  // the earlier call to JSON.parse() will have deeply parsed the returned contents.
+  // If that has happened, convert back to string.
+  if (typeof value === "object" && value !== null) {
+    // Special handling for contents parameter which should always be a string
+    if (argName === "contents") {
+      value = JSON.stringify(value);
+      return value;
+    } else {
+      throw new Error(
+        `Argument \`${argName}\` must be a string, not an object`,
+      );
+    }
+  }
+
+  if (!allowEmpty && !value.trim()) {
     throw new Error(`Argument ${argName} must not be empty or whitespace-only`);
   }
-  return args[argName];
+
+  if (typeof value !== "string") {
+    throw new Error(
+      `\`${argName}\` argument is required${allowEmpty ? "" : " and must not be empty or whitespace-only"}. (type string)`,
+    );
+  }
+
+  return value;
 }
 
 export function getOptionalStringArg(