fix: handle JSON contents of create_new_file

core/tools/parseArgs.ts

@@ -18,15 +18,38 @@ 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 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 arguments.
+  // If that has happened, convert back to string.
+  if (typeof value === "object" && value !== null) {
+    try {
+      value = JSON.stringify(value);
+      return value;
+    } catch (e) {
+      //Swallow this, because it might be fine later.
+    }
+  }
+
+  if (typeof value !== "string") {
+    throw new Error(
+      `\`${argName}\` argument is required${allowEmpty ? "" : " and must not be empty or whitespace-only"}. (type string)`,
+    );
+  }
+
+  if (!allowEmpty && !value.trim()) {
     throw new Error(`Argument ${argName} must not be empty or whitespace-only`);
   }
-  return args[argName];
+
+  return value;
 }
 
 export function getOptionalStringArg(

core/tools/parseArgs.vitest.ts

@@ -144,6 +144,50 @@ describe("getStringArg", () => {
       "`name` argument is required and must not be empty or whitespace-only. (type string)",
     );
   });
+
+  it("should convert parsed JSON object to string for contents parameter", () => {
+    // This simulates the case where JSON.parse() has converted a JSON string into an object
+    const args = { contents: { key: "value", number: 123 } };
+    const result = getStringArg(args, "contents");
+    expect(result).toBe('{"key":"value","number":123}');
+  });
+
+  it("should convert nested JSON object to string for contents parameter", () => {
+    const args = {
+      contents: {
+        user: {
+          name: "John",
+          details: {
+            age: 30,
+            preferences: ["coding", "reading"],
+          },
+        },
+      },
+    };
+    const result = getStringArg(args, "contents");
+    const expected =
+      '{"user":{"name":"John","details":{"age":30,"preferences":["coding","reading"]}}}';
+    expect(result).toBe(expected);
+  });
+
+  it("should convert JSON array to string for contents parameter", () => {
+    const args = { contents: ["item1", "item2", { key: "value" }] };
+    const result = getStringArg(args, "contents");
+    expect(result).toBe('["item1","item2",{"key":"value"}]');
+  });
+
+  it("should handle contents parameter that is already a string", () => {
+    const args = { contents: "already a string" };
+    const result = getStringArg(args, "contents");
+    expect(result).toBe("already a string");
+  });
+
+  it("should handle contents parameter that is null", () => {
+    const args = { contents: null };
+    expect(() => getStringArg(args, "contents")).toThrowError(
+      "`contents` argument is required and must not be empty or whitespace-only. (type string)",
+    );
+  });
 });
 
 describe("getOptionalStringArg", () => {