diff --git a/core/tools/implementations/createNewFile.ts b/core/tools/implementations/createNewFile.ts
index b6c626d7380..f08c56564d1 100644
--- a/core/tools/implementations/createNewFile.ts
+++ b/core/tools/implementations/createNewFile.ts
@@ -3,12 +3,15 @@ import { inferResolvedUriFromRelativePath } from "../../util/ideUtils";
 import { ToolImpl } from ".";
 import { throwIfFileIsSecurityConcern } from "../../indexing/ignore";
 import { getCleanUriPath, getUriPathBasename } from "../../util/uri";
-import { getStringArg } from "../parseArgs";
+import { getStringArg, stringifyFileContents } from "../parseArgs";
 import { ContinueError, ContinueErrorReason } from "../../util/errors";
 
 export const createNewFileImpl: ToolImpl = async (args, extras) => {
   const filepath = getStringArg(args, "filepath");
-  const contents = getStringArg(args, "contents", true);
+  const contents =
+    typeof args?.contents === "object" && args.contents !== null
+      ? stringifyFileContents(args.contents)
+      : getStringArg(args, "contents", true);
 
   const resolvedFileUri = await inferResolvedUriFromRelativePath(
     filepath,
diff --git a/core/tools/implementations/createNewFile.vitest.ts b/core/tools/implementations/createNewFile.vitest.ts
new file mode 100644
index 00000000000..a148d1c5c21
--- /dev/null
+++ b/core/tools/implementations/createNewFile.vitest.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("../../util/ideUtils", () => ({
+  inferResolvedUriFromRelativePath: async (path: string) =>
+    `file:///workspace/${path}`,
+}));
+
+import { createNewFileImpl } from "./createNewFile";
+
+function fakeExtras() {
+  const written: Record<string, string> = {};
+  const ide = {
+    fileExists: async () => false,
+    writeFile: async (uri: string, contents: string) => {
+      written[uri] = contents;
+    },
+    openFile: async () => {},
+    saveFile: async () => {},
+  };
+  return { written, extras: { ide } as any };
+}
+
+describe("createNewFileImpl", () => {
+  it("writes a JSON value sent as contents as indented JSON", async () => {
+    const { written, extras } = fakeExtras();
+    await createNewFileImpl(
+      {
+        filepath: "app/package.json",
+        contents: { name: "app", private: true },
+      },
+      extras,
+    );
+    expect(written["file:///workspace/app/package.json"]).toBe(
+      '{\n  "name": "app",\n  "private": true\n}',
+    );
+  });
+
+  it("writes string contents unchanged", async () => {
+    const { written, extras } = fakeExtras();
+    await createNewFileImpl(
+      { filepath: "src/a.ts", contents: "export const a = 1;\n" },
+      extras,
+    );
+    expect(written["file:///workspace/src/a.ts"]).toBe("export const a = 1;\n");
+  });
+});
diff --git a/core/tools/parseArgs.ts b/core/tools/parseArgs.ts
index 64a4f67d09a..673ea1dd099 100644
--- a/core/tools/parseArgs.ts
+++ b/core/tools/parseArgs.ts
@@ -62,6 +62,26 @@ export function coerceArgsToSchema(
   return coerced;
 }
 
+/**
+ * The contents of a file a tool is about to write, as text.
+ * Some models send the contents of a JSON file as a JSON value rather than a string,
+ * so the parsed arguments hold an object or array. Write it as indented JSON, the way
+ * the file would normally look, rather than on one line.
+ */
+export function stringifyFileContents(value: unknown): string {
+  if (typeof value === "string") {
+    return value;
+  }
+  if (value === null || value === undefined) {
+    return "";
+  }
+  try {
+    return JSON.stringify(value, null, 2) ?? String(value);
+  } catch {
+    return String(value);
+  }
+}
+
 export function getStringArg(
   args: any,
   argName: string,
diff --git a/core/tools/parseArgs.vitest.ts b/core/tools/parseArgs.vitest.ts
index e21254b686d..441d0d49aa2 100644
--- a/core/tools/parseArgs.vitest.ts
+++ b/core/tools/parseArgs.vitest.ts
@@ -7,6 +7,7 @@ import {
   getOptionalStringArg,
   getStringArg,
   safeParseToolCallArgs,
+  stringifyFileContents,
 } from "./parseArgs";
 
 describe("safeParseToolCallArgs", () => {
@@ -191,6 +192,20 @@ describe("getStringArg", () => {
   });
 });
 
+describe("stringifyFileContents", () => {
+  it("should write a JSON value as indented JSON", () => {
+    expect(stringifyFileContents({ name: "app", deps: ["a"] })).toBe(
+      '{\n  "name": "app",\n  "deps": [\n    "a"\n  ]\n}',
+    );
+  });
+
+  it("should keep a string unchanged and turn a missing value into empty text", () => {
+    expect(stringifyFileContents("const x = 1;\n")).toBe("const x = 1;\n");
+    expect(stringifyFileContents(undefined)).toBe("");
+    expect(stringifyFileContents(null)).toBe("");
+  });
+});
+
 describe("getOptionalStringArg", () => {
   it("should return undefined when argument is not provided", () => {
     const args = { otherArg: "test" };
diff --git a/gui/src/pages/gui/ToolCallDiv/CreateFile.test.tsx b/gui/src/pages/gui/ToolCallDiv/CreateFile.test.tsx
new file mode 100644
index 00000000000..5b4dd71604b
--- /dev/null
+++ b/gui/src/pages/gui/ToolCallDiv/CreateFile.test.tsx
@@ -0,0 +1,56 @@
+import { render, screen } from "@testing-library/react";
+import { ToolCallState } from "core";
+import { BuiltInToolNames } from "core/tools/builtIn";
+import { describe, expect, it, vi } from "vitest";
+import FunctionSpecificToolCallDiv from "./FunctionSpecificToolCallDiv";
+
+// StyledMarkdownPreview needs Redux; show the Markdown source it is given instead
+vi.mock("../../../components/StyledMarkdownPreview", () => ({
+  default: ({ source }: { source: string }) => (
+    <pre data-testid="preview">{source}</pre>
+  ),
+}));
+
+function createFileCall(contents: unknown): ToolCallState {
+  const args = { filepath: "app/package.json", contents };
+  return {
+    toolCallId: "call-1",
+    toolCall: {
+      id: "call-1",
+      type: "function",
+      function: {
+        name: BuiltInToolNames.CreateNewFile,
+        arguments: JSON.stringify(args),
+      },
+    },
+    status: "generated",
+    // The arguments are parsed deeply, so the contents of a .json file arrive as an object
+    parsedArgs: args,
+  };
+}
+
+describe("CreateFile", () => {
+  it("shows the contents of a JSON file instead of [object Object]", () => {
+    render(
+      <FunctionSpecificToolCallDiv
+        toolCallState={createFileCall({ name: "app", version: "1.0.0" })}
+        historyIndex={0}
+      />,
+    );
+    const source = screen.getByTestId("preview").textContent ?? "";
+    expect(source).not.toContain("[object Object]");
+    expect(source).toContain('{\n  "name": "app",\n  "version": "1.0.0"\n}');
+  });
+
+  it("shows string contents unchanged", () => {
+    render(
+      <FunctionSpecificToolCallDiv
+        toolCallState={createFileCall("export const x = 1;")}
+        historyIndex={0}
+      />,
+    );
+    expect(screen.getByTestId("preview").textContent).toContain(
+      "export const x = 1;",
+    );
+  });
+});
diff --git a/gui/src/pages/gui/ToolCallDiv/FunctionSpecificToolCallDiv.tsx b/gui/src/pages/gui/ToolCallDiv/FunctionSpecificToolCallDiv.tsx
index ed00060a4ad..83f1a5ea9b8 100644
--- a/gui/src/pages/gui/ToolCallDiv/FunctionSpecificToolCallDiv.tsx
+++ b/gui/src/pages/gui/ToolCallDiv/FunctionSpecificToolCallDiv.tsx
@@ -1,6 +1,7 @@
 import { ToolCallState } from "core";
 import { BuiltInToolNames } from "core/tools/builtIn";
 import { EditOperation } from "core/tools/definitions/multiEdit";
+import { stringifyFileContents } from "core/tools/parseArgs";
 import { CreateFile } from "./CreateFile";
 import { EditFile } from "./EditFile";
 import { FindAndReplaceDisplay } from "./FindAndReplace";
@@ -22,7 +23,7 @@ function FunctionSpecificToolCallDiv({
       return (
         <CreateFile
           relativeFilepath={args?.filepath ?? ""}
-          fileContents={args?.contents ?? ""}
+          fileContents={stringifyFileContents(args?.contents)}
           historyIndex={historyIndex}
         />
       );