diff --git a/.changeset/write-files-string-content.md b/.changeset/write-files-string-content.md
new file mode 100644
index 00000000..4afef09e
--- /dev/null
+++ b/.changeset/write-files-string-content.md
@@ -0,0 +1,5 @@
+---
+"@vercel/sandbox": patch
+---
+
+Accept `string` and `Uint8Array` in `writeFiles()` content, not just `Buffer`.
diff --git a/packages/vercel-sandbox/src/api-client/api-client.ts b/packages/vercel-sandbox/src/api-client/api-client.ts
index e87c392b..1f226a3e 100644
--- a/packages/vercel-sandbox/src/api-client/api-client.ts
+++ b/packages/vercel-sandbox/src/api-client/api-client.ts
@@ -460,7 +460,11 @@ export class APIClient extends BaseClient {
   async writeFiles(params: {
     sandboxId: string;
     cwd: string;
-    files: { path: string; content: Buffer; mode?: number }[];
+    files: {
+      path: string;
+      content: string | Uint8Array;
+      mode?: number;
+    }[];
     extractDir: string;
     signal?: AbortSignal;
   }) {
diff --git a/packages/vercel-sandbox/src/api-client/file-writer.test.ts b/packages/vercel-sandbox/src/api-client/file-writer.test.ts
index f293d117..facefbe7 100644
--- a/packages/vercel-sandbox/src/api-client/file-writer.test.ts
+++ b/packages/vercel-sandbox/src/api-client/file-writer.test.ts
@@ -29,9 +29,10 @@ describe("FileWriter", () => {
       name: "hello.txt",
       content: Buffer.from("Hello world"),
     });
-    writer.end();
+    const end = writer.end();
 
     const files = await extractFiles(writer.readable);
+    await end;
     expect(files.get("hello.txt")?.toString()).toBe("Hello world");
   });
 
@@ -42,9 +43,10 @@ describe("FileWriter", () => {
       name: "utf8.txt",
       content: Buffer.from(content),
     });
-    writer.end();
+    const end = writer.end();
 
     const files = await extractFiles(writer.readable);
+    await end;
     expect(files.get("utf8.txt")?.toString()).toBe(content);
   });
 
@@ -58,14 +60,71 @@ describe("FileWriter", () => {
       name: "b.txt",
       content: Buffer.from("file b"),
     });
-    writer.end();
+    const end = writer.end();
 
     const files = await extractFiles(writer.readable);
+    await end;
     expect(files.size).toBe(2);
     expect(files.get("a.txt")?.toString()).toBe("file a");
     expect(files.get("b.txt")?.toString()).toBe("file b");
   });
 
+  it("writes string content", async () => {
+    const writer = new FileWriter();
+    await writer.addFile({
+      name: "hello.txt",
+      content: "Hello world",
+    });
+    const end = writer.end();
+
+    const files = await extractFiles(writer.readable);
+    await end;
+    expect(files.get("hello.txt")?.toString()).toBe("Hello world");
+  });
+
+  it("writes multi-byte UTF-8 string content", async () => {
+    const writer = new FileWriter();
+    const content = "café ☕ — Grüße aus München 🌍 日本語テスト";
+    await writer.addFile({
+      name: "utf8.txt",
+      content,
+    });
+    const end = writer.end();
+
+    const files = await extractFiles(writer.readable);
+    await end;
+    expect(files.get("utf8.txt")?.toString()).toBe(content);
+  });
+
+  it("writes Uint8Array content", async () => {
+    const writer = new FileWriter();
+    const content = new TextEncoder().encode("Hello world");
+    await writer.addFile({
+      name: "hello.txt",
+      content,
+    });
+    const end = writer.end();
+
+    const files = await extractFiles(writer.readable);
+    await end;
+    expect(files.get("hello.txt")?.toString()).toBe("Hello world");
+  });
+
+  it("writes multi-byte UTF-8 Uint8Array content", async () => {
+    const writer = new FileWriter();
+    const text = "café ☕ — Grüße aus München 🌍 日本語テスト";
+    const content = new TextEncoder().encode(text);
+    await writer.addFile({
+      name: "utf8.txt",
+      content,
+    });
+    const end = writer.end();
+
+    const files = await extractFiles(writer.readable);
+    await end;
+    expect(files.get("utf8.txt")?.toString()).toBe(text);
+  });
+
   it("writes stream content with explicit size", async () => {
     const content = Buffer.from("streamed content");
     const writer = new FileWriter();
@@ -74,9 +133,10 @@ describe("FileWriter", () => {
       content: Readable.from(content),
       size: content.length,
     });
-    writer.end();
+    const end = writer.end();
 
     const files = await extractFiles(writer.readable);
+    await end;
     expect(files.get("stream.txt")?.toString()).toBe("streamed content");
   });
 });
diff --git a/packages/vercel-sandbox/src/api-client/file-writer.ts b/packages/vercel-sandbox/src/api-client/file-writer.ts
index 65e94d42..adbc2b4e 100644
--- a/packages/vercel-sandbox/src/api-client/file-writer.ts
+++ b/packages/vercel-sandbox/src/api-client/file-writer.ts
@@ -2,15 +2,15 @@ import zlib from "zlib";
 import tar, { type Pack } from "tar-stream";
 import { Readable } from "stream";
 
-interface FileBuffer {
+interface FileData {
   /**
    * The name (path) of the file to write.
    */
   name: string;
   /**
-   * The content of the file as a Buffer.
+   * The content of the file.
    */
-  content: Buffer;
+  content: string | Uint8Array;
   /**
    * The file mode (permissions) to set on the file.
    * For example, 0o755 for executable files.
@@ -62,12 +62,16 @@ export class FileWriter {
    * Returns a Promise resolved once the file is written in the
    * stream.
    */
-  async addFile(file: FileBuffer | FileStream) {
+  async addFile(file: FileData | FileStream) {
     return new Promise<void>((resolve, reject) => {
       const entry = this.pack.entry(
         "size" in file
           ? { name: file.name, size: file.size, mode: file.mode }
-          : { name: file.name, size: Buffer.byteLength(file.content), mode: file.mode },
+          : {
+              name: file.name,
+              size: Buffer.byteLength(file.content),
+              mode: file.mode,
+            },
         (error) => {
           if (error) {
             return reject(error);
diff --git a/packages/vercel-sandbox/src/sandbox.ts b/packages/vercel-sandbox/src/sandbox.ts
index caec91ce..2a895c50 100644
--- a/packages/vercel-sandbox/src/sandbox.ts
+++ b/packages/vercel-sandbox/src/sandbox.ts
@@ -703,11 +703,15 @@ export class Sandbox {
    * @example
    * // Write an executable script
    * await sandbox.writeFiles([
-   *   { path: "/usr/local/bin/myscript", content: Buffer.from("#!/bin/bash\necho hello"), mode: 0o755 }
+   *   { path: "/usr/local/bin/myscript", content: "#!/bin/bash\necho hello", mode: 0o755 }
    * ]);
    */
   async writeFiles(
-    files: { path: string; content: Buffer; mode?: number }[],
+    files: {
+      path: string;
+      content: string | Uint8Array;
+      mode?: number;
+    }[],
     opts?: { signal?: AbortSignal },
   ) {
     "use step";