diff --git a/src/files/loader.ts b/src/files/loader.ts
index 791c19d..c668f96 100644
--- a/src/files/loader.ts
+++ b/src/files/loader.ts
@@ -1,7 +1,3 @@
-import fs from "node:fs/promises";
-import path from "node:path";
-import fg from "fast-glob";
-
 export interface LoadFilesOptions {
   files?: Record<string, string>;
   uploadDirectory?: {
@@ -15,6 +11,41 @@ export interface FileEntry {
   content: Buffer;
 }
 
+type FastGlobFn = (
+  source: string | string[],
+  options?: import("fast-glob").Options,
+) => Promise<string[]>;
+
+// Lazy-loaded Node.js dependencies
+let cachedDeps: {
+  fs: typeof import("node:fs/promises");
+  path: typeof import("node:path");
+  fg: FastGlobFn;
+} | null = null;
+
+async function loadNodeDependencies() {
+  if (cachedDeps) {
+    return cachedDeps;
+  }
+
+  try {
+    const [fs, path, fgModule] = await Promise.all([
+      import("node:fs/promises"),
+      import("node:path"),
+      import("fast-glob"),
+    ]);
+    // fast-glob uses `export =` so dynamic import wraps it as { default: ... }
+    const fg = fgModule.default as FastGlobFn;
+    cachedDeps = { fs, path, fg };
+    return cachedDeps;
+  } catch {
+    throw new Error(
+      "uploadDirectory requires Node.js. " +
+        "In browser environments, use the 'files' option to provide file contents directly instead.",
+    );
+  }
+}
+
 /**
  * Stream files from inline definitions and/or a directory on disk.
  * Yields files one at a time to avoid loading everything into memory.
@@ -35,6 +66,7 @@ export async function* streamFiles(
 
   // Stream from directory (skip files already yielded from inline)
   if (options.uploadDirectory) {
+    const { fs, path, fg } = await loadNodeDependencies();
     const { source, include = "**/*" } = options.uploadDirectory;
     const absoluteSource = path.resolve(source);
 
@@ -66,6 +98,7 @@ export async function getFilePaths(
   const paths: string[] = [];
 
   if (options.uploadDirectory) {
+    const { path, fg } = await loadNodeDependencies();
     const { source, include = "**/*" } = options.uploadDirectory;
     const absoluteSource = path.resolve(source);
 
diff --git a/src/posix-path.test.ts b/src/posix-path.test.ts
new file mode 100644
index 0000000..e586937
--- /dev/null
+++ b/src/posix-path.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "vitest";
+import { posixJoin, posixResolve } from "./posix-path.js";
+
+describe("posixJoin", () => {
+  it("joins simple segments", () => {
+    expect(posixJoin("a", "b", "c")).toBe("a/b/c");
+  });
+
+  it("handles leading slash", () => {
+    expect(posixJoin("/workspace", "src", "file.ts")).toBe(
+      "/workspace/src/file.ts",
+    );
+  });
+
+  it("normalizes double slashes", () => {
+    expect(posixJoin("/workspace/", "/src")).toBe("/workspace/src");
+  });
+
+  it("resolves . segments", () => {
+    expect(posixJoin("a", ".", "b")).toBe("a/b");
+  });
+
+  it("resolves .. segments", () => {
+    expect(posixJoin("a", "b", "..", "c")).toBe("a/c");
+  });
+
+  it("handles empty segments", () => {
+    expect(posixJoin("a", "", "b")).toBe("a/b");
+  });
+});
+
+describe("posixResolve", () => {
+  it("resolves relative path against base", () => {
+    expect(posixResolve("/workspace", "src/file.ts")).toBe(
+      "/workspace/src/file.ts",
+    );
+  });
+
+  it("returns absolute path as-is (normalized)", () => {
+    expect(posixResolve("/workspace", "/other/file.ts")).toBe("/other/file.ts");
+  });
+
+  it("handles .. in relative path", () => {
+    expect(posixResolve("/workspace/src", "../file.ts")).toBe(
+      "/workspace/file.ts",
+    );
+  });
+
+  it("handles . in relative path", () => {
+    expect(posixResolve("/workspace", "./file.ts")).toBe("/workspace/file.ts");
+  });
+
+  it("normalizes the result", () => {
+    expect(posixResolve("/workspace//", "src//file.ts")).toBe(
+      "/workspace/src/file.ts",
+    );
+  });
+});
diff --git a/src/posix-path.ts b/src/posix-path.ts
new file mode 100644
index 0000000..9e26817
--- /dev/null
+++ b/src/posix-path.ts
@@ -0,0 +1,49 @@
+/**
+ * Browser-compatible POSIX path utilities.
+ * These functions handle forward-slash paths without Node.js dependencies.
+ */
+
+/**
+ * Join path segments with forward slashes.
+ * Normalizes the result to remove redundant slashes and resolve . and ..
+ */
+export function posixJoin(...segments: string[]): string {
+  const joined = segments.filter(Boolean).join("/");
+  return normalizePosixPath(joined);
+}
+
+/**
+ * Resolve a path against a base directory.
+ * If the path is absolute (starts with /), return it normalized.
+ * Otherwise, join it with the base and normalize.
+ */
+export function posixResolve(base: string, relativePath: string): string {
+  if (relativePath.startsWith("/")) {
+    return normalizePosixPath(relativePath);
+  }
+  return normalizePosixPath(`${base}/${relativePath}`);
+}
+
+/**
+ * Normalize a POSIX path by resolving . and .. segments and removing duplicate slashes.
+ */
+function normalizePosixPath(p: string): string {
+  const isAbsolute = p.startsWith("/");
+  const segments = p.split("/").filter((s) => s !== "" && s !== ".");
+  const result: string[] = [];
+
+  for (const segment of segments) {
+    if (segment === "..") {
+      if (result.length > 0 && result[result.length - 1] !== "..") {
+        result.pop();
+      } else if (!isAbsolute) {
+        result.push("..");
+      }
+    } else {
+      result.push(segment);
+    }
+  }
+
+  const normalized = result.join("/");
+  return isAbsolute ? `/${normalized}` : normalized || ".";
+}
diff --git a/src/sandbox/just-bash.ts b/src/sandbox/just-bash.ts
index 40f2d12..46b8223 100644
--- a/src/sandbox/just-bash.ts
+++ b/src/sandbox/just-bash.ts
@@ -45,11 +45,13 @@ export async function createJustBashSandbox(
   // Dynamic import to handle optional peer dependency
   let Bash: typeof import("just-bash").Bash;
   let OverlayFs: typeof import("just-bash").OverlayFs | undefined;
+  let nodePath: typeof import("node:path");
 
   try {
     const module = await import("just-bash");
     Bash = module.Bash;
     OverlayFs = module.OverlayFs;
+    nodePath = await import("node:path");
   } catch {
     throw new Error(
       'just-bash is not installed. Either install it with "npm install just-bash" or provide your own sandbox via the sandbox option.',
@@ -61,7 +63,9 @@ export async function createJustBashSandbox(
 
   if (options.overlayRoot && OverlayFs) {
     // Use OverlayFs for copy-on-write over a real directory
-    const overlay = new OverlayFs({ root: options.overlayRoot });
+    // Resolve to absolute path for OverlayFs
+    const absoluteRoot = nodePath.resolve(options.overlayRoot);
+    const overlay = new OverlayFs({ root: absoluteRoot });
     mountPoint = overlay.getMountPoint();
     bashEnv = new Bash({
       fs: overlay,
diff --git a/src/tool.ts b/src/tool.ts
index 655e485..ccbbcba 100644
--- a/src/tool.ts
+++ b/src/tool.ts
@@ -1,5 +1,5 @@
-import path from "node:path";
 import { getFilePaths, streamFiles } from "./files/loader.js";
+import { posixJoin } from "./posix-path.js";
 import {
   createJustBashSandbox,
   isJustBash,
@@ -99,7 +99,7 @@ export async function createBashTool(
         uploadDirectory: options.uploadDirectory,
       })) {
         batch.push({
-          path: path.posix.join(destination, file.path),
+          path: posixJoin(destination, file.path),
           content: file.content,
         });
 
@@ -120,9 +120,8 @@ export async function createBashTool(
 
     if (options.uploadDirectory && !options.files) {
       // Use OverlayFs for uploadDirectory (avoids loading all files into memory)
-      const overlayRoot = path.resolve(options.uploadDirectory.source);
       const result = await createJustBashSandbox({
-        overlayRoot,
+        overlayRoot: options.uploadDirectory.source,
       });
       sandbox = result;
 
@@ -152,7 +151,7 @@ export async function createBashTool(
         files: options.files,
         uploadDirectory: options.uploadDirectory,
       })) {
-        const absolutePath = path.posix.join(destination, file.path);
+        const absolutePath = posixJoin(destination, file.path);
         filesWithDestination[absolutePath] = file.content.toString("utf-8");
       }
 
diff --git a/src/tools/read-file.ts b/src/tools/read-file.ts
index 63f47b9..1933eec 100644
--- a/src/tools/read-file.ts
+++ b/src/tools/read-file.ts
@@ -1,6 +1,6 @@
-import nodePath from "node:path";
 import { tool } from "ai";
 import { z } from "zod";
+import { posixResolve } from "../posix-path.js";
 import type { Sandbox } from "../types.js";
 
 const readFileSchema = z.object({
@@ -20,7 +20,7 @@ export function createReadFileTool(options: CreateReadFileToolOptions) {
     description: "Read the contents of a file from the sandbox.",
     inputSchema: readFileSchema,
     execute: async ({ path }) => {
-      const resolvedPath = nodePath.posix.resolve(cwd, path);
+      const resolvedPath = posixResolve(cwd, path);
       const content = await sandbox.readFile(resolvedPath);
       return { content };
     },
diff --git a/src/tools/write-file.ts b/src/tools/write-file.ts
index 6a31363..12a9856 100644
--- a/src/tools/write-file.ts
+++ b/src/tools/write-file.ts
@@ -1,6 +1,6 @@
-import nodePath from "node:path";
 import { tool } from "ai";
 import { z } from "zod";
+import { posixResolve } from "../posix-path.js";
 import type { Sandbox } from "../types.js";
 
 const writeFileSchema = z.object({
@@ -22,7 +22,7 @@ export function createWriteFileTool(options: CreateWriteFileToolOptions) {
       "Write content to a file in the sandbox. Creates parent directories if needed.",
     inputSchema: writeFileSchema,
     execute: async ({ path, content }) => {
-      const resolvedPath = nodePath.posix.resolve(cwd, path);
+      const resolvedPath = posixResolve(cwd, path);
       await sandbox.writeFiles([{ path: resolvedPath, content }]);
       return { success: true };
     },