diff --git a/ts_config/README.md b/ts_config/README.md
index ec8fe55..c84120b 100644
--- a/ts_config/README.md
+++ b/ts_config/README.md
@@ -16,7 +16,7 @@ files in three different strictness tiers.
 ```jsonc
 // package.json
 "devDependencies": {
-  "@kuhyx/ts-config": "github:kuhyx/utils#ts_config-v0.1.0&path:/ts_config",
+  "@kuhyx/ts-config": "github:kuhyx/utils#ts_config-v0.1.1&path:/ts_config",
   "eslint": "10.10.0",
   "typescript": "6.0.3",
   "typescript-eslint": "8.70.0"
@@ -55,6 +55,24 @@ export default defineConfig({
 `jscpd.base.json` (jscpd has no extends). `stryker.config.mjs`:
 `import { strykerBase } from "@kuhyx/ts-config/stryker"; export default { ...strykerBase };`
 
+## Adopting the alias rule in an existing repo
+
+`base()` bans parent-relative imports, so a repo with dozens of `../` paths
+needs them rewritten before the rule can go on. That is a script, not a
+session's worth of hand edits:
+
+```sh
+node ~/src/utils/ts_config/scripts/alias-imports.mjs <repo-root> [src-dir]
+```
+
+Every `import` / `export … from` / `import()` specifier under `src/` that
+starts with `../` and resolves inside `src/` becomes `@/<path from src>`;
+`./sibling`, package imports and a `../` that leaves `src/` are left alone
+(the last one is the lint report's to surface). Extensions are carried over
+untouched. Idempotent. The repo still needs the alias itself: `paths` in
+tsconfig (see above) and `resolve.alias` in vite.config, which vitest reads.
+`test/alias-imports.test.js` runs it over a fixture tree.
+
 ## What is in it
 
 | Export | Content |
diff --git a/ts_config/package.json b/ts_config/package.json
index f453012..1e10850 100644
--- a/ts_config/package.json
+++ b/ts_config/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@kuhyx/ts-config",
-  "version": "0.1.0",
+  "version": "0.1.1",
   "private": false,
   "type": "module",
   "description": "The one lint/TypeScript bar every kuhyx TypeScript repo extends: a strict tsconfig base, ESLint flat-config presets (base, react, vitest, playwright, layer boundaries, a fast editor subset), and Knip / jscpd / Stryker bases. Plain JS, no build step -- consumable straight from a git tag.",
@@ -16,6 +16,7 @@
     "knip.base.json",
     "jscpd.base.json",
     "stryker.base.mjs",
+    "scripts",
     "README.md"
   ],
   "exports": {
diff --git a/ts_config/scripts/alias-imports.mjs b/ts_config/scripts/alias-imports.mjs
new file mode 100755
index 0000000..fa0a94d
--- /dev/null
+++ b/ts_config/scripts/alias-imports.mjs
@@ -0,0 +1,5 @@
+#!/usr/bin/env node
+// CLI shim; the logic and its tests live in lib/alias-imports.mjs.
+import { main } from "./lib/alias-imports.mjs";
+
+process.exitCode = main(process.argv.slice(2));
diff --git a/ts_config/scripts/lib/alias-imports.mjs b/ts_config/scripts/lib/alias-imports.mjs
new file mode 100644
index 0000000..5b9dc34
--- /dev/null
+++ b/ts_config/scripts/lib/alias-imports.mjs
@@ -0,0 +1,114 @@
+// Rewrite parent-relative imports under src/ to the `@/` alias.
+//
+// The preset bans `../` specifiers (imports.js): they encode the importer's
+// depth, and a model that guesses the depth wrong lands on the wrong module.
+// Turning the rule on in a repo with dozens of them by hand is the kind of
+// mechanical step that gets done slightly differently each time, so it is a
+// script: every `import`/`export … from "../x"` (and dynamic `import("../x")`)
+// whose target resolves under src/ becomes `@/<path from src>`. `./sibling`
+// and package imports are left alone, and so is a `../` that resolves OUTSIDE
+// src/ -- there is no alias for it, and the lint report is the right place
+// for that one to surface.
+//
+// The extension is carried over untouched: a repo on NodeNext resolution
+// needs its `.js` suffixes, a bundler repo has none, and this script has no
+// business changing which of the two it is.
+import fs from "node:fs";
+import path from "node:path";
+
+const SOURCE_EXTENSIONS = new Set([".cts", ".js", ".jsx", ".mts", ".ts", ".tsx"]);
+
+// A string literal that is an import/export specifier: preceded by `from`,
+// by `import` (side-effect form), or by `import(` (dynamic form).
+const SPECIFIER = /(\bfrom\s*|\bimport\s*\(\s*|\bimport\s+)(['"])(\.\.\/[^'"\n]*)\2/g;
+
+/** Every source file below `directory`, depth-first, sorted for a stable report. */
+export function listSourceFiles(directory) {
+  const out = [];
+  const entries = fs.readdirSync(directory, { withFileTypes: true }).toSorted((a, b) => a.name.localeCompare(b.name));
+  for (const entry of entries) {
+    const full = path.join(directory, entry.name);
+    if (entry.isDirectory()) {
+      out.push(...listSourceFiles(full));
+    } else if (SOURCE_EXTENSIONS.has(path.extname(entry.name))) {
+      out.push(full);
+    }
+  }
+  return out;
+}
+
+/**
+ * The alias form of one specifier, or null when it must stay as written.
+ *
+ * @param {string} file Absolute path of the importing file.
+ * @param {string} specifier The `../…` string as it appears in the source.
+ * @param {string} sourceDirectory Absolute path of the aliased directory.
+ */
+export function aliasFor(file, specifier, sourceDirectory) {
+  const resolved = path.resolve(path.dirname(file), specifier);
+  const fromSource = path.relative(sourceDirectory, resolved);
+  const outside = fromSource === "" || fromSource.startsWith("..") || path.isAbsolute(fromSource);
+  return outside ? null : `@/${fromSource.split(path.sep).join("/")}`;
+}
+
+/**
+ * Rewrite one file's text. Pure: returns the new text and how many
+ * specifiers changed, touches nothing on disk.
+ */
+export function rewriteSource(file, text, sourceDirectory) {
+  let count = 0;
+  const rewritten = text.replaceAll(SPECIFIER, (match, lead, quote, specifier) => {
+    const alias = aliasFor(file, specifier, sourceDirectory);
+    if (alias === null) {
+      return match;
+    }
+    count += 1;
+    return `${lead}${quote}${alias}${quote}`;
+  });
+  return { count, text: rewritten };
+}
+
+/**
+ * Rewrite every source file under `<root>/<source>` in place.
+ *
+ * @param {string} root Repo root.
+ * @param {string} [source] Aliased directory, relative to root.
+ * @returns {{ files: number, imports: number }} What changed.
+ */
+export function aliasImports(root, source = "src") {
+  const sourceDirectory = path.resolve(root, source);
+  if (!fs.existsSync(sourceDirectory)) {
+    throw new Error(`alias-imports: no such directory ${sourceDirectory}`);
+  }
+  let files = 0;
+  let imports = 0;
+  for (const file of listSourceFiles(sourceDirectory)) {
+    const before = fs.readFileSync(file, "utf8");
+    const { count, text } = rewriteSource(file, before, sourceDirectory);
+    if (count === 0) {
+      continue;
+    }
+    fs.writeFileSync(file, text);
+    files += 1;
+    imports += count;
+  }
+  return { files, imports };
+}
+
+/**
+ * CLI body: `alias-imports <repo-root> [src-dir]`. Returns the exit code
+ * and writes the one-line report, so the shim stays at two lines.
+ *
+ * @param {string[]} argv Arguments after the script name.
+ * @param {{ log: (line: string) => void }} [io]
+ */
+export function main(argv, io = console) {
+  const [root, source] = argv;
+  if (root === undefined) {
+    io.log("usage: alias-imports <repo-root> [src-dir=src]");
+    return 2;
+  }
+  const { files, imports } = aliasImports(root, source);
+  io.log(`alias-imports: rewrote ${imports} import(s) in ${files} file(s) under ${path.resolve(root, source ?? "src")}`);
+  return 0;
+}
diff --git a/ts_config/test/alias-imports.test.js b/ts_config/test/alias-imports.test.js
new file mode 100644
index 0000000..f4ae03e
--- /dev/null
+++ b/ts_config/test/alias-imports.test.js
@@ -0,0 +1,137 @@
+// The alias codemod against a fixture tree built in a temp dir: one-level,
+// two-level, `export … from`, dynamic import, single AND double quotes, a
+// `./sibling` that must stay, and a `../` that resolves OUTSIDE src/ (no
+// alias exists for it, so it must be left for the lint report).
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import { aliasFor, aliasImports, listSourceFiles, main, rewriteSource } from "../scripts/lib/alias-imports.mjs";
+
+const CLI = path.join(import.meta.dirname, "..", "scripts", "alias-imports.mjs");
+
+const FIXTURE = {
+  "src/components/deep/card.tsx": [
+    'import { api } from "../../services/api";',
+    "import type { User } from '../../types/user';",
+    'import { helper } from "./helper";',
+    'export { format } from "../../utils/format";',
+    'const lazy = () => import("../../utils/lazy");',
+    'import "../../styles/global.css";',
+    'import { outside } from "../../../tooling/outside";',
+    "export const card = { api, helper, lazy, outside };",
+  ].join("\n"),
+  "src/components/deep/helper.ts": 'export const helper = 1;\nimport "./nothing-to-do";\n',
+  "src/services/api.test.ts": 'import { api } from "./api";\nexport const t = api;\n',
+  "src/services/api.ts": 'import { format } from "../utils/format";\nexport const api = format;\n',
+  "src/services/notes.md": "not source: [x](../utils/format)\n",
+  "src/utils/format.ts": "export const format = 1;\n",
+  "tooling/outside.ts": "export const outside = 1;\n",
+};
+
+// One fixture tree per test, so a rewriting test cannot leak into the next.
+const state = { root: "" };
+const rootOf = () => state.root;
+const sourceOf = () => path.join(rootOf(), "src");
+const read = (relative) => fs.readFileSync(path.join(rootOf(), relative), "utf8");
+const collect = (lines) => ({ log: (line) => { lines.push(line); } });
+
+beforeEach(() => {
+  state.root = fs.mkdtempSync(path.join(os.tmpdir(), "alias-imports-"));
+  for (const [relative, content] of Object.entries(FIXTURE)) {
+    const full = path.join(rootOf(), relative);
+    fs.mkdirSync(path.dirname(full), { recursive: true });
+    fs.writeFileSync(full, content);
+  }
+});
+
+afterEach(() => {
+  fs.rmSync(rootOf(), { force: true, recursive: true });
+});
+
+describe("aliasFor", () => {
+  const from = (relative, specifier) => aliasFor(path.join(sourceOf(), relative), specifier, sourceOf());
+
+  it("maps a parent-relative target under src to @/", () => {
+    expect(from("a/b/c.ts", "../../x/y")).toBe("@/x/y");
+    expect(from("a/b/c.ts", "../../x/y.js")).toBe("@/x/y.js");
+  });
+
+  it("refuses targets outside src and src itself", () => {
+    expect(from("a/c.ts", "../../tooling/x")).toBeNull();
+    expect(from("a/c.ts", "..")).toBeNull();
+  });
+});
+
+describe("rewriteSource", () => {
+  it("rewrites every import form and keeps the quote style", () => {
+    const file = path.join(sourceOf(), "components/deep/card.tsx");
+    const { count, text } = rewriteSource(file, FIXTURE["src/components/deep/card.tsx"], sourceOf());
+    expect(count).toBe(5);
+    expect(text).toContain('import { api } from "@/services/api";');
+    expect(text).toContain("import type { User } from '@/types/user';");
+    expect(text).toContain('export { format } from "@/utils/format";');
+    expect(text).toContain('import("@/utils/lazy")');
+    expect(text).toContain('import "@/styles/global.css";');
+    // Untouched: the sibling and the import that leaves src/.
+    expect(text).toContain('from "./helper"');
+    expect(text).toContain('from "../../../tooling/outside"');
+  });
+
+  it("reports zero for a file with nothing to do", () => {
+    const file = path.join(sourceOf(), "components/deep/helper.ts");
+    expect(rewriteSource(file, FIXTURE["src/components/deep/helper.ts"], sourceOf())).toEqual({
+      count: 0,
+      text: FIXTURE["src/components/deep/helper.ts"],
+    });
+  });
+});
+
+describe("aliasImports", () => {
+  it("walks only source files and writes only changed ones", () => {
+    expect(listSourceFiles(sourceOf()).map((file) => path.relative(rootOf(), file))).toEqual([
+      "src/components/deep/card.tsx",
+      "src/components/deep/helper.ts",
+      "src/services/api.test.ts",
+      "src/services/api.ts",
+      "src/utils/format.ts",
+    ]);
+    expect(aliasImports(rootOf())).toEqual({ files: 2, imports: 6 });
+    expect(read("src/services/api.ts")).toContain('from "@/utils/format"');
+    expect(read("src/services/api.test.ts")).toBe(FIXTURE["src/services/api.test.ts"]);
+    expect(read("src/services/notes.md")).toBe(FIXTURE["src/services/notes.md"]);
+    // Idempotent: a second pass finds nothing.
+    expect(aliasImports(rootOf())).toEqual({ files: 0, imports: 0 });
+  });
+
+  it("honours a non-default source dir", () => {
+    fs.renameSync(sourceOf(), path.join(rootOf(), "lib"));
+    expect(aliasImports(rootOf(), "lib")).toEqual({ files: 2, imports: 6 });
+  });
+
+  it("fails loudly on a missing source dir", () => {
+    expect(() => aliasImports(rootOf(), "nope")).toThrow(/no such directory/);
+  });
+});
+
+describe("main", () => {
+  it("prints usage and exits 2 without a root", () => {
+    const lines = [];
+    expect(main([], collect(lines))).toBe(2);
+    expect(lines[0]).toMatch(/^usage:/);
+  });
+
+  it("reports the count and exits 0", () => {
+    const lines = [];
+    expect(main([rootOf()], collect(lines))).toBe(0);
+    expect(lines[0]).toBe(`alias-imports: rewrote 6 import(s) in 2 file(s) under ${sourceOf()}`);
+  });
+
+  it("runs as a CLI", () => {
+    const stdout = execFileSync(process.execPath, [CLI, rootOf()], { encoding: "utf8" });
+    expect(stdout).toContain("rewrote 6 import(s) in 2 file(s)");
+    expect(read("src/services/api.ts")).toContain("@/utils/format");
+  });
+});
diff --git a/ts_config/vitest.config.js b/ts_config/vitest.config.js
index 3899d1e..fcc7d55 100644
--- a/ts_config/vitest.config.js
+++ b/ts_config/vitest.config.js
@@ -3,7 +3,7 @@ import { defineConfig } from "vitest/config";
 export default defineConfig({
   test: {
     coverage: {
-      include: ["eslint/**/*.js", "stryker.base.mjs"],
+      include: ["eslint/**/*.js", "scripts/lib/**/*.mjs", "stryker.base.mjs"],
       provider: "v8",
       // The same bar as ts_core and web_ui: the package every repo's lint
       // runs through does not get to be the least-tested code in the tree.