diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index b0598b7..7218b83 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -22,3 +22,4 @@ jobs:
           node-version: ${{ matrix.node }}
       - run: npm install
       - run: npm run lint
+      - run: npm test
diff --git a/README.md b/README.md
index ec1c2f9..e21ca37 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ This package provides ESLint rules and configurations for **Hubspot Marketing We
 - [Stylelint Setup](#stylelint-setup)
 - [Cypress Setup](#cypress-setup)
 - [Accessibility Testing](#accessibility-testing-optional)
+- [Custom Rules](#custom-rules)
 - [Where to use it](#where-to-use-it)
 - [Using the Prettier Scripts](#using-the-prettier-scripts)
 - [Contributing](#contributing)
@@ -158,6 +159,16 @@ This package includes a shared accessibility testing setup using [cypress-axe](h
 
 `cy.checkAccessibility()` adds the `high-contrast` class to `body`, runs WCAG 2.2 Level AA rules only, and logs each violation with its id, help text, impact, element targets, and help URL. TypeScript types are included — no `tsconfig.json` changes required.
 
+## Custom Rules
+
+This package ships a `hs-web-team` ESLint plugin with custom rules that are active automatically when you use `wtConfig` or `wtBrowserConfig` — no extra setup needed.
+
+| Rule | Severity | Summary |
+|------|----------|---------|
+| `hs-web-team/no-reduce-accumulator-copy` | error | Disallows O(n²) accumulator copies (spread, concat) inside `reduce`/`reduceRight` |
+
+For examples and remediation guidance, see [examples/custom-rules.md](./examples/custom-rules.md).
+
 ## Where to use it
 
 This package provides multiple configurations:
diff --git a/bin/check-peer-deps.cjs b/bin/check-peer-deps.cjs
index a35f27d..fefc2e3 100644
--- a/bin/check-peer-deps.cjs
+++ b/bin/check-peer-deps.cjs
@@ -14,7 +14,7 @@ if (!projectRoot || projectRoot === path.resolve(__dirname, '..')) {
 const ownPkg = require('../package.json');
 
 const stylelintPeers = Object.entries(ownPkg.peerDependencies || {}).filter(([name]) =>
-  name.startsWith('stylelint')
+  name.startsWith('stylelint'),
 );
 
 // Extracts the minimum required major version from a semver range like ^17.0.0 or >=17.1.1.
diff --git a/browser.js b/browser.js
index d379599..8bf93e6 100644
--- a/browser.js
+++ b/browser.js
@@ -4,6 +4,7 @@ import tseslint from 'typescript-eslint';
 import reactPlugin from 'eslint-plugin-react';
 import reactHooksPlugin from 'eslint-plugin-react-hooks';
 import jsxA11yPlugin from 'eslint-plugin-jsx-a11y';
+import { hsWebTeamPlugin } from './plugins/hs-web-team/index.js';
 
 // Base rules adapted from the browser config
 const baseRules = {
@@ -63,7 +64,6 @@ const commonIgnores = [
   '**/build/**',
   '**/.next/**',
   '**/coverage/**',
-  'eslint.config.js',
 ];
 
 export default [
@@ -117,6 +117,7 @@ export default [
       ...reactRules,
     },
   },
+  hsWebTeamPlugin.configs.recommended,
   // TypeScript config
   ...tseslint.configs.recommended.map(config => ({
     ...config,
diff --git a/eslint.config.js b/eslint.config.js
new file mode 100644
index 0000000..6459fc6
--- /dev/null
+++ b/eslint.config.js
@@ -0,0 +1,16 @@
+import wtConfig from './index.js';
+
+export default [
+  ...wtConfig,
+  // Cypress utility files use globals (Cypress, cy) injected by the Cypress
+  // runtime — declare them so no-undef doesn't false-positive on this directory.
+  {
+    files: ['cypress/**'],
+    languageOptions: {
+      globals: {
+        Cypress: 'readonly',
+        cy: 'readonly',
+      },
+    },
+  },
+];
diff --git a/examples/custom-rules.md b/examples/custom-rules.md
new file mode 100644
index 0000000..9a1b431
--- /dev/null
+++ b/examples/custom-rules.md
@@ -0,0 +1,59 @@
+# Custom Rules (`hs-web-team` plugin)
+
+This package bundles a small set of custom ESLint rules under the `hs-web-team` plugin. They are active automatically when you spread `wtConfig` or `wtBrowserConfig` — no extra configuration required.
+
+---
+
+## `hs-web-team/no-reduce-accumulator-copy` ❌ error
+
+**Disallows O(n²) accumulator copies inside `reduce`/`reduceRight` callbacks.**
+
+Each iteration of `reduce` creates an entirely new object or array, making the total work proportional to n². This is rarely intentional and degrades quickly as the input grows.
+
+### What triggers it
+
+```js
+// ❌ Object spread — copies every key on every iteration
+const byId = items.reduce((acc, item) => ({ ...acc, [item.id]: item }), {});
+
+// ❌ Array spread — copies every element on every iteration
+const doubled = items.reduce((acc, n) => [...acc, n * 2], []);
+
+// ❌ acc.concat() — same problem, different syntax
+const flat = items.reduce((acc, arr) => acc.concat(arr), []);
+```
+
+### What to do instead
+
+```js
+// ✅ Object.groupBy / Object.fromEntries for grouping/indexing
+const byId = Object.fromEntries(items.map(item => [item.id, item]));
+
+// ✅ for...of with in-place mutation for accumulation
+const byId = {};
+for (const item of items) {
+  byId[item.id] = item;
+}
+
+// ✅ flatMap for flat-map patterns
+const doubled = items.flatMap(n => [n * 2]);
+
+// ✅ flat() for flattening
+const flat = items.flat();
+```
+
+### When `reduce` is fine
+
+The rule only fires when the **accumulator itself** is copied. Reads from the accumulator, or spreading non-accumulator values into it, are allowed:
+
+```js
+// ✅ Spreading a non-accumulator value into the result
+const merged = items.reduce((acc, item) => ({ ...acc, ...item.overrides }), base);
+//                                                         ^^^^^^^^^^^^^^ not the accumulator — fine
+
+// ✅ Accumulator mutation with no copy
+const counts = items.reduce((acc, item) => {
+  acc[item.type] = (acc[item.type] ?? 0) + 1;
+  return acc;
+}, {});
+```
diff --git a/index.js b/index.js
index 1c14b07..9257af9 100644
--- a/index.js
+++ b/index.js
@@ -1,6 +1,7 @@
 import js from '@eslint/js';
 import globals from 'globals';
 import tseslint from 'typescript-eslint';
+import { hsWebTeamPlugin } from './plugins/hs-web-team/index.js';
 
 // Base rules for all JavaScript files
 const baseRules = {
@@ -51,7 +52,6 @@ const commonIgnores = [
   '**/.serverless/**',
   '**/.webpack/**',
   '**/dist/**',
-  'eslint.config.js',
 ];
 
 export default [
@@ -68,6 +68,7 @@ export default [
     },
     rules: baseRules,
   },
+  hsWebTeamPlugin.configs.recommended,
   // TypeScript config - restrict to TypeScript files only
   ...tseslint.configs.recommended.map(config => ({
     ...config,
diff --git a/package.json b/package.json
index 65cb8a8..61bf4ac 100644
--- a/package.json
+++ b/package.json
@@ -2,6 +2,19 @@
   "name": "@hs-web-team/eslint-config-node",
   "version": "4.2.2",
   "description": "HubSpot Marketing WebTeam shared configurations for ESLint, Prettier, Stylelint, and Cypress",
+  "files": [
+    ".prettierrc.json",
+    ".stylelintrc.json",
+    "bin/",
+    "browser.js",
+    "cypress.config.cjs",
+    "cypress.config.d.ts",
+    "cypress/",
+    "docs/",
+    "examples/",
+    "index.js",
+    "plugins/"
+  ],
   "main": "index.js",
   "type": "module",
   "exports": {
@@ -31,8 +44,8 @@
   },
   "scripts": {
     "postinstall": "node bin/check-peer-deps.cjs",
-    "lint": "npx eslint -c ./index.js *.js --fix",
-    "test": "echo \"Error: no test specified\" && exit 1",
+    "lint": "npx eslint . --fix",
+    "test": "node --test",
     "prepare": "git rev-parse --git-dir > /dev/null 2>&1 && git config core.hooksPath .githooks || true"
   },
   "engines": {
diff --git a/plugins/hs-web-team/index.js b/plugins/hs-web-team/index.js
new file mode 100644
index 0000000..8e1593f
--- /dev/null
+++ b/plugins/hs-web-team/index.js
@@ -0,0 +1,18 @@
+import { noReduceAccumulatorCopy } from './rules/no-reduce-accumulator-copy.js';
+
+export const hsWebTeamPlugin = {
+  rules: {
+    'no-reduce-accumulator-copy': noReduceAccumulatorCopy,
+  },
+};
+
+// Self-reference allows the plugin to register itself via configs.recommended,
+// following the same pattern as typescript-eslint and eslint-plugin-react.
+hsWebTeamPlugin.configs = {
+  recommended: {
+    plugins: { 'hs-web-team': hsWebTeamPlugin },
+    rules: {
+      'hs-web-team/no-reduce-accumulator-copy': 'error',
+    },
+  },
+};
diff --git a/plugins/hs-web-team/rules/no-reduce-accumulator-copy.js b/plugins/hs-web-team/rules/no-reduce-accumulator-copy.js
new file mode 100644
index 0000000..f12f38b
--- /dev/null
+++ b/plugins/hs-web-team/rules/no-reduce-accumulator-copy.js
@@ -0,0 +1,90 @@
+const RECOMMENDATION =
+  'Prefer a for...of loop with in-place mutation, or a purpose-built method (flatMap, Object.groupBy, flat) if it fits your pattern.';
+
+function isReduceCall(node) {
+  if (!node) return false;
+
+  const callee = node.callee;
+  if (!callee || !callee.property || !callee.property.name) {
+    return false;
+  }
+
+  return (
+    callee.property.name === 'reduce' || callee.property.name === 'reduceRight'
+  );
+}
+
+function isReduceAccumulator(def) {
+  return (
+    def &&
+    def.type === 'Parameter' &&
+    def.node.parent &&
+    isReduceCall(def.node.parent) &&
+    def.node.params &&
+    def.node.params.length &&
+    // def.name is the Identifier AST node for the definition; this identity
+    // check confirms it is the first parameter (the accumulator), not a later one.
+    def.name === def.node.params[0]
+  );
+}
+
+export const noReduceAccumulatorCopy = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description:
+        'Disallow O(n²) accumulator copies (spread, concat) inside reduce/reduceRight callbacks',
+      recommended: true,
+    },
+    messages: {
+      AccumulatorSpread: `Spreading the reduce accumulator is O(n²). ${RECOMMENDATION}`,
+      AccumulatorShallowCopy: `Calling .{{ method }}() on the reduce accumulator is O(n²). ${RECOMMENDATION}`,
+    },
+    schema: [],
+  },
+
+  create(context) {
+    const { sourceCode } = context;
+
+    function checkIsAccumulatorSpread(node) {
+      if (!node.argument || node.argument.type !== 'Identifier') return;
+
+      const scope = sourceCode.getScope(node);
+      const spreadee = scope.set.get(node.argument.name);
+      const spreadeeDef =
+        spreadee && spreadee.defs && spreadee.defs.length && spreadee.defs[0];
+
+      if (isReduceAccumulator(spreadeeDef)) {
+        context.report({ node, messageId: 'AccumulatorSpread' });
+      }
+    }
+
+    function checkIsAccumulatorShallowCopy(node) {
+      if (
+        node.callee.type !== 'MemberExpression' ||
+        !node.callee.property ||
+        !node.callee.object ||
+        node.callee.object.type !== 'Identifier'
+      ) {
+        return;
+      }
+
+      const method = node.callee.property.name;
+      if (method !== 'concat') return;
+
+      const scope = sourceCode.getScope(node);
+      const variable = scope.set.get(node.callee.object.name);
+      const definition =
+        variable && variable.defs && variable.defs.length && variable.defs[0];
+
+      if (isReduceAccumulator(definition)) {
+        context.report({ node, messageId: 'AccumulatorShallowCopy', data: { method } });
+      }
+    }
+
+    return {
+      SpreadElement: checkIsAccumulatorSpread,
+      CallExpression: checkIsAccumulatorShallowCopy,
+    };
+  },
+};
diff --git a/tests/plugins/hs-web-team/no-reduce-accumulator-copy.test.js b/tests/plugins/hs-web-team/no-reduce-accumulator-copy.test.js
new file mode 100644
index 0000000..ef31e05
--- /dev/null
+++ b/tests/plugins/hs-web-team/no-reduce-accumulator-copy.test.js
@@ -0,0 +1,146 @@
+import { describe, it } from 'node:test';
+import { RuleTester } from 'eslint';
+import { noReduceAccumulatorCopy as rule } from '../../../plugins/hs-web-team/rules/no-reduce-accumulator-copy.js';
+
+const ruleTester = new RuleTester({
+  languageOptions: {
+    ecmaVersion: 2022,
+    sourceType: 'module',
+  },
+});
+
+describe('no-reduce-accumulator-copy', () => {
+  describe('valid', () => {
+    it('does not flag spreading a non-accumulator variable inside reduce', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [
+          // Spreading the current item (object), not the accumulator
+          { code: 'items.reduce((acc, item) => ({ ...item, extra: 1 }), {})' },
+          // Spreading the current item (array), not the accumulator
+          { code: 'items.reduce((acc, item) => [...item], [])' },
+        ],
+        invalid: [],
+      });
+    });
+
+    it('does not flag Object.fromEntries + flatMap (a preferred alternative)', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [
+          {
+            code: 'Object.fromEntries(items.flatMap(item => [[item.key, item.value]]))',
+          },
+        ],
+        invalid: [],
+      });
+    });
+
+    it('does not flag a for...of loop with push (a preferred alternative)', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [
+          {
+            code: `
+              const result = [];
+              for (const item of items) {
+                result.push(item.value);
+              }
+            `,
+          },
+        ],
+        invalid: [],
+      });
+    });
+
+    it('does not flag in-place mutation of the accumulator', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [
+          {
+            code: 'items.reduce((acc, item) => { acc[item.key] = item.value; return acc; }, {})',
+          },
+          {
+            code: 'items.reduce((acc, item) => { acc.push(item); return acc; }, [])',
+          },
+        ],
+        invalid: [],
+      });
+    });
+
+    it('does not flag spreading a variable with the same name outside a reduce', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [
+          {
+            code: `
+              const acc = { a: 1 };
+              const result = { ...acc, b: 2 };
+            `,
+          },
+        ],
+        invalid: [],
+      });
+    });
+  });
+
+  describe('invalid', () => {
+    it('flags object spread on the accumulator', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [],
+        invalid: [
+          {
+            code: 'items.reduce((acc, item) => ({ ...acc, [item.key]: item.value }), {})',
+            errors: [{ messageId: 'AccumulatorSpread' }],
+          },
+        ],
+      });
+    });
+
+    it('flags array spread on the accumulator', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [],
+        invalid: [
+          {
+            code: 'items.reduce((acc, item) => [...acc, item], [])',
+            errors: [{ messageId: 'AccumulatorSpread' }],
+          },
+        ],
+      });
+    });
+
+    it('flags acc.concat() on the accumulator', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [],
+        invalid: [
+          {
+            code: 'items.reduce((acc, item) => acc.concat(item), [])',
+            errors: [{ messageId: 'AccumulatorShallowCopy' }],
+          },
+        ],
+      });
+    });
+
+    it('flags object spread on the accumulator inside reduceRight', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [],
+        invalid: [
+          {
+            code: 'items.reduceRight((acc, item) => ({ ...acc, [item.key]: item.value }), {})',
+            errors: [{ messageId: 'AccumulatorSpread' }],
+          },
+        ],
+      });
+    });
+
+    it('flags multiple accumulator copies in the same reduce', () => {
+      ruleTester.run('no-reduce-accumulator-copy', rule, {
+        valid: [],
+        invalid: [
+          {
+            code: 'items.reduce((acc, item) => item.flag ? { ...acc, [item.key]: item.value } : acc.concat(item), [])',
+            errors: [
+              { messageId: 'AccumulatorSpread' },
+              { messageId: 'AccumulatorShallowCopy' },
+            ],
+          },
+        ],
+      });
+    });
+  });
+});