diff --git a/.gitignore b/.gitignore
index c8996328fe..3298ccde0c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -57,6 +57,23 @@ apps/site/.next
 apps/site/.map.ts
 apps/site/.source
 
+# Next.js agent-rules files, minted by `next dev` in apps/site (objectui#4160).
+# From next 16.3 (the #4094 bump; 16.2 bundled the docs but generated nothing),
+# `next dev` detects an AI coding agent from the environment (CLAUDECODE,
+# CURSOR, CODEX, AI_AGENT, ...) and writes AGENTS.md + CLAUDE.md next to the
+# app's next.config — see node_modules/next/dist/server/lib/generate-agent-files.js.
+# They appeared from merely RUNNING the app, were neither tracked nor ignored,
+# and the block's own text says "committing it with your work keeps the tree
+# clean": in a repo staged with `git add -A` by parallel agents that is a
+# contamination trap of the objectui#3430 shape. We ignore rather than commit —
+# the prose is tool-minted per next version, and apps/site/CLAUDE.md's
+# `@AGENTS.md` import would splice framework rules into this repo's own binding
+# instruction chain. `agentRules: false` in apps/site/next.config.mjs stops the
+# minting at the source; these two lines are the belt to that braces, because
+# the flag is upstream-owned and can be renamed or dropped by any next bump.
+/apps/site/AGENTS.md
+/apps/site/CLAUDE.md
+
 # Test artifacts
 test-screenshots
 test-results
diff --git a/apps/site/.gitignore b/apps/site/.gitignore
index 9e429e4985..c692d4a40d 100644
--- a/apps/site/.gitignore
+++ b/apps/site/.gitignore
@@ -23,4 +23,11 @@ yarn-error.log*
 # others
 .env*.local
 .vercel
-next-env.d.ts
\ No newline at end of file
+next-env.d.ts
+
+# generated by `next dev` when it detects an AI coding agent (objectui#4160) —
+# turned off at the source by `agentRules: false` in next.config.mjs, ignored
+# here and in the repo root .gitignore (which carries the reasoning) because
+# that flag is upstream-owned. Anchored: only this directory's own files.
+/AGENTS.md
+/CLAUDE.md
diff --git a/apps/site/next.config.mjs b/apps/site/next.config.mjs
index 30f375a77a..9684b411f3 100644
--- a/apps/site/next.config.mjs
+++ b/apps/site/next.config.mjs
@@ -5,6 +5,18 @@ const withMDX = createMDX();
 /** @type {import('next').NextConfig} */
 const config = {
   reactStrictMode: true,
+  // Do not let `next dev` mint AGENTS.md / CLAUDE.md in this directory
+  // (objectui#4160). From next 16.3 the dev server detects an AI coding agent
+  // from the environment and writes both files here
+  // (node_modules/next/dist/server/lib/generate-agent-files.js), carrying a
+  // managed block whose own text asks the reader to commit it. This repo's
+  // AGENTS.md / CLAUDE.md are hand-written and binding, and this app's
+  // CLAUDE.md would be a bare `@AGENTS.md` import splicing framework prose into
+  // that authority chain — so we opt out here and additionally gitignore both
+  // paths, since `agentRules` is upstream-owned and a later next release can
+  // rename or drop it. Both halves are pinned by
+  // `scripts/__tests__/site-next-agent-files-4160.test.ts`.
+  agentRules: false,
   // Every entry MUST be a declared dependency of this app (pinned by
   // `scripts/__tests__/site-playground-layout-registration-3904.test.ts`).
   // Next resolves each name as `<pkg>/package.json` FROM THIS APP'S DIRECTORY to
diff --git a/scripts/__tests__/site-next-agent-files-4160.test.ts b/scripts/__tests__/site-next-agent-files-4160.test.ts
new file mode 100644
index 0000000000..b372308d15
--- /dev/null
+++ b/scripts/__tests__/site-next-agent-files-4160.test.ts
@@ -0,0 +1,171 @@
+import { describe, expect, it } from 'vitest';
+import { spawnSync } from 'node:child_process';
+import { createRequire } from 'node:module';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+/**
+ * objectui#4160 — `next dev` mints agent-rules files into `apps/site`, and this
+ * repo must be in one of the two end states Next.js expects (ignored, or
+ * committed) rather than neither.
+ *
+ * What happens without this: from next 16.3 — the #4094 bump; on 16.2 the docs
+ * were bundled but nothing was generated — `next dev` calls
+ * `ensureAgentRulesForDev` (`node_modules/next/dist/server/lib/app-info-log.js`),
+ * which asks `@vercel/detect-agent` whether an AI coding agent is driving the
+ * session (CLAUDECODE / CURSOR / CODEX / AI_AGENT / ... in the environment — all
+ * true for the agents that work this repo) and then writes `AGENTS.md` and
+ * `CLAUDE.md` beside the app's `next.config`. They were neither tracked nor
+ * ignored, so they surfaced as `??` in `git status` from merely RUNNING the app,
+ * and the generated block says "committing it with your work keeps the tree
+ * clean" — an untracked file that appears on its own and asks to be committed is
+ * exactly what `git add -A` sweeps into an unrelated PR (the objectui#3430
+ * shape). `apps/site/CLAUDE.md` is a bare `@AGENTS.md` import, so committing it
+ * would also splice framework-owned prose into this repo's binding instruction
+ * chain.
+ *
+ * The fix has two independent halves and this file pins both, because each
+ * covers the other's failure mode:
+ *
+ *  - `agentRules: false` in `apps/site/next.config.mjs` — the upstream opt-out,
+ *    which stops the write instead of hiding it. Upstream-owned: a later next
+ *    release may rename or drop the key, and an unknown key in that
+ *    `z.strictObject` config schema only warns.
+ *  - the two paths gitignored (repo root `.gitignore` + `apps/site/.gitignore`,
+ *    matching how this repo ignores the app's other dev-time artifacts) — ours,
+ *    and still correct if the flag stops working or another entry point
+ *    (`create-next-app`, `@next/codemod agents-md`) writes the files.
+ *
+ * Reverse verification (direction predicted before running — the two halves fail
+ * DIFFERENTLY, which is the point):
+ *  - drop `agentRules: false` and re-run `next dev`: the files are minted again,
+ *    but `git status --short` stays clean because the ignore catches them;
+ *    the config case here goes red.
+ *  - drop the ignore lines: `next dev` mints nothing (the flag holds), so the
+ *    tree looks fine — nothing observable regresses until the flag stops
+ *    working. That silence is why the ignore is pinned here rather than left to
+ *    a dev-server run to notice.
+ *
+ * The filenames are upstream literals, so they are not hardcoded as the only
+ * source of truth: the last case reads them back out of the installed next and
+ * fails if it starts minting a name this repo does not ignore.
+ */
+
+const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '../..');
+const SITE_DIR = path.join(repoRoot, 'apps/site');
+const NEXT_CONFIG = path.join(SITE_DIR, 'next.config.mjs');
+
+/** Repo-relative paths `next dev` writes today. */
+const MINTED = ['apps/site/AGENTS.md', 'apps/site/CLAUDE.md'] as const;
+
+/** `git check-ignore` exits 0 when ignored, 1 when not, >1 on error. */
+function isIgnored(relPath: string): boolean {
+  const result = spawnSync('git', ['check-ignore', '-q', '--', relPath], {
+    cwd: repoRoot,
+    encoding: 'utf8',
+  });
+  if (result.status !== 0 && result.status !== 1) {
+    throw new Error(`git check-ignore failed for ${relPath}: ${result.stderr ?? ''}`);
+  }
+  return result.status === 0;
+}
+
+function isTracked(relPath: string): boolean {
+  const result = spawnSync('git', ['ls-files', '--error-unmatch', '--', relPath], {
+    cwd: repoRoot,
+    encoding: 'utf8',
+  });
+  return result.status === 0;
+}
+
+/**
+ * Directory of the `next` package as `apps/site` resolves it. Under pnpm's
+ * strict linker this is the app's own copy, which is the one `next dev` loads.
+ */
+function resolveNextDir(): string {
+  const require = createRequire(path.join(SITE_DIR, 'noop.js'));
+  return path.dirname(require.resolve('next/package.json'));
+}
+
+describe('objectui#4160 — the agent-rules files `next dev` mints in apps/site are ignored, not committed', () => {
+  it.each(MINTED)('%s is gitignored', (relPath) => {
+    expect(
+      isIgnored(relPath),
+      `${relPath} is minted by \`next dev\` (agent detected) and is not gitignored, so it ` +
+        'shows up as an untracked file after merely running the docs site — and its own text ' +
+        'invites the reader to commit it. Restore the entry in the repo root .gitignore.'
+    ).toBe(true);
+  });
+
+  it.each(MINTED)('%s is not tracked either — the decision was ignore, not commit', (relPath) => {
+    // The other end state Next.js offers. Committing it would pin tool-minted
+    // prose that the next `next` bump rewrites, inside this repo's instruction
+    // namespace; a tracked copy would also go dirty on every dev-server run.
+    expect(
+      isTracked(relPath),
+      `${relPath} is tracked. objectui#4160 chose to IGNORE these files: the block is minted ` +
+        "per next version and conflicts with this repo's own AGENTS.md authority chain. " +
+        'If that decision is being reversed, this case and the .gitignore entries go together.'
+    ).toBe(false);
+  });
+
+  it('turns the minting off at the source with `agentRules: false`', () => {
+    const source = fs.readFileSync(NEXT_CONFIG, 'utf8');
+    expect(
+      /^\s*agentRules:\s*false\s*,/m.test(source),
+      'apps/site/next.config.mjs no longer sets `agentRules: false`, so `next dev` writes ' +
+        'AGENTS.md and CLAUDE.md again on every run. The gitignore entries keep them out of ' +
+        '`git status`, but the files still land in the working tree.'
+    ).toBe(true);
+  });
+});
+
+describe('objectui#4160 — the pin still matches the installed next', () => {
+  const nextDir = resolveNextDir();
+  const generator = path.join(nextDir, 'dist/server/lib/generate-agent-files.js');
+  const configSchema = path.join(nextDir, 'dist/server/config-schema.js');
+
+  it('finds the generator this pin is about', () => {
+    // A zero-hit lookup would make the next case vacuously green — the
+    // empty-fixture trap. If next restructures its dist, this is the signal to
+    // re-check where the agent files come from, not something to delete.
+    expect(
+      fs.existsSync(generator),
+      `${path.relative(repoRoot, generator)} is gone. Re-verify whether \`next dev\` still ` +
+        'mints agent files (and under what names) before trusting the entries in .gitignore.'
+    ).toBe(true);
+  });
+
+  it('ignores every filename the installed next actually writes', () => {
+    // Read the names back out of upstream rather than trusting our own list: a
+    // next bump that renames AGENTS.md, or adds a third file, would otherwise
+    // re-expose the trap through a path nothing here covers.
+    const source = fs.readFileSync(generator, 'utf8');
+    const written = [...new Set(Array.from(source.matchAll(/'([\w.-]+\.md)'/g), (m) => m[1]))].sort();
+
+    expect(written.length, 'no .md filename literals found in the generator').toBeGreaterThan(0);
+
+    const uncovered = written.filter((name) => !isIgnored(`apps/site/${name}`));
+    expect(
+      uncovered,
+      `next ${JSON.stringify(written)} mints ${JSON.stringify(uncovered)} under apps/site, ` +
+        'which nothing in this repo ignores. Add the path(s) to the root .gitignore and to ' +
+        'apps/site/.gitignore.'
+    ).toEqual([]);
+  });
+
+  it('still accepts the `agentRules` opt-out key', () => {
+    // next validates next.config against a `z.strictObject`; an unknown key only
+    // WARNS at dev-server startup. So a bump that renames or removes this key
+    // silently turns the opt-out back on, and only the gitignore half survives.
+    const schema = fs.readFileSync(configSchema, 'utf8');
+    expect(
+      /\bagentRules\s*:/.test(schema),
+      'the installed next no longer declares `agentRules` in its config schema, so the opt-out ' +
+        'in apps/site/next.config.mjs is now an unrecognized key (a startup warning, not an ' +
+        'error) and the files are being minted again. Find the replacement flag; the gitignore ' +
+        'entries are the only thing holding until then.'
+    ).toBe(true);
+  });
+});