diff --git a/user.js/docs/CONVENTIONS.md b/user.js/docs/CONVENTIONS.md
index 1cde075..a9ce055 100644
--- a/user.js/docs/CONVENTIONS.md
+++ b/user.js/docs/CONVENTIONS.md
@@ -56,10 +56,15 @@ bundler/linter without discussing it first (see [AGENTS.md](../AGENTS.md) "Addit
   context. See the `networkixlan` toolbar lookup in `peeringdb-cp-consolidated-tools.src.js`.
 - **Sensitive-data redaction**: no secrets are logged or stored — there are no API tokens/credentials
   anywhere in the codebase (PeeringDB auth is same-origin session cookies).
-- **Documentation convention**: functions carry a JSDoc block with `Purpose:`, `Necessity:`, and an
-  `@ai Preserve ...` / `@ai Keep behavior stable and prefer minimal, localized edits.` tag flagging
-  behavior contracts — 900+ occurrences across the repo. This is a repo-specific convention for
-  guiding AI-assisted edits, documented in [AGENTS.md](../AGENTS.md).
+- **Documentation convention**: functions carry a JSDoc block with `Purpose:` and `Necessity:`
+  lines. An `@ai` tag, where present, states **one invariant specific to that function that a
+  plausible diff could silently violate**. Litmus test: *If the sentence could be pasted onto a
+  different function unchanged and still read true, it is not an @ai tag.* The repo-wide default —
+  keep behavior stable, prefer minimal localized edits — is stated once in
+  [AGENTS.md](../AGENTS.md) and applies to every function, tagged or not; do not restate it in
+  tags. Enforced by `tests/conventions-ai-tags.test.js`: the historical stock phrases are
+  denylisted, identical tag text may appear at most twice repo-wide, and tag prose must carry at
+  least 30 characters of substance.
 
 ## Testing
 
diff --git a/user.js/tests/conventions-ai-tags.test.js b/user.js/tests/conventions-ai-tags.test.js
new file mode 100644
index 0000000..37a163f
--- /dev/null
+++ b/user.js/tests/conventions-ai-tags.test.js
@@ -0,0 +1,107 @@
+'use strict';
+
+// Conformance tests for the @ai annotation convention. An @ai tag states ONE
+// invariant specific to its function that a plausible diff could silently
+// violate. Litmus (docs/CONVENTIONS.md): "If the sentence could be pasted onto
+// a different function unchanged and still read true, it is not an @ai tag."
+//
+// Enforcement rests on the observation that boilerplate is, by definition,
+// repeated -- so no similarity scoring is needed:
+//   (a) the seven historical stock phrases are denylisted outright (they were
+//       stripped repo-wide in favor of one global posture sentence in
+//       AGENTS.md; this blocks reintroduction with a precise error);
+//   (b) identical normalized @ai text may appear at most twice repo-wide
+//       (permits one deliberate twin pair, makes template reuse impossible);
+//   (c) tag prose must carry at least 30 characters of substance.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const ROOT = path.join(__dirname, '..');
+const SOURCE_FILES = [
+  'peeringdb-cp-consolidated-tools.src.js',
+  'peeringdb-fp-consolidated-tools.src.js',
+  'peeringdb-deskpro-tools.src.js',
+  'lib/admincom-common.js',
+  'lib/cp-name-normalization.js',
+  'lib/admincom-entity-exclusions.js',
+];
+
+// The seven historical stock phrases (exact text). These said nothing about
+// any particular function -- they restated the repo-wide editing posture now
+// stated once in AGENTS.md.
+const STOCK_PHRASES = [
+  'Keep behavior stable and prefer minimal, localized edits.',
+  'Preserve request retries/timeouts/error classification and payload assumptions.',
+  'Preserve selector contracts and idempotent DOM mutation behavior.',
+  'Preserve normalization/parsing rules and backward-compatible output formats.',
+  'Preserve shared storage/cache key contracts and TTL behavior.',
+  'Preserve execution ordering, locks, and route/module boundaries.',
+  'Preserve menu command registration behavior and gating on feature flag.',
+];
+
+const MAX_IDENTICAL_OCCURRENCES = 2;
+const MIN_PROSE_LENGTH = 30;
+
+// An @ai tag line in either comment form. The prose runs to end of line;
+// multi-line continuation is not part of the convention.
+const AI_TAG_REGEX = /^\s*(?:\*|\/\/)\s*@ai\s+(.*)$/;
+
+function normalize(prose) {
+  return prose.trim().replace(/\s+/g, ' ');
+}
+
+/** Collects every @ai tag with file, 1-based line, and normalized prose. */
+function collectTags() {
+  const tags = [];
+  for (const relPath of SOURCE_FILES) {
+    const lines = fs.readFileSync(path.join(ROOT, relPath), 'utf8').split(/\r?\n/);
+    for (let i = 0; i < lines.length; i += 1) {
+      const m = AI_TAG_REGEX.exec(lines[i]);
+      if (!m) continue;
+      tags.push({ file: relPath, line: i + 1, prose: normalize(m[1]) });
+    }
+  }
+  return tags;
+}
+
+test('@ai tag specificity', async (t) => {
+  const tags = collectTags();
+
+  await t.test('no @ai tag reuses a denylisted stock phrase', () => {
+    const stock = new Set(STOCK_PHRASES);
+    const bad = tags
+      .filter((tag) => stock.has(tag.prose))
+      .map((tag) => `${tag.file}:${tag.line} stock phrase: "${tag.prose}"`);
+    assert.deepEqual(bad, [],
+      `stock @ai phrases are banned -- the posture lives in AGENTS.md; ` +
+      `state this function's own invariant or drop the tag:\n${bad.join('\n')}`);
+  });
+
+  await t.test(`identical @ai text appears at most ${MAX_IDENTICAL_OCCURRENCES} times repo-wide`, () => {
+    const groups = new Map();
+    for (const tag of tags) {
+      if (!groups.has(tag.prose)) groups.set(tag.prose, []);
+      groups.get(tag.prose).push(`${tag.file}:${tag.line}`);
+    }
+    const bad = [];
+    for (const [prose, sites] of groups) {
+      if (sites.length > MAX_IDENTICAL_OCCURRENCES) {
+        bad.push(`"${prose}" appears ${sites.length}x:\n  ${sites.join('\n  ')}`);
+      }
+    }
+    assert.deepEqual(bad, [],
+      `boilerplate is, by definition, repeated -- an invariant shared by ` +
+      `${MAX_IDENTICAL_OCCURRENCES + 1}+ functions is not function-specific:\n${bad.join('\n')}`);
+  });
+
+  await t.test(`@ai prose carries at least ${MIN_PROSE_LENGTH} characters of substance`, () => {
+    const bad = tags
+      .filter((tag) => tag.prose.length < MIN_PROSE_LENGTH)
+      .map((tag) => `${tag.file}:${tag.line} too short (${tag.prose.length} chars): "${tag.prose}"`);
+    assert.deepEqual(bad, [],
+      `an @ai tag must state a real invariant, not a stub:\n${bad.join('\n')}`);
+  });
+});