diff --git a/.github/workflows/coherence.yml b/.github/workflows/coherence.yml
new file mode 100644
index 0000000..b9995ae
--- /dev/null
+++ b/.github/workflows/coherence.yml
@@ -0,0 +1,16 @@
+name: Coherence
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+      - run: node scripts/check-coherence.mjs
diff --git a/CLAUDE.md b/CLAUDE.md
index c40c6a2..2f960f2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -35,4 +35,7 @@ Non-negotiables:
   (cloud/self-hosted, fresh/upgrade, existing agents, subscriptions).
 - Findings rejected by Seb (`references/decisions.md`) are never
   re-raised unless the platform changed again on that point.
+- Every page exists twice: `<page>.mdx` at the repo root, published by
+  Mintlify, and `llm-gateway/docs/<page>.mdx`, its dead twin. Edit both, or
+  the change never reaches the site. The coherence check enforces it.
 - Invariants are machine-checked: `node scripts/check-coherence.mjs` must be GREEN before any issue is filed or any docs PR is pushed. Reversing a recorded decision requires the written derivation of the reversal protocol (see the docs-sync skill).
diff --git a/scripts/check-coherence.mjs b/scripts/check-coherence.mjs
index 379ef5a..3cb81f6 100644
--- a/scripts/check-coherence.mjs
+++ b/scripts/check-coherence.mjs
@@ -6,6 +6,10 @@
  *    must have its row in reference/environment-variables.mdx.
  * 2. ERROR-CODE invariant: any M### code cited in a docs page must
  *    have its page at errors/M###.mdx.
+ * 3. TWIN invariant: every page exists twice, at the repo root and under
+ *    llm-gateway/docs/. The root copy is the published one. The two must
+ *    stay identical, so that a correction made in the wrong file is caught
+ *    here instead of silently changing nothing on the site.
  *
  * Usage: node scripts/check-coherence.mjs
  * Exit 0 = GREEN, 1 = violations (each printed with file:line).
@@ -100,6 +104,32 @@ for (const f of files) {
   });
 }
 
+// --- Twin invariant --------------------------------------------------------
+// Mintlify publishes the root copy. The copy under llm-gateway/docs/ is dead
+// weight kept for now, and an edit made there alone changes nothing on the
+// site, so the two must match byte for byte.
+const TWIN_DIR = join(ROOT, "llm-gateway/docs");
+if (existsSync(TWIN_DIR)) {
+  const twins = walk(TWIN_DIR).map((f) => relative(TWIN_DIR, f));
+  for (const page of twins) {
+    const root = join(ROOT, page);
+    if (!existsSync(root)) {
+      violations.push(`llm-gateway/docs/${page}  has no twin at the repo root, so it is never published`);
+      continue;
+    }
+    if (readFileSync(root, "utf8") !== readFileSync(join(TWIN_DIR, page), "utf8")) {
+      violations.push(`${page}  differs from llm-gateway/docs/${page} — edit both, the root copy is the published one`);
+    }
+  }
+  for (const f of walk(ROOT)) {
+    const rel = relative(ROOT, f);
+    if (rel.startsWith("llm-gateway/") || !rel.endsWith(".mdx")) continue;
+    if (!existsSync(join(TWIN_DIR, rel))) {
+      violations.push(`${rel}  has no twin under llm-gateway/docs/`);
+    }
+  }
+}
+
 const unique = [...new Set(violations)];
 if (unique.length === 0) {
   console.log(`Coherence: GREEN (${files.length} pages, ${defined.size} vars defined)`);