diff --git a/src/model.ts b/src/model.ts
index 5499382..fe519b4 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -116,6 +116,14 @@ export function emptyArtifact(docPath: string): Artifact {
   };
 }
 
+/**
+ * INV-16 fail-safe: schemaVersion IS the major. A sidecar from a future major
+ * is render-only — a conforming writer never rewrites what it can't fully read.
+ */
+export function isNewerMajor(a: Pick<Artifact, "schemaVersion">): boolean {
+  return a.schemaVersion > SCHEMA_VERSION;
+}
+
 /** Stable, generated id (spec §6.3). */
 export function newId(prefix: string): string {
   return `${prefix}_${randomUUID()}`;
diff --git a/src/store.ts b/src/store.ts
index e8c467a..3d6c451 100644
--- a/src/store.ts
+++ b/src/store.ts
@@ -8,7 +8,7 @@
  */
 import * as fs from "node:fs";
 import * as path from "node:path";
-import { emptyArtifact, serializeArtifact, type Artifact } from "./model";
+import { SCHEMA_VERSION, emptyArtifact, isNewerMajor, serializeArtifact, type Artifact } from "./model";
 
 export class CoauthorStore {
   /**
@@ -67,6 +67,13 @@ export class CoauthorStore {
    */
   update(docPath: string, mutate: (artifact: Artifact) => void): Artifact {
     const artifact = this.load(docPath) ?? emptyArtifact(docPath);
+    if (isNewerMajor(artifact)) {
+      // INV-16 backstop: the controllers gate first (VersionGuard); this throw
+      // guarantees no missed write path can ever destroy a newer rung's data.
+      throw new Error(
+        `refusing to write ${docPath}: sidecar schemaVersion ${artifact.schemaVersion} > supported ${SCHEMA_VERSION} (INV-16)`,
+      );
+    }
     mutate(artifact);
     const referenced = new Set<string>([
       ...artifact.threads.map((t) => t.anchorId),
diff --git a/test/model.test.ts b/test/model.test.ts
index 3815baa..7baf525 100644
--- a/test/model.test.ts
+++ b/test/model.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect } from "vitest";
 import {
   SCHEMA_VERSION,
   emptyArtifact,
+  isNewerMajor,
   serializeArtifact,
   type Artifact,
 } from "../src/model";
@@ -103,6 +104,13 @@ describe("cross-rung identity (F5 SLICE-2)", () => {
   });
 });
 
+describe("isNewerMajor (F5 SLICE-2, INV-16)", () => {
+  it("is false for v1 and true for a newer major", () => {
+    expect(isNewerMajor(emptyArtifact("d.md"))).toBe(false);
+    expect(isNewerMajor({ schemaVersion: 2 })).toBe(true);
+  });
+});
+
 describe("unknown-field preservation (F5 SLICE-2, INV-15)", () => {
   it("a rewrite preserves unknown fields at every level, after known keys, sorted", () => {
     const foreign = {
diff --git a/test/store.test.ts b/test/store.test.ts
index 9cc8448..3a43ab3 100644
--- a/test/store.test.ts
+++ b/test/store.test.ts
@@ -107,3 +107,20 @@ describe("CoauthorStore", () => {
     expect(store.consumeSelfWrite(p)).toBe(false);
   });
 });
+
+describe("newer-major write refusal (F5 SLICE-2, INV-16 backstop)", () => {
+  it("update REFUSES to write a sidecar from a newer major, leaving bytes untouched", () => {
+    const store = new CoauthorStore(root);
+    const p = store.sidecarPath("docs/v2.md");
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    const v2 = { ...emptyArtifact("docs/v2.md"), schemaVersion: 2 };
+    fs.writeFileSync(p, serializeArtifact(v2), "utf8");
+    const before = fs.readFileSync(p, "utf8");
+    expect(() =>
+      store.update("docs/v2.md", (a) => {
+        a.threads = [];
+      }),
+    ).toThrow(/INV-16/);
+    expect(fs.readFileSync(p, "utf8")).toBe(before);
+  });
+});