diff --git a/packages/zero-cache/src/workers/connection.ts b/packages/zero-cache/src/workers/connection.ts
index 510e7f726..8f85bbfa7 100644
--- a/packages/zero-cache/src/workers/connection.ts
+++ b/packages/zero-cache/src/workers/connection.ts
@@ -16,7 +16,10 @@ import {
   type PongMessage,
   upstreamSchema,
 } from '../../../zero-protocol/src/mod.js';
-import {PROTOCOL_VERSION} from '../../../zero-protocol/src/protocol-version.js';
+import {
+  MIN_SERVER_SUPPORTED_PROTOCOL_VERSION,
+  PROTOCOL_VERSION,
+} from '../../../zero-protocol/src/protocol-version.js';
 import type {ConnectParams} from '../services/dispatcher/connect-params.js';
 import type {Mutagen} from '../services/mutagen/mutagen.js';
 import type {
@@ -102,14 +105,16 @@ export class Connection {
    */
   init() {
     if (
-      this.#protocolVersion !== PROTOCOL_VERSION &&
-      this.#protocolVersion !== PROTOCOL_VERSION - 1
+      this.#protocolVersion > PROTOCOL_VERSION ||
+      this.#protocolVersion < MIN_SERVER_SUPPORTED_PROTOCOL_VERSION
     ) {
       this.#closeWithError({
         kind: ErrorKind.VersionNotSupported,
-        message: `server supports v${
-          PROTOCOL_VERSION - 1
-        } and v${PROTOCOL_VERSION} protocols`,
+        message: `server is at sync protocol v${PROTOCOL_VERSION} and does not support v${
+          this.#protocolVersion
+        }. The ${
+          this.#protocolVersion > PROTOCOL_VERSION ? 'server' : 'client'
+        } must be updated to a newer release.`,
       });
     } else {
       const connectedMessage: ConnectedMessage = [
diff --git a/packages/zero-protocol/src/protocol-version.ts b/packages/zero-protocol/src/protocol-version.ts
index 407219b6f..2af2e2369 100644
--- a/packages/zero-protocol/src/protocol-version.ts
+++ b/packages/zero-protocol/src/protocol-version.ts
@@ -1,4 +1,8 @@
+import {assert} from '../../shared/src/asserts.js';
+
 /**
+ * The current `PROTOCOL_VERSION` of the code.
+ *
  * The `PROTOCOL_VERSION` encompasses both the wire-protocol of the `/sync/...`
  * connection between the browser and `zero-cache`, as well as the format of
  * the `AST` objects stored in both components (i.e. IDB and CVR).
@@ -7,13 +11,18 @@
  * accompanied by an increment of the `PROTOCOL_VERSION` and a new major
  * release. The server (`zero-cache`) must be deployed before clients start
  * running the new code.
- *
- * The contract for backwards compatibility is that a `zero-cache` supports
- * its current `PROTOCOL_VERSION` and the previous one (i.e.
- * `PROTOCOL_VERSION - 1`, which is necessary to support old clients when
- * the server is rolled out). This corresponds to supporting clients running
- * the current release and the previous (major) release. Any client connections
- * from earlier protocol versions are closed with a `VersionNotSupported`
- * error.
  */
 export const PROTOCOL_VERSION = 3;
+
+/**
+ * The minimum protocol version supported by the server. The contract for
+ * backwards compatibility is that a `zero-cache` supports the current
+ * `PROTOCOL_VERSION` and at least the previous one (i.e. `PROTOCOL_VERSION - 1`)
+ * if not earlier ones as well. This corresponds to supporting clients running
+ * the current release and the previous (major) release. Any client connections
+ * from protocol versions before `MIN_SERVER_SUPPORTED_PROTOCOL_VERSION` are
+ * closed with a `VersionNotSupported` error.
+ */
+export const MIN_SERVER_SUPPORTED_PROTOCOL_VERSION = 2;
+
+assert(MIN_SERVER_SUPPORTED_PROTOCOL_VERSION < PROTOCOL_VERSION);