diff --git a/packages/@aws-cdk/cdk/lib/app.ts b/packages/@aws-cdk/cdk/lib/app.ts
index ee861781761f0..f3e7c6a90ff8a 100644
--- a/packages/@aws-cdk/cdk/lib/app.ts
+++ b/packages/@aws-cdk/cdk/lib/app.ts
@@ -43,6 +43,7 @@ export class App extends Root {
     }
 
     const result: cxapi.SynthesizeResponse = {
+      version: cxapi.PROTO_RESPONSE_VERSION,
       stacks: this.synthesizeStacks(Object.keys(this.stacks)),
       runtime: this.collectRuntimeInformation()
     };
diff --git a/packages/@aws-cdk/cdk/test/test.app.ts b/packages/@aws-cdk/cdk/test/test.app.ts
index 9b8d7325e1203..177ac8682219d 100644
--- a/packages/@aws-cdk/cdk/test/test.app.ts
+++ b/packages/@aws-cdk/cdk/test/test.app.ts
@@ -69,7 +69,9 @@ export = {
     response.stacks.forEach(s => delete s.metadata);
     delete response.runtime;
 
-    test.deepEqual(response, { stacks:
+    test.deepEqual(response, {
+      version: '0.14.0',
+      stacks:
       [ { name: 'stack1',
           environment:
            { name: '12345/us-east-1',
diff --git a/packages/@aws-cdk/cx-api/lib/cxapi.ts b/packages/@aws-cdk/cx-api/lib/cxapi.ts
index 9835ffd765500..f0ef41a59256b 100644
--- a/packages/@aws-cdk/cx-api/lib/cxapi.ts
+++ b/packages/@aws-cdk/cx-api/lib/cxapi.ts
@@ -4,6 +4,24 @@
 
 import { Environment } from './environment';
 
+/**
+ * Bump this to the library version if and only if the CX protocol changes.
+ *
+ * We could also have used 1, 2, 3, ... here to indicate protocol versions, but
+ * those then still need to be mapped to software versions to be useful. So we
+ * might as well use the software version as protocol version and immediately
+ * generate a useful error message from this.
+ *
+ * Note the following:
+ *
+ * - The versions are not compared in a semver way, they are used as
+ *    opaque ordered tokens.
+ * - The version needs to be set to the NEXT releasable version when it's
+ *   updated (as the current verison in package.json has already been released!)
+ * - The request does not have versioning yet, only the response.
+ */
+export const PROTO_RESPONSE_VERSION = '0.14.0';
+
 export const OUTFILE_NAME = 'cdk.out';
 export const OUTDIR_ENV = 'CDK_OUTDIR';
 export const CONTEXT_ENV = 'CDK_CONTEXT_JSON';
@@ -21,6 +39,10 @@ export interface MissingContext {
 }
 
 export interface SynthesizeResponse {
+  /**
+   * Protocol version
+   */
+  version: string;
   stacks: SynthesizedStack[];
   runtime?: AppRuntime;
 }
diff --git a/packages/aws-cdk/bin/cdk.ts b/packages/aws-cdk/bin/cdk.ts
index 27a9e21451ab7..256a5d3bc6b61 100644
--- a/packages/aws-cdk/bin/cdk.ts
+++ b/packages/aws-cdk/bin/cdk.ts
@@ -9,6 +9,7 @@ import YAML = require('js-yaml');
 import minimatch = require('minimatch');
 import os = require('os');
 import path = require('path');
+import semver = require('semver');
 import util = require('util');
 import yargs = require('yargs');
 import cdkUtil = require('../lib/util');
@@ -469,7 +470,7 @@ async function initCommandLine() {
 
         const response = await fs.readJson(outfile);
         debug(response);
-        return response;
+        return versionCheckResponse(response);
       } finally {
         debug('Removing outdir', outdir);
         await fs.remove(outdir);
@@ -508,7 +509,37 @@ async function initCommandLine() {
         });
       }
     }
+  }
+
+  /**
+   * Look at the type of response we get and upgrade it to the latest expected version
+   */
+  function versionCheckResponse(response: cxapi.SynthesizeResponse): cxapi.SynthesizeResponse {
+    if (!response.version) {
+      // tslint:disable-next-line:max-line-length
+      throw new Error(`CDK Framework >= ${cxapi.PROTO_RESPONSE_VERSION} is required in order to interact with this version of the Toolkit.`);
+    }
+
+    const frameworkVersion = semver.coerce(response.version);
+    const toolkitVersion = semver.coerce(cxapi.PROTO_RESPONSE_VERSION);
+
+    // Should not happen, but I don't trust this library 100% either, so let's check for it to be safe
+    if (!frameworkVersion || !toolkitVersion) { throw new Error('SemVer library could not parse versions'); }
+
+    if (semver.gt(frameworkVersion, toolkitVersion)) {
+      throw new Error(`CDK Toolkit >= ${response.version} is required in order to interact with this program.`);
+    }
+
+    if (semver.lt(frameworkVersion, toolkitVersion)) {
+      // Toolkit protocol is newer than the framework version, and we KNOW the
+      // version. This is a scenario in which we could potentially do some
+      // upgrading of the response in the future.
+      //
+      // For now though, we simply reject old responses.
+      throw new Error(`CDK Framework >= ${cxapi.PROTO_RESPONSE_VERSION} is required in order to interact with this version of the Toolkit.`);
+    }
 
+    return response;
   }
 
   /**
diff --git a/packages/aws-cdk/package.json b/packages/aws-cdk/package.json
index 31ad390cfb164..c768a563aa33e 100644
--- a/packages/aws-cdk/package.json
+++ b/packages/aws-cdk/package.json
@@ -39,6 +39,7 @@
     "@types/request": "^2.47.1",
     "@types/uuid": "^3.4.3",
     "@types/yargs": "^8.0.3",
+    "@types/semver": "^5.5.0",
     "cdk-build-tools": "^0.13.0",
     "mockery": "^2.1.0",
     "pkglint": "^0.13.0"
@@ -58,6 +59,7 @@
     "promptly": "^0.2.0",
     "proxy-agent": "^3.0.1",
     "request": "^2.83.0",
+    "semver": "^5.5.0",
     "source-map-support": "^0.5.6",
     "yargs": "^9.0.1"
   },