diff --git a/test-apps/extending-blocks-guide-blocksbackend/aws-blocks/index.cdk.ts b/test-apps/extending-blocks-guide-blocksbackend/aws-blocks/index.cdk.ts
index 2443e5d34..05732c7ca 100644
--- a/test-apps/extending-blocks-guide-blocksbackend/aws-blocks/index.cdk.ts
+++ b/test-apps/extending-blocks-guide-blocksbackend/aws-blocks/index.cdk.ts
@@ -11,7 +11,7 @@
 import * as cdk from 'aws-cdk-lib';
 import { RemovalPolicies, Mixins } from 'aws-cdk-lib';
 import * as sqs from 'aws-cdk-lib/aws-sqs';
-import { BlocksBackend, SandboxDisableDeletionProtection } from '@aws-blocks/blocks/cdk';
+import { BlocksBackend, SandboxDisableDeletionProtection, registerConfig } from '@aws-blocks/blocks/cdk';
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
 import { getSandboxId } from './scripts/sandbox-id.js';
@@ -53,9 +53,9 @@ class MyExistingStack extends cdk.Stack {
       backendCDKPath: join(__dirname, 'index.ts'),
     });
 
-    // Wire IAM + env on the BlocksBackend's handler — same surface as BlocksStack.
+    // Wire IAM and runtime configuration on the BlocksBackend — same surface as BlocksStack.
     stack.externalQueue.grantSendMessages(stack.blocks.handler);
-    stack.blocks.handler.addEnvironment('EXTERNAL_QUEUE_URL', stack.externalQueue.queueUrl);
+    registerConfig(stack.blocks, 'EXTERNAL_QUEUE_URL', stack.externalQueue.queueUrl);
 
     new cdk.CfnOutput(stack, 'ApiUrl', { value: stack.blocks.apiUrl });
     new cdk.CfnOutput(stack, 'ExternalQueueUrl', { value: stack.externalQueue.queueUrl });
diff --git a/test-apps/extending-blocks-guide-blocksbackend/test/synth.test.ts b/test-apps/extending-blocks-guide-blocksbackend/test/synth.test.ts
index 94d703ff7..add8b4c7f 100644
--- a/test-apps/extending-blocks-guide-blocksbackend/test/synth.test.ts
+++ b/test-apps/extending-blocks-guide-blocksbackend/test/synth.test.ts
@@ -5,8 +5,8 @@
  * Synth-time validation for the BlocksBackend harness.
  *
  * Asserts that BlocksBackend.create produces the expected resources INSIDE
- * the user-owned outer stack (not in a separate stack), and that env vars
- * + IAM are wired correctly to the BlocksBackend's handler.
+ * the user-owned outer stack (not in a separate stack), and that runtime
+ * configuration + IAM are wired correctly to the BlocksBackend's handler.
  */
 import { test, describe, before } from 'node:test';
 import assert from 'node:assert';
@@ -29,15 +29,13 @@ describe('extending-blocks-guide-blocksbackend synth', () => {
     template = JSON.parse(readFileSync(join(SYNTH_DIR, files[0]), 'utf-8'));
   });
 
-  test('exactly one Lambda function (the BlocksBackend handler)', () => {
-    const lambdas = Object.values(template.Resources).filter(
-      (r: any) => r.Type === 'AWS::Lambda::Function'
-    );
-    assert.strictEqual(
-      lambdas.length,
-      1,
-      `expected 1 Lambda (BlocksBackend handler); got ${lambdas.length}.`
+  test('contains the BlocksBackend handler', () => {
+    const handler = Object.values(template.Resources).find(
+      (r: any) =>
+        r.Type === 'AWS::Lambda::Function' &&
+        r.Properties?.Environment?.Variables?.NODE_ENV === 'production'
     );
+    assert.ok(handler, 'could not find BlocksBackend handler Lambda');
   });
 
   test('exactly one API Gateway (managed by BlocksBackend)', () => {
@@ -54,7 +52,7 @@ describe('extending-blocks-guide-blocksbackend synth', () => {
     assert.strictEqual(queues.length, 1);
   });
 
-  test('BlocksBackend handler has EXTERNAL_QUEUE_URL injected by the outer stack', () => {
+  test('registers EXTERNAL_QUEUE_URL in the runtime config', () => {
     const handler: any = Object.values(template.Resources).find(
       (r: any) =>
         r.Type === 'AWS::Lambda::Function' &&
@@ -62,6 +60,15 @@ describe('extending-blocks-guide-blocksbackend synth', () => {
     );
     assert.ok(handler, 'could not find Blocks handler Lambda');
     const envVars = handler.Properties.Environment.Variables;
-    assert.ok('EXTERNAL_QUEUE_URL' in envVars, 'missing EXTERNAL_QUEUE_URL on BlocksBackend handler');
+    assert.ok('BLOCKS_CONFIG_BUCKET' in envVars, 'missing BLOCKS_CONFIG_BUCKET on handler');
+    assert.ok('BLOCKS_CONFIG_KEY' in envVars, 'missing BLOCKS_CONFIG_KEY on handler');
+    assert.ok(!('EXTERNAL_QUEUE_URL' in envVars), 'EXTERNAL_QUEUE_URL must use runtime config');
+
+    const configDeployment: any = Object.values(template.Resources).find(
+      (r: any) => r.Type === 'Custom::CDKBucketDeployment'
+    );
+    assert.ok(configDeployment, 'missing runtime config deployment');
+    const config = JSON.stringify(configDeployment.Properties.SourceMarkers);
+    assert.ok(config.includes('EXTERNAL_QUEUE_URL'), 'missing EXTERNAL_QUEUE_URL in runtime config');
   });
 });
diff --git a/test-apps/extending-blocks-guide/aws-blocks/index.cdk.ts b/test-apps/extending-blocks-guide/aws-blocks/index.cdk.ts
index 23d8adf6f..898215fa6 100644
--- a/test-apps/extending-blocks-guide/aws-blocks/index.cdk.ts
+++ b/test-apps/extending-blocks-guide/aws-blocks/index.cdk.ts
@@ -5,7 +5,7 @@ import * as cdk from 'aws-cdk-lib';
 import { RemovalPolicies, Mixins } from 'aws-cdk-lib';
 import * as sqs from 'aws-cdk-lib/aws-sqs';
 import * as dynamodb from 'aws-cdk-lib/aws-dynamodb';
-import { BlocksStack, SandboxDisableDeletionProtection } from '@aws-blocks/blocks/cdk';
+import { BlocksStack, SandboxDisableDeletionProtection, registerConfig } from '@aws-blocks/blocks/cdk';
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
 import { getSandboxId } from './scripts/sandbox-id.js';
@@ -38,7 +38,7 @@ Mixins.of(blocksStack).apply(new SandboxDisableDeletionProtection());
 // Pattern 1 fixture: an SQS queue we'll send to via raw SDK.
 const externalQueue = new sqs.Queue(blocksStack, 'external-queue');
 externalQueue.grantSendMessages(blocksStack.handler);
-blocksStack.handler.addEnvironment('EXTERNAL_QUEUE_URL', externalQueue.queueUrl);
+registerConfig(blocksStack, 'EXTERNAL_QUEUE_URL', externalQueue.queueUrl);
 
 // Pattern 2 fixtures: two DynamoDB tables that pretend to predate Blocks.
 // We pre-pinned the physical names above so KVStore.fromExisting and
@@ -67,7 +67,7 @@ new cdk.CfnOutput(blocksStack, 'StackNameOut', { value: stackName });
 
 // Surface the table names to runtime — *.fromExisting on the runtime side
 // reads them from process.env.
-blocksStack.handler.addEnvironment('BLOCKS_LEGACY_SESSIONS_TABLE', legacyTableName);
-blocksStack.handler.addEnvironment('BLOCKS_LEGACY_USERS_TABLE', legacyUsersTableName);
+registerConfig(blocksStack, 'BLOCKS_LEGACY_SESSIONS_TABLE', legacyTableName);
+registerConfig(blocksStack, 'BLOCKS_LEGACY_USERS_TABLE', legacyUsersTableName);
 
 cdk.Tags.of(blocksStack).add('blocks:purpose', 'extending-guide-validation');
diff --git a/test-apps/extending-blocks-guide/packages/bb-queue/src/index.cdk.ts b/test-apps/extending-blocks-guide/packages/bb-queue/src/index.cdk.ts
index 679df9f4a..23b944a06 100644
--- a/test-apps/extending-blocks-guide/packages/bb-queue/src/index.cdk.ts
+++ b/test-apps/extending-blocks-guide/packages/bb-queue/src/index.cdk.ts
@@ -3,7 +3,7 @@
 
 import { Duration } from 'aws-cdk-lib';
 import * as sqs from 'aws-cdk-lib/aws-sqs';
-import { Scope } from '@aws-blocks/core/cdk';
+import { Scope, registerConfig } from '@aws-blocks/core/cdk';
 import type { ScopeParent } from '@aws-blocks/core';
 
 export interface QueueOptions {
@@ -21,7 +21,7 @@ export class Queue extends Scope {
     });
 
     queue.grantSendMessages(this.handler);
-    this.handler.addEnvironment(`${envSafe(this.fullId)}_URL`, queue.queueUrl);
+    registerConfig(this, `${envSafe(this.fullId)}_URL`, queue.queueUrl);
   }
 }
 
diff --git a/test-apps/extending-blocks-guide/test/synth.test.ts b/test-apps/extending-blocks-guide/test/synth.test.ts
index 369f7ea3c..8027ddb36 100644
--- a/test-apps/extending-blocks-guide/test/synth.test.ts
+++ b/test-apps/extending-blocks-guide/test/synth.test.ts
@@ -6,10 +6,10 @@
  * template to assert each pattern produced (or *didn't* produce) the
  * expected resources.
  *
- * Pattern 1: external SQS queue + handler env var EXTERNAL_QUEUE_URL.
+ * Pattern 1: external SQS queue + runtime config EXTERNAL_QUEUE_URL.
  * Pattern 2: legacy DynamoDB table is created by us; KVStore.fromExisting
  *            does NOT add a second table (this asserts the bb-kv-store fix).
- * Pattern 3: bb-queue creates its own SQS queue, env var WORK_URL on handler.
+ * Pattern 3: bb-queue creates its own SQS queue, runtime config WORK_URL.
  */
 import { test, describe, before } from 'node:test';
 import assert from 'node:assert';
@@ -70,7 +70,7 @@ describe('extending-blocks-guide synth', () => {
     assert.strictEqual(queues.length, 2, `expected 2 SQS queues; got ${queues.length}`);
   });
 
-  test('handler has EXTERNAL_QUEUE_URL and BLOCKS_LEGACY_SESSIONS_TABLE env vars', () => {
+  test('registers external resource identifiers in the runtime config', () => {
     const handler: any = Object.values(template.Resources).find(
       (r: any) =>
         r.Type === 'AWS::Lambda::Function' &&
@@ -78,10 +78,17 @@ describe('extending-blocks-guide synth', () => {
     );
     assert.ok(handler, 'could not find Blocks handler Lambda');
     const envVars = handler.Properties.Environment.Variables;
-    assert.ok('EXTERNAL_QUEUE_URL' in envVars, 'missing EXTERNAL_QUEUE_URL on handler');
-    assert.ok('BLOCKS_LEGACY_SESSIONS_TABLE' in envVars, 'missing BLOCKS_LEGACY_SESSIONS_TABLE on handler');
-    // Pattern 3: bb-queue injects EXTENDING_GUIDE_WORK_URL.
-    const workKey = Object.keys(envVars).find(k => k.endsWith('_WORK_URL'));
-    assert.ok(workKey, 'missing *_WORK_URL on handler (Pattern 3 custom BB)');
+    assert.ok('BLOCKS_CONFIG_BUCKET' in envVars, 'missing BLOCKS_CONFIG_BUCKET on handler');
+    assert.ok('BLOCKS_CONFIG_KEY' in envVars, 'missing BLOCKS_CONFIG_KEY on handler');
+    assert.ok(!('EXTERNAL_QUEUE_URL' in envVars), 'EXTERNAL_QUEUE_URL must use runtime config');
+
+    const configDeployment: any = Object.values(template.Resources).find(
+      (r: any) => r.Type === 'Custom::CDKBucketDeployment'
+    );
+    assert.ok(configDeployment, 'missing runtime config deployment');
+    const config = JSON.stringify(configDeployment.Properties.SourceMarkers);
+    assert.ok(config.includes('EXTERNAL_QUEUE_URL'), 'missing EXTERNAL_QUEUE_URL in runtime config');
+    assert.ok(config.includes('BLOCKS_LEGACY_SESSIONS_TABLE'), 'missing legacy sessions table in runtime config');
+    assert.ok(config.includes('WORK_URL'), 'missing custom BB queue URL in runtime config');
   });
 });