diff --git a/.changes/aa10152c-0040-46b3-8551-ed63aca0c6ee.json b/.changes/aa10152c-0040-46b3-8551-ed63aca0c6ee.json
new file mode 100644
index 00000000..fcfa63c0
--- /dev/null
+++ b/.changes/aa10152c-0040-46b3-8551-ed63aca0c6ee.json
@@ -0,0 +1,8 @@
+{
+    "id": "aa10152c-0040-46b3-8551-ed63aca0c6ee",
+    "type": "bugfix",
+    "description": "Enable building this project on JDK21 by setting -Xjdk-release flag",
+    "issues": [
+        "https://github.com/smithy-lang/smithy-kotlin/issues/1295"
+    ]
+}
\ No newline at end of file
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 36e11d7b..43ec0386 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -7,10 +7,6 @@ on:
       - '!main'
 
 env:
-  BUILDER_VERSION: v0.8.22
-  BUILDER_SOURCE: releases
-  BUILDER_HOST: https://d19elf31gohf1l.cloudfront.net
-  PACKAGE_NAME: aws-crt-kotlin
   RUN: ${{ github.run_id }}-${{ github.run_number }}
 
 jobs:
@@ -19,6 +15,7 @@ jobs:
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
+
       - uses: actions/cache@v4
         with:
           path: |
@@ -27,18 +24,25 @@ jobs:
           key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
           restore-keys: |
             ${{ runner.os }}-gradle-
-      - name: Build and Test ${{ env.PACKAGE_NAME }}
+
+      - name: Configure JDK
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'corretto'
+          java-version: 17
+          cache: 'gradle'
+
+      - name: Build and Test
         run: |
-          python3 -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz?run=${{ env.RUN }}', 'builder.pyz')"
-          chmod a+x builder.pyz
           echo "kotlinWarningsAsErrors=true" >> $GITHUB_WORKSPACE/local.properties
-          ./builder.pyz build -p ${{ env.PACKAGE_NAME }}
+          ./gradlew allTests
 
   macos-compat:
     runs-on: macos-latest
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
+
       - uses: actions/cache@v4
         with:
           path: |
@@ -47,19 +51,42 @@ jobs:
           key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
           restore-keys: |
             ${{ runner.os }}-gradle-
-      - name: Build and Test ${{ env.PACKAGE_NAME }}
+
+      - name: Configure JDK
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'corretto'
+          java-version: 17
+          cache: 'gradle'
+
+      - name: Build and Test
         run: |
-          python3 -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz?run=${{ env.RUN }}', 'builder.pyz')"
-          chmod a+x builder.pyz
           echo "kotlinWarningsAsErrors=true" >> $GITHUB_WORKSPACE/local.properties
-          ./builder.pyz build -p ${{ env.PACKAGE_NAME }}
+          ./gradlew allTests
 
   windows-compat:
     runs-on: windows-latest
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
-      - name: Build and Test ${{ env.PACKAGE_NAME }}
+
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
+      - name: Configure JDK
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'corretto'
+          java-version: 17
+          cache: 'gradle'
+
+      - name: Build and Test
         run: |
-          python3 -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz?run=${{ env.RUN }}', 'builder.pyz')"
-          python3 builder.pyz build -p ${{ env.PACKAGE_NAME }}
+          echo "kotlinWarningsAsErrors=true" >> $GITHUB_WORKSPACE/local.properties
+          ./gradlew allTests
diff --git a/build.gradle.kts b/build.gradle.kts
index 248c6286..139832ce 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -41,6 +41,7 @@ subprojects {
     tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
         compilerOptions {
             jvmTarget.set(JvmTarget.JVM_1_8)
+            freeCompilerArgs.add("-Xjdk-release=1.8")
             freeCompilerArgs.add("-Xexpect-actual-classes")
         }
     }