diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index 228ae2341..cfd10cf01 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -4,7 +4,8 @@
       "Bash(find:*)",
       "Bash(java *)",
       "Bash(mvn *)",
-      "WebSearch"
+      "WebSearch",
+      "mcp__context7__query-docs"
     ]
   }
 }
diff --git a/.github/workflows/docker-ci.yml b/.github/workflows/docker-ci.yml
index 265c52c88..6584bc4b1 100644
--- a/.github/workflows/docker-ci.yml
+++ b/.github/workflows/docker-ci.yml
@@ -26,11 +26,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up JDK 25
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: liberica
-          java-version: 25
+          java-version: 21
           cache: maven
           cache-dependency-path: "**/pom.xml"
 
@@ -114,11 +114,11 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: Set up JDK 25
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: liberica
-          java-version: 25
+          java-version: 21
           cache: maven
           cache-dependency-path: "**/pom.xml"
 
diff --git a/Dockerfile b/Dockerfile
index 6a63eff29..000c2d3ec 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,8 +15,8 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #
 
-# Use JDK 2025.9 base image
-FROM pnoker/dc3-jdk:25
+# Use JDK 21 base image
+FROM pnoker/dc3-jdk:21
 LABEL dc3.author=pnoker
 LABEL dc3.author.email=pnokers.icloud.com
 
diff --git a/pom.xml b/pom.xml
index 82fb3a9c2..9d43aaebc 100644
--- a/pom.xml
+++ b/pom.xml
@@ -70,7 +70,7 @@
         <maven.deploy.skip>true</maven.deploy.skip>
 
         <!-- JDK Version -->
-        <java.version>25</java.version>
+        <java.version>21</java.version>
 
         <!-- DC3 Version -->
         <dc3.version>2026.4.29</dc3.version>
@@ -236,11 +236,11 @@
                         <configuration>
                             <rules>
                                 <requireJavaVersion>
-                                    <version>[25,26)</version>
+                                    <version>[21,)</version>
                                     <message>
-                                        This project requires JDK 25 to build.
+                                        This project targets Java 21 bytecode (via --release 21).
+                                        JDK 21 or newer is required to build.
                                         Current JDK: ${java.version}
-                                        Please install JDK 25 and set JAVA_HOME accordingly.
                                     </message>
                                 </requireJavaVersion>
                             </rules>