diff --git a/.github/workflows/build-common.yml b/.github/workflows/build-common.yml
index 839ec42c7db..acefc39ad0f 100644
--- a/.github/workflows/build-common.yml
+++ b/.github/workflows/build-common.yml
@@ -22,7 +22,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v5
@@ -46,7 +46,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Build and install local azure-monitor-opentelemetry-autoconfigure dependency
         if: env.AZURE_MONITOR_OPENTELEMETRY_AUTOCONFIGURE_SNAPSHOT != ''
@@ -90,7 +90,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Build and install local azure-monitor-opentelemetry-autoconfigure dependency
         if: env.AZURE_MONITOR_OPENTELEMETRY_AUTOCONFIGURE_SNAPSHOT != ''
@@ -160,7 +160,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Build and install local azure-monitor-opentelemetry-autoconfigure dependency
         if: env.AZURE_MONITOR_OPENTELEMETRY_AUTOCONFIGURE_SNAPSHOT != ''
@@ -228,11 +228,11 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - name: Set up Java 17
+      - name: Set up Java 21
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Build and install local azure-monitor-opentelemetry-autoconfigure dependency
         if: env.AZURE_MONITOR_OPENTELEMETRY_AUTOCONFIGURE_SNAPSHOT != ''
diff --git a/.github/workflows/codeql-daily.yml b/.github/workflows/codeql-daily.yml
index 292bb5575d4..929ccbc3da3 100644
--- a/.github/workflows/codeql-daily.yml
+++ b/.github/workflows/codeql-daily.yml
@@ -20,11 +20,11 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - name: Set up Java 17
+      - name: Set up Java 21
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v5
@@ -54,11 +54,11 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       
-      - name: Set up Java 17 (required for JNI compilation)
+      - name: Set up Java 21 (required for JNI compilation)
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Visual Studio Build Tools
         uses: microsoft/setup-msbuild@v2
diff --git a/.github/workflows/copilot-setup-steps.yml b/.github/workflows/copilot-setup-steps.yml
index 74c9d1a0f72..1d63185c226 100644
--- a/.github/workflows/copilot-setup-steps.yml
+++ b/.github/workflows/copilot-setup-steps.yml
@@ -31,7 +31,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v5
diff --git a/.github/workflows/owasp-dependency-check-daily.yml b/.github/workflows/owasp-dependency-check-daily.yml
index e1fe0f80df6..fda6b86cb56 100644
--- a/.github/workflows/owasp-dependency-check-daily.yml
+++ b/.github/workflows/owasp-dependency-check-daily.yml
@@ -13,11 +13,11 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - name: Set up Java 17
+      - name: Set up Java 21
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v5
diff --git a/.github/workflows/perf-test_daily.yml b/.github/workflows/perf-test_daily.yml
index b83cdc4678c..bd0708738c8 100644
--- a/.github/workflows/perf-test_daily.yml
+++ b/.github/workflows/perf-test_daily.yml
@@ -16,7 +16,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v5
diff --git a/buildSrc/src/main/kotlin/ai.java-conventions.gradle.kts b/buildSrc/src/main/kotlin/ai.java-conventions.gradle.kts
index d878e9a4496..7fef177d845 100644
--- a/buildSrc/src/main/kotlin/ai.java-conventions.gradle.kts
+++ b/buildSrc/src/main/kotlin/ai.java-conventions.gradle.kts
@@ -19,7 +19,7 @@ repositories {
 
 java {
   toolchain {
-    languageVersion.set(JavaLanguageVersion.of(17))
+    languageVersion.set(JavaLanguageVersion.of(21))
   }
 
   // See https://docs.gradle.org/current/userguide/upgrading_version_5.html, Automatic target JVM version
@@ -32,6 +32,10 @@ tasks.withType<JavaCompile>().configureEach {
   with(options) {
     release.set(8)
     compilerArgs.add("-Werror")
+    // We need to support compiling to Java 8.
+    // Suppress obsolete source/target warning added in JDK 21 while retaining -Werror for everything else.
+    // This only disables the 'options' lint category (e.g., the obsolete source/target messages).
+    compilerArgs.add("-Xlint:-options")
   }
 }
 
diff --git a/buildSrc/src/main/kotlin/ai.smoke-test.gradle.kts b/buildSrc/src/main/kotlin/ai.smoke-test.gradle.kts
index 6e0c264bf15..805acb06c4a 100644
--- a/buildSrc/src/main/kotlin/ai.smoke-test.gradle.kts
+++ b/buildSrc/src/main/kotlin/ai.smoke-test.gradle.kts
@@ -24,7 +24,7 @@ configurations["smokeTestRuntimeOnly"].extendsFrom(configurations.runtimeOnly.ge
 // FIXME (trask) copy-pasted from ai.java-conventions.gradle
 java {
   toolchain {
-    languageVersion.set(JavaLanguageVersion.of(17))
+    languageVersion.set(JavaLanguageVersion.of(21))
   }
 
   // See https://docs.gradle.org/current/userguide/upgrading_version_5.html, Automatic target JVM version
@@ -38,6 +38,10 @@ tasks.withType<JavaCompile>().configureEach {
   with(options) {
     release.set(8)
     compilerArgs.add("-Werror")
+    // We need to support compiling to Java 8.
+    // Suppress obsolete source/target warning added in JDK 21 while retaining -Werror for everything else.
+    // This only disables the 'options' lint category (e.g., the obsolete source/target messages).
+    compilerArgs.add("-Xlint:-options")
   }
 }
 
diff --git a/smoke-tests/apps/DiagnosticExtension/MockExtension/build.gradle.kts b/smoke-tests/apps/DiagnosticExtension/MockExtension/build.gradle.kts
index 9b7e7a601da..3b60c3d9236 100644
--- a/smoke-tests/apps/DiagnosticExtension/MockExtension/build.gradle.kts
+++ b/smoke-tests/apps/DiagnosticExtension/MockExtension/build.gradle.kts
@@ -10,7 +10,7 @@ dependencies {
 
 java {
   toolchain {
-    languageVersion.set(JavaLanguageVersion.of(17))
+    languageVersion.set(JavaLanguageVersion.of(21))
   }
 }
 
@@ -18,6 +18,10 @@ tasks.withType<JavaCompile>().configureEach {
   with(options) {
     release.set(8)
     compilerArgs.add("-Werror")
+    // We need to support compiling to Java 8.
+    // Suppress obsolete source/target warning added in JDK 21 while retaining -Werror for everything else.
+    // This only disables the 'options' lint category (e.g., the obsolete source/target messages).
+    compilerArgs.add("-Xlint:-options")
   }
 }
 
diff --git a/smoke-tests/apps/Diagnostics/JfrFileReader/build.gradle.kts b/smoke-tests/apps/Diagnostics/JfrFileReader/build.gradle.kts
index 65487e3da48..c60e970fd96 100644
--- a/smoke-tests/apps/Diagnostics/JfrFileReader/build.gradle.kts
+++ b/smoke-tests/apps/Diagnostics/JfrFileReader/build.gradle.kts
@@ -8,7 +8,7 @@ dependencies {
 
 java {
   toolchain {
-    languageVersion.set(JavaLanguageVersion.of(17))
+    languageVersion.set(JavaLanguageVersion.of(21))
   }
 }