diff --git a/.circleci/config.yml b/.circleci/config.yml
deleted file mode 100644
index 7ad8a15..0000000
--- a/.circleci/config.yml
+++ /dev/null
@@ -1,44 +0,0 @@
-version: 2.1
-
-workflows:
-  build:
-    jobs:
-      - build:
-          name: build_lowest
-          composer_args: --prefer-lowest
-      - build:
-          name: build_highest
-
-jobs:
-  build:
-    parameters:
-      composer_args:
-        type: string
-        default: ""
-    docker:
-      - image: skpr/php-circleci:8.1-v2-latest
-    working_directory: /data
-    steps:
-      - checkout
-      - restore_cache:
-          keys:
-            - deps-{{ arch }}
-      - run:
-          name: "Install Dependencies"
-          command: composer update --prefer-dist --no-progress --no-interaction <<parameters.composer_args>>
-      - save_cache:
-          key: deps-{{ arch }}
-          paths:
-            - vendor
-      - run:
-          name: "Lint"
-          command: ./bin/phpcs
-      - run:
-          name: "Test"
-          command: |
-            mkdir -p ~/phpunit
-            ./bin/phpunit --testsuite unit --log-junit ~/phpunit/phpunit.xml
-      - store_test_results:
-          path: ~/phpunit
-      - store_artifacts:
-          path: ~/phpunit
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
new file mode 100644
index 0000000..a8a48ff
--- /dev/null
+++ b/.github/workflows/build.yml
@@ -0,0 +1,44 @@
+name: 🏗 Build
+
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        prefer_lowest: ["", "--prefer-lowest"]
+        php: ["8.2", "8.5"]
+    container:
+      image: skpr/php-cli:${{ matrix.php }}-dev-v2-stable
+      options:
+        --pull always
+        --user 1001:1001
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+        with:
+          show-progress: false
+      - name: Composer Update
+        run: composer update --with-all-dependencies --prefer-dist --no-progress --no-interaction ${{ matrix.prefer_lowest }}
+      - name: PHPCS
+        # Convert emacs format (file:line:col: severity - message) to GitHub annotations (::severity file=,line=,col=::message)
+        run: |
+          ./bin/phpcs --report=emacs -q | sed -E 's/^([^:]+):([0-9]+):([0-9]+): (error|warning) - (.*)$/::\4 file=\1,line=\2,col=\3::\5/' || true
+          ./bin/phpcs --report=full
+      - name: PHPStan
+        run: ./bin/phpstan --error-format=github analyse
+      - name: Add PHPUnit matcher
+        run: |
+          cat << 'EOF' > .github/phpunit-failure.json
+          {"problemMatcher":[{"owner":"phpunit-failure","severity":"error","pattern":[{"regexp":"##teamcity\\[testFailed[^\\]]*message='([^']*)'[^\\]]*details='/data/([^:]+):(\\d+)","message":1,"file":2,"line":3}]}]}
+          EOF
+          echo "::add-matcher::.github/phpunit-failure.json"
+      - name: Run Tests
+        run: ./bin/phpunit --teamcity
+      - name: Remove PHPUnit matcher
+        if: always()
+        run: echo "::remove-matcher owner=phpunit-failure::"
diff --git a/.gitignore b/.gitignore
index 0d12a30..ee8bcb7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /bin
 /vendor
 /composer.lock
+.phpunit.cache
 .phpunit.result.cache
diff --git a/composer.json b/composer.json
index f74711c..153a2ea 100644
--- a/composer.json
+++ b/composer.json
@@ -9,16 +9,16 @@
     }
   ],
   "require": {
-    "php": "^8.1",
+    "php": "^8.2",
     "ext-json": "*",
-    "symfony/property-access": "^5.4|^6.2|^7.1",
-    "symfony/serializer": "^5.4|^6.2|^7.1"
+    "symfony/property-access": "^6.4.32|^7.4.8|^8.0.8",
+    "symfony/serializer": "^6.4.32|^7.4.8|^8.0.10"
   },
   "require-dev": {
-    "drupal/coder": "^8.3.16",
-    "phpunit/phpunit": "^9.5.28",
-    "phpspec/prophecy": "^1.16",
-    "symfony/phpunit-bridge": "^5.4|^6.2|^7.1"
+    "drupal/coder": "^9.0.0",
+    "phpspec/prophecy": "^1.26.1",
+    "phpstan/phpstan": "^2.1.55",
+    "phpunit/phpunit": "^11.5.55"
   },
   "autoload": {
     "psr-4": {"PNX\\Prometheus\\": "src/"}
diff --git a/phpcs.xml b/phpcs.xml
index cb9efcd..c0657de 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -1,26 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <ruleset name="php-prometheus">
   <description>PHP CodeSniffer configuration for PHP Prometheus</description>
-  <!-- folders to scan -->
   <file>./src</file>
   <file>./tests</file>
-  <!-- extensions to scan -->
   <arg name="extensions" value="php"/>
-  <!-- additional arguments -->
-  <arg name="report" value="full"/>
-  <!-- inherit from coder -->
-  <rule ref="./vendor/drupal/coder/coder_sniffer/Drupal"/>
-  <!-- Additional detailed sniff configuration -->
-  <!-- You can determine sniff names by running with -s flag -->
-  <!-- Example 1 - ignore Drupal.NamingConventions.ValidFunctionName.ScopeNotCamelCaps
-    <rule ref="Drupal.NamingConventions.ValidFunctionName.ScopeNotCamelCaps">
-      <severity>0</severity>
-    </rule>
-  // End example 1 -->
-  <!-- Example 2 - ignore rule for specific files
-    <rule ref="Drupal.NamingConventions.ValidVariableName.LowerCamelName">
-      <exclude-pattern>OfferNormalizerTrait.php</exclude-pattern>
-      <exclude-pattern>TimeOfUseValidationTrait.php</exclude-pattern>
-    </rule>
-  // End example 2 -->
+  <rule ref="Drupal"/>
 </ruleset>
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..0ce79d5
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,5 @@
+parameters:
+  level: 6
+  paths:
+    - src
+    - tests
diff --git a/phpunit.xml b/phpunit.xml
index e3c3655..0b802c0 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,34 +1,24 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!-- TODO set checkForUnintentionallyCoveredCode="true" once https://www.drupal.org/node/2626832 is resolved. -->
-<!-- PHPUnit expects functional tests to be run with either a privileged user
- or your current system user. See core/tests/README.md and
- https://www.drupal.org/node/2116263 for details.
--->
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" bootstrap="tests/bootstrap.php" colors="true" beStrictAboutTestsThatDoNotTestAnything="true" beStrictAboutOutputDuringTests="true" beStrictAboutChangesToGlobalState="true" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
-  <coverage>
-    <include>
-      <directory>./src</directory>
-    </include>
-    <exclude>
-      <directory suffix="Test.php">./</directory>
-      <directory suffix="TestBase.php">./</directory>
-    </exclude>
-  </coverage>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/11.5/phpunit.xsd"
+         bootstrap="tests/bootstrap.php"
+         cacheDirectory=".phpunit.cache"
+         colors="true"
+         beStrictAboutTestsThatDoNotTestAnything="true"
+         beStrictAboutOutputDuringTests="true"
+         beStrictAboutChangesToGlobalState="true">
   <php>
-    <!-- Set error reporting to E_ALL. -->
     <ini name="error_reporting" value="32767"/>
-    <!-- Do not limit the amount of memory tests take to run. -->
     <ini name="memory_limit" value="-1"/>
-    <const name="BOOTSTRAP_IS_PHPUNIT" value="true"/>
   </php>
   <testsuites>
     <testsuite name="unit">
       <directory>./tests/Unit</directory>
     </testsuite>
   </testsuites>
-  <listeners>
-    <listener class="Symfony\Bridge\PhpUnit\SymfonyTestsListener">
-    </listener>
-  </listeners>
-  <!-- Filter for coverage reports. -->
+  <source>
+    <include>
+      <directory>./src</directory>
+    </include>
+  </source>
 </phpunit>
diff --git a/src/Counter.php b/src/Counter.php
index 62829ee..9c59841 100644
--- a/src/Counter.php
+++ b/src/Counter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus;
 
 /**
@@ -24,10 +26,13 @@ public function getType(): string {
    *
    * @param mixed $value
    *   The value.
-   * @param array $labels
+   * @param array<string, string|int|float> $labels
    *   The list of key value label pairs.
+   *
+   * @throws \InvalidArgumentException
+   *   If the value is not a non-negative integer.
    */
-  public function set($value, array $labels = []) {
+  public function set(mixed $value, array $labels = []): void {
     if (!$this->isValidValue($value)) {
       throw new \InvalidArgumentException("A count value must be a positive integer.");
     }
@@ -44,7 +49,7 @@ public function set($value, array $labels = []) {
    * @return bool
    *   TRUE if the value is valid. FALSE otherwise.
    */
-  protected function isValidValue($value) {
+  protected function isValidValue(mixed $value): bool {
     return is_int($value) && $value >= 0;
   }
 
diff --git a/src/Gauge.php b/src/Gauge.php
index ab26ad9..fc688e6 100644
--- a/src/Gauge.php
+++ b/src/Gauge.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus;
 
 /**
@@ -24,10 +26,10 @@ public function getType(): string {
    *
    * @param mixed $value
    *   The value.
-   * @param array $labels
+   * @param array<string, string|int|float> $labels
    *   The list of key value label pairs.
    */
-  public function set($value, array $labels = []) {
+  public function set(mixed $value, array $labels = []): void {
     $key = $this->getKey($labels);
     $this->labelledValues[$key] = new LabelledValue($this->getName(), $value, $labels);
   }
diff --git a/src/LabelledValue.php b/src/LabelledValue.php
index c625b44..5b2de7a 100644
--- a/src/LabelledValue.php
+++ b/src/LabelledValue.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus;
 
 /**
@@ -14,24 +16,20 @@ class LabelledValue {
 
   /**
    * The metric value.
-   *
-   * @var mixed
    */
-  protected $value;
+  protected mixed $value;
 
   /**
    * The key value pairs of labels.
    *
-   * @var array
+   * @var array<string, string|int|float>
    */
-  protected $labels;
+  protected array $labels;
 
   /**
    * The name override for this labelled value.
-   *
-   * @var string
    */
-  protected $name;
+  protected string $name;
 
   /**
    * Value constructor.
@@ -40,10 +38,13 @@ class LabelledValue {
    *   The name override for this label.
    * @param mixed $value
    *   The metric value.
-   * @param array[] $labels
+   * @param array<string, string|int|float> $labels
    *   The key value pairs of labels.
+   *
+   * @throws \InvalidArgumentException
+   *   If the name or label names are invalid.
    */
-  public function __construct($name, $value, array $labels = []) {
+  public function __construct(string $name, mixed $value, array $labels = []) {
     $this->value = $value;
     foreach ($labels as $labelKey => $labelValue) {
       if (!preg_match(self::LABEL_NAME_REGEX, $labelKey)) {
@@ -63,14 +64,14 @@ public function __construct($name, $value, array $labels = []) {
    * @return mixed
    *   The Value.
    */
-  public function getValue() {
+  public function getValue(): mixed {
     return $this->value;
   }
 
   /**
    * Gets the Labels.
    *
-   * @return array[]
+   * @return array<string, string|int|float>
    *   The Labels.
    */
   public function getLabels(): array {
@@ -80,10 +81,10 @@ public function getLabels(): array {
   /**
    * Gets the Name override.
    *
-   * @return null|string
+   * @return string
    *   The Name.
    */
-  public function getName() {
+  public function getName(): string {
     return $this->name;
   }
 
diff --git a/src/Metric.php b/src/Metric.php
index 2468948..c5a00cb 100644
--- a/src/Metric.php
+++ b/src/Metric.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus;
 
 /**
@@ -14,24 +16,20 @@ abstract class Metric {
 
   /**
    * The metric name.
-   *
-   * @var string
    */
-  protected $name;
+  protected string $name;
 
   /**
    * The help message for the metric.
-   *
-   * @var string
    */
-  protected $help;
+  protected string|\Stringable $help;
 
   /**
    * The metric values.
    *
    * @var \PNX\Prometheus\LabelledValue[]
    */
-  protected $labelledValues = [];
+  protected array $labelledValues = [];
 
   /**
    * Metric constructor.
@@ -40,10 +38,10 @@ abstract class Metric {
    *   The metric namespace.
    * @param string $name
    *   The metric name.
-   * @param string $help
+   * @param string|\Stringable $help
    *   The help message for the metric.
    */
-  public function __construct(string $namespace, string $name, string $help) {
+  public function __construct(string $namespace, string $name, string|\Stringable $help) {
     $fullName = $namespace . '_' . $name;
     $this->validateName($fullName);
     $this->name = $fullName;
@@ -75,7 +73,7 @@ public function getName(): string {
    *   The Help.
    */
   public function getHelp(): string {
-    return $this->help;
+    return (string) $this->help;
   }
 
   /**
@@ -84,7 +82,7 @@ public function getHelp(): string {
    * @return \PNX\Prometheus\LabelledValue[]
    *   The array of values.
    */
-  public function getLabelledValues() {
+  public function getLabelledValues(): array {
     return array_values($this->labelledValues);
   }
 
@@ -93,8 +91,11 @@ public function getLabelledValues() {
    *
    * @param string $name
    *   The metric or label name.
+   *
+   * @throws \InvalidArgumentException
+   *   If the name is invalid.
    */
-  protected function validateName($name): void {
+  protected function validateName(string $name): void {
     if (!preg_match(self::METRIC_NAME_REGEX, $name)) {
       throw new \InvalidArgumentException("Invalid name: '" . $name . "'");
     }
@@ -103,13 +104,13 @@ protected function validateName($name): void {
   /**
    * Generates a unique key for the specified labels.
    *
-   * @param array $labels
+   * @param array<string, string|int|float> $labels
    *   The labels.
    *
    * @return string
    *   A unique key for the labels.
    */
-  protected function getKey(array $labels) {
+  protected function getKey(array $labels): string {
     return md5(json_encode($labels, JSON_FORCE_OBJECT));
   }
 
diff --git a/src/Serializer/MetricSerializerFactory.php b/src/Serializer/MetricSerializerFactory.php
index adb5123..44a4593 100644
--- a/src/Serializer/MetricSerializerFactory.php
+++ b/src/Serializer/MetricSerializerFactory.php
@@ -1,10 +1,13 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Serializer;
 
 use Symfony\Component\Serializer\NameConverter\CamelCaseToSnakeCaseNameConverter;
 use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
 use Symfony\Component\Serializer\Serializer;
+use Symfony\Component\Serializer\SerializerInterface;
 
 /**
  * Creates a serializer for metrics.
@@ -14,10 +17,10 @@ class MetricSerializerFactory {
   /**
    * Creates a new metric serializer.
    *
-   * @return \Symfony\Component\Serializer\Serializer
+   * @return \Symfony\Component\Serializer\SerializerInterface
    *   The serializer.
    */
-  public static function create() {
+  public static function create(): SerializerInterface {
     $encoders = [new PrometheusEncoder()];
     $objectNormalizer = new ObjectNormalizer(NULL, new CamelCaseToSnakeCaseNameConverter());
     $normalizers = [$objectNormalizer];
diff --git a/src/Serializer/PrometheusEncoder.php b/src/Serializer/PrometheusEncoder.php
index 44ba6d4..d404a46 100644
--- a/src/Serializer/PrometheusEncoder.php
+++ b/src/Serializer/PrometheusEncoder.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Serializer;
 
 use Symfony\Component\Serializer\Encoder\EncoderInterface;
@@ -14,14 +16,16 @@ class PrometheusEncoder implements EncoderInterface {
   /**
    * {@inheritdoc}
    */
-  public function supportsEncoding($format): bool {
+  public function supportsEncoding(string $format): bool {
     return $format == self::ENCODING;
   }
 
   /**
    * {@inheritdoc}
+   *
+   * @phpstan-ignore missingType.iterableValue
    */
-  public function encode($data, $format, array $context = []): string {
+  public function encode(mixed $data, string $format, array $context = []): string {
     $output = [];
     $output[] = '# HELP ' . $data['name'] . ' ' . $data['help'];
     $output[] = '# TYPE ' . $data['name'] . ' ' . $data['type'];
@@ -35,13 +39,13 @@ public function encode($data, $format, array $context = []): string {
   /**
    * Encode the labels as in the prometheus format.
    *
-   * @param array $labels
+   * @param array<string, string|int|float> $labels
    *   The labels.
    *
    * @return string
    *   The labels in prometheus format.
    */
-  protected function encodeLabels(array $labels) {
+  protected function encodeLabels(array $labels): string {
     if (empty($labels)) {
       return '';
     }
@@ -55,16 +59,17 @@ protected function encodeLabels(array $labels) {
   /**
    * Escape special characters in values.
    *
-   * @param string $value
+   * @param string|int|float $value
    *   The raw value.
    *
    * @return string
    *   The escaped value.
    */
-  protected function escapeValue($value) {
+  protected function escapeValue(string|int|float $value): string {
+    $value = (string) $value;
+    $value = str_replace("\\", "\\\\", $value);
     $value = str_replace("\"", "\\\"", $value);
     $value = str_replace("\n", "\\n", $value);
-    $value = str_replace("\\", "\\\\", $value);
     return $value;
   }
 
diff --git a/src/Summary.php b/src/Summary.php
index 3d3e2c7..50791a4 100644
--- a/src/Summary.php
+++ b/src/Summary.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus;
 
 /**
@@ -14,29 +16,32 @@ class Summary extends Metric {
 
   /**
    * The sum of all values.
-   *
-   * @var float
    */
-  protected $sum;
+  protected float $sum;
 
   /**
    * The count of all values.
-   *
-   * @var int
    */
-  protected $count;
+  protected int $count;
 
   /**
    * The metric label.
-   *
-   * @var string
    */
-  protected $label;
+  protected string $label;
 
   /**
    * Summary constructor.
+   *
+   * @param string $namespace
+   *   The metric namespace.
+   * @param string $name
+   *   The metric name.
+   * @param string $help
+   *   The help message for the metric.
+   * @param string $label
+   *   The label name for the summary buckets.
    */
-  public function __construct(string $namespace, string $name, string $help, $label) {
+  public function __construct(string $namespace, string $name, string $help, string $label) {
     parent::__construct($namespace, $name, $help);
     $this->sum = 0;
     $this->count = 0;
@@ -54,12 +59,15 @@ public function getType(): string {
   /**
    * Sets an array of summary values.
    *
-   * @param array $buckets
+   * @param array<int|float> $buckets
    *   The list of buckets.
-   * @param array $values
+   * @param array<int|float> $values
    *   The list of bucket values.
+   *
+   * @throws \InvalidArgumentException
+   *   If the number of buckets and values do not match.
    */
-  public function setValues(array $buckets, array $values) {
+  public function setValues(array $buckets, array $values): void {
     if (count($buckets) != count($values)) {
       throw new \InvalidArgumentException("The number of buckets and values must match.");
     }
@@ -74,7 +82,7 @@ public function setValues(array $buckets, array $values) {
   /**
    * {@inheritdoc}
    */
-  public function getLabelledValues() {
+  public function getLabelledValues(): array {
     $labelledValues = parent::getLabelledValues();
     $labelledValues[] = new LabelledValue($this->getName() . '_sum',
       $this->sum, []);
diff --git a/tests/Unit/CounterTest.php b/tests/Unit/CounterTest.php
index dae97c9..405c762 100644
--- a/tests/Unit/CounterTest.php
+++ b/tests/Unit/CounterTest.php
@@ -1,19 +1,23 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Tests\Unit;
 
+use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\TestCase;
 use PNX\Prometheus\Counter;
 
 /**
- * @coversDefaultClass \PNX\Prometheus\Counter
+ * Tests for the Counter class.
  */
+#[CoversClass(Counter::class)]
 class CounterTest extends TestCase {
 
   /**
-   * @covers ::set
+   * Tests setting counter values.
    */
-  public function testCounter() {
+  public function testCounter(): void {
 
     $counter = new Counter('foo', 'bar', 'Example counter help');
     $counter->set(89, ['baz' => 'wiz']);
@@ -29,9 +33,9 @@ public function testCounter() {
   }
 
   /**
-   * @covers ::__construct
+   * Tests that invalid counter names throw an exception.
    */
-  public function testInvalidCounter() {
+  public function testInvalidCounter(): void {
     $this->expectException(\InvalidArgumentException::class);
     new Counter('foo^&**', 'bar&*', 'Example counter help');
   }
diff --git a/tests/Unit/GaugeTest.php b/tests/Unit/GaugeTest.php
index 38e39a0..394761a 100644
--- a/tests/Unit/GaugeTest.php
+++ b/tests/Unit/GaugeTest.php
@@ -1,21 +1,23 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Tests\Unit;
 
+use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\TestCase;
 use PNX\Prometheus\Gauge;
 
 /**
- * @coversDefaultClass \PNX\Prometheus\Gauge
+ * Tests for the Gauge class.
  */
+#[CoversClass(Gauge::class)]
 class GaugeTest extends TestCase {
 
   /**
-   * @covers ::__construct
-   * @covers ::set
-   * @covers ::getLabelledValues
+   * Tests setting gauge values.
    */
-  public function testGauge() {
+  public function testGauge(): void {
     $gauge = new Gauge("foo", "bar", "A test gauge");
     $gauge->set(100, ['baz' => 'wiz']);
     $gauge->set(90, ['wobble' => 'wibble', 'bing' => 'bong']);
@@ -37,11 +39,8 @@ public function testGauge() {
 
   /**
    * Ensure a gauge with no values is valid.
-   *
-   * @covers ::__construct
-   * @covers ::getLabelledValues
    */
-  public function testGaugeNoValues() {
+  public function testGaugeNoValues(): void {
     $gauge = new Gauge("foo", "bar", "A test gauge");
     $this->assertEmpty($gauge->getLabelledValues());
   }
diff --git a/tests/Unit/Serializer/MetricSerializerTest.php b/tests/Unit/Serializer/MetricSerializerTest.php
index 24af838..0325038 100644
--- a/tests/Unit/Serializer/MetricSerializerTest.php
+++ b/tests/Unit/Serializer/MetricSerializerTest.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Tests\Unit\Serializer;
 
+use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\TestCase;
 use PNX\Prometheus\Counter;
 use PNX\Prometheus\Gauge;
@@ -9,14 +12,15 @@
 use PNX\Prometheus\Summary;
 
 /**
- * @coversDefaultClass \PNX\Prometheus\Serializer\MetricSerializerFactory
+ * Tests for the MetricSerializerFactory class.
  */
+#[CoversClass(MetricSerializerFactory::class)]
 class MetricSerializerTest extends TestCase {
 
   /**
-   * @covers ::create
+   * Tests serializing a gauge.
    */
-  public function testSerializeGauge() {
+  public function testSerializeGauge(): void {
     $serializer = MetricSerializerFactory::create();
     $gauge = $this->getTestGauge();
     $gaugeText = $serializer->serialize($gauge, 'prometheus');
@@ -25,9 +29,9 @@ public function testSerializeGauge() {
   }
 
   /**
-   * @covers ::create
+   * Tests serializing a counter.
    */
-  public function testSerializeCounter() {
+  public function testSerializeCounter(): void {
     $serializer = MetricSerializerFactory::create();
     $counter = $this->getTestCounter();
     $counterText = $serializer->serialize($counter, 'prometheus');
@@ -36,9 +40,9 @@ public function testSerializeCounter() {
   }
 
   /**
-   * @covers ::create
+   * Tests serializing a summary.
    */
-  public function testSerializeSummary() {
+  public function testSerializeSummary(): void {
     $serializer = MetricSerializerFactory::create();
     $summary = $this->getTestSummary();
     $summaryText = $serializer->serialize($summary, 'prometheus');
@@ -52,7 +56,7 @@ public function testSerializeSummary() {
    * @return \PNX\Prometheus\Gauge
    *   The gauge.
    */
-  protected function getTestGauge() {
+  protected function getTestGauge(): Gauge {
     $gauge = new Gauge("foo", "bar", "A test gauge");
     $gauge->set(100, ['baz' => 'wiz']);
     $gauge->set(90, ['wobble' => 'wibble', 'bing' => 'bong']);
@@ -66,19 +70,19 @@ protected function getTestGauge() {
    * @return \PNX\Prometheus\Counter
    *   The counter.
    */
-  protected function getTestCounter() {
+  protected function getTestCounter(): Counter {
     $counter = new Counter("foo", "bar", "A counter for testing");
     $counter->set(100, ['baz' => 'wiz']);
     return $counter;
   }
 
   /**
-   * Gets a counter for testing.
+   * Gets a summary for testing.
    *
    * @return \PNX\Prometheus\Summary
    *   The summary.
    */
-  protected function getTestSummary() {
+  protected function getTestSummary(): Summary {
     $summary = new Summary("foo", "bar", "Summary help text", 'baz');
 
     $buckets = [0, 0.25, 0.5, 0.75, 1];
diff --git a/tests/Unit/SummaryTest.php b/tests/Unit/SummaryTest.php
index af96eee..b773439 100644
--- a/tests/Unit/SummaryTest.php
+++ b/tests/Unit/SummaryTest.php
@@ -1,19 +1,23 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PNX\Prometheus\Tests\Unit;
 
+use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\TestCase;
 use PNX\Prometheus\Summary;
 
 /**
- * @coversDefaultClass \PNX\Prometheus\Summary
+ * Tests for the Summary class.
  */
+#[CoversClass(Summary::class)]
 class SummaryTest extends TestCase {
 
   /**
-   * @covers ::setValues
+   * Tests setting summary values.
    */
-  public function testSummary() {
+  public function testSummary(): void {
     $summary = new Summary("foo", "bar", "Summary help text", 'baz');
 
     $buckets = [0, 0.25, 0.5, 0.75, 1];
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 7ddf755..7fdaf7f 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -2,10 +2,10 @@
 
 /**
  * @file
- * Boostrap for PHPUnit.
+ * Bootstrap for PHPUnit.
  */
 
-assert_options(ASSERT_ACTIVE, FALSE);
+declare(strict_types=1);
 
 $autoloader = __DIR__ . '/../vendor/autoload.php';
 $loader = require $autoloader;