diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..2cd720b
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,24 @@
+# file generated with AI assistance: Claude Code - 2026-06-13 23:14:54 UTC
+name: Tests
+
+on:
+  push:
+    branches: [main, master, "feature/**"]
+  pull_request:
+
+jobs:
+  phpunit:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ["8.3", "8.4"]
+    name: PHPUnit (PHP ${{ matrix.php }})
+    steps:
+      - uses: actions/checkout@v4
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+      - run: composer install --no-interaction --no-progress
+      - run: vendor/bin/phpunit
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..cff2af8
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+# file generated with AI assistance: Claude Code - 2026-06-13 23:14:54 UTC
+/vendor/
+/composer.lock
+/.phpunit.cache/
diff --git a/composer.json b/composer.json
index 85d7ee3..6376b61 100644
--- a/composer.json
+++ b/composer.json
@@ -18,6 +18,9 @@
         "dmstr/api-platform-utils-bundle": "*",
         "dmstr/openapi-json-schema-bundle": "*"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^12.0"
+    },
     "autoload": {
         "psr-4": {
             "Dmstr\\ApiConfiguration\\": "src/"
diff --git a/docker-compose.yml b/docker-compose.yml
new file mode 100644
index 0000000..8086794
--- /dev/null
+++ b/docker-compose.yml
@@ -0,0 +1,15 @@
+# file generated with AI assistance: Claude Code - 2026-06-13 23:14:54 UTC
+# CLI-only test runner for this bundle — no MySQL/FPM required.
+# Usage (on host):
+#   docker compose run --rm php
+# Runs `composer install` then PHPUnit against tests/.
+services:
+  php:
+    build:
+      dockerfile_inline: |
+        FROM php:8.4-cli-alpine
+        COPY --from=composer:2 /usr/bin/composer /usr/bin/composer
+    working_dir: /repo
+    volumes:
+      - .:/repo
+    command: sh -c "composer install --no-interaction --no-progress && vendor/bin/phpunit"
diff --git a/phpunit.dist.xml b/phpunit.dist.xml
new file mode 100644
index 0000000..9dc4142
--- /dev/null
+++ b/phpunit.dist.xml
@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- file generated with AI assistance: Claude Code - 2026-06-13 23:14:54 UTC -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache">
+    <testsuites>
+        <testsuite name="default">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+    <source>
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/tests/Service/ApiExtensionRegistryTest.php b/tests/Service/ApiExtensionRegistryTest.php
new file mode 100644
index 0000000..abdaa35
--- /dev/null
+++ b/tests/Service/ApiExtensionRegistryTest.php
@@ -0,0 +1,82 @@
+<?php
+// file generated with AI assistance: Claude Code - 2026-06-13 23:14:54 UTC
+
+declare(strict_types=1);
+
+namespace Dmstr\ApiConfiguration\Tests\Service;
+
+use Dmstr\ApiConfiguration\Extension\ApiExtensionInterface;
+use Dmstr\ApiConfiguration\Service\ApiExtensionRegistry;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Unit tests for {@see ApiExtensionRegistry} — the in-memory map of pluggable
+ * API extensions keyed by their {@see ApiExtensionInterface::getName()}.
+ */
+final class ApiExtensionRegistryTest extends TestCase
+{
+    public function testRegisterAndGet(): void
+    {
+        $registry = new ApiExtensionRegistry();
+        $github = $this->extension('github');
+
+        $registry->register($github);
+
+        self::assertSame($github, $registry->get('github'));
+    }
+
+    public function testGetUnknownReturnsNull(): void
+    {
+        self::assertNull((new ApiExtensionRegistry())->get('does-not-exist'));
+    }
+
+    public function testHasReflectsRegistration(): void
+    {
+        $registry = new ApiExtensionRegistry();
+        self::assertFalse($registry->has('gitlab'));
+
+        $registry->register($this->extension('gitlab'));
+        self::assertTrue($registry->has('gitlab'));
+    }
+
+    public function testAllReturnsNameKeyedMap(): void
+    {
+        $registry = new ApiExtensionRegistry();
+        $github = $this->extension('github');
+        $gitlab = $this->extension('gitlab');
+        $registry->register($github);
+        $registry->register($gitlab);
+
+        self::assertSame(['github' => $github, 'gitlab' => $gitlab], $registry->all());
+    }
+
+    public function testGetNamesListsRegisteredKeys(): void
+    {
+        $registry = new ApiExtensionRegistry();
+        $registry->register($this->extension('github'));
+        $registry->register($this->extension('basecamp4'));
+
+        self::assertSame(['github', 'basecamp4'], $registry->getNames());
+    }
+
+    public function testRegisteringSameNameOverwrites(): void
+    {
+        $registry = new ApiExtensionRegistry();
+        $first = $this->extension('github');
+        $second = $this->extension('github');
+
+        $registry->register($first);
+        $registry->register($second);
+
+        self::assertSame($second, $registry->get('github'));
+        self::assertCount(1, $registry->all());
+    }
+
+    private function extension(string $name): ApiExtensionInterface
+    {
+        $extension = $this->createStub(ApiExtensionInterface::class);
+        $extension->method('getName')->willReturn($name);
+
+        return $extension;
+    }
+}