diff --git a/.claude/settings.json b/.claude/settings.json
new file mode 100644
index 0000000..bc91e4b
--- /dev/null
+++ b/.claude/settings.json
@@ -0,0 +1,10 @@
+{
+    "hooks": {
+        "PostToolUse": [
+            {
+                "matcher": "Edit|Write",
+                "command": "php bin/phpunit 2>&1 | tail -20"
+            }
+        ]
+    }
+}
diff --git a/.gitignore b/.gitignore
index 0857d02..7bb0d35 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@
 /var/
 /public/bundles/
 composer.lock
+/.phpunit.cache
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
new file mode 100644
index 0000000..b3d3649
--- /dev/null
+++ b/phpunit.xml.dist
@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/11.5/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache"
+>
+    <testsuites>
+        <testsuite name="unit">
+            <directory>tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory suffix=".php">src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/tests/Unit/Service/SvgRendererTest.php b/tests/Unit/Service/SvgRendererTest.php
new file mode 100644
index 0000000..346f0cb
--- /dev/null
+++ b/tests/Unit/Service/SvgRendererTest.php
@@ -0,0 +1,120 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Service;
+
+use App\Service\SvgRenderer;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(SvgRenderer::class)]
+final class SvgRendererTest extends TestCase
+{
+    private SvgRenderer $renderer;
+
+    protected function setUp(): void
+    {
+        $this->renderer = new SvgRenderer();
+    }
+
+    #[Test]
+    public function it_returns_a_valid_svg_element(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        $this->assertStringStartsWith('<svg', $svg);
+        $this->assertStringEndsWith('</svg>', $svg);
+    }
+
+    #[Test]
+    public function it_includes_accessibility_attributes(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        $this->assertStringContainsString('role="img"', $svg);
+        $this->assertStringContainsString('aria-label="Contribution graph"', $svg);
+    }
+
+    #[Test]
+    public function it_applies_dark_theme_background_color(): void
+    {
+        $svg = $this->renderer->render([], 'dark');
+
+        $this->assertStringContainsString('#0d1117', $svg);
+    }
+
+    #[Test]
+    public function it_applies_light_theme_background_color(): void
+    {
+        $svg = $this->renderer->render([], 'light');
+
+        $this->assertStringContainsString('#ffffff', $svg);
+    }
+
+    #[Test]
+    public function it_falls_back_to_dark_theme_for_unknown_theme_names(): void
+    {
+        $svg = $this->renderer->render([], 'unknown');
+
+        $this->assertStringContainsString('#0d1117', $svg);
+    }
+
+    #[Test]
+    public function it_shows_zero_contributions_when_no_data_is_provided(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        $this->assertStringContainsString('0 contributions in the last year', $svg);
+    }
+
+    #[Test]
+    public function it_displays_formatted_total_contribution_count(): void
+    {
+        $today = (new \DateTimeImmutable('today'))->format('Y-m-d');
+
+        $svg = $this->renderer->render([$today => 1234]);
+
+        $this->assertStringContainsString('1,234 contributions in the last year', $svg);
+    }
+
+    #[Test]
+    public function it_renders_all_53_week_columns(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        // MARGIN_X(28) + col_52 * STEP(13) = 704
+        $this->assertStringContainsString('x="704"', $svg);
+    }
+
+    #[Test]
+    public function it_renders_day_of_week_labels_matching_github(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        $this->assertStringContainsString('>Mon<', $svg);
+        $this->assertStringContainsString('>Wed<', $svg);
+        $this->assertStringContainsString('>Fri<', $svg);
+    }
+
+    #[Test]
+    public function it_renders_a_legend_with_less_and_more_labels(): void
+    {
+        $svg = $this->renderer->render([]);
+
+        $this->assertStringContainsString('>Less<', $svg);
+        $this->assertStringContainsString('>More<', $svg);
+    }
+
+    #[Test]
+    public function it_sums_contributions_from_multiple_dates(): void
+    {
+        $today     = (new \DateTimeImmutable('today'))->format('Y-m-d');
+        $yesterday = (new \DateTimeImmutable('yesterday'))->format('Y-m-d');
+
+        $svg = $this->renderer->render([$today => 3, $yesterday => 7]);
+
+        $this->assertStringContainsString('10 contributions in the last year', $svg);
+    }
+}