diff --git a/tests/Unit/RouterTest.php b/tests/Unit/RouterTest.php
new file mode 100644
index 0000000..a5584cd
--- /dev/null
+++ b/tests/Unit/RouterTest.php
@@ -0,0 +1,209 @@
+<?php
+
+namespace Tests\Unit;
+
+use Exception;
+use Libs\Neuron;
+use Libs\Router;
+use LogicException;
+use PDOException;
+use PHPUnit\Framework\Attributes\Test;
+use ReflectionProperty;
+use Tests\TestCase;
+use TypeError;
+
+/**
+ * RouterTest - DuckBrain test harness
+ *
+ * Cubre la respuesta de error que produce Router::defaultException(): el
+ * status derivado del code de la excepcion con guard de rango 400-599 y la
+ * negociacion por Accept entre representacion JSON y texto plano. Tambien la
+ * frontera de Router::apply(): que toda excepcion de la cadena (middlewares,
+ * callback final, notFound) llegue al $exceptionCallback deteniendo la
+ * ejecucion sin exit() y dejando intacto el flujo normal.
+ */
+final class RouterTest extends TestCase
+{
+    private array $serverBackup = [];
+
+    protected function setUp(): void
+    {
+        $this->serverBackup = $_SERVER;
+        $_SERVER['REQUEST_METHOD'] = 'GET';
+        unset($_SERVER['HTTP_ACCEPT']);
+        http_response_code(200);
+        $this->resetRouterState();
+    }
+
+    protected function tearDown(): void
+    {
+        $_SERVER = $this->serverBackup;
+        http_response_code(200);
+        $this->resetRouterState();
+    }
+
+    private function resetRouterState(): void
+    {
+        foreach (['get', 'post', 'put', 'patch', 'delete'] as $method) {
+            (new ReflectionProperty(Router::class, $method))->setValue(null, []);
+        }
+        Router::$params = new Neuron();
+        Router::$notFoundCallback = 'Libs\Router::defaultNotFound';
+        Router::$exceptionCallback = 'Libs\Router::defaultException';
+    }
+
+    private function render(\Throwable $exception): string
+    {
+        ob_start();
+        Router::defaultException($exception);
+
+        return ob_get_clean();
+    }
+
+    #[Test]
+    public function intCodeInHttpStatusRangeBecomesResponseStatus(): void
+    {
+        $output = $this->render(new Exception('The edad must be at least 18.', 422));
+
+        $this->assertSame(422, http_response_code());
+        $this->assertStringContainsString('The edad must be at least 18.', $output);
+    }
+
+    #[Test]
+    public function stringSqlStateCodeFallsBackToServerError(): void
+    {
+        $this->render(new PDOException('SQLSTATE[23000]: Integrity constraint violation', '23000'));
+
+        $this->assertSame(500, http_response_code(), 'A string SQLSTATE code must never reach http_response_code');
+    }
+
+    #[Test]
+    public function intCodeOutOfRangeFallsBackToServerError(): void
+    {
+        $this->render(new Exception('arbitrary code', 7));
+
+        $this->assertSame(500, http_response_code());
+    }
+
+    #[Test]
+    public function throwableWithoutMeaningfulCodeAnswersServerError(): void
+    {
+        $this->render(new TypeError('must be of the type int'));
+
+        $this->assertSame(500, http_response_code());
+    }
+
+    #[Test]
+    public function plainTextIsDefaultRepresentation(): void
+    {
+        $output = $this->render(new Exception('boom', 418));
+
+        $this->assertStringStartsWith("boom\n", $output);
+        $this->assertStringContainsString('#0', $output, 'Plain text must include the full trace');
+    }
+
+    #[Test]
+    public function jsonRepresentationWhenClientAcceptsJson(): void
+    {
+        $_SERVER['HTTP_ACCEPT'] = 'application/json';
+
+        $output = $this->render(new Exception('The e must be a valid email address.', 422));
+
+        $payload = json_decode($output, true);
+        $this->assertIsArray($payload, 'Accept: application/json must produce a JSON body');
+        $this->assertSame(422, http_response_code());
+        $this->assertSame('The e must be a valid email address.', $payload['error']);
+        $this->assertSame('Exception', $payload['exception']);
+        $this->assertArrayHasKey('file', $payload);
+        $this->assertArrayHasKey('line', $payload);
+        $this->assertIsArray($payload['trace']);
+        $this->assertIsString($payload['trace'][0], 'The trace must be serialized as readable lines, not raw frames');
+    }
+
+    #[Test]
+    public function middlewareExceptionStopsTheRemainingChain(): void
+    {
+        $log = [];
+        $caught = [];
+        Router::$exceptionCallback = function (\Throwable $exception) use (&$caught): void {
+            $caught[] = $exception;
+        };
+
+        Router::get('/chain', function () use (&$log): void {
+            $log[] = 'final';
+        });
+        Router::middleware(function () use (&$log): void {
+            $log[] = 'mw-late-registered';
+        });
+        Router::middleware(function () use (&$log): void {
+            throw new Exception('chain break', 422);
+        });
+        Router::middleware(function () use (&$log): void {
+            $log[] = 'mw-first-registered';
+        });
+
+        Router::apply('/chain');
+
+        // Callbacks run in reverse registration order: the last registered middleware
+        // ran first, the thrower stopped everything after it, and apply() returned
+        // normally (this line being reached proves there was no exit()).
+        $this->assertSame(['mw-first-registered'], $log);
+        $this->assertCount(1, $caught, 'The route chain must deliver exactly one exception to the callback');
+        $this->assertSame('chain break', $caught[0]->getMessage());
+        $this->assertSame(422, $caught[0]->getCode());
+    }
+
+    #[Test]
+    public function finalCallbackExceptionIsDeliveredToTheExceptionCallback(): void
+    {
+        $caught = [];
+        Router::$exceptionCallback = function (\Throwable $exception) use (&$caught): void {
+            $caught[] = get_class($exception) . ': ' . $exception->getMessage();
+        };
+
+        Router::get('/throwing', function (): void {
+            throw new LogicException('controller boom');
+        });
+
+        Router::apply('/throwing');
+
+        $this->assertSame(['LogicException: controller boom'], $caught);
+    }
+
+    #[Test]
+    public function notFoundCallbackExceptionReachesTheExceptionCallback(): void
+    {
+        $caught = [];
+        Router::$exceptionCallback = function (\Throwable $exception) use (&$caught): void {
+            $caught[] = $exception;
+        };
+        Router::$notFoundCallback = function (): void {
+            throw new Exception('not found exploded', 418);
+        };
+
+        Router::apply('/no-such-route');
+
+        $this->assertCount(1, $caught);
+        $this->assertSame('not found exploded', $caught[0]->getMessage());
+        $this->assertSame(418, $caught[0]->getCode());
+    }
+
+    #[Test]
+    public function normalFlowStillPrintsReturnedData(): void
+    {
+        Router::$exceptionCallback = function (): void {
+            $this->fail('The exception callback must not run on a successful request');
+        };
+
+        Router::get('/ok', function (): array {
+            return ['status' => 'ok'];
+        });
+
+        ob_start();
+        Router::apply('/ok');
+        $output = ob_get_clean();
+
+        $this->assertSame('{"status":"ok"}', $output);
+        $this->assertSame(200, http_response_code());
+    }
+}