diff --git a/.gitattributes b/.gitattributes
index 805a9fe..e888d34 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,8 +3,9 @@ tests/ export-ignore
 .github/ export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
-.phpcs.xml export-ignore
+.phpcs.xml.dist export-ignore
 phpunit.xml export-ignore
+phpstan.neon export-ignore
 
 # Auto detect text files and perform LF normalization
 * text=auto
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 65a1037..4295747 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,26 +14,26 @@ jobs:
         timeout-minutes: 15
         strategy:
             matrix:
-                php: [ '8.1', '8.2', '8.3', '8.4' ]
+                php: [ '8.2', '8.3', '8.4', '8.5' ]
                 dependency-version: [ '' ]
                 include:
-                    -   php: '8.1'
+                    -   php: '8.2'
                         dependency-version: '--prefer-lowest'
         steps:
-            -   uses: actions/checkout@v4
+            -   uses: actions/checkout@v7
             -   name: Setup PHP
                 uses: shivammathur/setup-php@v2
                 with:
                     php-version: ${{ matrix.php }}
                     coverage: none
             -   name: Cache Composer dependencies
-                uses: actions/cache@v4
+                uses: actions/cache@v6
                 with:
                     path: ~/.composer/cache
                     key: php-${{ matrix.php }}-composer-locked-${{ hashFiles('composer.lock') }}
                     restore-keys: php-${{ matrix.php }}-composer-locked-
             -   name: Install PHP dependencies
-                run: composer update ${{ matrix.dependency-version }} --prefer-dist --no-interaction --no-progress --no-suggest
+                run: composer update ${{ matrix.dependency-version }} --prefer-dist --no-interaction --no-progress
             -   name: PHPUnit
                 run: vendor/bin/phpunit
 
@@ -41,21 +41,21 @@ jobs:
         name: Coding standards
         runs-on: ubuntu-latest
         steps:
-            -   uses: actions/checkout@v4
+            -   uses: actions/checkout@v7
             -   name: Setup PHP
                 uses: shivammathur/setup-php@v2
                 with:
-                    php-version: 8.3
+                    php-version: 8.5
                     tools: cs2pr
                     coverage: none
             -   name: Cache Composer dependencies
-                uses: actions/cache@v4
+                uses: actions/cache@v6
                 with:
                     path: ~/.composer/cache
                     key: php-composer-locked-${{ hashFiles('composer.lock') }}
                     restore-keys: php-composer-locked-
             -   name: Install PHP dependencies
-                run: composer install --no-interaction --no-progress --no-suggest
+                run: composer install --no-interaction --no-progress
             -   name: PHP CS
                 run: vendor/bin/phpcs -q --no-colors --report=checkstyle | cs2pr
             -   name: PHPStan
diff --git a/README.md b/README.md
index f636df6..8b1409a 100644
--- a/README.md
+++ b/README.md
@@ -1,10 +1,10 @@
 All you need to log with [Bref](https://bref.sh) on AWS Lambda.
 
-Bref/Logger is a lightweight [PSR-3](https://www.php-fig.org/psr/psr-3/) logger for AWS Lambda. Messages are sent to `stderr` so that they end up in [CloudWatch](https://bref.sh/docs/environment/logs.html).
+Bref/Logger is a lightweight [PSR-3](https://www.php-fig.org/psr/psr-3/) logger for AWS Lambda. Messages are sent to `stderr` so that they end up in [CloudWatch](https://bref.sh/docs/environment/logs).
 
 ## Why?
 
-As explained in [the Bref documentation](https://bref.sh/docs/environment/logs.html), logging in AWS Lambda means logging to `stderr`. Logs written to `stderr` are automatically sent to [CloudWatch](https://aws.amazon.com/cloudwatch/), AWS' solution to collect and view logs.
+As explained in [the Bref documentation](https://bref.sh/docs/environment/logs), logging in AWS Lambda means logging to `stderr`. Logs written to `stderr` are automatically sent to [CloudWatch](https://aws.amazon.com/cloudwatch/), AWS' solution to collect and view logs.
 
 While classic loggers like [Monolog](https://github.com/Seldaek/monolog) work fine, this logger comes as a simpler and lighter alternative optimized for AWS Lambda. It does not require any configuration and currently contains a single class.
 
diff --git a/composer.json b/composer.json
index f7d2239..f57fc43 100644
--- a/composer.json
+++ b/composer.json
@@ -15,14 +15,14 @@
         }
     },
     "require": {
-        "php": ">=8.0",
+        "php": ">=8.2",
         "ext-json": "*",
         "psr/log": "^1.1 | ^2.0 | ^3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^10.5|^11|^12",
+        "phpunit/phpunit": "^11.5|^12|^13",
         "mnapoli/hard-mode": "^0.3",
-        "phpstan/phpstan": "^1"
+        "phpstan/phpstan": "^2"
     },
     "config": {
         "allow-plugins": {
diff --git a/phpstan.neon b/phpstan.neon
index 46b0f47..1494f51 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,5 +1,5 @@
 parameters:
-    level: 5
+    level: max
     paths:
         - src
         - tests
diff --git a/src/StderrLogger.php b/src/StderrLogger.php
index 844b4e2..6315595 100644
--- a/src/StderrLogger.php
+++ b/src/StderrLogger.php
@@ -3,7 +3,9 @@
 namespace Bref\Logger;
 
 use Psr\Log\AbstractLogger;
+use Psr\Log\InvalidArgumentException;
 use Psr\Log\LogLevel;
+use Stringable;
 use Throwable;
 
 /**
@@ -22,44 +24,39 @@ class StderrLogger extends AbstractLogger
         LogLevel::DEBUG => 1,
     ];
 
-    /** @var string */
-    private $logLevel;
-
-    /** @var string|null */
-    private $url;
-
-    /** @var resource|null */
+    /** @var resource|string The stream, or its URL until it is opened */
     private $stream;
 
     /**
      * @param string $logLevel The log level above which messages will be logged. Messages under this log level will be ignored.
      * @param resource|string $stream If unsure leave the default value.
      */
-    public function __construct(string $logLevel = LogLevel::INFO, $stream = 'php://stderr')
-    {
-        $this->logLevel = $logLevel;
-
-        if (is_resource($stream)) {
-            $this->stream = $stream;
-        } elseif (is_string($stream)) {
-            $this->url = $stream;
-        } else {
+    public function __construct(
+        private string $logLevel = LogLevel::INFO,
+        $stream = 'php://stderr',
+    ) {
+        if (! is_resource($stream) && ! is_string($stream)) {
             throw new \InvalidArgumentException('A stream must either be a resource or a string.');
         }
+        $this->stream = $stream;
     }
 
     /**
-     * {@inheritdoc}
+     * @param mixed $level
+     * @param string|Stringable $message
+     * @param array<mixed> $context
+     * @throws InvalidArgumentException If the log level is not one of the PSR-3 levels.
      */
     public function log($level, $message, array $context = []): void
     {
+        if (! is_string($level) || ! isset(self::LOG_LEVEL_MAP[$level])) {
+            throw new InvalidArgumentException('Unsupported log level: ' . var_export($level, true));
+        }
         if (self::LOG_LEVEL_MAP[$level] < self::LOG_LEVEL_MAP[$this->logLevel]) {
             return;
         }
 
-        $this->openStderr();
-
-        $message = $this->interpolate($message, $context);
+        $message = $this->interpolate((string) $message, $context);
 
         // Make sure everything is kept on one line to count as one record
         $displayMessage = str_replace(["\r\n", "\r", "\n"], ' ', $message);
@@ -89,41 +86,48 @@ public function log($level, $message, array $context = []): void
             $formattedMessage = "$requestId\t$formattedMessage";
         }
 
-        fwrite($this->stream, $formattedMessage);
+        fwrite($this->stream(), $formattedMessage);
     }
 
-    private function openStderr(): void
+    /**
+     * @return resource
+     */
+    private function stream()
     {
-        if ($this->stream !== null) {
-            return;
-        }
-        $this->stream = fopen($this->url, 'a');
-        if (! $this->stream) {
-            throw new \RuntimeException('Unable to open stream ' . $this->url);
+        if (is_string($this->stream)) {
+            $stream = fopen($this->stream, 'a');
+            if ($stream === false) {
+                throw new \RuntimeException('Unable to open stream ' . $this->stream);
+            }
+            $this->stream = $stream;
         }
+
+        return $this->stream;
     }
 
     /**
      * Interpolates context values into the message placeholders.
+     *
+     * @param array<mixed> $context
      */
     private function interpolate(string $message, array $context): string
     {
-        if (strpos($message, '{') === false) {
+        if (! str_contains($message, '{')) {
             return $message;
         }
 
         $replacements = [];
         foreach ($context as $key => $val) {
-            if ($val === null || is_scalar($val) || (\is_object($val) && method_exists($val, '__toString'))) {
-                $replacements["{{$key}}"] = $val;
+            if ($val === null || is_scalar($val) || $val instanceof Stringable) {
+                $replacements["{{$key}}"] = (string) $val;
             } elseif ($val instanceof \DateTimeInterface) {
                 $replacements["{{$key}}"] = $val->format(\DateTime::RFC3339);
             } elseif (\is_object($val)) {
-                $replacements["{{$key}}"] = '{object ' . \get_class($val) . '}';
+                $replacements["{{$key}}"] = '{object ' . $val::class . '}';
             } elseif (\is_resource($val)) {
                 $replacements["{{$key}}"] = '{resource}';
             } else {
-                $replacements["{{$key}}"] = json_encode($val);
+                $replacements["{{$key}}"] = (string) json_encode($val);
             }
         }
 
@@ -133,11 +137,9 @@ private function interpolate(string $message, array $context): string
     /**
      * Normalizes data for JSON serialization.
      *
-     * @param mixed $data
      * @param int $depth Current recursion depth
-     * @return mixed
      */
-    private function normalize($data, int $depth = 0)
+    private function normalize(mixed $data, int $depth = 0): mixed
     {
         $maxDepth = 9; // Similar to NormalizerFormatter's default
         $maxItems = 1000; // Similar to NormalizerFormatter's default
@@ -175,11 +177,11 @@ private function normalize($data, int $depth = 0)
                 return $data;
             }
 
-            if (method_exists($data, '__toString')) {
+            if ($data instanceof Stringable) {
                 return $data->__toString();
             }
 
-            if (get_class($data) === '__PHP_Incomplete_Class') {
+            if ($data instanceof \__PHP_Incomplete_Class) {
                 return new \ArrayObject($data);
             }
 
@@ -195,17 +197,19 @@ private function normalize($data, int $depth = 0)
 
     /**
      * Normalizes an exception for JSON serialization.
+     *
+     * @return array<string, mixed>
      */
     private function normalizeException(Throwable $e, int $depth = 0): array
     {
         $maxDepth = 9;
 
         if ($depth > $maxDepth) {
-            return ['class' => get_class($e), 'message' => 'Over ' . $maxDepth . ' levels deep, aborting normalization'];
+            return ['class' => $e::class, 'message' => 'Over ' . $maxDepth . ' levels deep, aborting normalization'];
         }
 
         $data = [
-            'class' => get_class($e),
+            'class' => $e::class,
             'message' => $e->getMessage(),
             'code' => $e->getCode(),
             'file' => $e->getFile() . ':' . $e->getLine(),
@@ -218,11 +222,8 @@ private function normalizeException(Throwable $e, int $depth = 0): array
         return $data;
     }
 
-    /**
-     * @param mixed $data
-     */
-    private function toJson($data): string
+    private function toJson(mixed $data): string
     {
-        return json_encode($data, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_PRESERVE_ZERO_FRACTION);
+        return json_encode($data, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_PRESERVE_ZERO_FRACTION | JSON_THROW_ON_ERROR);
     }
 }
diff --git a/tests/StderrLoggerTest.php b/tests/StderrLoggerTest.php
index 40df28a..32951e0 100644
--- a/tests/StderrLoggerTest.php
+++ b/tests/StderrLoggerTest.php
@@ -5,24 +5,30 @@
 use Bref\Logger\StderrLogger;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
+use Psr\Log\InvalidArgumentException;
 use Psr\Log\LogLevel;
+use RuntimeException;
+use Stringable;
 
 class StderrLoggerTest extends TestCase
 {
     /** @var resource */
     private $stream;
-    /** @var StderrLogger */
-    private $logger;
+    private StderrLogger $logger;
 
     public function setUp(): void
     {
         parent::setUp();
 
-        $this->stream = fopen('php://memory', 'a+');
+        $stream = fopen('php://memory', 'a+');
+        if ($stream === false) {
+            throw new RuntimeException('Unable to open a memory stream');
+        }
+        $this->stream = $stream;
         $this->logger = new StderrLogger(LogLevel::DEBUG, $this->stream);
     }
 
-    public function test_log_messages_format()
+    public function test_log_messages_format(): void
     {
         $this->logger->debug('Debug');
         $this->logger->info('Info');
@@ -47,7 +53,7 @@ public function test_log_messages_format()
         );
     }
 
-    public function test_logs_above_the_configured_log_level()
+    public function test_logs_above_the_configured_log_level(): void
     {
         $this->logger = new StderrLogger(LogLevel::WARNING, $this->stream);
         $this->logger->debug('Debug');
@@ -74,7 +80,7 @@ public function test_logs_above_the_configured_log_level()
      * @param mixed $contextValue
      */
     #[DataProvider('provideInterpolationExamples')]
-    public function test_log_messages_are_interpolated($contextValue, string $expectedMessage)
+    public function test_log_messages_are_interpolated($contextValue, string $expectedMessage): void
     {
         $this->logger->info('{foo}', [
             'foo' => $contextValue,
@@ -84,6 +90,9 @@ public function test_log_messages_are_interpolated($contextValue, string $expect
         $this->assertStringStartsWith('INFO	' . $expectedMessage . '	', $logs);
     }
 
+    /**
+     * @return list<array{mixed, string}>
+     */
     public static function provideInterpolationExamples(): array
     {
         $date = new \DateTime;
@@ -103,7 +112,7 @@ public static function provideInterpolationExamples(): array
         ];
     }
 
-    public function test_logs_with_context()
+    public function test_logs_with_context(): void
     {
         $this->logger->info('Test message', ['key' => 'value']);
 
@@ -114,7 +123,7 @@ public function test_logs_with_context()
         );
     }
 
-    public function test_multiline_message()
+    public function test_multiline_message(): void
     {
         $this->logger->error("Test\nmessage");
 
@@ -125,7 +134,7 @@ public function test_multiline_message()
         );
     }
 
-    public function test_with_exception()
+    public function test_with_exception(): void
     {
         $e = new \Exception('Test error');
         $this->logger->info('Test message', ['exception' => $e]);
@@ -136,7 +145,7 @@ public function test_with_exception()
         $this->assertStringContainsString('"message":"Test error"', $logs);
     }
 
-    public function test_lines_start_with_the_lambda_request_id()
+    public function test_lines_start_with_the_lambda_request_id(): void
     {
         $_SERVER['LAMBDA_REQUEST_ID'] = '8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2';
         try {
@@ -148,19 +157,46 @@ public function test_lines_start_with_the_lambda_request_id()
         $this->assertLogsMatch(<<<'LOGS'
 8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2	INFO	Test message	{"message":"Test message","level":"INFO"}
 
+LOGS
+        );
+    }
+
+    public function test_unsupported_log_levels_are_rejected(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        $this->logger->log('warn', 'Test message');
+    }
+
+    public function test_stringable_messages_are_logged(): void
+    {
+        $this->logger->info(new class implements Stringable {
+            public function __toString(): string
+            {
+                return 'Test message';
+            }
+        });
+
+        $this->assertLogsMatch(<<<'LOGS'
+INFO	Test message	{"message":"Test message","level":"INFO"}
+
 LOGS
         );
     }
 
     private function assertLogsMatch(string $expectedLog): void
     {
-        rewind($this->stream);
-        self::assertStringMatchesFormat($expectedLog, fread($this->stream, fstat($this->stream)['size']));
+        self::assertStringMatchesFormat($expectedLog, $this->getLogs());
     }
 
     private function getLogs(): string
     {
         rewind($this->stream);
-        return stream_get_contents($this->stream);
+        $logs = stream_get_contents($this->stream);
+        if ($logs === false) {
+            throw new RuntimeException('Unable to read the logs');
+        }
+
+        return $logs;
     }
 }
\ No newline at end of file