diff --git a/src/doctor/src/Diagnostics/ComposerAutoloadIsValid.php b/src/doctor/src/Diagnostics/ComposerAutoloadIsValid.php
index 8bd116d2a..0d60c34f6 100644
--- a/src/doctor/src/Diagnostics/ComposerAutoloadIsValid.php
+++ b/src/doctor/src/Diagnostics/ComposerAutoloadIsValid.php
@@ -4,12 +4,14 @@
 
 namespace Hypervel\Doctor\Diagnostics;
 
+use Hypervel\Contracts\Process\ProcessResult;
 use Hypervel\Doctor\Contracts\Fixable;
 use Hypervel\Doctor\Diagnostic;
 use Hypervel\Doctor\Results\DiagnosticResult;
 use Hypervel\Doctor\Results\FixResult;
 use Hypervel\Doctor\Results\Message;
 use Hypervel\Doctor\Support\Composer;
+use Hypervel\Doctor\Support\Details;
 use Hypervel\Foundation\PackageManifest;
 use Hypervel\Support\Facades\Process;
 
@@ -70,7 +72,7 @@ public function check(): DiagnosticResult
 
         return $this->fail('invalid')
             ->fixable()
-            ->withDetails(trim($process->errorOutput() !== '' ? $process->errorOutput() : $process->output()));
+            ->withDetails($this->failureDetails($process));
     }
 
     /**
@@ -89,9 +91,21 @@ public function fix(DiagnosticResult $result, ?string $option = null): FixResult
 
         if (! $process->successful()) {
             return $this->fixFailed('regeneration-failed')
-                ->withDetails(trim($process->errorOutput() !== '' ? $process->errorOutput() : $process->output()));
+                ->withDetails($this->failureDetails($process));
         }
 
         return $this->fixed('regenerated');
     }
+
+    /**
+     * Get the failure details for a Composer process.
+     */
+    private function failureDetails(ProcessResult $process): string
+    {
+        return Details::processOutput(
+            $process->output(),
+            $process->errorOutput(),
+            "Composer exited with code {$process->exitCode()} without output.",
+        );
+    }
 }
diff --git a/src/doctor/src/Diagnostics/QueueConnectionIsReachable.php b/src/doctor/src/Diagnostics/QueueConnectionIsReachable.php
index b1a5b816e..dd58b48b0 100644
--- a/src/doctor/src/Diagnostics/QueueConnectionIsReachable.php
+++ b/src/doctor/src/Diagnostics/QueueConnectionIsReachable.php
@@ -135,7 +135,10 @@ private function probeRedisQueue(array $configuration): void
     {
         $connection = $configuration['connection'] ?? 'default';
 
-        Redis::connection($connection)->ping();
+        // phpredis returns false instead of throwing for ERR replies such as "max number of clients reached".
+        if (Redis::connection($connection)->ping() === false) {
+            throw new RuntimeException("Redis connection [{$connection}] returned an error for PING.");
+        }
     }
 
     /**
diff --git a/src/doctor/src/Diagnostics/RedisConnectionsAreReachable.php b/src/doctor/src/Diagnostics/RedisConnectionsAreReachable.php
index 7cb232eee..8de1d3847 100644
--- a/src/doctor/src/Diagnostics/RedisConnectionsAreReachable.php
+++ b/src/doctor/src/Diagnostics/RedisConnectionsAreReachable.php
@@ -52,7 +52,10 @@ public function check(): DiagnosticResult
 
         foreach ($connections as $connection) {
             try {
-                Redis::connection($connection)->ping();
+                // phpredis returns false instead of throwing for ERR replies such as "max number of clients reached".
+                if (Redis::connection($connection)->ping() === false) {
+                    $failures[$connection] = 'The Redis server returned an error for PING.';
+                }
             } catch (CanceledException $exception) {
                 throw $exception;
             } catch (Throwable $e) {
diff --git a/src/doctor/src/Diagnostics/RequiredConfigurationValuesAreSet.php b/src/doctor/src/Diagnostics/RequiredConfigurationValuesAreSet.php
index 6ddb298bf..f0c09849f 100644
--- a/src/doctor/src/Diagnostics/RequiredConfigurationValuesAreSet.php
+++ b/src/doctor/src/Diagnostics/RequiredConfigurationValuesAreSet.php
@@ -60,12 +60,37 @@ private function missingValues(): array
         $required = $this->requiredValues();
 
         return [
+            ...$this->missingDefaultValues(),
             ...array_intersect_key($required, array_flip(Configured::missing(array_keys($required)))),
             ...$this->missingMercureValues(),
             ...$this->missingMailValues(),
         ];
     }
 
+    /**
+     * Get default driver keys without a usable value, mapped to the feature that needs them.
+     *
+     * The managers read these keys with typed getters, so a blank or null
+     * value fails at runtime instead of falling back to a default driver.
+     *
+     * @return array<string, string>
+     */
+    private function missingDefaultValues(): array
+    {
+        $defaults = [
+            'broadcasting.default' => 'default broadcaster',
+            'filesystems.default' => 'default filesystem disk',
+            'mail.default' => 'default mailer',
+            'queue.default' => 'default queue connection',
+        ];
+
+        return array_filter(
+            $defaults,
+            static fn (string $key): bool => Configured::string($key) === null,
+            ARRAY_FILTER_USE_KEY,
+        );
+    }
+
     /**
      * Get configuration keys required by the active drivers, mapped to the feature that needs them.
      *
@@ -88,7 +113,12 @@ private function requiredValues(): array
      */
     private function broadcastingValues(): array
     {
-        $connection = Configured::string('broadcasting.default', 'null');
+        $connection = Configured::string('broadcasting.default');
+
+        if ($connection === null) {
+            return [];
+        }
+
         $driver = Configured::string("broadcasting.connections.{$connection}.driver", $connection);
 
         $keys = match ($driver) {
@@ -107,7 +137,12 @@ private function broadcastingValues(): array
      */
     private function missingMercureValues(): array
     {
-        $connection = Configured::string('broadcasting.default', 'null');
+        $connection = Configured::string('broadcasting.default');
+
+        if ($connection === null) {
+            return [];
+        }
+
         $prefix = "broadcasting.connections.{$connection}";
 
         if (Configured::string("{$prefix}.driver", $connection) !== 'mercure') {
@@ -140,9 +175,9 @@ private function missingMercureValues(): array
      */
     private function filesystemValues(): array
     {
-        $disk = Configured::string('filesystems.default', 'local');
+        $disk = Configured::string('filesystems.default');
 
-        if (Configured::string("filesystems.disks.{$disk}.driver", $disk) !== 's3') {
+        if ($disk === null || Configured::string("filesystems.disks.{$disk}.driver", $disk) !== 's3') {
             return [];
         }
 
@@ -156,9 +191,16 @@ private function filesystemValues(): array
      */
     private function loggingValues(): array
     {
+        $default = Configured::string('logging.default');
+
+        // The default log channel is optional; without one, LogManager writes to its emergency logger.
+        if ($default === null) {
+            return [];
+        }
+
         $values = [];
 
-        foreach (ActiveDrivers::logChannels(Configured::string('logging.default', 'stack')) as $channel) {
+        foreach (ActiveDrivers::logChannels($default) as $channel) {
             $values = [...$values, ...$this->logChannelValues($channel)];
         }
 
@@ -204,9 +246,15 @@ private function logChannelValues(string $channel): array
      */
     private function missingMailValues(): array
     {
+        $default = Configured::string('mail.default');
+
+        if ($default === null) {
+            return [];
+        }
+
         $values = [];
 
-        foreach (ActiveDrivers::mailers(Configured::string('mail.default', 'log')) as $mailer) {
+        foreach (ActiveDrivers::mailers($default) as $mailer) {
             $values = [...$values, ...$this->missingMailerValues($mailer)];
         }
 
@@ -263,7 +311,12 @@ private function missingMailerValues(string $mailer): array
      */
     private function queueValues(): array
     {
-        $connection = Configured::string('queue.default', 'database');
+        $connection = Configured::string('queue.default');
+
+        if ($connection === null) {
+            return [];
+        }
+
         $values = [];
 
         foreach (ActiveDrivers::queueConnections($connection) as $name) {
diff --git a/src/doctor/src/Diagnostics/SessionDriverIsReachable.php b/src/doctor/src/Diagnostics/SessionDriverIsReachable.php
index fd1a774cf..6bd9b467f 100644
--- a/src/doctor/src/Diagnostics/SessionDriverIsReachable.php
+++ b/src/doctor/src/Diagnostics/SessionDriverIsReachable.php
@@ -114,7 +114,12 @@ private function probeDatabaseSessions(): void
      */
     private function probeRedisSessions(): void
     {
-        Redis::connection(Configured::string('session.connection', SessionManager::DEFAULT_REDIS_CONNECTION))->ping();
+        $connection = Configured::string('session.connection', SessionManager::DEFAULT_REDIS_CONNECTION);
+
+        // phpredis returns false instead of throwing for ERR replies such as "max number of clients reached".
+        if (Redis::connection($connection)->ping() === false) {
+            throw new RuntimeException("Redis connection [{$connection}] returned an error for PING.");
+        }
     }
 
     /**
diff --git a/src/doctor/src/Support/ComposerJson.php b/src/doctor/src/Support/ComposerJson.php
index 8f5b12e96..3a8af6920 100644
--- a/src/doctor/src/Support/ComposerJson.php
+++ b/src/doctor/src/Support/ComposerJson.php
@@ -9,13 +9,6 @@
 
 class ComposerJson
 {
-    /**
-     * The decoded composer.json payload.
-     *
-     * @var null|array<string, mixed>
-     */
-    private ?array $contents = null;
-
     /**
      * Determine whether the application has a composer.json file.
      */
@@ -41,9 +34,11 @@ public function phpConstraint(): ?string
      */
     public function requiredExtensions(): array
     {
+        $contents = $this->contents() ?? [];
+
         return $this->extensionsFrom([
-            ...$this->packageNames(Arr::get($this->contents() ?? [], 'require', [])),
-            ...$this->packageNames(Arr::get($this->contents() ?? [], 'require-dev', [])),
+            ...$this->packageNames(Arr::get($contents, 'require', [])),
+            ...$this->packageNames(Arr::get($contents, 'require-dev', [])),
         ]);
     }
 
@@ -62,14 +57,13 @@ public function suggestedExtensions(): array
     /**
      * Get the decoded composer.json contents.
      *
+     * Read on every call because this class is auto-singletoned for the worker
+     * lifetime, so a memoized copy would go stale between Doctor runs.
+     *
      * @return null|array<string, mixed>
      */
     public function contents(): ?array
     {
-        if ($this->contents !== null) {
-            return $this->contents;
-        }
-
         if (! $this->exists()) {
             return null;
         }
@@ -81,7 +75,7 @@ public function contents(): ?array
         }
 
         /** @var array<string, mixed> $decoded */
-        return $this->contents = $decoded;
+        return $decoded;
     }
 
     /**
diff --git a/tests/Doctor/Feature/Diagnostics/ComposerAutoloadIsValidTest.php b/tests/Doctor/Feature/Diagnostics/ComposerAutoloadIsValidTest.php
index 3d1767e88..5fe77d19f 100644
--- a/tests/Doctor/Feature/Diagnostics/ComposerAutoloadIsValidTest.php
+++ b/tests/Doctor/Feature/Diagnostics/ComposerAutoloadIsValidTest.php
@@ -108,4 +108,23 @@ public function testRegeneratesComposerAutoloadFilesWhenFixed(): void
 
         $this->assertSame('pass', $fix->status->value);
     }
+
+    public function testExplainsComposerFailuresWithoutOutput(): void
+    {
+        $basePath = $this->basePath;
+        mkdir($basePath . '/vendor');
+        file_put_contents($basePath . '/composer.json', '{}');
+        file_put_contents($basePath . '/vendor/autoload.php', '<?php');
+
+        Process::fake([
+            '*' => Process::result(exitCode: 137),
+        ]);
+
+        $check = (new ComposerAutoloadIsValid)->check();
+        $fix = (new ComposerAutoloadIsValid)->fix($check);
+
+        $this->assertSame('fail', $check->status->value);
+        $this->assertSame('Composer exited with code 137 without output.', $check->details);
+        $this->assertSame('Composer exited with code 137 without output.', $fix->details);
+    }
 }
diff --git a/tests/Doctor/Feature/Diagnostics/PhpVersionSatisfiesComposerRequirementTest.php b/tests/Doctor/Feature/Diagnostics/PhpVersionSatisfiesComposerRequirementTest.php
index 9049414b6..96b25183c 100644
--- a/tests/Doctor/Feature/Diagnostics/PhpVersionSatisfiesComposerRequirementTest.php
+++ b/tests/Doctor/Feature/Diagnostics/PhpVersionSatisfiesComposerRequirementTest.php
@@ -104,6 +104,18 @@ public function testRechecksInstalledDependencyRequirementsAfterMetadataChanges(
         $this->assertSame('pass', $diagnostic->check()->status->value);
     }
 
+    public function testRechecksRootRequirementsAfterComposerJsonChanges(): void
+    {
+        file_put_contents($this->basePath . '/composer.json', '{"require":{"php":">=8.4"}}');
+        $diagnostic = $this->app->make(PhpVersionSatisfiesComposerRequirement::class);
+
+        $this->assertSame('pass', $diagnostic->check()->status->value);
+
+        file_put_contents($this->basePath . '/composer.json', '{"require":{"php":">=99"}}');
+
+        $this->assertSame('fail', $diagnostic->check()->status->value);
+    }
+
     #[DataProvider('phpVersions')]
     public function testChecksDistributionAndPrereleaseVersions(string $version, string $constraint, string $status): void
     {
diff --git a/tests/Doctor/Feature/Diagnostics/QueueConnectionIsReachableTest.php b/tests/Doctor/Feature/Diagnostics/QueueConnectionIsReachableTest.php
index f37067931..da54578d8 100644
--- a/tests/Doctor/Feature/Diagnostics/QueueConnectionIsReachableTest.php
+++ b/tests/Doctor/Feature/Diagnostics/QueueConnectionIsReachableTest.php
@@ -8,7 +8,9 @@
 use Hypervel\Contracts\Queue\Queue as QueueContract;
 use Hypervel\Doctor\Diagnostics\QueueConnectionIsReachable;
 use Hypervel\Doctor\Results\Link;
+use Hypervel\Redis\RedisProxy;
 use Hypervel\Support\Facades\Queue;
+use Hypervel\Support\Facades\Redis;
 use Hypervel\Support\Facades\Schema;
 use Hypervel\Tests\Doctor\TestCase;
 use Hypervel\Tests\Queue\Fixtures\IntegerQueueName;
@@ -104,6 +106,20 @@ public static function failoverConnections(): array
         ];
     }
 
+    public function testReportsRedisQueuesWhenTheServerRejectsThePing(): void
+    {
+        config([
+            'queue.default' => 'redis',
+            'queue.connections.redis' => ['driver' => 'redis', 'connection' => 'queue'],
+        ]);
+        Redis::shouldReceive('connection')->with('queue')->andReturn(m::mock(RedisProxy::class, ['ping' => false]));
+
+        $result = (new QueueConnectionIsReachable)->check();
+
+        $this->assertSame('fail', $result->status->value);
+        $this->assertStringContainsString('Redis connection [queue] returned an error for PING.', $result->details);
+    }
+
     public function testBuiltInNullConnectionNeedsNoConfigurationRecord(): void
     {
         config(['queue.default' => 'null', 'queue.connections' => []]);
diff --git a/tests/Doctor/Feature/Diagnostics/RedisConnectionsAreReachableTest.php b/tests/Doctor/Feature/Diagnostics/RedisConnectionsAreReachableTest.php
index 1a32e709f..b7a5e1394 100644
--- a/tests/Doctor/Feature/Diagnostics/RedisConnectionsAreReachableTest.php
+++ b/tests/Doctor/Feature/Diagnostics/RedisConnectionsAreReachableTest.php
@@ -7,8 +7,10 @@
 use Hypervel\Contracts\Foundation\Application;
 use Hypervel\Doctor\Diagnostics\RedisConnectionsAreReachable;
 use Hypervel\Doctor\Results\Link;
+use Hypervel\Redis\RedisProxy;
 use Hypervel\Support\Facades\Redis;
 use Hypervel\Tests\Doctor\TestCase;
+use Mockery as m;
 use RuntimeException;
 
 class RedisConnectionsAreReachableTest extends TestCase
@@ -135,6 +137,23 @@ public function testSkipsWhenRedisIsOnlyScaffoldedAndUnused(): void
         $this->assertSame('skip', $result->status->value);
     }
 
+    public function testReportsConnectionsWhoseServerRejectsThePing(): void
+    {
+        config([
+            'cache.default' => 'redis',
+            'cache.stores.redis' => [
+                'driver' => 'redis',
+                'connection' => 'default',
+            ],
+        ]);
+        Redis::shouldReceive('connection')->with('default')->andReturn(m::mock(RedisProxy::class, ['ping' => false]));
+
+        $result = (new RedisConnectionsAreReachable)->check();
+
+        $this->assertSame('fail', $result->status->value);
+        $this->assertStringContainsString('The Redis server returned an error for PING.', $result->details);
+    }
+
     public function testLinksToRedisConfigurationWhenAnActiveConnectionCannotBeReached(): void
     {
         config([
diff --git a/tests/Doctor/Feature/Diagnostics/RequiredConfigurationValuesAreSetTest.php b/tests/Doctor/Feature/Diagnostics/RequiredConfigurationValuesAreSetTest.php
index d0a616af9..17de03bac 100644
--- a/tests/Doctor/Feature/Diagnostics/RequiredConfigurationValuesAreSetTest.php
+++ b/tests/Doctor/Feature/Diagnostics/RequiredConfigurationValuesAreSetTest.php
@@ -19,6 +19,41 @@ public function testPassesWithTheDefaultConfiguration(): void
         $this->assertSame('pass', $result->status->value);
     }
 
+    #[DataProvider('requiredDefaults')]
+    public function testReportsBlankRequiredDefaultsInsteadOfAssumingADriver(string $key, string $feature): void
+    {
+        config([$key => '']);
+
+        $result = (new RequiredConfigurationValuesAreSet)->check();
+
+        $this->assertSame('fail', $result->status->value);
+        $this->assertSame("- {$key} ({$feature})", $result->details);
+    }
+
+    /**
+     * Provide required default keys and the features that need them.
+     */
+    public static function requiredDefaults(): array
+    {
+        return [
+            ['broadcasting.default', 'default broadcaster'],
+            ['filesystems.default', 'default filesystem disk'],
+            ['mail.default', 'default mailer'],
+            ['queue.default', 'default queue connection'],
+        ];
+    }
+
+    public function testDoesNotAssumeAStackChannelWithoutADefaultLogChannel(): void
+    {
+        config([
+            'logging.default' => '',
+            'logging.channels.stack' => ['driver' => 'stack', 'channels' => ['slack']],
+            'logging.channels.slack' => ['driver' => 'slack', 'url' => null],
+        ]);
+
+        $this->assertSame('pass', (new RequiredConfigurationValuesAreSet)->check()->status->value);
+    }
+
     public function testReportsMissingCredentialsForTheActivePusherBroadcastConnection(): void
     {
         config([
diff --git a/tests/Doctor/Feature/Diagnostics/SessionDriverIsReachableTest.php b/tests/Doctor/Feature/Diagnostics/SessionDriverIsReachableTest.php
index 9d20f330b..86d83079a 100644
--- a/tests/Doctor/Feature/Diagnostics/SessionDriverIsReachableTest.php
+++ b/tests/Doctor/Feature/Diagnostics/SessionDriverIsReachableTest.php
@@ -7,6 +7,7 @@
 use Hypervel\Database\Schema\Builder;
 use Hypervel\Doctor\Diagnostics\SessionDriverIsReachable;
 use Hypervel\Doctor\Results\Link;
+use Hypervel\Redis\RedisProxy;
 use Hypervel\Support\Facades\File;
 use Hypervel\Support\Facades\Redis;
 use Hypervel\Support\Facades\Schema;
@@ -116,6 +117,17 @@ public static function redisConnections(): array
         return [[null, 'session'], ['', 'session'], ['custom', 'custom']];
     }
 
+    public function testReportsRedisSessionsWhenTheServerRejectsThePing(): void
+    {
+        config(['session.driver' => 'redis', 'session.connection' => 'session']);
+        Redis::shouldReceive('connection')->with('session')->andReturn(m::mock(RedisProxy::class, ['ping' => false]));
+
+        $result = (new SessionDriverIsReachable)->check();
+
+        $this->assertSame('fail', $result->status->value);
+        $this->assertSame('Redis connection [session] returned an error for PING.', $result->details);
+    }
+
     public function testDatabaseSessionsKeepTheDefaultSchemaConnection(): void
     {
         config(['session.driver' => 'database', 'session.connection' => null, 'session.table' => 'custom_sessions']);