diff --git a/src/Commands/Refresh.php b/src/Commands/Refresh.php
index 01167e2..dada513 100644
--- a/src/Commands/Refresh.php
+++ b/src/Commands/Refresh.php
@@ -8,6 +8,7 @@
 use Illuminate\Console\Command;
 use Illuminate\Contracts\Cache\Repository as CacheRepository;
 use Illuminate\Support\Facades\Cache;
+use Illuminate\Support\Str;
 
 final class Refresh extends Command
 {
@@ -117,17 +118,34 @@ protected function resolveBases(): array
         $args = (array) $this->argument('bases');
 
         if ($args !== []) {
-            return array_values(array_map('strtoupper', $args));
+            return $this->normalizeBases($args);
         }
 
         $configured = config('rates.bases');
 
         if (is_array($configured) && $configured !== []) {
-            return array_values(array_filter($configured, 'is_string'));
+            return $this->normalizeBases($configured);
         }
 
         $default = config('rates.base');
 
-        return is_string($default) ? [$default] : [];
+        return is_string($default) ? $this->normalizeBases([$default]) : [];
+    }
+
+    /**
+     * Normalize base strings for cache-key consistency.
+     *
+     * @param  array<int, mixed>  $bases
+     * @return array<int, string>
+     */
+    protected function normalizeBases(array $bases): array
+    {
+        return array_values(array_filter(
+            array_map(
+                fn (mixed $base): ?string => is_string($base) ? (string) Str::of($base)->trim()->upper() : null,
+                $bases,
+            ),
+            fn (?string $base): bool => $base !== null && $base !== '',
+        ));
     }
 }
diff --git a/src/Commands/Status.php b/src/Commands/Status.php
index d516eb8..04146f2 100644
--- a/src/Commands/Status.php
+++ b/src/Commands/Status.php
@@ -8,6 +8,7 @@
 use Illuminate\Console\Command;
 use Illuminate\Support\Carbon;
 use Illuminate\Support\Facades\Cache;
+use Illuminate\Support\Str;
 
 final class Status extends Command
 {
@@ -84,11 +85,28 @@ protected function resolveBases(): array
         $args = (array) $this->argument('bases');
 
         if ($args !== []) {
-            return array_values(array_map('strtoupper', $args));
+            return $this->normalizeBases($args);
         }
 
         $configured = config('rates.bases');
 
-        return is_array($configured) ? array_values(array_filter($configured, 'is_string')) : [];
+        return is_array($configured) ? $this->normalizeBases($configured) : [];
+    }
+
+    /**
+     * Normalize base strings for cache-key consistency.
+     *
+     * @param  array<int, mixed>  $bases
+     * @return array<int, string>
+     */
+    protected function normalizeBases(array $bases): array
+    {
+        return array_values(array_filter(
+            array_map(
+                fn (mixed $base): ?string => is_string($base) ? (string) Str::of($base)->trim()->upper() : null,
+                $bases,
+            ),
+            fn (?string $base): bool => $base !== null && $base !== '',
+        ));
     }
 }
diff --git a/src/Concerns/InteractsWithCache.php b/src/Concerns/InteractsWithCache.php
index 115a146..1a448cc 100644
--- a/src/Concerns/InteractsWithCache.php
+++ b/src/Concerns/InteractsWithCache.php
@@ -49,11 +49,27 @@ public function flush(Currency|string|null $base = null): void
         $store = $this->cacheStore();
 
         if ($base !== null) {
-            $store->forget($this->cacheKey($this->resolveBase($base)));
+            $key = $this->cacheKey($this->resolveBase($base));
+
+            $store->forget($key);
+            $this->forgetCacheKey($store, $key);
 
             return;
         }
 
+        $keys = $store->get($this->cacheIndexKey(), []);
+
+        if (is_array($keys)) {
+            foreach ($keys as $key) {
+                if (is_string($key)) {
+                    $store->forget($key);
+                }
+            }
+        }
+
+        $store->forget($this->cacheIndexKey());
+
+        // Clear legacy entries created before cache keys were indexed.
         foreach (Currency::cases() as $currency) {
             $store->forget($this->cacheKey($currency->value));
         }
@@ -69,6 +85,8 @@ protected function cacheRate(CacheRepository $store, string $key, Rate $rate): v
         $this->applyFreshUntil($rate);
 
         $store->put($key, $rate, $this->staleTtl());
+
+        $this->rememberCacheKey($store, $key);
     }
 
     /**
@@ -171,6 +189,58 @@ protected function cacheKey(string $base): string
         return "{$prefix}:{$base}";
     }
 
+    /**
+     * Track cache keys so flush() can clear custom string bases too.
+     */
+    protected function rememberCacheKey(CacheRepository $store, string $key): void
+    {
+        $indexKey = $this->cacheIndexKey();
+        $keys = $store->get($indexKey, []);
+
+        if (! is_array($keys)) {
+            $keys = [];
+        }
+
+        if (! in_array($key, $keys, true)) {
+            $keys[] = $key;
+        }
+
+        $store->forever($indexKey, array_values($keys));
+    }
+
+    /**
+     * Remove a key from the tracked cache-key index.
+     */
+    protected function forgetCacheKey(CacheRepository $store, string $key): void
+    {
+        $indexKey = $this->cacheIndexKey();
+        $keys = $store->get($indexKey, []);
+
+        if (! is_array($keys)) {
+            return;
+        }
+
+        $keys = array_values(array_filter($keys, fn (mixed $cachedKey): bool => $cachedKey !== $key));
+
+        if ($keys === []) {
+            $store->forget($indexKey);
+
+            return;
+        }
+
+        $store->forever($indexKey, $keys);
+    }
+
+    /**
+     * Build the key used to index all cached rate keys.
+     */
+    protected function cacheIndexKey(): string
+    {
+        $prefix = config('rates.cache.prefix', 'rates');
+
+        return "{$prefix}:__keys";
+    }
+
     /**
      * Get the stale TTL in seconds (total time before eviction from cache).
      */
diff --git a/src/Concerns/ResolvesDrivers.php b/src/Concerns/ResolvesDrivers.php
index 787f0f1..7e3a615 100644
--- a/src/Concerns/ResolvesDrivers.php
+++ b/src/Concerns/ResolvesDrivers.php
@@ -20,6 +20,7 @@
 use AtoBeach\Rates\Rate;
 use Closure;
 use Illuminate\Support\Str;
+use InvalidArgumentException;
 
 /**
  * Driver-resolution machinery, modelled on Illuminate\Support\Manager.
@@ -113,11 +114,19 @@ protected function resolveBase(Currency|string|null $base): string
             return $base->value;
         }
 
-        if ($base !== null) {
-            return $base;
+        $base ??= config('rates.base', 'USD');
+
+        if (! is_string($base)) {
+            throw new InvalidArgumentException('Base currency must be a string or Currency enum.');
+        }
+
+        $base = (string) Str::of($base)->trim()->upper();
+
+        if (! preg_match('/^[A-Z]{3}$/', $base)) {
+            throw new InvalidArgumentException("Invalid base currency [{$base}]. Expected a three-letter ISO 4217 code.");
         }
 
-        return config('rates.base', 'USD');
+        return $base;
     }
 
     /**
diff --git a/src/Conversion.php b/src/Conversion.php
index 18417eb..51b3511 100644
--- a/src/Conversion.php
+++ b/src/Conversion.php
@@ -30,7 +30,7 @@ public function __construct(
         private Currency|string|null $from = null,
         private ?Rate $rate = null,
     ) {
-        $this->amount = (string) $amount;
+        $this->amount = self::normalizeNumber($amount);
     }
 
     /**
@@ -41,6 +41,8 @@ public function __construct(
      */
     public static function bcround(string $value, int $precision): string
     {
+        $value = self::normalizeNumber($value);
+
         if ($precision < 0) {
             $precision = 0;
         }
@@ -132,6 +134,42 @@ public function toMany(array $currencies): array
         return $result;
     }
 
+    /**
+     * Normalize input to the plain decimal format expected by bcmath.
+     */
+    private static function normalizeNumber(float|int|string $value): string
+    {
+        if (is_float($value)) {
+            if (! is_finite($value)) {
+                throw ConversionException::invalidAmount((string) $value);
+            }
+
+            $value = (string) Str::of(sprintf('%.14F', $value))->rtrim('0')->rtrim('.');
+        }
+
+        $value = (string) Str::of((string) $value)->trim();
+
+        if ($value === '') {
+            throw ConversionException::invalidAmount($value);
+        }
+
+        if (str_starts_with($value, '+')) {
+            $value = mb_substr($value, 1);
+        }
+
+        if (str_starts_with($value, '.')) {
+            $value = '0'.$value;
+        } elseif (str_starts_with($value, '-.')) {
+            $value = '-0'.mb_substr($value, 1);
+        }
+
+        if (! preg_match('/^-?(?:\d+|\d+\.\d+)$/', $value)) {
+            throw ConversionException::invalidAmount($value);
+        }
+
+        return $value === '-0' ? '0' : $value;
+    }
+
     /**
      * Resolve the Rate instance needed for conversion.
      *
diff --git a/src/Drivers/ApilayerCurrencyData.php b/src/Drivers/ApilayerCurrencyData.php
index 9b9bae4..d22470d 100644
--- a/src/Drivers/ApilayerCurrencyData.php
+++ b/src/Drivers/ApilayerCurrencyData.php
@@ -18,7 +18,9 @@ class ApilayerCurrencyData extends HttpDriver
      */
     public function url(string $base): string
     {
-        return "https://api.apilayer.com/currency_data/live?source={$base}";
+        return 'https://api.apilayer.com/currency_data/live?'.http_build_query([
+            'source' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -45,6 +47,10 @@ protected function hydrate(Rate $rate, Fluent $data): Rate
             return $rate;
         }
 
+        if (! $this->matchesRequestedBase($rate, $data->source, 'source')) {
+            return $rate;
+        }
+
         $rate->source = 'Apilayer Currency Data';
         $rate->timestamp = $data->timestamp;
 
diff --git a/src/Drivers/CurrencyLayer.php b/src/Drivers/CurrencyLayer.php
index a14d6f3..926dc28 100644
--- a/src/Drivers/CurrencyLayer.php
+++ b/src/Drivers/CurrencyLayer.php
@@ -19,7 +19,10 @@ public function url(string $base): string
     {
         $key = config('rates.currencylayer.key');
 
-        return "https://api.currencylayer.com/live?access_key={$key}&source={$base}";
+        return 'https://api.currencylayer.com/live?'.http_build_query([
+            'access_key' => (string) $key,
+            'source' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -36,6 +39,10 @@ protected function hydrate(Rate $rate, Fluent $data): Rate
             return $rate;
         }
 
+        if (! $this->matchesRequestedBase($rate, $data->source, 'source')) {
+            return $rate;
+        }
+
         $rate->source = 'CurrencyLayer';
         $rate->timestamp = $data->timestamp;
 
diff --git a/src/Drivers/Driver.php b/src/Drivers/Driver.php
index 23df359..d67bced 100644
--- a/src/Drivers/Driver.php
+++ b/src/Drivers/Driver.php
@@ -6,7 +6,9 @@
 
 use AtoBeach\Rates\Contracts\Driver as DriverContract;
 use AtoBeach\Rates\Rate;
+use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Fluent;
+use Illuminate\Support\Str;
 
 abstract class Driver implements DriverContract
 {
@@ -116,4 +118,30 @@ protected function unpackPairQuotes(array $quotes, string $source): array
 
         return $rates;
     }
+
+    /**
+     * Ensure the upstream response is quoted against the requested base.
+     */
+    protected function matchesRequestedBase(Rate $rate, mixed $actual, string $field = 'base'): bool
+    {
+        if (! is_string($actual)) {
+            return true;
+        }
+
+        $actual = (string) Str::of($actual)->trim()->upper();
+
+        if ($actual === '') {
+            return true;
+        }
+
+        if ($actual === $rate->base) {
+            return true;
+        }
+
+        $provider = static::PROVIDER ?? static::class;
+
+        Log::warning("[{$provider}] Response {$field} ({$actual}) differs from requested base ({$rate->base}); ignoring response.");
+
+        return false;
+    }
 }
diff --git a/src/Drivers/EuropeanCentralBank.php b/src/Drivers/EuropeanCentralBank.php
index fef1de5..bef7cd5 100644
--- a/src/Drivers/EuropeanCentralBank.php
+++ b/src/Drivers/EuropeanCentralBank.php
@@ -66,14 +66,28 @@ public function update(Command $command): void
             (string) Str::of($this->getDatabasePath())->basename()
         );
 
-        $response = Http::withOptions(['sink' => $path])->get(
-            $this->getDatabaseUrl()
-        );
+        $temporaryPath = tempnam($root, 'rates-ecb-');
 
         throw_if(
-            $response->failed(),
-            new RuntimeException('Failed to download European Central Bank reference file. Response: '.$response->body())
+            $temporaryPath === false,
+            new RuntimeException('Failed to create a temporary European Central Bank reference file.')
+        );
+
+        $response = Http::withOptions(['sink' => $temporaryPath])->get(
+            $this->getDatabaseUrl()
         );
+
+        if ($response->failed()) {
+            @unlink($temporaryPath);
+
+            throw new RuntimeException('Failed to download European Central Bank reference file. Response: '.$response->body());
+        }
+
+        if (! @rename($temporaryPath, $path)) {
+            @unlink($temporaryPath);
+
+            throw new RuntimeException('Failed to replace European Central Bank reference file.');
+        }
     }
 
     /**
diff --git a/src/Drivers/ExchangeRateApi.php b/src/Drivers/ExchangeRateApi.php
index 603a541..1aa9009 100644
--- a/src/Drivers/ExchangeRateApi.php
+++ b/src/Drivers/ExchangeRateApi.php
@@ -17,10 +17,10 @@ class ExchangeRateApi extends HttpDriver
     public function url(string $base): string
     {
         if ($key = config('rates.exchange_rate_api.key')) {
-            return "https://v6.exchangerate-api.com/v6/{$key}/latest/{$base}";
+            return 'https://v6.exchangerate-api.com/v6/'.rawurlencode((string) $key).'/latest/'.rawurlencode($base);
         }
 
-        return "https://open.er-api.com/v6/latest/{$base}";
+        return 'https://open.er-api.com/v6/latest/'.rawurlencode($base);
     }
 
     /**
@@ -28,6 +28,10 @@ public function url(string $base): string
      */
     protected function hydrate(Rate $rate, Fluent $data): Rate
     {
+        if (! $this->matchesRequestedBase($rate, $data->base_code ?? $data->base)) {
+            return $rate;
+        }
+
         $rate->source = 'ExchangeRate-API';
         $rate->rates = (array) ($data->conversion_rates ?? $data->rates ?? []);
         $rate->timestamp = $data->time_last_update_unix;
diff --git a/src/Drivers/ExchangeRateHost.php b/src/Drivers/ExchangeRateHost.php
index 59d6169..67f9f94 100644
--- a/src/Drivers/ExchangeRateHost.php
+++ b/src/Drivers/ExchangeRateHost.php
@@ -19,7 +19,10 @@ public function url(string $base): string
     {
         $key = config('rates.exchangerate_host.key');
 
-        return "https://api.exchangerate.host/live?access_key={$key}&source={$base}";
+        return 'https://api.exchangerate.host/live?'.http_build_query([
+            'access_key' => (string) $key,
+            'source' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -36,6 +39,10 @@ protected function hydrate(Rate $rate, Fluent $data): Rate
             return $rate;
         }
 
+        if (! $this->matchesRequestedBase($rate, $data->source, 'source')) {
+            return $rate;
+        }
+
         $rate->source = 'exchangerate.host';
         $rate->timestamp = $data->timestamp;
 
diff --git a/src/Drivers/FawazAhmed.php b/src/Drivers/FawazAhmed.php
index c66ab34..228541e 100644
--- a/src/Drivers/FawazAhmed.php
+++ b/src/Drivers/FawazAhmed.php
@@ -17,7 +17,7 @@ class FawazAhmed extends HttpDriver
      */
     public function url(string $base): string
     {
-        $code = Str::lower($base);
+        $code = rawurlencode(Str::lower($base));
 
         return "https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/v1/currencies/{$code}.json";
     }
diff --git a/src/Drivers/Fixer.php b/src/Drivers/Fixer.php
index aee51fc..531f72f 100644
--- a/src/Drivers/Fixer.php
+++ b/src/Drivers/Fixer.php
@@ -19,7 +19,10 @@ public function url(string $base): string
     {
         $key = config('rates.fixer.key');
 
-        return "https://data.fixer.io/api/latest?access_key={$key}&base={$base}";
+        return 'https://data.fixer.io/api/latest?'.http_build_query([
+            'access_key' => (string) $key,
+            'base' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -36,6 +39,10 @@ protected function hydrate(Rate $rate, Fluent $data): Rate
             return $rate;
         }
 
+        if (! $this->matchesRequestedBase($rate, $data->base)) {
+            return $rate;
+        }
+
         $rate->source = 'Fixer';
         $rate->date = $data->date;
         $rate->timestamp = $data->timestamp;
diff --git a/src/Drivers/Frankfurter.php b/src/Drivers/Frankfurter.php
index c40919c..d72e8e3 100644
--- a/src/Drivers/Frankfurter.php
+++ b/src/Drivers/Frankfurter.php
@@ -16,7 +16,9 @@ class Frankfurter extends HttpDriver
      */
     public function url(string $base): string
     {
-        return "https://api.frankfurter.dev/v1/latest?from={$base}";
+        return 'https://api.frankfurter.dev/v1/latest?'.http_build_query([
+            'from' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -24,6 +26,10 @@ public function url(string $base): string
      */
     protected function hydrate(Rate $rate, Fluent $data): Rate
     {
+        if (! $this->matchesRequestedBase($rate, $data->base)) {
+            return $rate;
+        }
+
         $rate->source = 'European Central Bank';
         $rate->date = $data->date;
         $rate->rates = (array) ($data->rates ?? []);
diff --git a/src/Drivers/OpenExchangeRates.php b/src/Drivers/OpenExchangeRates.php
index b3b9c84..6101aa0 100644
--- a/src/Drivers/OpenExchangeRates.php
+++ b/src/Drivers/OpenExchangeRates.php
@@ -5,7 +5,6 @@
 namespace AtoBeach\Rates\Drivers;
 
 use AtoBeach\Rates\Rate;
-use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Fluent;
 
 class OpenExchangeRates extends HttpDriver
@@ -19,7 +18,10 @@ public function url(string $base): string
     {
         $appId = config('rates.openexchangerates.app_id');
 
-        return "https://openexchangerates.org/api/latest.json?app_id={$appId}&base={$base}";
+        return 'https://openexchangerates.org/api/latest.json?'.http_build_query([
+            'app_id' => (string) $appId,
+            'base' => $base,
+        ], '', '&', PHP_QUERY_RFC3986);
     }
 
     /**
@@ -27,6 +29,10 @@ public function url(string $base): string
      */
     protected function hydrate(Rate $rate, Fluent $data): Rate
     {
+        if (! $this->matchesRequestedBase($rate, $data->base)) {
+            return $rate;
+        }
+
         $rate->source = 'Open Exchange Rates';
         $rate->timestamp = $data->timestamp;
         $rate->rates = (array) ($data->rates ?? []);
@@ -35,15 +41,6 @@ protected function hydrate(Rate $rate, Fluent $data): Rate
             $rate->date = gmdate('Y-m-d', $data->timestamp);
         }
 
-        // Free-tier Open Exchange Rates keys always return USD regardless of
-        // the requested base. Align the Rate with the response's actual base
-        // and log so the caller knows their requested base was ignored.
-        if ($data->base && $data->base !== $rate->base) {
-            Log::warning("[openexchangerates] Response base ({$data->base}) differs from requested base ({$rate->base}); upgrade plan to change base.");
-
-            $rate->base = $data->base;
-        }
-
         return $rate;
     }
 }
diff --git a/src/Exceptions/ConversionException.php b/src/Exceptions/ConversionException.php
index 617eed6..0dd6739 100644
--- a/src/Exceptions/ConversionException.php
+++ b/src/Exceptions/ConversionException.php
@@ -20,4 +20,9 @@ public static function fetchFailed(string $base): static
     {
         return new static("Failed to fetch exchange rates for base [{$base}].");
     }
+
+    public static function invalidAmount(string $amount): static
+    {
+        return new static("Invalid conversion amount [{$amount}].");
+    }
 }
diff --git a/src/Rate.php b/src/Rate.php
index ab5da02..fa4ba7c 100644
--- a/src/Rate.php
+++ b/src/Rate.php
@@ -5,7 +5,6 @@
 namespace AtoBeach\Rates;
 
 use Illuminate\Contracts\Support\Arrayable;
-use Illuminate\Support\Arr;
 use Illuminate\Support\Carbon;
 use Illuminate\Support\Str;
 use Illuminate\Support\Traits\Macroable;
@@ -93,9 +92,7 @@ public static function make(array $attributes = []): static
      */
     public function isEmpty(): bool
     {
-        $data = Arr::except($this->toArray(), ['base', 'driver']);
-
-        return empty(array_filter($data));
+        return $this->rates === [];
     }
 
     /**
diff --git a/src/RatesManager.php b/src/RatesManager.php
index 451f882..c7160df 100644
--- a/src/RatesManager.php
+++ b/src/RatesManager.php
@@ -10,6 +10,7 @@
 use AtoBeach\Rates\Exceptions\DriverDoesNotExistException;
 use AtoBeach\Rates\Exceptions\RatesUnavailableException;
 use Illuminate\Support\Traits\Macroable;
+use InvalidArgumentException;
 
 class RatesManager
 {
@@ -47,6 +48,12 @@ public function setDriver(Driver $driver): static
      */
     public function setFallbacks(array $fallbacks): static
     {
+        foreach ($fallbacks as $fallback) {
+            if (! $fallback instanceof Driver) {
+                throw new InvalidArgumentException('Fallback drivers must implement '.Driver::class.'.');
+            }
+        }
+
         $this->fallbacksOverride = array_values($fallbacks);
 
         return $this;
diff --git a/tests/ApilayerCurrencyDataTest.php b/tests/ApilayerCurrencyDataTest.php
index c2b4ef1..dece4fa 100644
--- a/tests/ApilayerCurrencyDataTest.php
+++ b/tests/ApilayerCurrencyDataTest.php
@@ -93,3 +93,26 @@
 
     Http::assertSent(fn ($request) => $request->hasHeader('apikey', 'test-key-123'));
 });
+
+it('falls back when the response source differs from the requested base', function (): void {
+    Http::fake([
+        'api.apilayer.com/*' => Http::response([
+            'success' => true,
+            'timestamp' => 1_704_153_601,
+            'source' => 'USD',
+            'quotes' => ['USDEUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new ApilayerCurrencyData);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/CacheTest.php b/tests/CacheTest.php
index c7ab55a..d14c2bb 100644
--- a/tests/CacheTest.php
+++ b/tests/CacheTest.php
@@ -292,6 +292,27 @@
     Rates::get('EUR');  // cache miss → resolve (was flushed)
 });
 
+it('flushes custom string bases outside the currency enum', function (): void {
+    config([
+        'rates.cache.enabled' => true,
+        'rates.cache.store' => 'array',
+    ]);
+
+    $driver = fakeRateDriver();
+    $driver->shouldReceive('get')->twice()->with('XBT')->andReturnUsing(fn (string $base): Rate => Rate::make([
+        'base' => $base,
+        'driver' => $driver::class,
+        'rates' => ['USD' => 50000.0],
+    ]));
+
+    Rates::setDriver($driver);
+    Rates::setFallbacks([]);
+
+    Rates::get('xbt');
+    Rates::flush();
+    Rates::get('XBT');
+});
+
 it('flush is a no-op when caching is disabled', function (): void {
     config(['rates.cache.enabled' => false]);
 
diff --git a/tests/ConversionTest.php b/tests/ConversionTest.php
index 3973c13..65d1e0b 100644
--- a/tests/ConversionTest.php
+++ b/tests/ConversionTest.php
@@ -116,6 +116,26 @@
     $conversion->to(Currency::GBP);
 })->throws(ConversionException::class, 'No source currency specified.');
 
+it('throws a conversion exception for invalid amount strings', function (): void {
+    $rate = Rate::make([
+        'base' => 'USD',
+        'driver' => 'test',
+        'rates' => ['EUR' => 0.92],
+    ]);
+
+    $rate->convert('1e3', to: Currency::EUR);
+})->throws(ConversionException::class, 'Invalid conversion amount [1e3].');
+
+it('normalizes decimal amount strings for bcmath', function (): void {
+    $rate = Rate::make([
+        'base' => 'USD',
+        'driver' => 'test',
+        'rates' => ['EUR' => 0.92],
+    ]);
+
+    expect($rate->convert('.5', to: Currency::EUR))->toBe('0.46');
+});
+
 it('uses bcmath for precise multiplication', function (): void {
     // This test verifies that we don't lose precision due to
     // floating-point multiplication. 0.1 + 0.2 != 0.3 in IEEE 754,
diff --git a/tests/CurrencyLayerTest.php b/tests/CurrencyLayerTest.php
index 5b1bed3..ab46a8e 100644
--- a/tests/CurrencyLayerTest.php
+++ b/tests/CurrencyLayerTest.php
@@ -5,9 +5,11 @@
 namespace AtoBeach\Rates\Tests;
 
 use AtoBeach\Rates\Drivers\CurrencyLayer;
+use AtoBeach\Rates\Drivers\Driver;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
 
 it('it can process fluent response', function (): void {
     Http::fake([
@@ -44,3 +46,26 @@
         ],
     ]);
 });
+
+it('falls back when the response source differs from the requested base', function (): void {
+    Http::fake([
+        'api.currencylayer.com/*' => Http::response([
+            'success' => true,
+            'timestamp' => 1_704_153_601,
+            'source' => 'USD',
+            'quotes' => ['USDEUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new CurrencyLayer);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/EuropeanCentralBankTest.php b/tests/EuropeanCentralBankTest.php
index 2a2b06a..d49864e 100644
--- a/tests/EuropeanCentralBankTest.php
+++ b/tests/EuropeanCentralBankTest.php
@@ -8,7 +8,10 @@
 use AtoBeach\Rates\Drivers\EuropeanCentralBank;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
+use Illuminate\Console\Command;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
+use RuntimeException;
 
 it('can update reference file', function (): void {
     config([
@@ -24,6 +27,30 @@
     expect(database_path('ecb/eurofxref-daily.xml'))->toBeFile();
 });
 
+it('preserves the existing reference file when update fails', function (): void {
+    $path = tempnam(sys_get_temp_dir(), 'rates-ecb-test-');
+
+    file_put_contents($path, 'original');
+
+    config([
+        'rates.ecb.local.path' => $path,
+        'rates.ecb.local.url' => 'http://example.com/fail',
+    ]);
+
+    Http::fake([
+        'http://example.com/fail' => Http::response('failed', 500),
+    ]);
+
+    try {
+        (new EuropeanCentralBank)->update(m::mock(Command::class));
+    } catch (RuntimeException $exception) {
+        expect(file_get_contents($path))->toBe('original');
+        @unlink($path);
+
+        throw $exception;
+    }
+})->throws(RuntimeException::class);
+
 it('can read from the reference file for the euro base', function (): void {
     config(['rates.base' => 'EUR']);
     config(['rates.driver' => 'ecb']);
diff --git a/tests/ExchangeRateApiTest.php b/tests/ExchangeRateApiTest.php
index 5061db7..f772608 100644
--- a/tests/ExchangeRateApiTest.php
+++ b/tests/ExchangeRateApiTest.php
@@ -4,10 +4,12 @@
 
 namespace AtoBeach\Rates\Tests;
 
+use AtoBeach\Rates\Drivers\Driver;
 use AtoBeach\Rates\Drivers\ExchangeRateApi;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
 
 it('it can process fluent response', function (): void {
     Http::fake([
@@ -55,3 +57,26 @@
     expect((new ExchangeRateApi)->url('USD'))
         ->toEqual('https://open.er-api.com/v6/latest/USD');
 });
+
+it('falls back when the response base differs from the requested base', function (): void {
+    Http::fake([
+        'open.er-api.com/*' => Http::response([
+            'result' => 'success',
+            'base_code' => 'USD',
+            'time_last_update_unix' => 1_704_153_601,
+            'rates' => ['EUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new ExchangeRateApi);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/ExchangeRateHostTest.php b/tests/ExchangeRateHostTest.php
index 653579c..046a26e 100644
--- a/tests/ExchangeRateHostTest.php
+++ b/tests/ExchangeRateHostTest.php
@@ -4,10 +4,12 @@
 
 namespace AtoBeach\Rates\Tests;
 
+use AtoBeach\Rates\Drivers\Driver;
 use AtoBeach\Rates\Drivers\ExchangeRateHost;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
 
 it('it can process fluent response', function (): void {
     Http::fake([
@@ -42,3 +44,26 @@
         ],
     ]);
 });
+
+it('falls back when the response source differs from the requested base', function (): void {
+    Http::fake([
+        'api.exchangerate.host/*' => Http::response([
+            'success' => true,
+            'timestamp' => 1_704_153_601,
+            'source' => 'USD',
+            'quotes' => ['USDEUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new ExchangeRateHost);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/FixerTest.php b/tests/FixerTest.php
index 7d5c380..ce6cb54 100644
--- a/tests/FixerTest.php
+++ b/tests/FixerTest.php
@@ -29,12 +29,12 @@
     Rates::setDriver(new Fixer);
     Rates::setFallbacks([]);
 
-    $rate = Rates::get();
+    $rate = Rates::get('EUR');
 
     expect($rate)->toBeInstanceOf(Rate::class);
 
     expect($rate->toArray())->toEqual([
-        'base' => 'USD',
+        'base' => 'EUR',
         'driver' => Fixer::class,
         'provider' => 'fixer',
         'source' => 'Fixer',
@@ -71,3 +71,27 @@
 
     expect(Rates::get())->toBeInstanceOf(Rate::class);
 });
+
+it('falls back when the response base differs from the requested base', function (): void {
+    Http::fake([
+        'data.fixer.io/*' => Http::response([
+            'success' => true,
+            'timestamp' => 1_704_153_601,
+            'base' => 'USD',
+            'date' => '2024-01-02',
+            'rates' => ['EUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new Fixer);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/FrankfurterTest.php b/tests/FrankfurterTest.php
index e123836..0d57c84 100644
--- a/tests/FrankfurterTest.php
+++ b/tests/FrankfurterTest.php
@@ -4,10 +4,12 @@
 
 namespace AtoBeach\Rates\Tests;
 
+use AtoBeach\Rates\Drivers\Driver;
 use AtoBeach\Rates\Drivers\Frankfurter;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
 
 it('it can process fluent response', function (): void {
     Http::fake([
@@ -42,3 +44,48 @@
         ],
     ]);
 });
+
+it('falls back when the response base differs from the requested base', function (): void {
+    Http::fake([
+        'api.frankfurter.dev/*' => Http::response([
+            'amount' => 1.0,
+            'base' => 'USD',
+            'date' => '2024-01-02',
+            'rates' => ['EUR' => 0.9123],
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setDriver(new Frankfurter);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
+
+it('falls back when a successful response has no rates', function (): void {
+    Http::fake([
+        'api.frankfurter.dev/*' => Http::response([
+            'amount' => 1.0,
+            'base' => 'USD',
+            'date' => '2024-01-02',
+        ]),
+    ]);
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('USD')->andReturn(Rate::make([
+        'base' => 'USD',
+        'driver' => 'fallback',
+        'rates' => ['EUR' => 0.9123],
+    ]));
+
+    Rates::setDriver(new Frankfurter);
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get()->driver)->toBe('fallback');
+});
diff --git a/tests/OpenExchangeRatesTest.php b/tests/OpenExchangeRatesTest.php
index 585989b..9024085 100644
--- a/tests/OpenExchangeRatesTest.php
+++ b/tests/OpenExchangeRatesTest.php
@@ -4,10 +4,13 @@
 
 namespace AtoBeach\Rates\Tests;
 
+use AtoBeach\Rates\Drivers\Driver;
 use AtoBeach\Rates\Drivers\OpenExchangeRates;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
+use Illuminate\Support\Facades\Cache;
 use Illuminate\Support\Facades\Http;
+use Mockery as m;
 
 it('it can process fluent response', function (): void {
     Http::fake([
@@ -43,3 +46,35 @@
         ],
     ]);
 });
+
+it('falls back and does not cache under the requested base when the response base differs', function (): void {
+    config([
+        'rates.cache.enabled' => true,
+        'rates.cache.store' => 'array',
+    ]);
+
+    Http::fake([
+        'openexchangerates.org/*' => Http::response([
+            'timestamp' => 1_704_153_601,
+            'base' => 'USD',
+            'rates' => ['EUR' => 0.9123],
+        ]),
+    ]);
+
+    Rates::setDriver(new OpenExchangeRates);
+    Rates::setFallbacks([]);
+
+    expect(Rates::get('EUR'))->toBeNull();
+    expect(Cache::store('array')->get('rates:EUR'))->toBeNull();
+
+    $fallback = m::mock(Driver::class);
+    $fallback->shouldReceive('get')->once()->with('EUR')->andReturn(Rate::make([
+        'base' => 'EUR',
+        'driver' => 'fallback',
+        'rates' => ['USD' => 1.0961],
+    ]));
+
+    Rates::setFallbacks([$fallback]);
+
+    expect(Rates::get('EUR')->driver)->toBe('fallback');
+});
diff --git a/tests/RateTest.php b/tests/RateTest.php
index 9b97cb1..352d997 100644
--- a/tests/RateTest.php
+++ b/tests/RateTest.php
@@ -52,6 +52,18 @@
     expect($rate->isEmpty())->toBeTrue();
 });
 
+it('returns empty when only metadata is present', function (): void {
+    $rate = Rate::make([
+        'base' => 'USD',
+        'driver' => 'test',
+        'source' => 'metadata only',
+        'date' => '2024-01-02',
+        'timestamp' => 1_704_153_601,
+    ]);
+
+    expect($rate->isEmpty())->toBeTrue();
+});
+
 it('does not return empty', function (): void {
     $rate = new Rate;
 
diff --git a/tests/RatesTest.php b/tests/RatesTest.php
index 36271e0..8c15c4f 100644
--- a/tests/RatesTest.php
+++ b/tests/RatesTest.php
@@ -9,6 +9,7 @@
 use AtoBeach\Rates\Exceptions\RatesUnavailableException;
 use AtoBeach\Rates\Facades\Rates;
 use AtoBeach\Rates\Rate;
+use InvalidArgumentException;
 use Mockery as m;
 
 it('falls back to the next driver when the primary returns null', function (): void {
@@ -65,3 +66,25 @@
 
     Rates::getOrFail();
 })->throws(RatesUnavailableException::class, 'No driver in the chain produced rates for base [GBP].');
+
+it('normalizes base strings before resolving drivers', function (): void {
+    $driver = m::mock(Driver::class);
+    $driver->shouldReceive('get')->once()->with('USD')->andReturn(Rate::make([
+        'base' => 'USD',
+        'driver' => 'test',
+        'rates' => ['EUR' => 0.92],
+    ]));
+
+    Rates::setDriver($driver);
+    Rates::setFallbacks([]);
+
+    expect(Rates::get(' usd '))->toBeInstanceOf(Rate::class);
+});
+
+it('rejects invalid base strings', function (): void {
+    Rates::get('USD&foo=bar');
+})->throws(InvalidArgumentException::class, 'Invalid base currency [USD&FOO=BAR]. Expected a three-letter ISO 4217 code.');
+
+it('validates fallback overrides immediately', function (): void {
+    Rates::setFallbacks(['not-a-driver']);
+})->throws(InvalidArgumentException::class);