diff --git a/src/Providers/ProviderRegistry.php b/src/Providers/ProviderRegistry.php
index 00935042..d1bcff01 100644
--- a/src/Providers/ProviderRegistry.php
+++ b/src/Providers/ProviderRegistry.php
@@ -38,12 +38,12 @@ class ProviderRegistry implements WithHttpTransporterInterface
     /**
      * @var array<string, class-string<ProviderInterface>> Mapping of provider IDs to class names.
      */
-    private array $providerClassNames = [];
+    private array $registeredIdsToClassNames = [];
 
     /**
-     * @var array<class-string<ProviderInterface>, true> Set of registered class names for fast lookup.
+     * @var array<class-string<ProviderInterface>, string> Mapping of provider class names to IDs.
      */
-    private array $registeredClassNames = [];
+    private array $registeredClassNamesToIds = [];
 
     /**
      * @var array<class-string<ProviderInterface>, RequestAuthenticationInterface> Mapping of provider class names to
@@ -109,8 +109,8 @@ public function registerProvider(string $className): void
             $this->setRequestAuthenticationForProvider($className, $this->providerAuthenticationInstances[$className]);
         }
 
-        $this->providerClassNames[$metadata->getId()] = $className;
-        $this->registeredClassNames[$className] = true;
+        $this->registeredIdsToClassNames[$metadata->getId()] = $className;
+        $this->registeredClassNamesToIds[$className] = $metadata->getId();
     }
 
     /**
@@ -122,7 +122,7 @@ public function registerProvider(string $className): void
      */
     public function getRegisteredProviderIds(): array
     {
-        return array_keys($this->providerClassNames);
+        return array_keys($this->registeredIdsToClassNames);
     }
 
     /**
@@ -135,8 +135,8 @@ public function getRegisteredProviderIds(): array
      */
     public function hasProvider(string $idOrClassName): bool
     {
-        return isset($this->providerClassNames[$idOrClassName]) ||
-            isset($this->registeredClassNames[$idOrClassName]);
+        return isset($this->registeredIdsToClassNames[$idOrClassName]) ||
+            isset($this->registeredClassNamesToIds[$idOrClassName]);
     }
 
     /**
@@ -144,19 +144,26 @@ public function hasProvider(string $idOrClassName): bool
      *
      * @since 0.1.0
      *
-     * @param string $id The provider ID.
+     * @param string|class-string<ProviderInterface> $idOrClassName The provider ID or class name.
      * @return string The provider class name.
      * @throws InvalidArgumentException If the provider is not registered.
      */
-    public function getProviderClassName(string $id): string
+    public function getProviderClassName(string $idOrClassName): string
     {
-        if (!isset($this->providerClassNames[$id])) {
-            throw new InvalidArgumentException(
-                sprintf('Provider not registered: %s', $id)
-            );
+        // If it's already a class name, return it
+        if (isset($this->registeredClassNamesToIds[$idOrClassName])) {
+            return $idOrClassName;
+        }
+
+        // If it's a registered ID, return its class name
+        if (isset($this->registeredIdsToClassNames[$idOrClassName])) {
+            return $this->registeredIdsToClassNames[$idOrClassName];
         }
 
-        return $this->providerClassNames[$id];
+        // Not found
+        throw new InvalidArgumentException(
+            sprintf('Provider not registered: %s', $idOrClassName)
+        );
     }
 
     /**
@@ -171,17 +178,13 @@ public function getProviderClassName(string $id): string
     public function getProviderId(string $idOrClassName): string
     {
         // If it's already an ID, return it
-        if (isset($this->providerClassNames[$idOrClassName])) {
+        if (isset($this->registeredIdsToClassNames[$idOrClassName])) {
             return $idOrClassName;
         }
 
-        // If it's a class name, find its ID
-        if (isset($this->registeredClassNames[$idOrClassName])) {
-            foreach ($this->providerClassNames as $id => $className) {
-                if ($className === $idOrClassName) {
-                    return $id;
-                }
-            }
+        // If it's a registered class name, return its ID
+        if (isset($this->registeredClassNamesToIds[$idOrClassName])) {
+            return $this->registeredClassNamesToIds[$idOrClassName];
         }
 
         // Not found
@@ -225,7 +228,7 @@ public function findModelsMetadataForSupport(ModelRequirements $modelRequirement
     {
         $results = [];
 
-        foreach ($this->providerClassNames as $providerId => $className) {
+        foreach ($this->registeredIdsToClassNames as $providerId => $className) {
             $providerResults = $this->findProviderModelsMetadataForSupport($providerId, $modelRequirements);
             if (!empty($providerResults)) {
                 // Use static method from ProviderInterface
@@ -337,7 +340,7 @@ public function bindModelDependencies(ModelInterface $modelInstance): void
     private function resolveProviderClassName(string $idOrClassName): string
     {
         // Handle both ID and class name
-        $className = $this->providerClassNames[$idOrClassName] ?? $idOrClassName;
+        $className = $this->registeredIdsToClassNames[$idOrClassName] ?? $idOrClassName;
 
         if (!$this->hasProvider($idOrClassName)) {
             throw new InvalidArgumentException(
@@ -359,7 +362,7 @@ public function setHttpTransporter(HttpTransporterInterface $httpTransporter): v
         $this->setHttpTransporterOriginal($httpTransporter);
 
         // Make sure all registered providers have the HTTP transporter hooked up as needed.
-        foreach ($this->providerClassNames as $className) {
+        foreach ($this->registeredIdsToClassNames as $className) {
             $this->setHttpTransporterForProvider($className, $httpTransporter);
         }
     }
diff --git a/tests/unit/Providers/ProviderRegistryTest.php b/tests/unit/Providers/ProviderRegistryTest.php
index e0cae3bc..6c08fbae 100644
--- a/tests/unit/Providers/ProviderRegistryTest.php
+++ b/tests/unit/Providers/ProviderRegistryTest.php
@@ -51,6 +51,11 @@ public function testRegisterProviderWithValidProvider(): void
         $this->assertTrue($this->registry->hasProvider('mock'));
         $this->assertTrue($this->registry->hasProvider(MockProvider::class));
         $this->assertEquals(MockProvider::class, $this->registry->getProviderClassName('mock'));
+        $this->assertEquals('mock', $this->registry->getProviderId(MockProvider::class));
+
+        // Ensure calling the lookup functions with a value in the correct format simply returns it as is.
+        $this->assertEquals(MockProvider::class, $this->registry->getProviderClassName(MockProvider::class));
+        $this->assertEquals('mock', $this->registry->getProviderId('mock'));
     }
 
     /**
@@ -105,6 +110,19 @@ public function testGetProviderClassNameWithUnregisteredProvider(): void
         $this->registry->getProviderClassName('nonexistent');
     }
 
+    /**
+     * Tests getProviderId with unregistered provider.
+     *
+     * @return void
+     */
+    public function testGetProviderIdWithUnregisteredProvider(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Provider not registered: ' . InvalidArgumentException::class);
+
+        $this->registry->getProviderId(InvalidArgumentException::class);
+    }
+
     /**
      * Tests isProviderConfigured with registered provider.
      *