diff --git a/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php b/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
index bd2a272ea..ecc26b724 100644
--- a/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
+++ b/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
@@ -10,6 +10,8 @@
 use Tempest\Http\Session\SessionId;
 use Tempest\Http\Session\SessionIdResolver;
 
+use function Tempest\Support\str;
+
 final readonly class HeaderSessionIdResolver implements SessionIdResolver
 {
     public function __construct(
diff --git a/tests/Fixtures/Controllers/HeaderWithUnderscoresController.php b/tests/Fixtures/Controllers/HeaderWithUnderscoresController.php
new file mode 100644
index 000000000..a0616b99b
--- /dev/null
+++ b/tests/Fixtures/Controllers/HeaderWithUnderscoresController.php
@@ -0,0 +1,16 @@
+<?php
+
+namespace Tests\Tempest\Fixtures\Controllers;
+
+use Tempest\Http\Request;
+use Tempest\Http\Responses\Ok;
+use Tempest\Router\Get;
+
+final class HeaderWithUnderscoresController
+{
+    #[Get('/header-with-underscores')]
+    public function __invoke(Request $request): Ok
+    {
+        return new Ok()->addHeader('tempest_session_id', $request->headers->get('tempest_session_id'));
+    }
+}
diff --git a/tests/Integration/Http/SessionFromHeaderTest.php b/tests/Integration/Http/HeaderSessionIdResolverTest.php
similarity index 77%
rename from tests/Integration/Http/SessionFromHeaderTest.php
rename to tests/Integration/Http/HeaderSessionIdResolverTest.php
index adb9d3b5b..41ffec6d4 100644
--- a/tests/Integration/Http/SessionFromHeaderTest.php
+++ b/tests/Integration/Http/HeaderSessionIdResolverTest.php
@@ -9,19 +9,26 @@
 use Tempest\Http\Method;
 use Tempest\Http\Request;
 use Tempest\Http\Session\Config\FileSessionConfig;
+use Tempest\Http\Session\Resolvers\HeaderSessionIdResolver;
 use Tempest\Http\Session\Session;
+use Tempest\Http\Session\SessionIdResolver;
 use Tests\Tempest\Integration\FrameworkIntegrationTestCase;
 
 /**
  * @internal
  */
-final class SessionFromHeaderTest extends FrameworkIntegrationTestCase
+final class HeaderSessionIdResolverTest extends FrameworkIntegrationTestCase
 {
     public function test_resolving_session_from_header(): void
     {
+        $this->container->singleton(
+            SessionIdResolver::class,
+            $this->container->get(HeaderSessionIdResolver::class),
+        );
+
         $this->container->config(new FileSessionConfig(
-            path: 'test_sessions',
             expiration: Duration::hours(2),
+            path: 'test_sessions',
         ));
 
         $this->setSessionId('session_a');
diff --git a/tests/Integration/Route/RequestTest.php b/tests/Integration/Route/RequestTest.php
index 5648c567c..01d8f2357 100644
--- a/tests/Integration/Route/RequestTest.php
+++ b/tests/Integration/Route/RequestTest.php
@@ -106,6 +106,19 @@ public function test_custom_request_test(): void
         $this->assertEquals('test-title test-text', $response->body);
     }
 
+    public function test_headers_with_underscores(): void
+    {
+        $this->http
+            ->get(
+                uri: '/header-with-underscores',
+                headers: [
+                    'tempest_session_id' => 'test',
+                ],
+            )
+            ->assertOk()
+            ->assertHeaderMatches('tempest_session_id', 'test');
+    }
+
     public function test_generic_request_can_map_to_custom_request(): void
     {
         $response = $this->http