diff --git a/CHANGELOG.md b/CHANGELOG.md
index 22ad2b37..e77f0621 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 
 * [BC Break] Remove the `providerClass` argument of `#[CompletionProvider]`. Use `provider:`, which takes the same class-string and is now the first positional argument.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
+* Fix OIDC discovery rejecting issuers with a trailing slash (e.g. Authentik, Auth0).
 
 0.8.0
 -----
diff --git a/src/Server/Transport/Http/OAuth/OidcDiscovery.php b/src/Server/Transport/Http/OAuth/OidcDiscovery.php
index 2f246852..a0c0cd61 100644
--- a/src/Server/Transport/Http/OAuth/OidcDiscovery.php
+++ b/src/Server/Transport/Http/OAuth/OidcDiscovery.php
@@ -146,8 +146,10 @@ public function discover(string $issuer): array
      */
     private function fetchMetadata(string $issuer): array
     {
-        $issuer = rtrim($issuer, '/');
-        $parsed = parse_url($issuer);
+        // The trailing slash is dropped to build discovery URLs (RFC 8414 §3.1),
+        // but the issuer is matched verbatim (RFC 8414 §3.3, OIDC Discovery §4.3).
+        $discoveryIssuer = rtrim($issuer, '/');
+        $parsed = parse_url($discoveryIssuer);
 
         if (false === $parsed || !isset($parsed['scheme'], $parsed['host'])) {
             throw new RuntimeException(\sprintf('Invalid issuer URL: %s', $issuer));
@@ -170,7 +172,7 @@ private function fetchMetadata(string $issuer): array
             // 2. OIDC path insertion
             $discoveryUrls[] = $baseUrl.'/.well-known/openid-configuration'.$path;
             // 3. OIDC path appending
-            $discoveryUrls[] = $issuer.'/.well-known/openid-configuration';
+            $discoveryUrls[] = $discoveryIssuer.'/.well-known/openid-configuration';
         } else {
             // For issuer URLs without path components
             $discoveryUrls[] = $baseUrl.'/.well-known/oauth-authorization-server';
diff --git a/tests/Unit/Server/Transport/Http/OAuth/OidcDiscoveryTest.php b/tests/Unit/Server/Transport/Http/OAuth/OidcDiscoveryTest.php
index 29c86d6f..fd86d02e 100644
--- a/tests/Unit/Server/Transport/Http/OAuth/OidcDiscoveryTest.php
+++ b/tests/Unit/Server/Transport/Http/OAuth/OidcDiscoveryTest.php
@@ -293,6 +293,88 @@ public function testIssuerWithoutPathUsesStandardWellKnownEndpoints(): void
         $this->assertSame('https://auth.example.com/.well-known/openid-configuration', $requestedUrls[1]);
     }
 
+    #[TestDox('issuer with path and trailing slash is discovered and matched verbatim')]
+    public function testIssuerWithPathAndTrailingSlash(): void
+    {
+        $this->skipIfPsrHttpClientIsMissing();
+
+        $factory = new Psr17Factory();
+        $requestedUrls = [];
+        $issuer = 'https://auth.example.com/application/o/mcp/';
+        $validMetadata = [
+            'issuer' => $issuer,
+            'authorization_endpoint' => 'https://auth.example.com/application/o/authorize/',
+            'token_endpoint' => 'https://auth.example.com/application/o/token/',
+            'jwks_uri' => 'https://auth.example.com/application/o/mcp/jwks/',
+            'code_challenge_methods_supported' => ['S256'],
+        ];
+
+        $httpClient = $this->createMock(ClientInterface::class);
+        $httpClient->expects($this->exactly(3))
+            ->method('sendRequest')
+            ->willReturnCallback(static function (RequestInterface $request) use ($factory, &$requestedUrls, $validMetadata): ResponseInterface {
+                $requestedUrls[] = (string) $request->getUri();
+
+                if (3 !== \count($requestedUrls)) {
+                    return $factory->createResponse(404);
+                }
+
+                return $factory->createResponse(200)->withBody(
+                    $factory->createStream(json_encode($validMetadata, \JSON_THROW_ON_ERROR)),
+                );
+            });
+
+        $discovery = new OidcDiscovery(
+            httpClient: $httpClient,
+            requestFactory: $factory,
+        );
+
+        $metadata = $discovery->discover($issuer);
+
+        $this->assertSame($issuer, $metadata['issuer']);
+        $this->assertSame('https://auth.example.com/.well-known/oauth-authorization-server/application/o/mcp', $requestedUrls[0]);
+        $this->assertSame('https://auth.example.com/.well-known/openid-configuration/application/o/mcp', $requestedUrls[1]);
+        $this->assertSame('https://auth.example.com/application/o/mcp/.well-known/openid-configuration', $requestedUrls[2]);
+    }
+
+    #[TestDox('issuer without path but with trailing slash is discovered and matched verbatim')]
+    public function testIssuerWithoutPathWithTrailingSlash(): void
+    {
+        $this->skipIfPsrHttpClientIsMissing();
+
+        $factory = new Psr17Factory();
+        $requestedUrls = [];
+        $issuer = 'https://auth.example.com/';
+        $validMetadata = [
+            'issuer' => $issuer,
+            'authorization_endpoint' => 'https://auth.example.com/authorize',
+            'token_endpoint' => 'https://auth.example.com/oauth/token',
+            'jwks_uri' => 'https://auth.example.com/.well-known/jwks.json',
+            'code_challenge_methods_supported' => ['S256'],
+        ];
+
+        $httpClient = $this->createMock(ClientInterface::class);
+        $httpClient->expects($this->once())
+            ->method('sendRequest')
+            ->willReturnCallback(static function (RequestInterface $request) use ($factory, &$requestedUrls, $validMetadata): ResponseInterface {
+                $requestedUrls[] = (string) $request->getUri();
+
+                return $factory->createResponse(200)->withBody(
+                    $factory->createStream(json_encode($validMetadata, \JSON_THROW_ON_ERROR)),
+                );
+            });
+
+        $discovery = new OidcDiscovery(
+            httpClient: $httpClient,
+            requestFactory: $factory,
+        );
+
+        $metadata = $discovery->discover($issuer);
+
+        $this->assertSame($issuer, $metadata['issuer']);
+        $this->assertSame(['https://auth.example.com/.well-known/oauth-authorization-server'], $requestedUrls);
+    }
+
     private function skipIfPsrHttpClientIsMissing(): void
     {
         if (!interface_exists(ClientInterface::class)) {