diff --git a/src/Bridge/AccessToken.php b/src/Bridge/AccessToken.php
index 7d87e3da..6885bb13 100644
--- a/src/Bridge/AccessToken.php
+++ b/src/Bridge/AccessToken.php
@@ -2,11 +2,15 @@
 
 namespace Laravel\Passport\Bridge;
 
+use DateTimeImmutable;
+use Laravel\Passport\Passport;
+use Lcobucci\JWT\Token;
 use League\OAuth2\Server\Entities\AccessTokenEntityInterface;
 use League\OAuth2\Server\Entities\ClientEntityInterface;
 use League\OAuth2\Server\Entities\Traits\AccessTokenTrait;
 use League\OAuth2\Server\Entities\Traits\EntityTrait;
 use League\OAuth2\Server\Entities\Traits\TokenEntityTrait;
+use RuntimeException;
 
 class AccessToken implements AccessTokenEntityInterface
 {
@@ -30,4 +34,85 @@ public function __construct(?string $userIdentifier, array $scopes, ClientEntity
 
         $this->setClient($client);
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    private function convertToJWT(): Token
+    {
+        $this->initJwtConfiguration();
+
+        return $this->jwtConfiguration->builder()
+            ->withHeader('kid', $this->determineKid())
+            ->permittedFor($this->getClient()->getIdentifier())
+            ->identifiedBy($this->getIdentifier())
+            ->issuedAt(new DateTimeImmutable())
+            ->canOnlyBeUsedAfter(new DateTimeImmutable())
+            ->expiresAt($this->getExpiryDateTime())
+            ->relatedTo($this->getSubjectIdentifier())
+            ->withClaim('scopes', $this->getScopes())
+            ->getToken($this->jwtConfiguration->signer(), $this->jwtConfiguration->signingKey());
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function toString(): string
+    {
+        return $this->convertToJWT()->toString();
+    }
+
+    /**
+     * Determine the 'kid' (key ID) for the JWT header, based on the public key's JWK thumbprint.
+     * See RFC 7638: https://tools.ietf.org/html/rfc7638.
+     */
+    protected function determineKid(): string
+    {
+        $pem = $this->getPublicKeyMaterial();
+
+        $res = openssl_pkey_get_public($pem);
+        if ($res === false) {
+            throw new RuntimeException('Invalid public key material');
+        }
+
+        $details = openssl_pkey_get_details($res);
+        if ($details === false || $details['type'] !== OPENSSL_KEYTYPE_RSA) {
+            throw new RuntimeException('Only RSA public keys are supported');
+        }
+
+        // Base64url helpers
+        $b64url = fn (string $bin) => rtrim(strtr(base64_encode($bin), '+/', '-_'), '=');
+
+        // Build minimal JWK (kty, n, e)
+        $jwk = [
+            'e' => $b64url($details['rsa']['e']),
+            'kty' => 'RSA',
+            'n' => $b64url($details['rsa']['n']),
+        ];
+
+        // Canonical JSON: keys sorted, no spaces or escapes that change semantics
+        ksort($jwk);
+        $json = json_encode($jwk, JSON_UNESCAPED_SLASHES);
+
+        // RFC 7638 thumbprint = SHA-256 over canonical JSON, base64url-encoded
+        $thumb = hash('sha256', $json, true);
+
+        return $b64url($thumb);
+    }
+
+    /**
+     * Find the public key material from config or file.
+     */
+    private function getPublicKeyMaterial(): string
+    {
+        if ($keyMaterial = config('passport.public_key')) {
+            return $keyMaterial;
+        }
+
+        if (file_exists($publicKey = Passport::keyPath('oauth-public.key'))) {
+            return file_get_contents($publicKey);
+        }
+
+        throw new RuntimeException('No public key available');
+    }
 }