diff --git a/src/Contracts/CancellationClientInterface.php b/src/Contracts/CancellationClientInterface.php
new file mode 100644
index 0000000..8a7df9d
--- /dev/null
+++ b/src/Contracts/CancellationClientInterface.php
@@ -0,0 +1,19 @@
+<?php
+
+// SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+declare(strict_types=1);
+
+namespace LibreCodeCoop\NfsePHP\Contracts;
+
+/**
+ * Explicit cancellation contract for the official e101101 reason code and description.
+ */
+interface CancellationClientInterface
+{
+    /**
+     * @param string $codigoMotivo Official TSCodJustCanc value: 1, 2 or 9.
+     */
+    public function cancelWithReason(string $chaveAcesso, string $codigoMotivo, string $motivo): bool;
+}
diff --git a/src/Http/NfseClient.php b/src/Http/NfseClient.php
index 6e90928..4e8afe4 100644
--- a/src/Http/NfseClient.php
+++ b/src/Http/NfseClient.php
@@ -9,6 +9,7 @@
 
 use LibreCodeCoop\NfsePHP\Config\CertConfig;
 use LibreCodeCoop\NfsePHP\Config\EnvironmentConfig;
+use LibreCodeCoop\NfsePHP\Contracts\CancellationClientInterface;
 use LibreCodeCoop\NfsePHP\Contracts\DecisionNfseIssuerInterface;
 use LibreCodeCoop\NfsePHP\Contracts\DpsLookupInterface;
 use LibreCodeCoop\NfsePHP\Contracts\EventLookupInterface;
@@ -42,7 +43,7 @@
  * Communicates with the SEFIN gateway to issue, query, and cancel NFS-e.
  * All requests carry a signed DPS XML payload.
  */
-class NfseClient implements NfseClientInterface, DecisionNfseIssuerInterface, DpsLookupInterface, EventLookupInterface, EventRegistrationInterface
+class NfseClient implements NfseClientInterface, CancellationClientInterface, DecisionNfseIssuerInterface, DpsLookupInterface, EventLookupInterface, EventRegistrationInterface
 {
     private readonly string $baseUrl;
     private readonly XmlSignerInterface $signer;
@@ -240,7 +241,17 @@ public function registerEventXml(string $chaveAcesso, string $eventXml): EventRe
 
     public function cancel(string $chaveAcesso, string $motivo): bool
     {
-        $eventoXml = $this->buildCancelEventXml($chaveAcesso, $motivo);
+        return $this->cancelWithReason($chaveAcesso, '1', $motivo);
+    }
+
+    #[\Override]
+    public function cancelWithReason(string $chaveAcesso, string $codigoMotivo, string $motivo): bool
+    {
+        $codigoMotivo = trim($codigoMotivo);
+        $motivo = trim($motivo);
+        $this->assertCancellationReason($codigoMotivo, $motivo);
+
+        $eventoXml = $this->buildCancelEventXml($chaveAcesso, $codigoMotivo, $motivo);
 
         try {
             $this->registerEventXml($chaveAcesso, $eventoXml);
@@ -409,7 +420,7 @@ private function assertEventXmlMatchesAccessKey(string $eventXml, string $chaveA
         }
     }
 
-    private function buildCancelEventXml(string $chaveAcesso, string $motivo): string
+    private function buildCancelEventXml(string $chaveAcesso, string $codigoMotivo, string $motivo): string
     {
         $doc = new \DOMDocument('1.0', 'UTF-8');
         $doc->formatOutput = false;
@@ -430,13 +441,29 @@ private function buildCancelEventXml(string $chaveAcesso, string $motivo): strin
 
         $e101101 = $doc->createElement('e101101');
         $e101101->appendChild($doc->createElement('xDesc', 'Cancelamento de NFS-e'));
-        $e101101->appendChild($doc->createElement('cMotivo', '1'));
+        $e101101->appendChild($doc->createElement('cMotivo', $codigoMotivo));
         $e101101->appendChild($doc->createElement('xMotivo', $motivo));
         $infPedReg->appendChild($e101101);
 
         return $doc->saveXML($doc->documentElement) ?: '';
     }
 
+    private function assertCancellationReason(string $codigoMotivo, string $motivo): void
+    {
+        if (!in_array($codigoMotivo, ['1', '2', '9'], true)) {
+            throw new \InvalidArgumentException('Cancellation reason code must be one of: 1, 2, 9.');
+        }
+
+        $length = preg_match_all('/./us', $motivo, $characters);
+        if ($length === false) {
+            throw new \InvalidArgumentException('Cancellation reason description must be valid UTF-8.');
+        }
+
+        if ($length < 15 || $length > 255) {
+            throw new \InvalidArgumentException('Cancellation reason description must contain between 15 and 255 characters.');
+        }
+    }
+
     /**
      * @param array<string, mixed> $response
      */
diff --git a/tests/Unit/Http/NfseClientTest.php b/tests/Unit/Http/NfseClientTest.php
index 31c180a..191e70d 100644
--- a/tests/Unit/Http/NfseClientTest.php
+++ b/tests/Unit/Http/NfseClientTest.php
@@ -380,6 +380,62 @@ public function testCancelReturnsTrueOnSuccess(): void
         self::assertStringContainsString('<xMotivo>Cancelamento a pedido do tomador</xMotivo>', $eventoXml);
     }
 
+    public function testCancelWithReasonUsesRequestedOfficialReasonCode(): void
+    {
+        self::$server->setResponseOfPath(
+            '/SefinNacional/nfse/abc-123/eventos',
+            new Response('{}', ['Content-Type' => 'application/json'], 200)
+        );
+
+        $client = $this->makeClient($this->signer);
+        self::assertTrue($client->cancelWithReason('abc-123', '9', 'Outro motivo válido'));
+
+        $request = self::$server->getLastRequest();
+        self::assertNotNull($request);
+        $payload = json_decode($request->getInput(), true, 512, JSON_THROW_ON_ERROR);
+        $compressedXml = base64_decode((string) $payload['pedidoRegistroEventoXmlGZipB64'], true);
+        self::assertNotFalse($compressedXml);
+        $eventoXml = gzdecode($compressedXml);
+        self::assertNotFalse($eventoXml);
+        self::assertStringContainsString('<cMotivo>9</cMotivo>', $eventoXml);
+        self::assertStringContainsString('<xMotivo>Outro motivo válido</xMotivo>', $eventoXml);
+    }
+
+    public function testCancelWithReasonAcceptsOfficialDescriptionBoundaries(): void
+    {
+        self::$server->setResponseOfPath(
+            '/SefinNacional/nfse/abc-123/eventos',
+            new Response('{}', ['Content-Type' => 'application/json'], 200)
+        );
+
+        $client = $this->makeClient($this->signer);
+        self::assertTrue($client->cancelWithReason('abc-123', '1', str_repeat('a', 15)));
+        self::assertTrue($client->cancelWithReason('abc-123', '2', str_repeat('b', 255)));
+    }
+
+    /**
+     * @dataProvider invalidCancellationReasons
+     */
+    public function testCancelWithReasonRejectsInvalidContractBeforeNetwork(string $code, string $description): void
+    {
+        $client = $this->makeClient($this->signer);
+
+        $this->expectException(\InvalidArgumentException::class);
+        $client->cancelWithReason('abc-123', $code, $description);
+    }
+
+    /**
+     * @return array<string, array{string, string}>
+     */
+    public static function invalidCancellationReasons(): array
+    {
+        return [
+            'unknown reason code' => ['3', str_repeat('a', 15)],
+            'description too short' => ['1', str_repeat('a', 14)],
+            'description too long' => ['1', str_repeat('a', 256)],
+        ];
+    }
+
     public function testEmitRejectsMalformedBase64NfseXmlWithoutPhpWarning(): void
     {
         $payload = json_encode([
diff --git a/tests/Unit/Xml/OfficialEventSchemaValidationTest.php b/tests/Unit/Xml/OfficialEventSchemaValidationTest.php
index d9cfd58..99abbad 100644
--- a/tests/Unit/Xml/OfficialEventSchemaValidationTest.php
+++ b/tests/Unit/Xml/OfficialEventSchemaValidationTest.php
@@ -46,6 +46,35 @@ public function testValidatorRejectsUnknownEventElement(): void
         self::assertNotSame([], $this->validator->validate($invalidXml));
     }
 
+    /**
+     * @dataProvider cancellationReasonLengthCases
+     */
+    public function testCancellationReasonLengthMatchesOfficialSchema(int $length, bool $valid): void
+    {
+        $xml = str_replace(
+            'Erro de emissao confirmado pelo prestador',
+            str_repeat('a', $length),
+            $this->cancellationXml(),
+        );
+
+        $errors = $this->validator->validate($xml);
+
+        $valid ? self::assertSame([], $errors) : self::assertNotSame([], $errors);
+    }
+
+    /**
+     * @return array<string, array{int, bool}>
+     */
+    public static function cancellationReasonLengthCases(): array
+    {
+        return [
+            '14 rejected' => [14, false],
+            '15 accepted' => [15, true],
+            '255 accepted' => [255, true],
+            '256 rejected' => [256, false],
+        ];
+    }
+
     public function testValidatorRejectsWrongEventElementOrder(): void
     {
         $invalidXml = str_replace(