diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..c6ff171
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,16 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [4.8.0] 2026-09-04
+
+### Added
+
+- Add `invoices.paymentSummary` to get the related-document object needed to build a payment complement (complemento de pago): installment number, previous balance, and taxes prorated to the paid amount.
+
+### Fixed
+
+- Return API error codes as strings and only normalize numeric API error codes.
diff --git a/VERSION.md b/VERSION.md
index 969152c..f37088d 100644
--- a/VERSION.md
+++ b/VERSION.md
@@ -1,3 +1,8 @@
+4.7.1
+
+## Fixed
+- Return numeric API error codes as strings.
+
 4.7.0
 
 ## Added
diff --git a/src/Exceptions/FacturapiException.php b/src/Exceptions/FacturapiException.php
index 958dadd..ba47d15 100644
--- a/src/Exceptions/FacturapiException.php
+++ b/src/Exceptions/FacturapiException.php
@@ -48,9 +48,22 @@ public function getRawBody(): ?string
 		return $this->rawBody;
 	}
 
-	public function getErrorCode(): mixed
+	public function getErrorCode(): ?string
 	{
-		return is_array($this->errorData) ? ($this->errorData['code'] ?? null) : null;
+		if (!is_array($this->errorData) || !array_key_exists('code', $this->errorData)) {
+			return null;
+		}
+
+		$code = $this->errorData['code'];
+		if ($code === null) {
+			return null;
+		}
+
+		if (is_string($code)) {
+			return $code;
+		}
+
+		return is_int($code) || is_float($code) ? (string) $code : null;
 	}
 
 	public function getErrorPath(): ?string
diff --git a/src/Resources/Invoices.php b/src/Resources/Invoices.php
index d2b6b8e..a95eef0 100644
--- a/src/Resources/Invoices.php
+++ b/src/Resources/Invoices.php
@@ -39,6 +39,25 @@ public function retrieve( $id ): mixed {
 		}
 	}
 
+	/**
+	 * Gets the information needed to add this invoice as a related document in a
+	 * payment complement (complemento de pago): the installment number according
+	 * to the payment history, the previous balance, and the invoice tax breakdown
+	 * prorated to the amount being paid.
+	 *
+	 * @param string $id Invoice ID.
+	 * @param float $amount Amount being paid, expressed in the invoice currency. Cannot exceed the outstanding balance.
+	 * @return mixed JSON-decoded related document summary.
+	 * @throws FacturapiException
+	 */
+	public function paymentSummary( $id, $amount ): mixed {
+		try {
+			return json_decode( $this->executeGetRequest( $this->getRequestUrl( $id . '/payment-summary', array( 'amount' => $amount ) ) ) );
+		} catch ( FacturapiException $e ) {
+			throw $e;
+		}
+	}
+
 	/**
 	 * Create an Invoice in your organization
 	 *
diff --git a/tests/Http/ErrorHandlingTest.php b/tests/Http/ErrorHandlingTest.php
index d22e7d2..61df2e0 100644
--- a/tests/Http/ErrorHandlingTest.php
+++ b/tests/Http/ErrorHandlingTest.php
@@ -89,4 +89,20 @@ public function testNonJsonErrorsStillExposeRawBody(): void
             self::assertSame($rawBody, $exception->getRawBody());
         }
     }
+
+    public function testNumericApiErrorCodesAreConvertedToStrings(): void
+    {
+        $httpClient = new FakeHttpClient(
+            new Response(400, ['Content-Type' => 'application/json'], '{"code": 400}')
+        );
+
+        $invoices = new Invoices('sk_test_abc123', ['httpClient' => $httpClient]);
+
+        try {
+            $invoices->create(['customer' => []]);
+            self::fail('Expected FacturapiException to be thrown.');
+        } catch (FacturapiException $exception) {
+            self::assertSame('400', $exception->getErrorCode());
+        }
+    }
 }
diff --git a/tests/Resources/InvoicesTest.php b/tests/Resources/InvoicesTest.php
index 817ff0e..9274bcd 100644
--- a/tests/Resources/InvoicesTest.php
+++ b/tests/Resources/InvoicesTest.php
@@ -141,4 +141,32 @@ public function testDownloadZipRequestReturnsBinaryContentsFromExpectedPath(): v
         self::assertSame('GET', $request->getMethod());
         self::assertSame('https://www.facturapi.io/v2/invoices/zip-requests/zip_123/zip', (string) $request->getUri());
     }
+
+    public function testPaymentSummaryUsesExpectedPathAndQueryParameter(): void
+    {
+        $summary = [
+            'uuid' => '6CF6CE33-1BD2-4F88-A443-33013C069169',
+            'installment' => 1,
+            'last_balance' => 100.0,
+            'total' => 100.0,
+            'currency' => 'MXN',
+            'amount' => 58.0,
+            'taxes' => [
+                ['base' => 50.0, 'rate' => 0.16, 'type' => 'IVA', 'factor' => 'Tasa', 'withholding' => false],
+            ],
+        ];
+        $httpClient = new FakeHttpClient(new Response(200, [], json_encode($summary)));
+        $invoices = new Invoices('sk_test_abc123', ['httpClient' => $httpClient]);
+
+        $result = $invoices->paymentSummary('inv_123', 58.0);
+
+        self::assertSame('6CF6CE33-1BD2-4F88-A443-33013C069169', $result->uuid);
+        self::assertSame(1, $result->installment);
+        self::assertSame(50.0, (float) $result->taxes[0]->base);
+
+        $request = $httpClient->requests()[0];
+        self::assertSame('GET', $request->getMethod());
+        self::assertSame('https://www.facturapi.io/v2/invoices/inv_123/payment-summary?amount=58', (string) $request->getUri());
+        self::assertSame('Basic ' . base64_encode('sk_test_abc123:'), $request->getHeaderLine('Authorization'));
+    }
 }