diff --git a/src/Expectations/OppositeExpectation.php b/src/Expectations/OppositeExpectation.php
index 22162b5a..4f99c89f 100644
--- a/src/Expectations/OppositeExpectation.php
+++ b/src/Expectations/OppositeExpectation.php
@@ -798,13 +798,11 @@ final readonly class OppositeExpectation
 
         $exporter = Exporter::default();
 
-        $toString = fn (mixed $argument): string => $exporter->shortenedExport($argument);
-
         throw new ExpectationFailedException(sprintf(
             'Expecting %s not %s %s.',
-            $toString($this->original->value),
+            $exporter->shortenedExport($this->original->value),
             strtolower((string) preg_replace('/(?<!\ )[A-Z]/', ' $0', $name)),
-            implode(' ', array_map(fn (mixed $argument): string => $toString($argument), $arguments)),
+            implode(' ', array_map(fn (mixed $argument): string => $exporter->export($argument), $arguments)),
         ));
     }
 
diff --git a/src/Support/Exporter.php b/src/Support/Exporter.php
index 44367c08..6fd89c53 100644
--- a/src/Support/Exporter.php
+++ b/src/Support/Exporter.php
@@ -86,4 +86,17 @@ final readonly class Exporter
 
         return (string) preg_replace(array_keys($map), array_values($map), $this->exporter->shortenedExport($value));
     }
+
+    /**
+     * Exports a value into a full single-line string without truncation.
+     */
+    public function export(mixed $value): string
+    {
+        $map = [
+            '#\\\n\s*#' => '',
+            '# Object \(\.{3}\)#' => '',
+        ];
+
+        return (string) preg_replace(array_keys($map), array_values($map), $this->exporter->export($value));
+    }
 }
diff --git a/tests/Unit/Expectations/OppositeExpectation.php b/tests/Unit/Expectations/OppositeExpectation.php
index 448ebf6d..856a38e6 100644
--- a/tests/Unit/Expectations/OppositeExpectation.php
+++ b/tests/Unit/Expectations/OppositeExpectation.php
@@ -14,3 +14,17 @@ it('throw expectation failed exception with array argument', function (): void {
 
     $expectation->throwExpectationFailedException('toBe', ['bar']);
 })->throws(ExpectationFailedException::class, "Expecting 'foo' not to be 'bar'.");
+
+it('does not truncate long string arguments in error message', function (): void {
+    $expectation = new OppositeExpectation(expect('foo'));
+
+    $longMessage = 'Very long error message. Very long error message. Very long error message.';
+
+    $expectation->throwExpectationFailedException('toBe', [$longMessage]);
+})->throws(ExpectationFailedException::class, 'Very long error message. Very long error message. Very long error message.');
+
+it('does not truncate custom error message when using not()', function (): void {
+    $longMessage = 'This is a very detailed custom error message that should not be truncated in the output.';
+
+    expect(true)->not()->toBeTrue($longMessage);
+})->throws(ExpectationFailedException::class, 'This is a very detailed custom error message that should not be truncated in the output.');