diff --git a/plugin/codecov/src/Exception/CoverageReportNotWritten.php b/plugin/codecov/src/Exception/CoverageReportNotWritten.php
new file mode 100644
index 00000000..18b5b7df
--- /dev/null
+++ b/plugin/codecov/src/Exception/CoverageReportNotWritten.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Testo\Codecov\Exception;
+
+/**
+ * Thrown when a coverage report cannot be written to its target path.
+ *
+ * @api
+ */
+final class CoverageReportNotWritten extends \RuntimeException
+{
+    /**
+     * @param string $path The directory or file that could not be written.
+     * @param string $reason What the filesystem reported; empty when it said nothing.
+     */
+    public static function directory(string $path, string $reason): self
+    {
+        return new self(self::format('Unable to create the coverage report directory', $path, $reason));
+    }
+
+    /**
+     * @param string $path The directory or file that could not be written.
+     * @param string $reason What the filesystem reported; empty when it said nothing.
+     */
+    public static function file(string $path, string $reason): self
+    {
+        return new self(self::format('Unable to write the coverage report file', $path, $reason));
+    }
+
+    private static function format(string $action, string $path, string $reason): string
+    {
+        return $reason === ''
+            ? \sprintf('%s `%s`.', $action, $path)
+            : \sprintf('%s `%s`: %s.', $action, $path, $reason);
+    }
+}
diff --git a/plugin/codecov/src/Internal/CoverageCollector.php b/plugin/codecov/src/Internal/CoverageCollector.php
index ac77b418..44e8c34f 100644
--- a/plugin/codecov/src/Internal/CoverageCollector.php
+++ b/plugin/codecov/src/Internal/CoverageCollector.php
@@ -7,6 +7,7 @@
 use Internal\Container\Attribute\ScopeShared;
 use Internal\Destroy\Destroyable;
 use Psr\EventDispatcher\EventDispatcherInterface;
+use Testo\Codecov\Exception\CoverageReportNotWritten;
 use Testo\Codecov\Result\CoverageResult;
 use Testo\Codecov\Report\CoverageReport;
 use Testo\Core\Context\SuiteResult;
@@ -59,10 +60,20 @@ public function destroy(): void
             ? $this->cache->value->withSourceRoot($this->sourceRoot)
             : $this->cache->value;
 
+        # One unwritable target must not cost the other reports, so the first failure is rethrown last.
+        $failure = null;
         foreach ($this->reports as $report) {
-            $report->generate($result);
+            try {
+                $report->generate($result);
+            } catch (CoverageReportNotWritten $e) {
+                $failure ??= $e;
+                continue;
+            }
+
             $this->dispatcher?->dispatch(new ReportFileGenerated($report->info()));
         }
+
+        $failure === null or throw $failure;
     }
 
     /**
diff --git a/plugin/codecov/src/Internal/ReportFile.php b/plugin/codecov/src/Internal/ReportFile.php
new file mode 100644
index 00000000..018138e1
--- /dev/null
+++ b/plugin/codecov/src/Internal/ReportFile.php
@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Testo\Codecov\Internal;
+
+use Testo\Codecov\Exception\CoverageReportNotWritten;
+
+/**
+ * Filesystem writes shared by the coverage reports.
+ *
+ * @internal
+ * @psalm-internal Testo\Codecov
+ */
+final class ReportFile
+{
+    /**
+     * Writes the file, creating its parent directory when missing.
+     *
+     * @throws CoverageReportNotWritten
+     */
+    public static function write(string $path, string $content): void
+    {
+        self::directory(\dirname($path));
+
+        \error_clear_last();
+        @\file_put_contents($path, $content) === false and throw CoverageReportNotWritten::file(
+            $path,
+            self::lastError(),
+        );
+    }
+
+    /**
+     * Creates the directory with its parents when missing.
+     *
+     * @throws CoverageReportNotWritten
+     */
+    public static function directory(string $path): void
+    {
+        \error_clear_last();
+        # The second `is_dir()` covers a directory created concurrently between the check and `mkdir()`.
+        \is_dir($path) || @\mkdir($path, 0o755, true) || \is_dir($path) or throw CoverageReportNotWritten::directory(
+            $path,
+            self::lastError(),
+        );
+    }
+
+    /**
+     * The last PHP warning without its `function(): ` prefix, e.g. `Permission denied`.
+     */
+    private static function lastError(): string
+    {
+        $message = \error_get_last()['message'] ?? '';
+
+        return \preg_replace('/^\w+\([^)]*\): /', '', $message) ?? $message;
+    }
+}
diff --git a/plugin/codecov/src/Report/CloverReport.php b/plugin/codecov/src/Report/CloverReport.php
index 49359b38..a186f5ca 100644
--- a/plugin/codecov/src/Report/CloverReport.php
+++ b/plugin/codecov/src/Report/CloverReport.php
@@ -6,6 +6,7 @@
 
 use Internal\Path;
 use Testo\Codecov\Internal\BranchCoverageAggregator;
+use Testo\Codecov\Internal\ReportFile;
 use Testo\Codecov\Result\CoverageResult;
 use Testo\Codecov\Result\FileCoverage;
 use Testo\Codecov\Result\LineStatus;
@@ -75,9 +76,7 @@ public function generate(CoverageResult $result): void
         $xml->endElement(); // coverage
         $xml->endDocument();
 
-        $dir = \dirname($this->outputPath);
-        \is_dir($dir) or \mkdir($dir, 0o755, true);
-        \file_put_contents($this->outputPath, $xml->outputMemory());
+        ReportFile::write($this->outputPath, $xml->outputMemory());
     }
 
     #[\Override]
diff --git a/plugin/codecov/src/Report/CoberturaReport.php b/plugin/codecov/src/Report/CoberturaReport.php
index 1adb7a56..94af2a89 100644
--- a/plugin/codecov/src/Report/CoberturaReport.php
+++ b/plugin/codecov/src/Report/CoberturaReport.php
@@ -6,6 +6,7 @@
 
 use Internal\Path;
 use Testo\Codecov\Internal\BranchCoverageAggregator;
+use Testo\Codecov\Internal\ReportFile;
 use Testo\Codecov\Result\CoverageResult;
 use Testo\Codecov\Result\FileCoverage;
 use Testo\Codecov\Result\LineStatus;
@@ -82,9 +83,7 @@ public function generate(CoverageResult $result): void
         $xml->endElement(); // coverage
         $xml->endDocument();
 
-        $dir = \dirname($this->outputPath);
-        \is_dir($dir) or \mkdir($dir, 0o755, true);
-        \file_put_contents($this->outputPath, $xml->outputMemory());
+        ReportFile::write($this->outputPath, $xml->outputMemory());
     }
 
     #[\Override]
diff --git a/plugin/codecov/src/Report/CoverageReport.php b/plugin/codecov/src/Report/CoverageReport.php
index f6804b88..32b6ab5d 100644
--- a/plugin/codecov/src/Report/CoverageReport.php
+++ b/plugin/codecov/src/Report/CoverageReport.php
@@ -4,6 +4,7 @@
 
 namespace Testo\Codecov\Report;
 
+use Testo\Codecov\Exception\CoverageReportNotWritten;
 use Testo\Codecov\Result\CoverageResult;
 use Testo\Core\Report\ReportInfo;
 
@@ -14,6 +15,9 @@
  */
 interface CoverageReport
 {
+    /**
+     * @throws CoverageReportNotWritten When the target path cannot be written.
+     */
     public function generate(CoverageResult $result): void;
 
     /**
diff --git a/plugin/codecov/src/Report/PhpUnitXmlReport.php b/plugin/codecov/src/Report/PhpUnitXmlReport.php
index 93f00c05..dd2aca98 100644
--- a/plugin/codecov/src/Report/PhpUnitXmlReport.php
+++ b/plugin/codecov/src/Report/PhpUnitXmlReport.php
@@ -5,6 +5,7 @@
 namespace Testo\Codecov\Report;
 
 use Internal\Path;
+use Testo\Codecov\Internal\ReportFile;
 use Testo\Codecov\Result\CoverageResult;
 use Testo\Codecov\Result\FileCoverage;
 use Testo\Codecov\Result\LineStatus;
@@ -46,7 +47,7 @@ public function generate(CoverageResult $result): void
     {
         $sourceRoot = (string) Path::create($result->sourceRoot ?? (string) \getcwd());
 
-        \is_dir($this->outputDir) or \mkdir($this->outputDir, 0o755, true);
+        ReportFile::directory($this->outputDir);
 
         $entries = [];
         $totalStmts = 0;
@@ -189,7 +190,7 @@ private function writeIndex(array $entries, string $sourceRoot, int $totalStmts,
         $xml->endElement(); // phpunit
         $xml->endDocument();
 
-        \file_put_contents($this->outputDir . '/index.xml', $xml->outputMemory());
+        ReportFile::write($this->outputDir . '/index.xml', $xml->outputMemory());
     }
 
     /**
@@ -200,9 +201,6 @@ private function writeFile(array $entry): void
         $fileCoverage = $entry['fileCoverage'];
         $outputPath = $this->outputDir . '/' . $entry['href'];
 
-        $dir = \dirname($outputPath);
-        \is_dir($dir) or \mkdir($dir, 0o755, true);
-
         $xml = self::newXmlWriter();
         $xml->startDocument('1.0', 'UTF-8');
 
@@ -242,6 +240,6 @@ private function writeFile(array $entry): void
         $xml->endElement(); // phpunit
         $xml->endDocument();
 
-        \file_put_contents($outputPath, $xml->outputMemory());
+        ReportFile::write($outputPath, $xml->outputMemory());
     }
 }
diff --git a/plugin/codecov/tests/Unit/Internal/CoverageCollectorTest.php b/plugin/codecov/tests/Unit/Internal/CoverageCollectorTest.php
index 968c15ac..0ffb2f5b 100644
--- a/plugin/codecov/tests/Unit/Internal/CoverageCollectorTest.php
+++ b/plugin/codecov/tests/Unit/Internal/CoverageCollectorTest.php
@@ -8,6 +8,7 @@
 use Testo\Application\Internal\EventDispatcher;
 use Testo\Assert;
 use Testo\Codecov\Covers;
+use Testo\Codecov\Exception\CoverageReportNotWritten;
 use Testo\Codecov\Internal\CoverageCollector;
 use Testo\Codecov\Report\CoverageReport;
 use Testo\Codecov\Result\CoverageResult;
@@ -23,6 +24,7 @@
 use Testo\Core\Value\Status;
 use Testo\Data\MultipleResult;
 use Testo\Event\Report\ReportFileGenerated;
+use Testo\Expect;
 use Testo\Test;
 
 #[Test]
@@ -100,6 +102,32 @@ static function (ReportFileGenerated $event) use (&$seen): void {
         Assert::same($seen[1]->format, 'cobertura');
     }
 
+    public function unwritableReportDoesNotStopTheOthers(): never
+    {
+        $dispatcher = new EventDispatcher();
+        $seen = [];
+        $dispatcher->addListener(
+            ReportFileGenerated::class,
+            static function (ReportFileGenerated $event) use (&$seen): void {
+                $seen[] = $event->info->format;
+            },
+        );
+        $collector = new CoverageCollector(
+            [self::createReport('broken', fail: true), self::createReport('clover')],
+            null,
+            $dispatcher,
+        );
+
+        Expect::exception(CoverageReportNotWritten::class)
+            ->withMessage('Unable to write the coverage report file `/tmp/broken/index.xml`: Permission denied.');
+
+        try {
+            $collector->destroy();
+        } finally {
+            Assert::same($seen, ['clover']);
+        }
+    }
+
     private static function suiteOf(TestResult $result): SuiteResult
     {
         return new SuiteResult(
@@ -111,9 +139,9 @@ private static function suiteOf(TestResult $result): SuiteResult
     /**
      * @param non-empty-string $format Also spells the card's path, so two reports stay distinguishable.
      */
-    private static function createReport(string $format = 'stub'): CoverageReport
+    private static function createReport(string $format = 'stub', bool $fail = false): CoverageReport
     {
-        return new class($format) implements CoverageReport {
+        return new class($format, $fail) implements CoverageReport {
             public ?CoverageResult $result = null;
 
             /**
@@ -121,11 +149,16 @@ private static function createReport(string $format = 'stub'): CoverageReport
              */
             public function __construct(
                 private readonly string $format,
+                private readonly bool $fail,
             ) {}
 
             #[\Override]
             public function generate(CoverageResult $result): void
             {
+                $this->fail and throw CoverageReportNotWritten::file(
+                    (string) $this->info()->path,
+                    'Permission denied',
+                );
                 $this->result = $result;
             }
 
diff --git a/plugin/codecov/tests/Unit/Internal/ReportFileTest.php b/plugin/codecov/tests/Unit/Internal/ReportFileTest.php
new file mode 100644
index 00000000..0c4e9c73
--- /dev/null
+++ b/plugin/codecov/tests/Unit/Internal/ReportFileTest.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Codecov\Unit\Internal;
+
+use Testo\Assert;
+use Testo\Codecov\Covers;
+use Testo\Codecov\Exception\CoverageReportNotWritten;
+use Testo\Codecov\Internal\ReportFile;
+use Testo\Expect;
+use Testo\Lifecycle\AfterTest;
+use Testo\Lifecycle\BeforeTest;
+use Testo\Test;
+
+#[Test]
+#[Covers(ReportFile::class)]
+#[Covers(CoverageReportNotWritten::class)]
+final class ReportFileTest
+{
+    private string $root;
+
+    #[BeforeTest]
+    public function createRoot(): void
+    {
+        $this->root = \dirname(__DIR__, 2) . '/runtime/report_file_' . \uniqid();
+        \mkdir($this->root);
+    }
+
+    #[AfterTest]
+    public function removeRoot(): void
+    {
+        $items = new \RecursiveIteratorIterator(
+            new \RecursiveDirectoryIterator($this->root, \FilesystemIterator::SKIP_DOTS),
+            \RecursiveIteratorIterator::CHILD_FIRST,
+        );
+        foreach ($items as $item) {
+            $item->isDir() ? \rmdir($item->getPathname()) : \unlink($item->getPathname());
+        }
+        \rmdir($this->root);
+    }
+
+    public function writesFileCreatingMissingParents(): void
+    {
+        $path = $this->root . '/a/b/report.xml';
+
+        ReportFile::write($path, '<coverage/>');
+
+        Assert::same(\file_get_contents($path), '<coverage/>');
+    }
+
+    public function directoryUnderFileNamesThePath(): never
+    {
+        \touch($this->root . '/file');
+        $dir = $this->root . '/file/reports';
+
+        Expect::exception(CoverageReportNotWritten::class)
+            ->withMessageContaining("Unable to create the coverage report directory `{$dir}`: ");
+
+        ReportFile::directory($dir);
+    }
+
+    public function fileOverDirectoryNamesThePath(): never
+    {
+        $path = $this->root . '/report.xml';
+        \mkdir($path);
+
+        Expect::exception(CoverageReportNotWritten::class)
+            ->withMessageContaining("Unable to write the coverage report file `{$path}`: ");
+
+        ReportFile::write($path, '<coverage/>');
+    }
+
+    public function silentFailureStillNamesThePath(): void
+    {
+        $e = CoverageReportNotWritten::file('/out/report.xml', '');
+
+        Assert::same($e->getMessage(), 'Unable to write the coverage report file `/out/report.xml`.');
+    }
+}
diff --git a/skills/testo-coverage/SKILL.md b/skills/testo-coverage/SKILL.md
index 333f26b3..8c181d0e 100644
--- a/skills/testo-coverage/SKILL.md
+++ b/skills/testo-coverage/SKILL.md
@@ -160,6 +160,10 @@ the table is what a consumer switches on.
 consumer opens: for a report that fills a directory, the index inside it rather than the directory; for
 one that uploads its data, whatever URL it lands on.
 
+A report that cannot write its target throws `Testo\Codecov\Exception\CoverageReportNotWritten`, naming
+the path and the filesystem's reason. The remaining reports are still written, then the run exits
+non-zero. A custom report should throw the same exception on a failed write.
+
 ```php
 final readonly class MyReport implements CoverageReport
 {
@@ -177,6 +181,7 @@ final readonly class MyReport implements CoverageReport
 ## Pitfalls
 
 - No coverage written? Check the active Xdebug mode includes `coverage` — set it via `xdebug.mode`, `-d xdebug.mode=coverage`, or `XDEBUG_MODE=coverage` (or load PCOV). Testo skips the driver if neither is available.
+- A report landed in the project root under a name like `C:\Users\...`? A Windows path reached a Linux PHP (WSL, Docker): it has no `/`, so it reads as a relative file name. Pass the path as the interpreter sees it (`/mnt/c/...`).
 - `clover.xml` empty? Suite-level finder probably excludes the `src` directory you expected — verify the `FinderConfig` covers it.
 - Don't enable coverage in benchmark suites — it falsifies timings.
 - Coverage under `#[RunInFiber]` costs an extra driver stop/start per suspension (the window is closed