diff --git a/src/Rule/Rules/Composer/Psr4DirectoryExistsRule.php b/src/Rule/Rules/Composer/Psr4DirectoryExistsRule.php
index 900c0944..633cdca0 100644
--- a/src/Rule/Rules/Composer/Psr4DirectoryExistsRule.php
+++ b/src/Rule/Rules/Composer/Psr4DirectoryExistsRule.php
@@ -28,7 +28,7 @@ public function __construct(
 
     public function evaluateProject(string $basePath, Architecture $architecture, array $skipPaths = []): ?RuleViolation
     {
-        $composerFile = rtrim($basePath, '/') . '/composer.json';
+        $composerFile = Path::normalise(rtrim($basePath, '/') . '/composer.json', canonicalise: true);
 
         if (! file_exists($composerFile)) {
             return $this->violation(
diff --git a/src/Rule/Rules/Composer/Psr4EmptyNamespacePrefixRule.php b/src/Rule/Rules/Composer/Psr4EmptyNamespacePrefixRule.php
index 58424005..e8194d04 100644
--- a/src/Rule/Rules/Composer/Psr4EmptyNamespacePrefixRule.php
+++ b/src/Rule/Rules/Composer/Psr4EmptyNamespacePrefixRule.php
@@ -9,9 +9,9 @@
 use Boundwize\StructArmed\Rule\ComposerJsonRuleInterface;
 use Boundwize\StructArmed\Rule\MultipleProjectRuleViolationInterface;
 use Boundwize\StructArmed\Rule\RuleViolation;
+use Boundwize\StructArmed\Util\Path;
 
 use function array_keys;
-use function file_exists;
 use function is_array;
 use function is_string;
 use function rtrim;
@@ -38,18 +38,14 @@ public function evaluateProject(string $basePath, Architecture $architecture, ar
      */
     public function evaluateProjectAll(string $basePath, Architecture $architecture, array $skipPaths = []): array
     {
-        $composerFile = rtrim($basePath, '/') . '/composer.json';
-
-        if (! file_exists($composerFile)) {
-            return [];
-        }
-
         $composer = $this->psr4PathResolver->composerConfig($basePath);
 
         if ($composer === null) {
             return [];
         }
 
+        $composerFile = Path::normalise(rtrim($basePath, '/') . '/composer.json', canonicalise: true);
+
         $violations = [];
 
         foreach (['autoload', 'autoload-dev'] as $section) {
diff --git a/src/Rule/Rules/Composer/Psr4RootPathRule.php b/src/Rule/Rules/Composer/Psr4RootPathRule.php
index 5fb93676..4b131441 100644
--- a/src/Rule/Rules/Composer/Psr4RootPathRule.php
+++ b/src/Rule/Rules/Composer/Psr4RootPathRule.php
@@ -11,7 +11,6 @@
 use Boundwize\StructArmed\Rule\RuleViolation;
 use Boundwize\StructArmed\Util\Path;
 
-use function file_exists;
 use function is_array;
 use function is_string;
 use function rtrim;
@@ -36,18 +35,14 @@ public function evaluateProject(string $basePath, Architecture $architecture, ar
      */
     public function evaluateProjectAll(string $basePath, Architecture $architecture, array $skipPaths = []): array
     {
-        $composerFile = rtrim($basePath, '/') . '/composer.json';
-
-        if (! file_exists($composerFile)) {
-            return [];
-        }
-
         $composer = $this->psr4PathResolver->composerConfig($basePath);
 
         if ($composer === null) {
             return [];
         }
 
+        $composerFile = Path::normalise(rtrim($basePath, '/') . '/composer.json', canonicalise: true);
+
         $violations         = [];
         $normalisedBasePath = Path::normalise($basePath, canonicalise: true);
 
diff --git a/src/Rule/Rules/Composer/Psr4SourcePathsRule.php b/src/Rule/Rules/Composer/Psr4SourcePathsRule.php
index 8f8b2fe2..8f84fce5 100644
--- a/src/Rule/Rules/Composer/Psr4SourcePathsRule.php
+++ b/src/Rule/Rules/Composer/Psr4SourcePathsRule.php
@@ -34,7 +34,7 @@ public function __construct(
 
     public function evaluateProject(string $basePath, Architecture $architecture, array $skipPaths = []): ?RuleViolation
     {
-        $composerFile = rtrim($basePath, '/') . '/composer.json';
+        $composerFile = Path::normalise(rtrim($basePath, '/') . '/composer.json', canonicalise: true);
 
         if (! file_exists($composerFile)) {
             return $this->violation(
diff --git a/tests/Rule/Composer/Psr4ComposerFilePathCanonicalisationTest.php b/tests/Rule/Composer/Psr4ComposerFilePathCanonicalisationTest.php
new file mode 100644
index 00000000..9d36125b
--- /dev/null
+++ b/tests/Rule/Composer/Psr4ComposerFilePathCanonicalisationTest.php
@@ -0,0 +1,78 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Boundwize\StructArmed\Tests\Rule\Composer;
+
+use Boundwize\StructArmed\Architecture;
+use Boundwize\StructArmed\Rule\Rules\Composer\Psr4EmptyNamespacePrefixRule;
+use Boundwize\StructArmed\Rule\Rules\Composer\Psr4RootPathRule;
+use Boundwize\StructArmed\Tests\Support\TemporaryDirectoryCleanupTrait;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+
+use function file_put_contents;
+use function realpath;
+use function str_replace;
+
+#[CoversClass(Psr4EmptyNamespacePrefixRule::class)]
+#[CoversClass(Psr4RootPathRule::class)]
+final class Psr4ComposerFilePathCanonicalisationTest extends TestCase
+{
+    use TemporaryDirectoryCleanupTrait;
+
+    public function testEmptyNamespacePrefixRuleReportsCanonicalisedComposerFilePath(): void
+    {
+        $basePath = $this->makeTempProject(<<<'JSON'
+{
+    "autoload": {
+        "psr-4": {
+            "": "src/"
+        }
+    }
+}
+JSON);
+
+        $violations = (new Psr4EmptyNamespacePrefixRule())->evaluateProjectAll(
+            $basePath . '/.',
+            Architecture::define()
+        );
+
+        $this->assertCount(1, $violations);
+        $this->assertSame($this->expectedComposerFilePath($basePath), $violations[0]->file);
+    }
+
+    public function testRootPathRuleReportsCanonicalisedComposerFilePath(): void
+    {
+        $basePath = $this->makeTempProject(<<<'JSON'
+{
+    "autoload": {
+        "psr-4": {
+            "App\\": "."
+        }
+    }
+}
+JSON);
+
+        $violations = (new Psr4RootPathRule())->evaluateProjectAll(
+            $basePath . '/.',
+            Architecture::define()
+        );
+
+        $this->assertCount(1, $violations);
+        $this->assertSame($this->expectedComposerFilePath($basePath), $violations[0]->file);
+    }
+
+    private function expectedComposerFilePath(string $basePath): string
+    {
+        return str_replace('\\', '/', (string) realpath($basePath)) . '/composer.json';
+    }
+
+    private function makeTempProject(string $composerJson): string
+    {
+        $basePath = $this->makeTemporaryDirectory('structarmed-psr4-composer-file-path');
+        file_put_contents($basePath . '/composer.json', $composerJson);
+
+        return $basePath;
+    }
+}
diff --git a/tests/Rule/Composer/Psr4ComposerFilePathNormalisationTest.php b/tests/Rule/Composer/Psr4ComposerFilePathNormalisationTest.php
new file mode 100644
index 00000000..10929a3f
--- /dev/null
+++ b/tests/Rule/Composer/Psr4ComposerFilePathNormalisationTest.php
@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Boundwize\StructArmed\Tests\Rule\Composer;
+
+use Boundwize\StructArmed\Architecture;
+use Boundwize\StructArmed\Rule\Rules\Composer\Psr4DirectoryExistsRule;
+use Boundwize\StructArmed\Rule\Rules\Composer\Psr4SourcePathsRule;
+use Boundwize\StructArmed\Rule\RuleViolation;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(Psr4DirectoryExistsRule::class)]
+#[CoversClass(Psr4SourcePathsRule::class)]
+final class Psr4ComposerFilePathNormalisationTest extends TestCase
+{
+    private const WINDOWS_STYLE_MISSING_BASE_PATH = 'C:\structarmed-missing-fixture\app';
+
+    public function testDirectoryExistsRuleReportsForwardSlashesForWindowsStyleBasePath(): void
+    {
+        $violation = (new Psr4DirectoryExistsRule())->evaluateProject(
+            self::WINDOWS_STYLE_MISSING_BASE_PATH,
+            Architecture::define()
+        );
+
+        $this->assertInstanceOf(RuleViolation::class, $violation);
+        $this->assertSame('C:/structarmed-missing-fixture/app/composer.json', $violation->file);
+    }
+
+    public function testSourcePathsRuleReportsForwardSlashesForWindowsStyleBasePath(): void
+    {
+        $violation = (new Psr4SourcePathsRule(null))->evaluateProject(
+            self::WINDOWS_STYLE_MISSING_BASE_PATH,
+            Architecture::define()
+        );
+
+        $this->assertInstanceOf(RuleViolation::class, $violation);
+        $this->assertSame('C:/structarmed-missing-fixture/app/composer.json', $violation->file);
+    }
+}