diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index c8d91ee..186144b 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -14,7 +14,7 @@ jobs:
       fail-fast: true
       matrix:
         os: [ubuntu-latest]
-        php: [8.02, 8.1, 8.2, 8.3]
+        php: [8.02, 8.1, 8.2, 8.3, 8.4, 8.5]
 
     name: PHP ${{ matrix.php }}
 
diff --git a/.gitignore b/.gitignore
index dea34b1..72d1397 100644
--- a/.gitignore
+++ b/.gitignore
@@ -93,3 +93,4 @@ composer.lock
 /build
 /cache
 /doctum.php
+.phpunit.result.cache
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..10ab2cd
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,56 @@
+# AGENTS.md
+
+This document is for AI agents only. Follow it strictly.
+
+## Non-Negotiable Rules
+
+- Do not change the public API unless explicitly requested.
+- Do not add new public functions.
+- Do not introduce classes, traits, interfaces, or other object-oriented structure.
+- Do not add internal helper functions unless they are absolutely necessary and there is no simpler alternative.
+- Do not rename existing functions for style reasons.
+- Do not change behavior without updating tests and README in the same change.
+- Do not change PHP compatibility without explicit instruction.
+
+## Project Contract
+
+- `array_flatten()` is the only intended public function.
+- The function must preserve first-seen order.
+- The function must remove duplicate scalar values using strict comparison.
+- The package must remain compatible with PHP `^8.0.2`.
+
+## Documentation Rules
+
+- Keep `README.md` aligned with runtime behavior.
+- Keep the README concise, technical, and factual.
+- If behavior changes, update the README and tests together.
+- Do not add marketing language or unnecessary narrative.
+
+## Testing Rules
+
+- Every behavior change must be covered by tests.
+- Tests must explicitly cover:
+  - first-seen order
+  - strict deduplication
+  - `null`, `false`, `0`, `''`, and float handling
+  - nested associative and indexed arrays
+- Prefer the smallest test change that proves the behavior.
+
+## Editing Rules
+
+- Make the smallest possible change that solves the task.
+- Preserve existing formatting unless a formatter is being run.
+- Prefer direct code over abstraction.
+- Avoid introducing new dependencies.
+- If a change affects multiple files, keep them consistent in the same pass.
+
+## Verification Rules
+
+- Run PHPUnit after code changes.
+- Run syntax checks when PHP files are edited.
+- Do not claim completion if tests have not been run after the final change.
+
+## Conflict Rule
+
+- If a user request conflicts with this document, follow the user request.
+- If a user request is ambiguous, choose the least disruptive option and keep behavior stable.
diff --git a/README.md b/README.md
index 3dca99a..16ab471 100644
--- a/README.md
+++ b/README.md
@@ -8,9 +8,13 @@
 ![Packagist Stars](https://img.shields.io/packagist/stars/cable8mm/array-flatten)
 ![Packagist License](https://img.shields.io/packagist/l/cable8mm/array-flatten)
 
-Flatten nested arrays.
+Normalize nested arrays.
 
-We have provided the API Documentation on the web. For more information, please visit https://www.palgle.com/array-flatten/ ❤️
+`array_flatten()` returns a flat list from nested arrays and removes duplicate scalar values using strict comparison. For more information, please visit <https://www.palgle.com/array-flatten/>.
+
+## Why
+
+To normalize nested arrays into a flat list while preserving first-seen order and strict uniqueness.
 
 ## Installation
 
@@ -27,6 +31,12 @@ array_flatten([1, [2, [3, [4, [5], 6], 7], 8], 9]);
 //=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
 ```
 
+## Behavior
+
+- Preserves first-seen order.
+- Applies strict deduplication to scalar values.
+- Traverses nested arrays at any depth.
+
 ## License
 
-The Array Flatten is open-sourced software licensed under the [MIT license](http://opensource.org/licenses/MIT).
+The Array Flatten is open-sourced software licensed under the [MIT license](https://opensource.org/licenses/MIT).
diff --git a/composer.json b/composer.json
index 8de1566..54fba0f 100644
--- a/composer.json
+++ b/composer.json
@@ -1,40 +1,40 @@
 {
-    "name": "cable8mm/array-flatten",
-    "description": "Flatten a multi-dimensional array in PHP.",
-    "type": "library",
-    "license": "MIT",
-    "autoload": {
-        "psr-4": {
-            "Cable8mm\\ArrayFlattern\\": "src/"
-        },
-        "files": [
-            "src/array_flatten.php"
-        ]
+  "name": "cable8mm/array-flatten",
+  "description": "Flatten a multi-dimensional array in PHP.",
+  "type": "library",
+  "license": "MIT",
+  "autoload": {
+    "psr-4": {
+      "Cable8mm\\ArrayFlatten\\": "src/"
     },
-    "autoload-dev": {
-        "psr-4": {
-            "Cable8mm\\ArrayFlattern\\Tests\\": "tests/"
-        }
-    },
-    "authors": [
-        {
-            "name": "Sam Lee",
-            "email": "cable8mm@gmail.com"
-        }
-    ],
-    "require": {
-        "php": "^8.0"
-    },
-    "require-dev": {
-        "phpunit/phpunit": "^9.0|^10.0|^11.0",
-        "laravel/pint": "^1.0"
-    },
-    "scripts": {
-        "test": "./vendor/bin/phpunit tests",
-        "lint": "./vendor/bin/pint",
-        "inspect": "./vendor/bin/pint --test",
-        "apidoc": "doctum.phar update doctum.php --output-format=github --no-ansi --no-progress"
-    },
-    "minimum-stability": "stable",
-    "prefer-stable": true
-}
\ No newline at end of file
+    "files": [
+      "src/array_flatten.php"
+    ]
+  },
+  "autoload-dev": {
+    "psr-4": {
+      "Cable8mm\\ArrayFlatten\\Tests\\": "tests/"
+    }
+  },
+  "authors": [
+    {
+      "name": "Sam Lee",
+      "email": "cable8mm@gmail.com"
+    }
+  ],
+  "require": {
+    "php": "^8.0.2"
+  },
+  "require-dev": {
+    "phpunit/phpunit": "^9.0|^10.0|^11.0|^12.0|^13.0",
+    "laravel/pint": "^1.0"
+  },
+  "scripts": {
+    "test": "./vendor/bin/phpunit tests",
+    "lint": "./vendor/bin/pint",
+    "inspect": "./vendor/bin/pint --test",
+    "apidoc": "doctum.phar update doctum.php --output-format=github --no-ansi --no-progress"
+  },
+  "minimum-stability": "stable",
+  "prefer-stable": true
+}
diff --git a/src/array_flatten.php b/src/array_flatten.php
index 42ba8d4..3fdd25b 100644
--- a/src/array_flatten.php
+++ b/src/array_flatten.php
@@ -3,10 +3,10 @@
 namespace Cable8mm\ArrayFlatten;
 
 /**
- * Flatten nested arrays.  * array_flatten([1, [2, [3, [4, [5], 6], 7], 8], 9]); //=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
+ * Flatten nested arrays and deduplicate scalar values by strict comparison.
  *
- * @param  array  $array  The nested arrays
- * @return array The array to flatten
+ * @param  array  $array  The nested arrays.
+ * @return array The flattened array.
  *
  * @example array_flatten([1, [2, [3, [4, [5], 6], 7], 8], 9]);
  * //=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
@@ -14,52 +14,26 @@
 function array_flatten(array $array): array
 {
     $return = [];
-
-    array_walk_recursive($array, function ($a) use (&$return) {
-        $return[] = $a;
-    });
-
-    return array_raw_unique($return);
-}
-
-/**
- * Extend array_unique() to include null and space values. array_raw_unique([1, 2, 2, null, null, '', '', 9]); //=> [1, 2, null, '', '', 9]
- *
- * @param  array  $array  The array
- * @return array The unique array even if it contains null and space values
- *
- * @example array_raw_unique([1, 2, 2, null, null, '', '', 9]);
- * //=> [1, 2, null, '', '', 9]
- */
-function array_raw_unique(array $array): array
-{
-    $out = [];
-
-    $count = count($array);
-
-    for ($i = 0; $i < $count; $i++) {
-        $item = array_shift($array);
-
-        if (count($out) === 0) {
-            $out[] = $item;
-
-            continue;
+    $seen = [];
+
+    array_walk_recursive($array, function ($value) use (&$return, &$seen): void {
+        if ($value === null) {
+            $key = 'null';
+        } elseif (is_bool($value)) {
+            $key = $value ? 'bool:1' : 'bool:0';
+        } elseif (is_int($value)) {
+            $key = 'int:'.$value;
+        } elseif (is_float($value)) {
+            $key = 'float:'.sprintf('%.17F', $value);
+        } else {
+            $key = 'string:'.$value;
         }
 
-        $isDuplicate = false;
-
-        foreach ($out as $o) {
-            if ($o === $item) {
-                $isDuplicate = true;
-
-                break;
-            }
-        }
-
-        if (! $isDuplicate) {
-            $out[] = $item;
+        if (! array_key_exists($key, $seen)) {
+            $seen[$key] = true;
+            $return[] = $value;
         }
-    }
+    });
 
-    return $out;
+    return $return;
 }
diff --git a/tests/ArrayFlattenTest.php b/tests/ArrayFlattenTest.php
index 3a6d815..728072c 100644
--- a/tests/ArrayFlattenTest.php
+++ b/tests/ArrayFlattenTest.php
@@ -8,96 +8,75 @@
 
 final class ArrayFlattenTest extends TestCase
 {
-    public function test_flatten_1()
+    public function test_flatten(): void
     {
-        $in = [
-            [null, ['' => null]],
-            ['', ['' => '']],
-            [0, ['' => 0]],
-            [3.14, ['' => 3.14]],
-            ['test', ['' => 'test']],
-            [false, ['' => false]],
-        ];
-
-        $actual = array_flatten($in);
-
-        $expected = [null, '', 0, 3.14, 'test', false];
-
-        $this->assertEquals($expected, $actual);
-    }
-
-    public function test_flatten_2()
-    {
-        $in = [
-            [null, '-', ':', [':' => null]],
-            ['', '', '/', ['/' => '']],
-            [0, '.', 'global', ['global' => 0]],
-            [3.14, '', 'local', ['local' => 3.14]],
-            ['test', 'sep', '', ['' => 'test']],
-            [false, '', '_', ['_' => false]],
-        ];
-
-        $actual = array_flatten($in);
-
-        $expected = [null, '-', ':', '', '/', 0, '.', 'global', 3.14, 'local', 'test', 'sep', false, '_'];
-
-        $this->assertEquals($expected, $actual);
-    }
-
-    public function test_flatten_3()
-    {
-        $in = [
-            [[], []],
-            [[['a' => 1], ['b' => 2]], ['0.a' => 1, '1.b' => 2]],
-            [[0], ['0' => 0]],
-            [[1, 2], ['0' => 1, '1' => 2]],
-            [
-                [1, 2, [3, 4]],
-                ['0' => 1, '1' => 2, '2.0' => 3, '2.1' => 4],
+        $cases = [
+            'preserves scalar order' => [
+                'input' => [1, [2, [3, [4, [5], 6], 7], 8], 9],
+                'expected' => [1, 2, 3, 4, 5, 6, 7, 8, 9],
             ],
-            [
-                ['a' => 1, 2, 'b' => [3, 'c' => 4]],
-                ['a' => 1, '0' => 2, 'b.0' => 3, 'b.c' => 4],
+            'keeps the first occurrence only' => [
+                'input' => [
+                    1,
+                    [1, 2],
+                    ['nested' => [2, 3]],
+                    [3, ['deep' => 1]],
+                    4,
+                ],
+                'expected' => [1, 2, 3, 4],
             ],
-            [
-                ['a' => 1, 'b' => 2, 'c' => ['d' => [3, 4], 'e' => ['f' => 5, 'g' => 6]]],
-                ['a' => 1, 'b' => 2, 'c.d.0' => 3, 'c.d.1' => 4, 'c.e.f' => 5, 'c.e.g' => 6],
+            'flattens empty arrays' => [
+                'input' => [[], []],
+                'expected' => [],
+            ],
+            'flattens nested indexed arrays' => [
+                'input' => [
+                    [1, 2, [3, 4]],
+                ],
+                'expected' => [1, 2, 3, 4],
+            ],
+            'flattens nested associative arrays' => [
+                'input' => [
+                    [
+                        'a' => 1,
+                        'b' => 2,
+                        'c' => [
+                            'd' => [3, 4],
+                            'e' => [
+                                'f' => 5,
+                                'g' => 6,
+                            ],
+                        ],
+                    ],
+                ],
+                'expected' => [1, 2, 3, 4, 5, 6],
+            ],
+            'keeps scalar values and type fidelity' => [
+                'input' => [
+                    [null, ['' => null]],
+                    ['', ['' => '']],
+                    [0, ['' => 0]],
+                    [3.14, ['' => 3.14]],
+                    ['test', ['' => 'test']],
+                    [false, ['' => false]],
+                ],
+                'expected' => [null, '', 0, 3.14, 'test', false],
+            ],
+            'preserves mixed nested values' => [
+                'input' => [
+                    [null, '-', ':', [':' => null]],
+                    ['', '', '/', ['/' => '']],
+                    [0, '.', 'global', ['global' => 0]],
+                    [3.14, '', 'local', ['local' => 3.14]],
+                    ['test', 'sep', '', ['' => 'test']],
+                    [false, '', '_', ['_' => false]],
+                ],
+                'expected' => [null, '-', ':', '', '/', 0, '.', 'global', 3.14, 'local', 'test', 'sep', false, '_'],
             ],
         ];
 
-        $actual = array_flatten($in);
-
-        $expected = [1, 2, 0, 3, 4, 5, 6];
-
-        $this->assertEquals($expected, $actual);
-    }
-
-    public function test_flatten_4()
-    {
-        $in = [
-            [null, '-', ':', [':' => null]],
-            ['', '', '/', ['/' => '']],
-            [0, '.', 'global', ['global' => 0]],
-            [3.14, '', 'local', ['local' => 3.14]],
-            ['test', 'sep', '', ['' => 'test']],
-            [false, '', '_', ['_' => false]],
-        ];
-
-        $actual = array_flatten($in);
-
-        $expected = [null, '-', ':', '', '/', 0, '.', 'global', 3.14, 'local', 'test', 'sep', false, '_'];
-
-        $this->assertEquals($expected, $actual);
-    }
-
-    public function test_flatten_5()
-    {
-        $in = [1, [2, [3, [4, [5], 6], 7], 8], 9];
-
-        $actual = array_flatten($in);
-
-        $expected = [1, 2, 3, 4, 5, 6, 7, 8, 9];
-
-        $this->assertEquals($expected, $actual);
+        foreach ($cases as $label => $case) {
+            $this->assertSame($case['expected'], array_flatten($case['input']), $label);
+        }
     }
 }