diff --git a/.github/workflows/code_coverage.yaml b/.github/workflows/code_coverage.yaml
new file mode 100644
index 0000000..c04db4c
--- /dev/null
+++ b/.github/workflows/code_coverage.yaml
@@ -0,0 +1,46 @@
+# https://help.github.com/en/categories/automating-your-workflow-with-github-actions
+
+name: "Code Coverage"
+
+on:
+  pull_request:
+  push:
+    branches:
+      - "main"
+
+permissions:
+  contents: "read"
+
+concurrency:
+  group: "${{ github.workflow }}-${{ github.ref }}"
+  cancel-in-progress: true
+
+jobs:
+  code-coverage:
+    name: "Code Coverage"
+
+    runs-on: "ubuntu-latest"
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v7"
+
+      - name: "Install PHP"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          coverage: "pcov"
+          php-version: "8.5"
+
+      - name: "Install dependencies"
+        uses: "ramsey/composer-install@v4"
+
+      - name: "Tests with coverage"
+        run: "vendor/bin/phpunit --coverage-clover build/logs/clover.xml"
+
+      - name: "Upload to Codecov"
+        uses: "codecov/codecov-action@v7"
+        with:
+          token: "${{ secrets.CODECOV_TOKEN }}" # not required for public repos
+          files: "build/logs/clover.xml"
+          flags: "unittests"
+          fail_ci_if_error: true
diff --git a/CLAUDE.md b/CLAUDE.md
index 8a0e01e..714cc67 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -23,7 +23,9 @@ vendor/bin/phpcs                       # coding standard (brainbits/phpcs-standa
 
 CI (`.github/workflows/build.yml`) runs composer validate/audit, phpcs and phpstan on PHP 8.5 and the tests on
 PHP 8.4 and 8.5 with lowest and highest dependencies. phpcs only runs with the highest dependencies; the lowest
-coding standard version reports different violations.
+coding standard version reports different violations. `.github/workflows/code_coverage.yaml` uploads coverage (pcov,
+PHP 8.5) to Codecov. Line coverage is 100 %; `#[CoversFunction]` only counts lines of the named function, so internal
+helpers need their own `Internal*Test`.
 
 ## How functions are built
 
diff --git a/README.md b/README.md
index 1c08867..a5fc0ee 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # fnc
 
+[![Coverage Status](https://img.shields.io/codecov/c/github/temp/fnc)](https://app.codecov.io/gh/temp/fnc)
+
 A [Ramda](https://ramdajs.com/)-inspired functional programming library for PHP.
 
 All functions live in the `Fnc` namespace, are plain functions (no classes) and most of them are
@@ -30,8 +32,8 @@ $adultNames([
 composer require temp/fnc
 ```
 
-The package was called `temp/fn` up to 1.0, so versions 1.0 and older are only available under that name. `temp/fnc`
-replaces `temp/fn`, Composer never installs both. The namespace `Fnc` is unchanged.
+The package was called `temp/fn` up to 1.0 and is abandoned in favour of `temp/fnc`, which also provides the older
+versions. `temp/fnc` replaces `temp/fn`, Composer never installs both. The namespace `Fnc` is unchanged.
 
 Requires PHP 8.4 or later. The functions are loaded via Composer's `files` autoloading, so importing them with
 `use function Fnc\…;` is all you need.
diff --git a/tests/Unit/Curry5Test.php b/tests/Unit/Curry5Test.php
index 3723311..316381e 100644
--- a/tests/Unit/Curry5Test.php
+++ b/tests/Unit/Curry5Test.php
@@ -33,5 +33,6 @@ public function testCurry3(): void
         $this->assertEquals(17, $sum4(5, 10, -4)(4, 2));
         $this->assertEquals(17, $sum4(5, 10, -4, 4, 2));
         $this->assertEquals(17, $sum4(5, 10, -4, 4, 2, 8));
+        $this->assertEquals(17, $sum4(5, 10, -4, 4)(2));
     }
 }
diff --git a/tests/Unit/InternalAcceptsArgsTest.php b/tests/Unit/InternalAcceptsArgsTest.php
new file mode 100644
index 0000000..f084dc5
--- /dev/null
+++ b/tests/Unit/InternalAcceptsArgsTest.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FncTests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversFunction;
+use PHPUnit\Framework\TestCase;
+
+use function Fnc\_acceptsArgs;
+use function Fnc\curry2;
+
+// phpcs:disable SlevomatCodingStandard.Functions.UnusedParameter.UnusedParameter
+
+#[CoversFunction('Fnc\_acceptsArgs')]
+final class InternalAcceptsArgsTest extends TestCase
+{
+    public function testClosure(): void
+    {
+        $fn = static fn ($a, $b = null) => $a;
+
+        $this->assertTrue(_acceptsArgs($fn, 1));
+        $this->assertTrue(_acceptsArgs($fn, 2));
+        $this->assertFalse(_acceptsArgs($fn, 3));
+    }
+
+    public function testCurriedClosureUsesRegisteredArity(): void
+    {
+        $curried = curry2(static fn ($a, $b) => $a);
+
+        $this->assertTrue(_acceptsArgs($curried, 2));
+        $this->assertFalse(_acceptsArgs($curried, 3));
+    }
+
+    public function testVariadic(): void
+    {
+        $this->assertTrue(_acceptsArgs(static fn (...$args) => $args, 5));
+    }
+
+    public function testInternalFunctionCountsRequiredParametersOnly(): void
+    {
+        // trim() has an optional $characters parameter that must not receive a key
+        $this->assertTrue(_acceptsArgs('trim', 1));
+        $this->assertFalse(_acceptsArgs('trim', 2));
+    }
+}
diff --git a/tests/Unit/InternalReflectCallableTest.php b/tests/Unit/InternalReflectCallableTest.php
new file mode 100644
index 0000000..445dcc4
--- /dev/null
+++ b/tests/Unit/InternalReflectCallableTest.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FncTests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversFunction;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+use function Fnc\_reflectCallable;
+
+#[CoversFunction('Fnc\_reflectCallable')]
+final class InternalReflectCallableTest extends TestCase
+{
+    public function testCallable(): void
+    {
+        $this->assertSame('trim', _reflectCallable('trim')->getName());
+    }
+
+    public function testNonCallableThrows(): void
+    {
+        $this->expectException(RuntimeException::class);
+
+        _reflectCallable('not a function');
+    }
+}
diff --git a/tests/Unit/SplitTest.php b/tests/Unit/SplitTest.php
index 71b3662..a08c88f 100644
--- a/tests/Unit/SplitTest.php
+++ b/tests/Unit/SplitTest.php
@@ -4,6 +4,7 @@
 
 namespace FncTests\Unit;
 
+use InvalidArgumentException;
 use PHPUnit\Framework\Attributes\CoversFunction;
 use PHPUnit\Framework\TestCase;
 
@@ -82,4 +83,11 @@ public function testWithMultipleMixedElements(): void
 
         $this->assertSame(['foo', 'bar', '1', '2.5'], $result);
     }
+
+    public function testEmptySeparatorThrows(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        split('', 'foo');
+    }
 }