diff --git a/bin/pest b/bin/pest
index 10a65dd0..5c34c8c6 100755
--- a/bin/pest
+++ b/bin/pest
@@ -151,6 +151,12 @@ use Symfony\Component\Console\Output\ConsoleOutput;
     // mode gates).
     \Pest\Support\XdebugGuard::maybeDrop($rootPath);
 
+    // Restarts PHP with `pcov.directory=<root>` when `--tia` is active and
+    // pcov is loaded, so the driver never instruments anything outside the
+    // project (vendor, system includes). Idempotent — guarded by an env
+    // sentinel so a single round-trip is enough.
+    \Pest\Support\PcovGuard::maybeRestart($rootPath);
+
     $input = new ArgvInput;
 
     $testSuite = TestSuite::getInstance(
diff --git a/src/Support/PcovGuard.php b/src/Support/PcovGuard.php
new file mode 100644
index 00000000..7ba6dcf2
--- /dev/null
+++ b/src/Support/PcovGuard.php
@@ -0,0 +1,182 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pest\Support;
+
+/**
+ * Re-execs the PHP process with `pcov.directory` pinned to the project
+ * root so pcov never instruments anything outside it (vendor, system
+ * includes, etc.).
+ *
+ * pcov reads `pcov.directory` once, on the first file it instruments —
+ * setting it via `ini_set()` from inside the test runner is too late
+ * for files already compiled by Composer's autoloader. Restarting the
+ * process with `-dpcov.directory=<root>` from the very top of `bin/pest`
+ * means *every* file pcov sees is filtered correctly.
+ *
+ * Only fires when ALL of these hold:
+ *   1. The pcov extension is loaded.
+ *   2. `--tia` is present in argv (plain `pest` runs are unaffected).
+ *   3. The current `pcov.directory` differs from the project root.
+ *   4. We are not already the restarted process — guarded by an env
+ *      sentinel so a single round-trip is enough.
+ *
+ * Modelled after {@see XdebugGuard}: the same "check before doing real
+ * work in `bin/pest`" position, the same conservative gating around
+ * `--tia`. They are independent — both can fire on the same invocation
+ * (the user has pcov *and* xdebug loaded), in which case Xdebug is
+ * dropped first and the pcov restart inherits the slimmer process.
+ *
+ * @internal
+ */
+final class PcovGuard
+{
+    private const string ENV_RESTARTED = 'PEST_PCOV_GUARD_RESTARTED';
+
+    /**
+     * Call as early as possible after Composer autoload, before any
+     * Pest class beyond the autoloader is touched. Idempotent and
+     * defensive — returns silently when pcov isn't installed, when the
+     * INI is already correct, or when we've already restarted.
+     */
+    public static function maybeRestart(string $projectRoot): void
+    {
+        if (! extension_loaded('pcov')) {
+            return;
+        }
+
+        if (getenv(self::ENV_RESTARTED) === '1') {
+            return;
+        }
+
+        $argv = is_array($_SERVER['argv'] ?? null) ? $_SERVER['argv'] : [];
+
+        if (! self::hasTiaFlag($argv)) {
+            return;
+        }
+
+        $desired = self::normalise($projectRoot);
+        $current = self::normalise((string) ini_get('pcov.directory'));
+
+        if ($current === $desired) {
+            return;
+        }
+
+        self::restart($projectRoot, $argv);
+    }
+
+    /**
+     * @param  array<int, mixed>  $argv
+     */
+    private static function hasTiaFlag(array $argv): bool
+    {
+        foreach ($argv as $value) {
+            if (is_string($value) && $value === '--tia') {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * Spawns a child PHP process inheriting our stdin/stdout/stderr and
+     * exits with its status. `pcntl_exec` would be the cleanest path
+     * (replaces the current process, no double-buffering) but it isn't
+     * available on Windows or in environments that disable it; the
+     * `proc_open` fallback works everywhere PHP runs.
+     *
+     * @param  array<int, mixed>  $argv
+     */
+    private static function restart(string $projectRoot, array $argv): void
+    {
+        $script = self::scriptArgv($argv);
+
+        if ($script === null) {
+            return;
+        }
+
+        $env = self::inheritEnv();
+        $env[self::ENV_RESTARTED] = '1';
+
+        $command = array_merge(
+            [PHP_BINARY, '-d', 'pcov.directory='.$projectRoot],
+            $script,
+        );
+
+        if (function_exists('pcntl_exec')) {
+            // `pcntl_exec` returns false on failure and replaces the
+            // process on success — no `exit` needed in the success path.
+            // Pass the env explicitly because pcntl_exec doesn't inherit
+            // by default.
+            $binary = array_shift($command);
+
+            if (is_string($binary)) {
+                @pcntl_exec($binary, $command, $env);
+            }
+
+            // If we're still here, pcntl_exec failed; fall through.
+        }
+
+        $proc = @proc_open(
+            $command,
+            [STDIN, STDOUT, STDERR],
+            $pipes,
+            null,
+            $env,
+        );
+
+        if (! is_resource($proc)) {
+            return;
+        }
+
+        $exitCode = proc_close($proc);
+
+        exit($exitCode === -1 ? 1 : $exitCode);
+    }
+
+    /**
+     * Reconstructs the argv we want the child process to receive: the
+     * script path followed by every original argument. Returns null
+     * when argv is malformed and we can't safely restart.
+     *
+     * @param  array<int, mixed>  $argv
+     * @return list<string>|null
+     */
+    private static function scriptArgv(array $argv): ?array
+    {
+        $out = [];
+
+        foreach ($argv as $value) {
+            if (! is_string($value)) {
+                return null;
+            }
+
+            $out[] = $value;
+        }
+
+        return $out === [] ? null : $out;
+    }
+
+    /**
+     * @return array<string, string>
+     */
+    private static function inheritEnv(): array
+    {
+        $env = [];
+
+        foreach (getenv() as $name => $value) {
+            if (is_string($name) && is_string($value)) {
+                $env[$name] = $value;
+            }
+        }
+
+        return $env;
+    }
+
+    private static function normalise(string $path): string
+    {
+        return rtrim($path, '/\\');
+    }
+}