twigphp/Twig · error · LogicException

Unable to add function

Error message

Unable to add function "%s" as extensions have already been initialized.

What it means

Like addExtension, adding a Twig function is only allowed before extensions are initialized. After initialization the function table is built and frozen (and may be cached), so ExtensionSet::addFunction throws this LogicException for late additions.

Solutions

Call addFunction right after constructing the Environment, before rendering anything.
Alternatively wrap the function in a proper Extension class registered at construction time.
Create a new Environment if late registration is truly required.
In Symfony, register the function via a TwigExtension service so the framework adds it at boot.

Example fix

// before
$twig = new Environment($loader);
$twig->render('page.twig');
$twig->addFunction(new TwigFunction('price', fn($v) => $v * 1.2));

// after
$twig = new Environment($loader);
$twig->addFunction(new TwigFunction('price', fn($v) => $v * 1.2));
$twig->render('page.twig');

Defensive patterns

Strategy: try-catch

Validate before calling

// guard: register before initialization
$functionsAdded = false;
function addFunctionEarly(\Twig\Environment $twig, \Twig\TwigFunction $fn): void {
    global $functionsAdded;
    if ($functionsAdded) { throw new \LogicException('Too late to add functions'); }
}

Try / catch

try { $twig->addFunction($fn); } catch (\LogicException $e) {
    if (str_contains($e->getMessage(), 'already been initialized')) {
        // move to bootstrap or recreate the environment
    } else { throw $e; }
}

Prevention

Add all functions right after Environment construction.
Wrap functions in an Extension class so the framework registers them at boot.
Do not render any template before finishing Twig setup.
In tests, build a fresh Environment whenever functions change.

When it happens

Trigger: Calling $twig->addFunction(...) after the environment has been used (render/loadTemplate/getTokenParsers) which triggers lazy initialization; also calling it on an ExtensionSet already flagged initialized.

Common situations: Defining twig functions in a listener that fires post-boot; adding functions conditionally inside a loop that runs after first render; tests adding functions per-test on a shared Environment.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/5a92b2627e279104. Report an issue: GitHub.

Appendix: source

Thrown at src/ExtensionSet.php:169

        } else {
            $class = $extension::class;
        }

        if ($this->initialized) {
            throw new \LogicException(\sprintf('Unable to register extension "%s" as extensions have already been initialized.', $class));
        }

        if (isset($this->extensions[$class])) {
            throw new \LogicException(\sprintf('Unable to register extension "%s" as it is already registered.', $class));
        }

        $this->extensions[$class] = $extension;
    }

    public function addFunction(TwigFunction $function): void
    {
        if ($this->initialized) {
            throw new \LogicException(\sprintf('Unable to add function "%s" as extensions have already been initialized.', $function->getName()));
        }

        $this->staging->addFunction($function);
    }

    /**
     * @return TwigFunction[]
     */
    public function getFunctions(): array
    {
        if (!$this->initialized) {
            $this->initExtensions();
        }

        return $this->functions;
    }

    public function getFunction(string $name): ?TwigFunction

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