What causes "Invalid JSON: :error" in octobercms/october?

Fix the syntax error named in the JsonException message (it includes the position)

octobercms/october · warning · ValidationException

Invalid JSON: :error

Error message

Invalid JSON: :error

What it means

The backend composer.json editor (Updates > Manage projects) saves through manage_onSaveComposer. It runs json_decode($content, JSON_THROW_ON_ERROR) and converts any JsonException into a ValidationException on the content field with the message 'Invalid JSON: <parser error>'. File::put(base_path('composer.json'), ...) only runs after the content parses cleanly.

Source

Thrown at modules/system/controllers/updates/HasComposerEditor.php:38

        $widget->getField('content')->value(File::get($composerFile));

        $this->vars['widget'] = $widget;

        return $this->makePartial('composer_form');
    }

    /**
     * manage_onSaveComposer validates and saves the composer.json file
     */
    public function manage_onSaveComposer()
    {
        $content = post('Composer')['content'] ?? '';

        try {
            json_decode($content, flags: JSON_THROW_ON_ERROR);
        }
        catch (\JsonException $ex) {
            throw new ValidationException(['content' => __("Invalid JSON: :error", [
                'error' => $ex->getMessage()
            ])]);
        }

        File::put(base_path('composer.json'), $content);

        Flash::success(__("Composer file updated successfully."));
    }

    /**
     * makeComposerFormWidget creates a form widget for the composer editor
     */
    protected function makeComposerFormWidget()
    {
        $config = $this->makeConfig([
            'alias' => 'formComposerEditor',
            'arrayName' => 'Composer',
            'model' => new \Model,

View on GitHub (pinned to b608633a7e)

Solutions

Fix the syntax error named in the JsonException message (it includes the position)
Validate before saving: JSON.parse the content client-side, or run it through jsonlint
For intentionally empty content, save a minimal valid document such as {} instead of blank text

Example fix

// before
{
    "require": {
        "october/system": "^3.0",
    }
}

// after
{
    "require": {
        "october/system": "^3.0"
    }
}

Defensive patterns

Strategy: validation

Validate before calling

// server-side, before saving
try { json_decode($content, flags: JSON_THROW_ON_ERROR); } catch (\JsonException $ex) { /* block save, show $ex->getMessage() */ }

// client-side
try { JSON.parse(content); } catch (e) { /* block submit */ }

Prevention

Never allow blank submissions; substitute {} for empty content
Run JSON.parse in the editor before enabling the save button
Remember composer.json is strict JSON: no trailing commas, no single quotes, no comments

When it happens

Trigger: Saving composer.json with a trailing comma, single-quoted strings, unquoted keys, smart quotes, an unterminated object, or an empty string (post('Composer')['content'] defaults to '' which also fails json_decode with 'Syntax error').

Common situations: Hand-editing composer.json in the UI and forgetting JSON strictness; pasting YAML-ish config; submitting the form with a cleared textarea.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of octobercms/october@b608633a7e (2026-08-21). Data as JSON: /api/errors/ea46c1ef6e8b585c. Report an issue: GitHub.