OpenRouter ToolCallMap throws TypeError on malformed JSON arguments instead of a handled provider error

[ruttydm]

I found a crash path in prism-php/prism v0.99.22 when mapping OpenRouter tool calls.

Prism\Prism\Providers\OpenRouter\Maps\ToolCallMap::map() does this:

arguments: json_decode((string) $toolCall['function']['arguments'], true),

If the model returns malformed JSON for function.arguments, json_decode(..., true) returns null. That null is then passed to Prism\Prism\ValueObjects\ToolCall, whose constructor expects array|string for $arguments.

The result is a raw PHP TypeError instead of a handled Prism exception.

Minimal repro

Prism\Prism\Providers\OpenRouter\Maps\ToolCallMap::map([
    [
        'id' => '1',
        'function' => [
            'name' => 'demo',
            'arguments' => '{',
        ],
    ],
]);

This throws:

TypeError: Prism\Prism\ValueObjects\ToolCall::__construct(): Argument #3 ($arguments) must be of type array|string, null given

Expected

Malformed tool-call JSON should be handled gracefully, for example by:

• preserving the raw string, or
• throwing a Prism-level provider/parsing exception with context

Actual

A low-level TypeError escapes the mapping layer.

Because tool arguments are model-generated, malformed JSON is a normal failure mode and should not crash the mapper this way.