OpenAI: Missing `additionalProperties: false` in tool parameters when strict mode is enabled

[Frostech0]

Description

When using tools with OpenAI's strict mode enabled, the API returns a 400 error because additionalProperties: false is not included in the tool's parameter schema.

OpenAI's function calling documentation explicitly requires additionalProperties: false at every level of the parameter object when strict mode is active.

Reproduction

Using the tool definition from the Prism docs — OpenAI provider ("Combining Tools with Structured Output"):

$weatherTool = Tool::as('get_weather')
    ->for('Get current weather for a location')
    ->withStringParameter('location', 'The city and state')
    ->using(fn (string $location): string => "Weather in {$location}: 72°F, sunny")
    ->withProviderOptions([
        'strict' => true,
    ]);

$response = Prism::structured()
    ->using('openai', 'gpt-4o')
    ->withSchema($schema)
    ->withTools([$weatherTool])
    ->withMaxSteps(3)
    ->withPrompt('What is the weather in San Francisco and should I wear a coat?')
    ->asStructured();

Error

OpenAI Error [400]: invalid_request_error - Invalid schema for function 'get_weather': In context=(), 'additionalProperties' is required to be supplied and to be false.

Root Cause

In src/Providers/OpenAI/Maps/ToolMap.php, the parameter schema is built as:

'parameters' => [
    'type' => 'object',
    'properties' => $tool->parametersAsArray(),
    'required' => $tool->requiredParameters(),
],

When strict is set to true, OpenAI requires additionalProperties: false to be present in the schema, but Prism does not add it.

Suggested Fix

Add additionalProperties: false conditionally when strict mode is enabled:

'parameters' => [
    'type' => 'object',
    'properties' => $tool->parametersAsArray(),
    'required' => $tool->requiredParameters(),
    ...($tool->providerOptions('strict') ? [
        'additionalProperties' => false,
    ] : []),
],

Environment

• Prism version: v0.99.21
• PHP version: 8.4.1
• Provider: OpenAI (gpt-4o)