AnthropicTextGenerationModel fails on tool function call if it has empty function arguments for input

[saarnilauri]

AnthropicTextGenerationModel fails when sending function calls with empty or null arguments to Anthropic API, resulting in "Bad Request (400) - messages.X.content.Y.tool_use.input: Input should be a valid dictionary" error.

• Provider: Anthropic Claude (Sonnet 4.5)
• Affected file: php-ai-client/src/ProviderImplementations/Anthropic/AnthropicTextGenerationModel.php

When an AI function/tool is called without parameters (see the example ability: list-block-patterns ability that accepts optional parameters only), the FunctionCall object may contain:

• args = null
• args = [] (empty array)

When preparing the request for Anthropic API in getMessagePartData() method (line ~346), the code directly passes these values:

if ($type->isFunctionCall()) {
    $functionCall = $part->getFunctionCall();
    return [
        'type' => 'tool_use',
        'id' => $functionCall->getId(),
        'name' => $functionCall->getName(),
        'input' => $functionCall->getArgs(), // ❌ Can be null or []
    ];
}

Anthropic API Requirements:

• The tool_use.input field MUST be a valid dictionary/object
• Empty arrays [] are rejected
• null values are rejected
• Only {} (empty object) or populated objects are accepted

Proposed solution

Modify getMessagePartData() method in AnthropicTextGenerationModel.php to ensure input is always a valid object:

if ($type->isFunctionCall()) {
    $functionCall = $part->getFunctionCall();
    if (!$functionCall) {
        throw new RuntimeException(
            'The function_call typed message part must contain a function call.'
        );
    }
    
    $args = $functionCall->getArgs();
    
    // ✅ Ensure args is always a valid object for Anthropic API
    if ($args === null || (is_array($args) && empty($args))) {
        $args = new \stdClass(); // Serializes to {} in JSON
    }
    
    return [
        'type' => 'tool_use',
        'id' => $functionCall->getId(),
        'name' => $functionCall->getName(),
        'input' => $args,
    ];
}

The example ability to see what is the real case input:

<?php
/**
 * List Block Patterns Ability implementation.
 *
 * @package ValuDev\Abilities
 */

declare(strict_types=1);

namespace ValuDev\Abilities;

use ValuDev\Abstracts\Abstract_Valu_Ability;
use WP_Block_Patterns_Registry;

/**
 * List available block patterns ability.
 */
class List_Block_Patterns_Ability extends Abstract_Valu_Ability {

    /**
     * {@inheritDoc}
     */
    protected function input_schema(): array {
        return array(
            'type'                 => array( 'object', 'null' ),
            'properties'           => array(
                'category' => array(
                    'type'        => 'string',
                    'description' => __( 'Filter patterns by category slug (optional)', 'valu-dev-mcp' ),
                ),
            ),
            'additionalProperties' => false,
        );
    }

    /**
     * {@inheritDoc}
     */
    protected function output_schema(): array {
        return array(
            'type'       => 'object',
            'properties' => array(
                'success'  => array(
                    'type'        => 'boolean',
                    'description' => __( 'Whether the patterns were retrieved successfully', 'valu-dev-mcp' ),
                ),
                'patterns' => array(
                    'type'        => 'array',
                    'description' => __( 'List of available block patterns', 'valu-dev-mcp' ),
                    'items'       => array(
                        'type'       => 'object',
                        'properties' => array(
                            'id'          => array(
                                'type'        => array( 'integer', 'null' ),
                                'description' => __( 'Pattern post ID (null for registered patterns)', 'valu-dev-mcp' ),
                            ),
                            'slug'        => array(
                                'type'        => 'string',
                                'description' => __( 'Pattern slug for use with get-block-pattern-raw and get-block-pattern-parsed', 'valu-dev-mcp' ),
                            ),
                            'title'       => array(
                                'type'        => 'string',
                                'description' => __( 'Pattern title', 'valu-dev-mcp' ),
                            ),
                            'description' => array(
                                'type'        => 'string',
                                'description' => __( 'Pattern description', 'valu-dev-mcp' ),
                            ),
                        ),
                    ),
                ),
                'count'    => array(
                    'type'        => 'integer',
                    'description' => __( 'Total number of patterns returned', 'valu-dev-mcp' ),
                ),
                'message'  => array(
                    'type'        => 'string',
                    'description' => __( 'Status message', 'valu-dev-mcp' ),
                ),
            ),
            'additionalProperties' => false,
        );
    }

   // and the code continues …
       
}