Fix empty tool calls and arguments in OpenAI compatible models

[Jameswlepage]

Summary

Fixes critical API compatibility issues when AI models make function/tool calls with no parameters. The SDK was generating invalid payloads rejected by OpenAI and Anthropic APIs.

Changes

• Fix empty tool_calls handling: Only include tool_calls field when array contains items (OpenAI rejects empty arrays)
• Fix empty arguments encoding: Convert empty PHP arrays to objects before JSON encoding (APIs expect {} not [] for empty parameters)
• Add test coverage: New tests for both edge cases to prevent regression

Technical Details

The issue manifests in two ways:

1. OpenAI API Error: Invalid 'messages[X].tool_calls': empty array. Expected at least one item

   • Occurs when tool_calls: [] is included in messages
   • Fixed by conditionally including the field only when non-empty

2. Anthropic API Error: messages.X.content.0.tool_use.input: Input should be a valid dictionary

   • Occurs when empty arguments encode as "[]" instead of "{}"
   • Fixed by converting empty arrays to objects before JSON encoding

Testing

# Run new tests
./vendor/bin/phpunit --filter="testGetMessagePartToolCallDataEmptyArguments|testPrepareMessagesParamNoToolCalls"

# Verify all tests pass
composer run phpunit  # 878 tests, 3547 assertions ✓
composer run phpstan  # No errors ✓

Impact

• Enables WordPress Abilities API integration (many parameterless functions)
• Fixes production failures with function calling
• Maintains full backward compatibility

Related

• Community reports: OpenAI empty tool_calls error
• Similar issues reported with Anthropic's Claude API in August 2024

[Copilot]

Pull Request Overview

This PR fixes critical compatibility issues with OpenAI and Anthropic APIs when AI models make function/tool calls with empty parameters. The changes ensure proper JSON encoding and conditional field inclusion to prevent API rejections.

• Fix empty tool_calls array handling by only including the field when non-empty
• Fix empty arguments encoding to produce {} instead of [] for parameterless function calls
• Add comprehensive test coverage for both edge cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModel.php	Implements conditional tool_calls inclusion and empty arguments object conversion
tests/unit/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModelTest.php	Adds test cases for empty tool calls and empty arguments scenarios

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[Copilot]

[nitpick] The (object) array() conversion can be simplified to new \stdClass() for better readability and clarity of intent when creating an empty object.

Suggested change

	if (is_array($args) && empty($args)) {
	$args = (object) array();
	$args = new \stdClass();

[felixarntz]

@Jameswlepage Thanks, great catch!

For the empty tool_calls, this makes total sense. About the other change, I have a follow up question - technically always making it an empty object doesn't seem right (unless this is truly the only way to make it work against the OpenAI API).

[felixarntz]

This one is tricky. Technically speaking, for JSON-encoding this should still only be turned into stdClass when the expected input shape is object. While that's often the case, it only depends on the schema - it could also be array, in which case leaving it a PHP array would be correct.

What happens if the parameters JSON schema specified for the function expects an actual array (indexed/numeric)? Does the OpenAI API always fail if it's an empty array? If so, that would seem like a flaw in the API (although, if that's the case, of course we still need to fix / work around it in our SDK).

[felixarntz]

See my above comment, the problem is in PHP it's undefined whether this is an empty associative array (object in JSON schema) or an empty indexed/numeric array (array in JSON schema). Just to clarify what I meant above.

[felixarntz]

PHPCS errors here:

Suggested change

	$model = $this->createModel();
	$prepared = $model->exposePrepareMessagesParam([$message], null);
	$model = $this->createModel();
	$prepared = $model->exposePrepareMessagesParam([$message], null);