Feature Request: Add Support for OpenAI/Azure Responses API

The current implementation only supports the Chat Completions API. This prevents the use of newer reasoning models such as gpt-5-pro, which are only available through the Responses API.

I would like to request support for the OpenAI/Azure Responses API, including automatic routing based on model capabilities.

⸻

Problem

When using Azure OpenAI with a GPT-5 Pro deployment, requests fail with the following error:

Invalid API request
The requested operation is unsupported.

Investigation shows that the library always sends requests to the Chat Completions endpoint:

POST /chat/completions

However, Azure GPT-5 Pro only supports the Responses API and does not support Chat Completions.

As a result, users cannot use GPT-5 Pro even though the deployment and credentials are configured correctly.

⸻

Current Behavior

The implementation assumes all chat-capable models support Chat Completions:

$client->chat()->create([
'model' => $model,
'messages' => $messages,
]);

This works for:

* GPT-4o
* GPT-4.1
* GPT-5 Mini
* GPT-5

but fails for:

* GPT-5 Pro

⸻

Expected Behavior

The library should support both APIs:

Chat Completions

POST /chat/completions

Responses API

POST /responses

The framework should be able to route requests to the appropriate API depending on model capabilities.

Example:

if ($modelCapabilities->supportsResponsesOnly()) {
$client->responses()->create(...);
} else {
$client->chat()->create(...);
}

⸻

Proposed Solution

Option 1: Native Responses API Support

Introduce a dedicated Responses API client:

$client->responses()->create([
'model' => 'gpt-5-pro',
'input' => $messages,
]);

This gives users direct access to the newer API surface.

⸻

Option 2: Automatic Capability Routing

Add model capability detection and transparently choose the correct endpoint.

Example:

$client->generate([
'model' => 'gpt-5-pro',
'messages' => $messages,
]);

Internally:

if ($model->supportsResponses()) {
return $responsesClient->create(...);
}
return $chatClient->create(...);

This provides the best developer experience and future-proofs the implementation as more models migrate to Responses API.

⸻

Additional Considerations

The Responses API introduces features that are becoming the primary interface for modern reasoning models:

* GPT-5 Pro support
* Future reasoning model compatibility
* Unified tool calling
* Structured outputs
* Multi-modal input/output
* Reasoning controls

Supporting only Chat Completions will increasingly limit compatibility with newer OpenAI and Azure OpenAI models.

⸻

Environment

Provider

Azure OpenAI

API Version

2024-12-01-preview

Working Model

gpt-5-mini

Failing Model

gpt-5-pro

Error

Invalid API request
The requested operation is unsupported.

⸻

Benefit

Adding Responses API support would enable compatibility with GPT-5 Pro and future reasoning-focused models while maintaining backward compatibility with existing Chat Completions integrations.