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+# PSR-AI: Common Interface for AI Interaction in PHP
+
+## Table Of Contents
+
+- [1. Introduction](#1-introduction)
+- [2. Conventions Used In This Document](#2-conventions-used-in-this-document)
+- [3. Definitions](#3-definitions)
+- [4. Basic Principles](#4-basic-principles)
+- [5. The AI Client Abstraction](#5-the-ai-client-abstraction)
+  - [5.1 The Client Interface](#51-the-client-interface)
+  - [5.2 The Message Interface](#52-the-message-interface)
+  - [5.3 The Stream Interface](#53-the-stream-interface)
+  - [5.4 The Tool Interface](#54-the-tool-interface)
+  - [5.5 The Response Interface](#55-the-response-interface)
+- [6. Example Usage](#6-example-usage)
+- [Appendix A. ABNF Definitions](#appendix-a)
+- [Appendix B. References](#appendix-b)
+
+## 1. Introduction
+
+The purpose of this PSR is to define a common interface for interacting with artificial intelligence models in PHP, such as large language models, multimodal models, and reasoning engines.
+
+This PSR aims to standardize how PHP applications and frameworks:
+- Send messages to AI models.
+- Receive structured and streamed responses.
+- Register callable tools or functions for model invocation.
+- Maintain interoperability with the [Model Context Protocol (MCP)](https://modelcontextprotocol.io/).
+
+As with prior PSRs (such as PSR-3 and PSR-18), the goal is not to mandate a particular implementation, but to provide an interoperable interface layer allowing frameworks, SDKs, and providers to integrate seamlessly.
+
+## 2. Conventions Used In This Document
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119).
+
+## 3. Definitions
+
+- **Model**: A computational system capable of processing input and producing structured or natural-language output. Examples include OpenAI's GPT models, Anthropic's Claude, or local models served via Ollama.
+- **Client**: An implementation of the `Psr\Ai\ClientInterface` responsible for communicating with a model provider or local runtime.
+- **Message**: A discrete input or output unit representing a message exchanged with the model. A message MAY include role metadata (e.g., `system`, `user`, `assistant`) and MAY encapsulate data objects, code, or binary content.
+- **Stream**: A sequence of partial output tokens, chunks, or structured updates emitted during a model's response generation.
+- **Tool**: A callable function or operation that the model MAY invoke during reasoning or message generation. Tools MUST be discoverable and serializable in a format compatible with MCP.
+- **Response**: The final or intermediate result from a model call. Responses MAY include text, structured data, tool calls, or cost metrics.
+- **Session**: A series of related interactions forming a context (e.g., a multi-turn chat or reasoning session). Session handling is out of scope for this PSR, but MAY be implemented on top of it.
+
+## 4. Basic Principles
+
+1. All AI interactions MUST occur via a `Psr\Ai\ClientInterface`.
+2. Model inputs and outputs MUST be represented by standardized `MessageInterface` and `ResponseInterface` objects.
+3. Streaming outputs MUST conform to the `StreamInterface` contract, ensuring consistent event-driven consumption.
+4. Callable tools MUST implement the `ToolInterface` and MUST be compatible with MCP serialization.
+5. Implementations SHOULD remain framework-agnostic and transport-neutral.
+6. The PSR MUST NOT prescribe any particular provider API, HTTP format, or runtime behavior.
+
+## 5. The AI Client Abstraction
+
+### 5.1. The Client Interface
+
+The ClientInterface represents the entry point for AI interaction.
+
+An implementation MUST provide at least the following methods:
+
+```php
+namespace Psr\Ai;
+
+interface ClientInterface
+{
+    /**
+     * @param MessageInterface|array<MessageInterface> $message
+     */
+    public function complete(MessageInterface|array $message): ResponseInterface;
+
+    /**
+     * @param MessageInterface|array<MessageInterface> $message
+     */
+    public function stream(MessageInterface|array $message): StreamInterface;
+
+    public function invokeTool(ToolInterface $tool, array $arguments): mixed;
+}
+```
+
+**Rules:**
+- `complete()` MUST perform a synchronous model call and return a finalized `ResponseInterface`.
+- `stream()` MUST return a `StreamInterface` that yields incremental output as available.
+- `invokeTool()` MUST execute a callable tool in the same process or over MCP.
+
+### 5.2. The Message Interface
+
+The MessageInterface defines a structured input or output exchanged between client and model.
+
+```php
+namespace Psr\Ai;
+
+interface MessageInterface
+{
+    public function role(): string;
+    public function content(): string|array;
+    public function metadata(): array;
+}
+```
+
+**Rules**:
+- `role()` MUST return a string such as system, user, or assistant.
+- `content()` MAY contain plain text or structured data (e.g., JSON, array).
+- Implementations MAY define message classes (e.g., `UserMessage`, `AssistantMessage`) for typed interactions.
+- Message instances MUST be immutable once created.
+
+### 5.3. The Stream Interface
+
+The StreamInterface enables incremental consumption of model output.
+
+```php
+namespace Psr\Ai;
+
+interface StreamInterface extends \Traversable
+{
+    public function onToken(callable $callback): void;
+    public function onError(callable $callback): void;
+    public function onComplete(callable $callback): void;
+}
+```
+
+**Rules:**
+- Implementations MUST emit partial output (e.g., tokens, chunks) through `onToken()`.
+- `onError()` MUST emit any transport or model errors.
+- `onComplete()` MUST be called when the stream has ended.
+- Streams SHOULD be compatible with PSR-7 stream semantics where applicable.
+
+### 5.4. The Tool Interface 
+
+The ToolInterface standardizes callable functions the model may request via tool calling.
+
+```php
+namespace Psr\Ai;
+
+interface ToolInterface
+{
+    public function name(): string;
+    public function description(): ?string;
+    public function schema(): array;
+    public function execute(array $arguments): mixed;
+}
+```
+
+**Rules:**
+- Tools MUST define their input/output schema for validation and serialization.
+- Implementations SHOULD align schema definitions with the [Model Context Protocol](https://modelcontextprotocol.io/).
+- `execute()` MUST return serializable output or throw an exception.
+
+### 5.5. The Response Interface
+
+The ResponseInterface provides a normalized representation of model responses.
+
+```php
+namespace Psr\Ai;
+
+interface ResponseInterface
+{
+    public function message(): MessageInterface;
+    public function metadata(): array;
+}
+```
+
+**Rules:**
+- The `message()` MUST represent the model's returned message.
+- `metadata()` SHOULD include token usage, cost, latency, or provider-specific data.
+- Responses MAY include nested tool calls or structured outputs.
+
+## 6. Example Usage
+
+```php
+use Psr\Ai\{ClientInterface, MessageInterface};
+
+$message = new UserMessage('What is the capital of France?');
+
+$response = $client->complete($message);
+
+echo $response->message()->content(); // "Paris"
+```
+
+Streaming example:
+
+```php
+$stream = $client->stream(new UserMessage('Write a haiku about PHP.'));
+
+$stream->onToken(fn($token) => echo $token);
+$stream->onComplete(fn() => echo "\nDone.");
+```
+
+Tool registration example:
+
+```php
+class WeatherTool implements ToolInterface
+{
+    public function name(): string
+    {
+        return 'getWeather';
+    }
+
+    public function schema(): array
+    {
+        return [
+            'location' => 'string',
+        ];
+    }
+
+    public function execute(array $args): array
+    {
+        return [
+            'temp' => 18,
+            'unit' => 'C',
+        ];
+    }
+}
+```
+
+## Appendix A. ABNF Definitions
+
+
+The ABNF below defines general communication semantics:
+
+```
+message = role ":" content
+role    = "system" / "user" / "assistant" / "tool"
+content = *CHAR
+```
+
+Streaming token flow:
+
+```
+stream = 1*(token / event)
+token  = 1*CHAR
+event  = "[" event-type ":" data "]"
+```
+
+## Appendix B. References
+
+- [RFC 2119](https://tools.ietf.org/html/rfc2119). Key words for use in RFCs to indicate requirement levels. Defines "MUST", "SHOULD", "MAY", etc., for consistent interpretation of requirements.
+- [RFC 5234](https://tools.ietf.org/html/rfc5234). Augmented Backus-Naur Form (ABNF). Defines the ABNF grammar notation used in Appendix A.
+- [PSR-3: Logger Interface](https://www.php-fig.org/psr/psr-3/). Reference for a widely adopted interface standard, demonstrating framework-agnostic design.
+- [PSR-7: HTTP Message Interface](https://www.php-fig.org/psr/psr-7/). Provides streaming and message abstraction patterns referenced in StreamInterface.
+- [PSR-14: Event Dispatcher](https://www.php-fig.org/psr/psr-14/). Illustrates middleware/event pipelines for streaming token handling.
+- [PSR-18: HTTP Client Interface](https://www.php-fig.org/psr/psr-18/). Example of a provider-agnostic client abstraction.
+- [Model Context Protocol (MCP)](https://modelcontextprotocol.io/). Defines a cross-language protocol for tools, structured data, and context integration in AI workflows.
+- [OpenAI API](https://platform.openai.com/docs/api-reference/introduction). Industry-standard example of a large language model API with streaming and tool-calling features.
+- [Anthropic API.](https://docs.anthropic.com/). Provides examples of structured LLM interactions and multi-turn session concepts.
+- [Ollama](https://ollama.com/). Demonstrates locally-hosted model integrations for PHP and other ecosystems.
+- [Fluent Interface Pattern](https://en.wikipedia.org/wiki/Fluent_interface). References common design patterns used in PHP SDKs and streaming APIs.
+- [PHP Iterables](https://www.php.net/manual/en/language.types.iterable.php). Defines iterable types used in StreamInterface and message pipelines.
+- [PHP Resources](https://www.php.net/manual/en/language.types.resource.php). Guidance on representing external streams or handles in PHP.
+- [PHP Callables](https://www.php.net/manual/en/language.types.callable.php). Used in ToolInterface::execute() for callable definitions.
+- [Late Static Binding in PHP](https://www.php.net/manual/en/language.oop5.late-static-bindings.php). Covers use of static in message and response class hierarchies.
+- [DeFacto PHPDoc Specification](http://www.phpdoc.org/docs/latest/index.html). Serves as a reference for structured documentation and interface design.