add nthropic Messages Pass-Through Architecture

litellm/llms/anthropic/experimental_pass_through/architecture.md

@@ -0,0 +1,51 @@
+# Anthropic Messages Pass-Through Architecture
+
+## Request Flow
+
+```mermaid
+flowchart TD
+    A[litellm.anthropic.messages.acreate] --> B{Provider?}
+    
+    B -->|anthropic| C[AnthropicMessagesConfig]
+    B -->|azure_ai| D[AzureAnthropicMessagesConfig]
+    B -->|bedrock invoke| E[BedrockAnthropicMessagesConfig]
+    B -->|vertex_ai| F[VertexAnthropicMessagesConfig]
+    B -->|Other providers| G[LiteLLMAnthropicMessagesAdapter]
+    
+    C --> H[Direct Anthropic API]
+    D --> I[Azure AI Foundry API]
+    E --> J[Bedrock Invoke API]
+    F --> K[Vertex AI API]
+    
+    G --> L[translate_anthropic_to_openai]
+    L --> M[litellm.completion]
+    M --> N[Provider API]
+    N --> O[translate_openai_response_to_anthropic]
+    O --> P[Anthropic Response Format]
+    
+    H --> P
+    I --> P
+    J --> P
+    K --> P
+```
+
+## Adapter Flow (Non-Native Providers)
+
+```mermaid
+sequenceDiagram
+    participant User
+    participant Handler as anthropic_messages_handler
+    participant Adapter as LiteLLMAnthropicMessagesAdapter
+    participant LiteLLM as litellm.completion
+    participant Provider as Provider API
+
+    User->>Handler: Anthropic Messages Request
+    Handler->>Adapter: translate_anthropic_to_openai()
+    Note over Adapter: messages, tools, thinking,<br/>output_format → response_format
+    Adapter->>LiteLLM: OpenAI Format Request
+    LiteLLM->>Provider: Provider-specific Request
+    Provider->>LiteLLM: Provider Response
+    LiteLLM->>Adapter: OpenAI Format Response
+    Adapter->>Handler: translate_openai_response_to_anthropic()
+    Handler->>User: Anthropic Messages Response
+```