Add ANTHROPIC_BASE_URL support for Claude account subscription

- AnthropicClient now uses base_url param or ANTHROPIC_BASE_URL env var
  to connect to custom endpoints (subscription proxies, self-hosted, etc.)
- Add ANTHROPIC_BASE_URL to .env.example
- Document custom base URL usage in README

https://claude.ai/code/session_01AbNbJYL7gHy47BQ9BJJTAv

.env.example

@@ -1,5 +1,6 @@
 # LLM Providers (set the one you use)
 ANTHROPIC_API_KEY=
+ANTHROPIC_BASE_URL=
 OPENAI_API_KEY=
 GOOGLE_API_KEY=
 XAI_API_KEY=

README.md

@@ -130,6 +130,11 @@ export OPENROUTER_API_KEY=...      # OpenRouter
 export ALPHA_VANTAGE_API_KEY=...   # Alpha Vantage
 ```
 
+To use a Claude account subscription (or proxy) instead of a direct API key, set a custom base URL:
+```bash
+export ANTHROPIC_BASE_URL=https://your-custom-endpoint.example.com
+```
+
 For local models, configure Ollama with `llm_provider: "ollama"` in your config.
 
 Alternatively, copy `.env.example` to `.env` and fill in your keys:

tradingagents/llm_clients/anthropic_client.py

@@ -1,3 +1,4 @@
+import os
 from typing import Any, Optional
 
 from langchain_anthropic import ChatAnthropic
@@ -24,7 +25,12 @@ class NormalizedChatAnthropic(ChatAnthropic):
 
 
 class AnthropicClient(BaseLLMClient):
-    """Client for Anthropic Claude models."""
+    """Client for Anthropic Claude models.
+
+    Supports custom base URLs via the base_url parameter or ANTHROPIC_BASE_URL
+    environment variable, enabling use with Claude account subscriptions
+    or proxy endpoints instead of direct API key access.
+    """
 
     def __init__(self, model: str, base_url: Optional[str] = None, **kwargs):
         super().__init__(model, base_url, **kwargs)
@@ -33,6 +39,11 @@ class AnthropicClient(BaseLLMClient):
         """Return configured ChatAnthropic instance."""
         llm_kwargs = {"model": self.model}
 
+        # Resolve base URL: explicit param > env var > default Anthropic API
+        base_url = self.base_url or os.environ.get("ANTHROPIC_BASE_URL")
+        if base_url:
+            llm_kwargs["anthropic_api_url"] = base_url
+
         for key in _PASSTHROUGH_KWARGS:
             if key in self.kwargs:
                 llm_kwargs[key] = self.kwargs[key]