Fall back to Claude Code subscription when ANTHROPIC_API_KEY is unset

[aarshvir]

Summary

If a user has no ANTHROPIC_API_KEY but is already logged into the Claude Code CLI locally, this PR routes Anthropic model calls through claude-agent-sdk so the run uses their Claude Code subscription instead of erroring out with ValueError: Anthropic API key not found.

Today the only way to use a Claude model is to pay for the Anthropic API on top of any existing Claude Code / Pro / Max subscription. After this change, a Claude Code login is enough.

What changed

• New src/llm/claude_code.py — ChatClaudeCode, a minimal LangChain BaseChatModel that wraps claude_agent_sdk.query(). Concatenates messages, pulls SystemMessage into the SDK's system_prompt, runs the async call via asyncio.run (or a thread if a loop is already running), and returns the assistant text as an AIMessage. Prints a one-time stderr notice on first use.
• src/llm/models.py — the Anthropic branch of get_model() no longer raises on missing key. It lazy-imports ChatClaudeCode and returns it; if claude-agent-sdk isn't installed, the error message now tells the user to either set the key or install the SDK.
• src/llm/models.py — LLMModel.has_json_mode() returns False for Anthropic when ANTHROPIC_API_KEY is unset, so call_llm skips with_structured_output and falls into the existing markdown/brace JSON-extraction path that already serves Gemini and DeepSeek. No changes to call_llm itself.
• pyproject.toml — adds claude-agent-sdk = "^0.1.0".
• .env.example + README.md — short notes documenting the fallback.

Design notes

• Scoped to Anthropic. Other provider branches still raise on missing keys — we don't claim a Claude Code login can substitute for OpenAI, Groq, etc.
• Lazy import. Users with ANTHROPIC_API_KEY set never import the SDK, so it's effectively a soft dependency.
• No changes to call_llm. Reuses the existing text-parsing fallback path.
• CLI scope. The check in has_json_mode() reads os.environ rather than the web-app's per-request api_keys dict. If a web caller passes the key only via api_keys (not env), structured output is skipped and JSON extraction kicks in — still produces correct output, just slightly less efficient. Happy to thread api_keys into the check if you'd prefer.

Test plan

• poetry lock && poetry install cleanly resolves claude-agent-sdk
• With key (regression): ANTHROPIC_API_KEY set → poetry run python src/main.py --ticker AAPL picking Claude Opus 4.7 runs via ChatAnthropic as before
• Without key (the new behavior): ANTHROPIC_API_KEY unset, claude CLI logged in → same command prints the one-time notice and produces normal trading signals
• SDK not installed: pip uninstall claude-agent-sdk and unset the key → fails fast with the updated error message
• Non-Anthropic untouched: OpenAI/Groq paths unchanged with or without keys
• Backtester: poetry run python src/backtester.py --ticker AAPL under the fallback path survives the LangGraph loop

🤖 Generated with Claude Code