feat: auto-detect provider from common environment variables

[dwmkerr]

Summary

Should terminal-ai auto-detect the provider when common environment variables like OPENAI_API_KEY or ANTHROPIC_API_KEY are set, without requiring explicit configuration?

Research: How Other AI CLI Tools Handle This

Tool	Env Vars	Default Model	Auto-Detect?
llm (Simon Willison)	Custom key storage	gpt-4o-mini	No - plugin-based
shell-gpt	OPENAI_API_KEY	gpt-4o	OpenAI-first
mods (Charm)	Standard vars	gpt-4o	Config-first, then env
aichat	Standard vars	openai:gpt-4o	Yes - prefix-based

Proposed Implementation

Environment variables to auto-detect:

Variable	Provider	Default Model
OPENAI_API_KEY	OpenAI	gpt-4o
ANTHROPIC_API_KEY	Anthropic	claude-sonnet-4-20250514

Priority order:

1. Explicit config (~/.ai/config.yaml)
2. AI_* environment variables
3. Common provider env vars (OPENAI_API_KEY, etc.)

Task List

• Load .env files from current directory
• Auto-detect OPENAI_API_KEY and use OpenAI with gpt-4o default
• Auto-detect ANTHROPIC_API_KEY and use Anthropic with claude-sonnet-4-20250514 default
• Add --no-dotenv flag to disable .env loading
• Add --no-env-detect flag to disable auto-detection of common env vars
• Add AI_NO_DOTENV=1 env var to disable .env loading
• Add AI_NO_ENV_DETECT=1 env var to disable auto-detection
• Update docs with new behavior
• Add tests for env var detection

Open Questions

• Should auto-detection warn users when using defaults?
• What happens when multiple provider keys are set? (Propose: priority order OpenAI > Anthropic)

References

• aichat: https://github.com/sigoden/aichat
• mods: https://github.com/charmbracelet/mods
• shell-gpt: https://github.com/TheR1D/shell_gpt
• llm: https://github.com/simonw/llm