Environment variables - Aster

Environment variables sit between CLI flags and aster.yaml: CLI flags > environment > aster.yaml > defaults. Keys are usually set in .env files rather than the shell; both work the same once loaded.

Model and provider

Variable	Does
`ASTER_API_KEY`	Shared fallback key; a var named for the endpoint beats it
`ASTER_BASE_URL`	Override the endpoint; outranks aster.yaml
`ASTER_MODEL`	Override the model; outranks aster.yaml
`ASTER_EFFORT`	Reasoning budget: off, low, medium, high, xhigh, max, ultra
`ASTER_MAX_TOKENS`	Completion cap; `off` lifts the cap
`ASTER_SEED`	Sampling seed
`ASTER_TIMEOUT_SECS`	Per-request timeout
`ASTER_MAX_RETRIES`	Retry count for failed requests
`ASTER_DEADLINE_SECS`	Whole-turn deadline
`ASTER_VISION_MODEL`	Model used to describe images
`ASTER_PRICE_PROMPT_PER_M`, `ASTER_PRICE_COMPLETION_PER_M`	Manual pricing for cost display

Agent behavior

Variable	Does
`ASTER_MAX_TOOL_ROUNDS`	Cap on tool rounds per turn (default 60)
`ASTER_COMMAND_TIMEOUT`	Per-command timeout in seconds (default 300)
`ASTER_COMPACT_BUDGET`	Context budget that triggers compaction (default 192000 chars)
`ASTER_GOAL_MAX_TURNS`	Cap for `/goal` loops (default 20)
`ASTER_ROUTER_TIER`	`cheap`, `balanced`, or `strong` for `model: auto`

Web and browser

Variable	Does
`ASTER_WEB_SEARCH`	`1` gives the agent web search by default
`FIRECRAWL_API_KEY`	Enables `web crawl`
`CONTEXT_DEV_API_KEY`	Enables `crawl`, `sitemap`, `screenshot`
`CLOUDFLARE_API_ID`, `CLOUDFLARE_API_TOKEN`	Alternative keys for `crawl`
`ASTER_NO_BROWSER`	`1`: `open_preview` prints the URL instead of opening it

Review

Variable	Does
`ASTER_VERIFY_MODEL`	Override `review.verify_model`
`ASTER_HYPOTHESIS_MODEL`	Override `review.hypothesis_model`
`ASTER_VERIFY_CONCURRENCY`	Parallel verifier count
`ASTER_ASTGREP_RULES`	Extra ast-grep rules path

Runtime and misc

Variable	Does
`ASTER_TELEGRAM_TOKEN`, `ASTER_REMOTE_USERS`	Remote control
`ASTER_EDITOR`	Editor opened by config/edit commands
`ASTER_UI_DIR`	Serve a custom UI directory
`ASTER_MCP_EXTRA`	Extra MCP servers config
`ASTER_NO_UPDATE_CHECK`	`1` skips the release check
`OTEL_EXPORTER_OTLP_ENDPOINT`	OpenTelemetry export

.env.example in the main repository lists them with notes. aster key list --all shows which are set and where they came from.