env.md

title	Environment variables
description	Every ASTER_* variable, what it does, and where it sits in the precedence order.

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_REASONING_EFFORT	Alias for ASTER_EFFORT
ASTER_MAX_TOKENS	Completion cap; off lifts the cap
ASTER_SEED	Sampling seed; off disables fixing it
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 for models that cannot see them
ASTER_PRICE_PROMPT_PER_M, ASTER_PRICE_COMPLETION_PER_M	Manual pricing for cost display
ASTER_PROMPT_CACHE	off disables explicit prompt-cache breakpoints
ASTER_CATALOG_URL	Override the model catalog URL

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_LANGUAGE	Language every reply is written in; unset follows the user
ASTER_GOAL_MAX_TURNS	Cap for /goal loops (default 20)
ASTER_ROUTER_TIER	cheap, balanced, or strong for model: auto
ASTER_AGENT_MAX_CONCURRENT	Sub-agents running at the same time
ASTER_AGENT_MAX_PER_TURN	Sub-agent fan-out allowed per turn
ASTER_AGENT_TIMEOUT	Per-sub-agent timeout in seconds
ASTER_COLLECTOR_MODEL	Model that merges sub-agent reports
ASTER_LEARN	0 turns off skill learning

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
ASTER_WEBMCP_CDP_URL	Chrome remote debugging URL for the webmcp server

Review

Variable	Does
ASTER_VERIFY_MODEL	Override review.verify_model
ASTER_HYPOTHESIS_MODEL	Override review.hypothesis_model
ASTER_VERIFY_CONCURRENCY	Parallel verifier count
ASTER_ANALYZERS	Analyzers for review, comma separated: semgrep, ast-grep; empty means the model alone
ASTER_ASTGREP_RULES	Extra ast-grep rules path
ASTER_REPO	Repo name shown in review reports when none is detected

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.