Errors & limits

Errors are JSON: {"error": "…"}, sometimes with extra fields. Launch responses:

Status	`error`	Meaning	What to do
`400`	`invalid_json`	The body isn't a JSON object.	Fix the request.
`400`	`proxy_required`	The launch has no `proxy`. Every session requires one.	Add your proxy (formats).
`400`		The browser rejected a launch option — an unparseable `proxy`, an unknown Camoufox `config` property or an unsupported `os`. The message says which.	Fix the option; don't retry as is.
`401`	`missing_api_key / invalid_api_key`	No key, or an unknown or revoked key.	Check `x-api-key`.
`402`	`subscription_inactive`	Your account has no active plan.	Check your plan in the dashboard.
`403`	`engine_not_allowed`	Your plan doesn't include this browser.	Use another browser or upgrade.
`404`	`not_found`	Unknown endpoint, e.g. a misspelled browser.	Check the path.
`429`	`rate_limited`	Too many launches per second or minute for your plan. Includes `rps_limit` and `rpm_limit`.	Wait for `Retry-After` seconds.
`429`	`threads_exceeded`	This launch would exceed your threads. Includes `threads`, `in_use` and `requested_weight`.	Close a session or wait; `Retry-After: 1`.
`429`	`daily_quota_exhausted`	Shared threads: today's pool is used up. Includes the budget and usage.	`Retry-After` counts down to midnight UTC.
`429`	`hourly_quota_exhausted`	Shared threads: the last 60 minutes used 25% of your daily pool. Includes the budget and usage.	Wait; the window rolls forward continuously. `Retry-After: 60`.
`500`		The browser failed to start, e.g. the proxy is unreachable or Camoufox couldn't look up the proxy's location.	Check the proxy, then retry.
`503`	`shared_capacity_full`	Shared threads: the shared fleet is full right now. Dedicated threads are never refused for this.	Retry after `Retry-After` (5 s).
`503`	`fleet_unavailable / starting_up / route_unavailable`	The fleet is momentarily full.	Retry with backoff.
`503`	`version_unavailable`	No server has the requested `version` right now. Includes `available`, the majors you can pick.	Pick one from `available`.

Connecting

A WebSocket connect can fail with 401/403 (wrong key — only the key that launched the session may connect), 404 (the session has ended, or the 60-second connect window passed), 409 (the session already has a connection) or 502 {"error":"upstream_unreachable"} (the server running the browser didn't answer — rare). After a 502, launch a new session rather than reconnecting to the same wsUrl; the failed session isn't billed.

Limits

Threads — concurrent sessions, weighted by type (see Sessions & billing).
Launch rate — per second and per minute, set by your plan. Only launches that reach the fleet count; refused launches don't.
Daily pool — shared threads only: threads × browser-minutes per day, resetting at midnight UTC.
Rolling hour — shared threads only: 25% of the daily pool in any 60 minutes.
Session length — 24 h at most; overall_timeout is capped at it.
Connect window — 60 seconds from launch.