Tail or fetch runtime logs for a Fly.io app programmatically
domain: fly.io · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
CLI tail: run `fly logs --app myapp` to stream real-time logs (add `--no-tail` to fetch recent logs and exit)
CLI filtering: add `--region <region>` to filter by region or `--machine <machine-id>` to filter by a specific machine
CLI JSON output: add `--json` for newline-delimited JSON log objects instead of text
HTTP API: GET https://api.fly.io/api/v1/apps/{app_name}/logs with Authorization: Bearer <personal access token>
HTTP API historical logs: add query params such as ?start_time=<RFC3339> and optionally region or instance filters; parse newline-delimited JSON
NATS real-time access (experimental): subscribe to subject pattern logs.{app}.{region}.{instance} via the internal NATS endpoint nats://[fdaa::3]:4223, authenticating with org slug + personal access token
Docs: https://fly.io/docs/flyctl/logs/ and https://fly.io/docs/monitoring/logs-api-options/
Known gotchas
NATS log streaming is experimental, only delivers logs from the moment of subscription (no history), and requires handling reconnects/backpressure
HTTP API instance/region filters are documented as sometimes flaky; verify results in automation
Historical log retention via the HTTP API is limited (~15 days); older logs are permanently inaccessible
Machine IDs are transient across redeploys/restarts; scripts filtering by machine must refresh IDs
NATS subjects support wildcards: logs.myapp.lhr.* for all instances in a region, logs.myapp.*.* for everything
`fly logs` streams by default; use --no-tail in CI to avoid a command that never exits
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