FastAPI is API-first.
The first failed handshake hits the client SDK.
FastAPI agencies running async Python on Uvicorn behind Caddy or nginx deal with Caddyfile parse errors after a config change that disable automatic HTTPS across every site on the host, Fly.io managed certificate provisioning that breaks when DNS providers are switched mid-deploy, and per-WebSocket subdomain SSL for LLM streaming endpoints with independent renewal cycles. Merlonix monitors SSL and DNS so client SDK calls don't return TLS handshake errors before you know the cert expired.
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- Check cadence (Agency)
- 1 min
- SSL pre-expiry alert
- 30 days
- Independent DNS resolvers
- 3
- Vendors watched
- 11
Where FastAPI agencies get caught out
Three failure modes specific to FastAPI deployments on Caddy, Fly.io, and WebSocket-heavy LLM streaming setups.
FastAPI agencies running Uvicorn behind Caddy or nginx deal with Caddyfile parse errors after config changes that block auto-HTTPS across every site on the host, Fly.io managed certs that break when DNS is migrated mid-deploy without re-issuing the custom domain, and per-WebSocket subdomain SSL gaps when the streaming endpoint is added after the HTTP API and Certbot is not re-run.
Caddyfile typo stops auto-HTTPS
A Caddyfile parse error blocks cert renewal across every site on the host
A FastAPI agency adds a new client API to an existing Caddy host by editing the Caddyfile and running caddy reload. The reload fails with a parse error on line 47 — a missing closing brace in a snippet definition. The engineer runs caddy validate to confirm the syntax error and notes a fix is needed, but does not flag it as urgent because the existing three sites continue serving over HTTPS using the previously-loaded config. Six weeks pass. The first existing client cert reaches its 60-day Caddy renewal window. Caddy attempts to renew but the in-memory config is stale and the on-disk config is broken, so the renewal logs show ambiguous errors. The cert expires in another 30 days; Caddy continues serving the expired cert. The first client's mobile app starts returning ssl.SSLCertVerificationError on Python requests calls. The agency engineer realizes the original parse error was never fixed.
DNS switch breaks Fly.io cert renewal
Switching DNS mid-deploy breaks the Fly.io managed cert when the ACME validation record is not migrated
A FastAPI agency runs a client API on Fly.io with a custom domain api.client.com originally configured against Cloudflare DNS. The Fly.io managed certificate was issued and is renewing automatically through Fly.io's ACME integration. A compliance change requires moving DNS to Route53. The agency migrates the zone successfully and verifies the api.client.com CNAME resolves correctly through Route53. The cert continues working for the next 60 days until the Fly.io renewal cycle attempts ACME validation and the validation challenge fails — Fly.io is looking for a TXT record that was not migrated because Cloudflare's zone export did not include the validation TXT (it had been auto-cleaned by Fly.io after the original issuance). The Fly.io cert expires. The agency learns about it from the client's on-call engineer who noticed Python requests calls failing with SSL: CERTIFICATE_VERIFY_FAILED on every API call from their data pipeline.
WebSocket subdomain ships with no cert
A ws.* streaming subdomain serves a self-signed cert because Certbot was never run for it
A FastAPI agency builds an LLM streaming proxy. The product team launches the HTTP API at api.client.com on day one with a Let's Encrypt cert provisioned via Certbot --nginx during the original deploy. Day eight, the streaming WebSocket endpoint at ws.client.com is added to handle real-time partial completions. The agency engineer adds the nginx server block but the deploy script's Certbot invocation is hardcoded to api.client.com. The WebSocket service starts listening on 443 using nginx's default snakeoil certificate. The browser WebSocket client fails the TLS handshake with NET::ERR_CERT_AUTHORITY_INVALID. The agency assumes the issue is in the FastAPI WebSocket code and spends four hours debugging Uvicorn's WebSocket support before checking the nginx config and seeing the missing Certbot certificate. By the time the issue is fixed, the demo with the prospective client has ended.
How it works
SSL and DNS monitoring for FastAPI agencies across Caddy auto-HTTPS, Fly.io managed certs, and Certbot on nginx for WebSocket-heavy LLM streaming setups.
Merlonix monitors SSL expiry and DNS A record integrity across every FastAPI application subdomain — HTTP APIs, WebSocket streaming endpoints, OpenAPI docs hosts, and per-customer key subdomains — and catches renewal failures caused by Caddyfile parse errors, Fly.io cert breakage from DNS migrations, or unregistered WebSocket subdomains before client SDK calls return TLS handshake errors.
01
Add FastAPI application domains — apex, api.*, ws.*, stream.*, docs.*, and per-customer keys.* subdomains — with DNS TXT record verification
Verify ownership with a DNS TXT record on the apex domain. All subdomains under that apex — api.*, ws.*, stream.*, docs.*, and any per-customer subdomain — are added without additional verification. Monitoring every FastAPI subdomain from a single apex registration ensures that WebSocket and streaming subdomains added during product expansion are covered alongside the primary HTTP API. Under two minutes per client.
02
A record and CNAME integrity monitoring across Caddy VPS IPs, Fly.io app endpoints, and Cloud Run / App Runner managed domains
Three independent DNS resolvers check every A record and CNAME delegation on every monitoring interval. When a FastAPI deployment moves from Cloudflare DNS to Route53 mid-cert-cycle, the new resolution path is validated across all monitored subdomains. When a Fly.io app is recreated and the .fly.dev hostname changes, the CNAME mismatch surfaces immediately. DNS change monitoring provides the audit trail confirming a DNS provider migration completed correctly across every FastAPI application subdomain.
03
SSL monitoring 30 days before expiry across Caddy auto-HTTPS certs, Certbot-issued nginx certs, Fly.io managed certs, and Cloud Run domain mappings
Full SSL chain validation on every FastAPI application subdomain — HTTP APIs, WebSocket endpoints, streaming subdomains, and OpenAPI docs hosts. An expiry alert fires 30 days before the certificate expires — enough lead time to identify whether the failure is a Caddyfile parse error blocking auto-cert renewal, a Fly.io cert broken by a DNS provider migration, or a WebSocket subdomain missing from Certbot registration. Catches Caddy and Certbot renewal failures before client SDK code starts returning TLS handshake errors.
04
Vendor status for FastAPI hosting providers to distinguish infrastructure incidents from application SSL and DNS configuration failures
Merlonix monitors AWS, Google Cloud, Fly.io, Hetzner, and DigitalOcean status pages alongside client SSL and DNS. When a Fly.io regional incident causes API failures across multiple FastAPI client deployments simultaneously, you see the vendor event — not a cluster of individual SSL alerts that each require separate investigation to determine whether the root cause is a Fly.io regional outage or a Caddyfile parse error blocking auto-renewal.
What the numbers mean for FastAPI agencies
Monitoring built for FastAPI agencies where one client product means an HTTP API, a WebSocket streaming endpoint, an OpenAPI docs host, and per-customer key subdomains — each a separate SSL surface.
FastAPI agencies managing Caddy auto-HTTPS across multi-site VPS deployments, Fly.io managed certs across multi-app rosters, and Certbot on nginx for WebSocket-heavy LLM streaming endpoints need monitoring that covers every configured subdomain — because a Caddyfile parse error after a config change is silent until the next 60-day cert renewal cycle and the production Python client starts returning ssl.SSLCertVerificationError.
< 10 min
Time from DNS change to alert — catches FastAPI VPS IP changes during agency consolidation, Fly.io app endpoint rotations, and DNS provider migrations from Cloudflare to Route53 before clients see SSL errors
30 days
SSL expiry warning lead time — enough time to identify a Caddyfile parse error blocking auto-renewal, a Fly.io cert broken by a DNS provider migration, or a WebSocket subdomain missing from Certbot registration, and correct it before the certificate expires
11 vendors
Upstream services monitored — AWS, Google Cloud, Fly.io, Hetzner, and DigitalOcean included to distinguish provider outages from FastAPI application SSL and DNS failures requiring Caddy or Certbot configuration changes
250 assets
Maximum monitored domains on the Agency plan — covers HTTP APIs, WebSocket streaming endpoints, OpenAPI docs hosts, and per-customer subdomains across a full FastAPI client portfolio
Pricing
Flat monthly fee. Every FastAPI subdomain and WebSocket endpoint included.
No per-subdomain charges. No per-WebSocket fees. Pick the tier that fits your FastAPI client and endpoint count and monitor every HTTP API, streaming endpoint, and docs host without billing surprises.
Starter
For individual FastAPI developers managing a small client portfolio on Caddy or Fly.io.
$19/ month
- 15 monitored assets
- 3 seats
- 5 min check cadence
- SSL + DNS + vendor monitoring
- Email + Slack alerts
Team
For FastAPI agencies managing multi-subdomain deployments with WebSocket streaming.
$79/ month
- 60 monitored assets
- 10 seats
- 1 min check cadence
- SSL + DNS + vendor monitoring
- Email + Slack alerts
Agency
For agencies with a full FastAPI client roster including LLM streaming and per-customer subdomains.
$199/ month
- 250 monitored assets
- Unlimited seats
- 1 min check cadence
- SSL + DNS + vendor monitoring
- Email + Slack alerts
Compliance
For regulated-vertical teams that need continuous, audit-ready evidence.
$699/ month
- 500 monitored assets
- Unlimited seats
- 1 min check cadence
- SSL + DNS + vendor monitoring
- Email + Slack alerts
Know when a FastAPI WebSocket subdomain or Caddy auto-HTTPS cert is about to expire.
Add your first FastAPI client domain in under two minutes. HTTP APIs, WebSocket streaming endpoints, and OpenAPI docs hosts are monitored from the same dashboard. 14-day trial, no card required.