## Thinking Path
> - Paperclip orchestrates AI agents for zero-human companies
> - Humans interact with the system through a web UI that authenticates
a session and then issues mutations against the board
> - A CSRF-style guard (`boardMutationGuard`) protects those mutations
by requiring the request origin match a trusted set built from the
`Host` / `X-Forwarded-Host` header
> - Behind certain reverse proxies, neither header matches the public
URL — TLS terminates at the edge and the inbound `Host` carries an
internal service name (cluster-local hostname, IP, or an Ingress backend
reference)
> - Mutations from legitimate browser sessions then fail with `403 Board
mutation requires trusted browser origin`
> - `PAPERCLIP_PUBLIC_URL` is already the canonical "what operators told
us the public URL is" value — it's used by better-auth and `config.ts`
> - This pull request adds it to the trusted-origin set when set, so
browsers reaching the legit public URL aren't blocked
## What Changed
- `server/src/middleware/board-mutation-guard.ts` — parse
`PAPERCLIP_PUBLIC_URL` and add its origin to the trusted set in
`trustedOriginsForRequest`. Additive only.
## Verification
- `PAPERCLIP_PUBLIC_URL=https://example.com pnpm start` then issue a
mutation from a browser pointed at `https://example.com`: 200, as
before. From an unrecognized origin: 403, as before.
- Without `PAPERCLIP_PUBLIC_URL` set: behavior is unchanged.
## Risks
Low. Additive only. The default dev origins and the
`Host`/`X-Forwarded-Host`-derived origins continue to be trusted; this
just adds the operator-configured public URL on top.
## Model Used
Claude Opus 4.6 (1M context), extended thinking mode.
## Checklist
- [x] Thinking path traces from project context to this change
- [x] Model used specified
- [x] Tests run locally and pass
- [x] CI green
- [x] Greptile review addressed
Jannes Stubbemann
f460f744ef