## Thinking Path
> - Paperclip is the open source app people use to manage AI agents for
work
> - The server derives each spawned agent's `PAPERCLIP_API_URL` from
`authPublicBaseUrl` via `choosePrimaryRuntimeApiUrl` →
`buildPaperclipEnv`
> - At startup, `rewriteLocalUrlPort` rewrote the port of the configured
`auth.publicBaseUrl` to the internal listen port
> - The rewrite was applied to *any* explicit-port URL, not just
loopback ones — so an external base URL on a non-default port (e.g. a
Tailscale Serve listener on `:8443`) got clobbered to the internal HTTP
port `:3101`
> - `https://host:3101` (HTTPS scheme against the plaintext HTTP port)
is unreachable, and that dead value propagated to every spawned agent's
`PAPERCLIP_API_URL`
> - This pull request preserves explicit external base URLs at startup
while keeping the worktree path's intended per-worktree port rewrite
> - The benefit is that agents following the documented `curl
"$PAPERCLIP_API_URL/..."` pattern no longer hit a dead endpoint
## Linked Issues or Issue Description
No public GitHub issue; describing inline (bug report).
**Summary:** at server startup, `rewriteLocalUrlPort` corrupts an
explicit external `auth.publicBaseUrl`, leaking a dead
`PAPERCLIP_API_URL` to spawned agents.
**Steps to reproduce:**
1. Configure `auth.publicBaseUrl = https://<host>:8443` (an external
listener on a non-default port, e.g. Tailscale Serve).
2. Start the server (internal listen port `3101`).
3. Inspect a spawned agent run's env:
`PAPERCLIP_API_URL=https://<host>:3101`.
**Expected:** the agent-facing URL points at a reachable origin.
**Actual:** `curl "$PAPERCLIP_API_URL/..."` → `http_code=000` (HTTPS
against the plaintext HTTP port; TLS handshake fails). The fleet stays
healthy only because the runtime falls through its candidate list, but
any agent following the documented curl pattern silently hits a dead
endpoint first.
Related open PRs in the same area (dedup — none merged; this is a
smaller, targeted fix with regression tests):
- Refs #9916 (PAPERCLIP_RUNTIME_API_URL precedence + authPublicBaseUrl
port preservation)
- Refs #7342 (preserve explicit authPublicBaseUrl during startup,
GH#7341)
- Refs #9228 (prefer reachable runtime API URLs for local adapters)
## What Changed
- New `server/src/url-utils.ts` with two intent-revealing helpers
(single source of truth):
- `rewriteUrlPort` — rewrite any explicit-port URL to a new port.
- `rewriteLoopbackUrlPort` — rewrite **only** loopback hosts; explicit
external URLs survive untouched.
- `isLoopbackHost` — bracket-tolerant so a URL hostname form `[::1]`
matches.
- `server/src/index.ts` (startup, the bug): `authPublicBaseUrl` now uses
`rewriteLoopbackUrlPort`, so an external Serve URL keeps its port.
Nested helper copies removed in favor of the shared module.
- `server/src/worktree-config.ts` (worktree path): uses `rewriteUrlPort`
— **behavior unchanged**; a worktree still advertises its own server
port even on a non-loopback host (this is intended and asserted by the
existing worktree suite).
- `server/src/url-utils.test.ts`: regression coverage for both helpers.
- Updated one stale assertion in
`server-startup-feedback-export.test.ts` that had encoded the old
(buggy) external-host rewrite at startup.
## Verification
- `vitest run src/url-utils.test.ts
src/__tests__/worktree-config.test.ts
src/__tests__/server-startup-feedback-export.test.ts` → **33 passed**;
the only local failure is a pre-existing, environment-coupled test
(`derives trusted origins…`) that leaks the dev machine's real Tailscale
identity into an origins list and passes in CI (it is unrelated to this
change — its `authPublicBaseUrl` is loopback and rewrites identically
before/after).
- `npm run typecheck` (`tsc --noEmit`) → **clean, exit 0**.
- PR CI: Build, Typecheck + Release Registry, serialized server suites,
and `review` gate green.
## Risks
Low risk. The only behavioral change is at startup: an explicit
*external* base URL on a non-default port is no longer rewritten to the
internal listen port (the bug). Loopback/worktree behavior is unchanged.
No schema/migration changes.
## Model Used
Claude Opus 4.8, 1M context (`claude-opus-4-8[1m]`), extended thinking,
with tool use / code execution (Claude Code).
## Checklist
- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [ ] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge
---------
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Eric Brookfield