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fix(web): move the app origin off 6276, bound the publish route

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Copilot review round 2 on #2100, plus a collision the review did not find.

The default app-origin port was 6276 — which is already
RUNNER_OAUTH_CALLBACK_DEFAULT_PORT, the CLI/TUI loopback OAuth callback.
That port is fixed precisely so an OAuth app can pre-register
http://127.0.0.1:6276/oauth/callback, which makes it the one listener here
that cannot move when something takes it first. A running --web would hold
it and break a later --cli/--tui OAuth flow at its registered redirect URI,
and docs/mcp-app-review.md has those two running side by side. This
listener has a documented dynamic fallback, so this listener yields: the
default is now 6278 (6277 skipped — v1's retired proxy port, which the
migration guide tells people to stop forwarding). A test asserts the two
defaults stay distinct.

Review comments:

- POST /api/app-document now applies hono's bodyLimit BEFORE parsing. The
  8MiB check ran after c.req.json() had buffered and parsed the whole
  request, and bounded only `html` — so bulk in `csp`, or in keys the route
  never reads, was fully materialized before the 413.
- `csp` is validated as an HTTP header value and length-bounded. The
  controller hands it to res.writeHead() verbatim, where Node throws
  ERR_INVALID_CHAR synchronously — inside the handler for a later,
  unrelated request, outside this route's error handling.
- The app-origin port now propagates everywhere the other two do: the CLI
  --print-handoff port-forward command (and its test), the Dockerfile's ENV
  and EXPOSE, and the remote-review SSH recipe in docs/mcp-app-review.md.
  Without it an app declaring _meta.ui.domain is unreachable through every
  documented remote workflow.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>
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cliffhall 2026-08-24 00:07:15 -04:00 • committed by GitHub
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9
clients/web/README.md
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@ -155,8 +155,11 @@ Inspector owns no domain infrastructure — it cannot serve
not parsed, matched, or reserved — declare whatever your production host
expects.
- The Inspector answers with a real HTTP origin on loopback:
`http://<host>:<MCP_APP_ORIGIN_PORT>`, default **`6276`** (beside the web
port `6274` and the sandbox port `6275`, so the three forward together).
`http://<host>:<MCP_APP_ORIGIN_PORT>`, default **`6278`** — in the same
`627x` family as the web port `6274` and the sandbox port `6275`, so the
three forward together. It is **not** `6276`: that is the fixed loopback
OAuth callback the CLI and TUI listen on, which OAuth apps pre-register and
which therefore cannot move. `6277` is skipped as v1's retired proxy port.
- The app document is served from there under an unguessable path, its
per-app CSP delivered as a real response **header** (stronger than the
`<meta>` the `srcdoc` path relies on) plus a `frame-ancestors` that admits
@ -186,7 +189,7 @@ default (opaque-origin) way and logs a console warning naming `_meta.ui.domain`.
Losing the real origin degrades what the app can reach; losing the app itself
would be worse.
**Forward `6276` too** if you need this off loopback (a container, a tunnel).
**Forward `6278` too** if you need this off loopback (a container, a tunnel).
As with the sandbox port, a taken port falls back to an OS-assigned one with a
loud warning — the app still renders, but the origin an app's backend was told
to allowlist is then wrong, which is what the warning tells you.