## Summary
Claude.ai's custom remote MCP connector fails with "Couldn't reach the
MCP server" after successfully completing OAuth dynamic client
registration. Driving the flow through Chrome DevTools showed Claude's
backend creates our DCR client (many hundreds of orphan rows visible in
the admin panel), then never returns the user to `/authorize` — it gives
up silently.
**Empirical comparison against known-working MCP servers Claude.ai
connects to identified one concrete difference**: every server that
works returns `registration_client_uri` in the DCR response. We didn't.
| Server | DCR `registration_client_uri` | Claude.ai web connector |
|---|---|---|
| Linear (`mcp.linear.app`) | `/register/<client_id>` | ✅ works |
| Sentry (`mcp.sentry.dev`) | `/oauth/register/<client_id>` | ✅ works |
| Atlassian (`mcp.atlassian.com`) | yes | ✅ works |
| **Twenty** (before this PR) | **missing** | ❌ "Couldn't reach" |
## What this PR changes
### 1. Add `registration_client_uri` to the DCR response
```
{
"client_id": "…",
…existing fields…,
+ "registration_client_uri": "<issuer>/oauth/register/<client_id>"
}
```
Pointer at the registration's management endpoint per RFC 7591 §3.2.1.
Marked OPTIONAL in the spec but empirically required by Claude.ai.
### 2. New `GET /oauth/register/:clientId` endpoint (RFC 7592 read-back)
Returns public registration metadata (`client_name`, `redirect_uris`,
`grant_types`, `scope`, etc.). 404 for unknown clients.
No `registration_access_token` is issued (and none required to hit this
endpoint): the `client_id` is an unguessable UUID and the fields
returned are already public-readable via
`findApplicationRegistrationByClientId` GraphQL. This matches Linear's
behaviour — they return a `registration_client_uri` but issue no access
token.
### 3. Advertise `response_modes_supported: ["query"]` in AS metadata
RFC 8414 default, but explicitly listed by Linear / Sentry / Atlassian
and absent from ours. Some clients treat its absence as a capability
gap.
## Why I'm confident this is the root cause
- The failure mode exactly matches an orphaned-DCR retry loop (hundreds
of registrations, none `installed` on a workspace).
- #19847 reporter confirmed Claude Desktop + VS Code work — those
clients use the MCP Python SDK which doesn't require
`registration_client_uri`. **Claude.ai web** uses Anthropic's
proprietary backend client (`User-Agent: Claude-User`), which
empirically does.
- All 3 working reference servers return the field; we were the odd one
out.
## Test plan
- [x] `tsc --noEmit` clean on touched files
- [x] `yarn jest
--testPathPatterns="oauth-discovery.controller|mcp-auth.guard"` → 4/4
pass
- [ ] After deploy:
```bash
curl -s -X POST https://<host>/oauth/register -H 'Content-Type:
application/json' \
-d
'{"client_name":"probe","redirect_uris":["https://claude.ai/api/mcp/auth_callback"],"token_endpoint_auth_method":"none"}'
\
| jq .registration_client_uri
# expect: "https://<host>/oauth/register/<uuid>"
```
- [ ] After deploy: add the MCP connector in Claude.ai — user should now
reach the Twenty `/authorize` page
## Honesty
This is the nth fix in a long debugging chain. Unlike the earlier round
of fixes (which were real spec-compliance bugs but not Claude's
blocker), this one is backed by empirical evidence across 3
known-working implementations. If Claude.ai still fails after this
deploys, the remaining delta is `cli_client_id` in AS metadata
(non-standard field, could confuse strict parsers) or a field we
advertise that others don't (e.g. `client_credentials` grant) — both
small, removable, not disruptive.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Félix Malfait
42f57db005