[BUG] MCP OAuth: DCR client_name "Claude Code (<server>)" is rejected by servers that validate RFC 7591 metadata strictly (Calendly MCP → 400 invalid_client_metadata); recurrence of #59445 / #73293, still present in 2.1.271

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary
Adding a remote HTTP MCP server that requires Dynamic Client Registration fails at the registration step because Claude Code sends client_name as Claude Code (<serverName>). Servers that restrict client_name to alphanumerics, hyphens and spaces reject the parentheses. Calendly's MCP server is one such server, and it is DCR-only (its docs state that console-issued client credentials are not supported), so there is no supported path to connect it from Claude Code today.

This is the same defect as #59445 (May) and #73293 (July, "recurrence"). Both were closed by the inactivity bot without a maintainer response, the latter on 2026-09-04. It is still reproducible on 2.1.271. #74768 shows the same shape from Claude Desktop ("Claude Desktop (...)" rejected by Figma's registration endpoint), so it is not Calendly-specific.

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Environment
Windows 11 Pro 26200; Claude Code CLI 2.1.271 (standalone, outside the Desktop app); server added with --scope user.

What Should Happen?

Expected
Registration succeeds. Per the earlier reports, the identical request with "client_name":"Claude Code calendly" (no parentheses) registers successfully, so the value is the only blocker.

Error Messages/Logs

HTTP 400 with body: `{"error":"invalid_client_metadata","errors":{"name":["may only contain alphanumeric characters, hyphens, and spaces."]}}`

Steps to Reproduce

Steps to reproduce

1. claude mcp add --transport http --scope user calendly https://mcp.calendly.com
2. claude mcp login calendly (or /mcp → authenticate)
3. Registration request to Calendly's DCR endpoint is rejected before the browser opens.

Claude Model

Not sure / Multiple models

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.271

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Suggested fix (any one of these would unblock)

1. Change the default client_name to a value that satisfies the common strict validators, e.g. Claude Code - <serverName> or Claude Code <serverName>. RFC 7591 leaves the charset to the server, and "alphanumerics, hyphen, space" is a common conservative rule.
2. On 400 invalid_client_metadata, retry once with a sanitized client_name (strip everything outside [A-Za-z0-9 -]).
3. Add a --client-name option (or clientName in the server entry) so users can set it explicitly.

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Why the documented alternatives do not help
--client-id / --client-secret pre-registration: Calendly's MCP docs say DCR is required and portal-issued client IDs are not supported for the MCP endpoint; the MCP scopes are attached server-side to DCR-created clients only. A portal app with the https://localhost/ redirect was tried here and fails downstream with an invalid-scope error, consistent with the docs.

There is no --client-name flag or client-metadata override in claude mcp add, and none in settings, so the value cannot be changed by the user.

The claude.ai custom-connector path works for Calendly, but for Desktop Code-tab users it is exposed to the schema-stripping-on-resume bug in #91589, which is why we moved every other connector to local user-scope HTTP servers. Calendly is the one server that cannot follow.

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Notes
2.1.271's changelog entry about MCP OAuth client-registration handling (consent denial, redirect-URI reuse, concurrent writes) is about registration caching and does not change the client_name value; confirmed still failing on that build. Happy to test a build; the reproduction takes under a minute.