Epidemic Sound MCP connector: OAuth fails with "Client not found" — client_id set to user email instead of registered app ID

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?

When connecting the Epidemic Sound MCP connector via claude.ai Connectors settings, the OAuth flow fails with:

"Da ist leider etwas schiefgelaufen. Client nicht gefunden." ("Something went wrong. Client not found.")

Root cause observed in the OAuth redirect URL:
The client_id parameter is set to the user's personal email address (e.g. user@example.com) instead of a registered OAuth application ID.

Example URL:

https://login.epidemicsound.com/auth/realms/accounts/protocol/openid-connect/auth
  ?response_type=code
  &client_id=<user-email>
  &redirect_uri=https://claude.ai/api/mcp/auth_callback
  &scope=email+openid+offline_access
  &resource=https://www.epidemicsound.com/a/mcp-service/mcp

Epidemic Sound's auth server correctly rejects this request because no OAuth client is registered under an email address — client_id must be the application ID that Epidemic Sound issued to Anthropic.

Steps to reproduce:

1. Go to claude.ai → Settings → Connectors
2. Find "Epidemic Sound" and click Connect
3. OAuth redirect fires to login.epidemicsound.com with client_id = user's email
4. Epidemic Sound returns "Client not found"

Environment: Claude desktop app (Code tab), MCP connector URL: https://www.epidemicsound.com/a/mcp-service/mcp, Date: 2026-09-25

What Should Happen?

The OAuth flow should complete successfully. The client_id parameter in the authorization URL should be the registered OAuth application ID that Epidemic Sound assigned to Anthropic — not the end user's email address.

Error Messages/Logs

OAuth redirect URL (sensitive values redacted):
https://login.epidemicsound.com/auth/realms/accounts/protocol/openid-connect/auth?response_type=code&client_id=<USER-EMAIL>&redirect_uri=https%3A%2F%2Fclaude.ai%2Fapi%2Fmcp%2Fauth_callback&code_challenge=REDACTED&code_challenge_method=S256&state=REDACTED&scope=email+openid+offline_access&resource=https%3A%2F%2Fwww.epidemicsound.com%2Fa%2Fmcp-service%2Fmcp&prompt=consent

Epidemic Sound error page: "Da ist leider etwas schiefgelaufen. Client nicht gefunden." (Client not found.)

Steps to Reproduce

1. Go to claude.ai → Settings → Connectors
2. Search for "Epidemic Sound" and click Connect
3. The OAuth flow redirects to login.epidemicsound.com
4. Epidemic Sound returns "Client not found" error

The client_id in the redirect URL is set to the user's email address instead of a registered OAuth application ID.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

N/A – bug is in claude.ai connector infrastructure, not Claude Code CLI

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_