[BUG] MCP OAuth: resource parameter is double-encoded when the server URL has a query string (Supabase returns 422)

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?

What happens

Adding a remote HTTP MCP server whose URL contains a query string makes the OAuth
authorize request fail. The resource parameter is encoded wrongly: the ? is
encoded twice and the & between query parameters is not encoded at all.

Supabase's authorization server responds with:
422 {"message": "resource: Resource must be a valid MCP endpoint"}

The same server URL authenticates fine with VS Code's built-in MCP client.

Config (.mcp.json)

{
"mcpServers": {
"supabase": {
"type": "http",
"url": "https://mcp.supabase.com/mcp?project_ref=<ref>&features=docs%2Caccount%2Cdatabase%2Cdebugging%2Cdevelopment%2Cfunctions%2Cbranching"
}
}
}

Steps to reproduce

1. Add the config above (any Supabase project ref).
2. Run /mcp, select the server, choose Authenticate.
3. The browser opens the authorize URL and Supabase returns 422.

Authorize URL Claude Code generates (session values redacted)

https://api.supabase.com/v1/oauth/authorize?response_type=code&client_id=<redacted>&...
&resource=https://mcp.supabase.com/mcp%253Fproject_ref=<ref>&features=docs%252Caccount%252C...

• %253F is ? double-encoded, so the server decodes it to

https://mcp.supabase.com/mcp%3Fproject_ref=..., which is not a valid endpoint.

• &features=... is not encoded, so it is split off as a separate top-level

parameter of the authorize URL instead of staying inside resource.

Expected

resource is the server URL encoded exactly once, e.g.
resource=https%3A%2F%2Fmcp.supabase.com%2Fmcp%3Fproject_ref%3D<ref>%26features%3D...
(or the canonical resource from the server's protected-resource metadata).

Also observed

After failed attempts, re-authenticating the same server name returned
{"message": "Unrecognized client_id"}, which suggests a stale dynamic client
registration is reused. Renaming the server works around that part.

Environment

• Claude Code VS Code extension, version: <fill in>
• macOS 26 (Darwin 25.6.0)
• MCP server: https://mcp.supabase.com/mcp (hosted Supabase MCP)

Workaround

Use a Supabase personal access token in headers.Authorization instead of OAuth.

What Should Happen?

Claude should not deform the URL, the URL works fine with VS codes native mcp.

Error Messages/Logs

Steps to Reproduce

What happens

Adding a remote HTTP MCP server whose URL contains a query string makes the OAuth
authorize request fail. The resource parameter is encoded wrongly: the ? is
encoded twice and the & between query parameters is not encoded at all.

Supabase's authorization server responds with:
422 {"message": "resource: Resource must be a valid MCP endpoint"}

The same server URL authenticates fine with VS Code's built-in MCP client.

Config (.mcp.json)

{
"mcpServers": {
"supabase": {
"type": "http",
"url": "https://mcp.supabase.com/mcp?project_ref=<ref>&features=docs%2Caccount%2Cdatabase%2Cdebugging%2Cdevelopment%2Cfunctions%2Cbranching"
}
}
}

Steps to reproduce

1. Add the config above (any Supabase project ref).
2. Run /mcp, select the server, choose Authenticate.
3. The browser opens the authorize URL and Supabase returns 422.

Authorize URL Claude Code generates (session values redacted)

https://api.supabase.com/v1/oauth/authorize?response_type=code&client_id=<redacted>&...
&resource=https://mcp.supabase.com/mcp%253Fproject_ref=<ref>&features=docs%252Caccount%252C...

• %253F is ? double-encoded, so the server decodes it to

https://mcp.supabase.com/mcp%3Fproject_ref=..., which is not a valid endpoint.

• &features=... is not encoded, so it is split off as a separate top-level

parameter of the authorize URL instead of staying inside resource.

Expected

resource is the server URL encoded exactly once, e.g.
resource=https%3A%2F%2Fmcp.supabase.com%2Fmcp%3Fproject_ref%3D<ref>%26features%3D...
(or the canonical resource from the server's protected-resource metadata).

Also observed

After failed attempts, re-authenticating the same server name returned
{"message": "Unrecognized client_id"}, which suggests a stale dynamic client
registration is reused. Renaming the server works around that part.

Environment

• Claude Code VS Code extension, version: <fill in>
• macOS 26 (Darwin 25.6.0)
• MCP server: https://mcp.supabase.com/mcp (hosted Supabase MCP)

Workaround

Use a Supabase personal access token in headers.Authorization instead of OAuth.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.296

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

.