Claude Desktop Enterprise - Snowflake MCP Connector - MCP Client Invalid Error

### MCP server URL

Snowflake

### Where does the issue occur?

When auth finishes and redirects back to Claude.ai

### Transport used

stdio

### Client registration type

My server does not have auth

### SSE only: Does your server return the expected status code for POST requests?

Not sure

### When did you last reproduce this?

2026-05-05 8:21am PT

### Browser and OS

Chrome

### Describe the issue

Title: mcp_client_invalid error on Snowflake OAuth connector — Claude never reaches Snowflake token endpoint
Body:
Environment

Claude Desktop v1.5354.0.0 (Windows Store install)
Snowflake OAuth Custom security integration
Reproduced on both Claude Desktop and claude.ai web

Problem
After completing the OAuth authorization flow, the connector fails with:
oauth_error=invalid_client&error_code=mcp_client_invalid
What I verified on the Snowflake side

- Security integration is enabled with correct OAUTH_REDIRECT_URI (https://claude.ai/api/mcp/auth_callback)
- Client ID and secret are correct and freshly rotated
- Snowflake's token endpoint accepts the credentials when tested directly via PowerShell
- Snowflake login history shows zero inbound attempts from Anthropic's IPs during the OAuth flow
- Issue persists after completely removing and re-adding the connector
- Issue persists after recreating the security integration with a new name
- Reproduced across both Claude Desktop and claude.ai web

Key finding
Snowflake's login history shows no token exchange attempts from Anthropic's IPs at all. This means the failure is happening inside Claude's OAuth flow before it ever reaches Snowflake's token endpoint. The credentials are not being sent — or are being sent incorrectly — by Anthropic's servers.
Reference IDs for Anthropic to trace

ofid_9157800ff38c47e2
ofid_3e37f507b8569471

### Issue details

```shell
Problem
After completing the OAuth authorization flow, the connector fails with:
oauth_error=invalid_client&error_code=mcp_client_invalid
What I verified on the Snowflake side

- Security integration is enabled with correct OAUTH_REDIRECT_URI (https://claude.ai/api/mcp/auth_callback)
- Client ID and secret are correct and freshly rotated
- Snowflake's token endpoint accepts the credentials when tested directly via PowerShell
- Snowflake login history shows zero inbound attempts from Anthropic's IPs during the OAuth flow
- Issue persists after completely removing and re-adding the connector
- Issue persists after recreating the security integration with a new name
- Reproduced across both Claude Desktop and claude.ai web

Key finding
Snowflake's login history shows no token exchange attempts from Anthropic's IPs at all. This means the failure is happening inside Claude's OAuth flow before it ever reaches Snowflake's token endpoint. The credentials are not being sent — or are being sent incorrectly — by Anthropic's servers.
Reference IDs for Anthropic to trace

ofid_9157800ff38c47e2
ofid_3e37f507b8569471
```

### Expected behavior

Complete the connection to Snowflake MCP

### Logs from your server

```shell
ofid_9157800ff38c47e2
ofid_3e37f507b8569471
```

### Additional context

_No response_