Claude.ai custom connector fails to connect to Splunk MCP Server using OAuth static credentials

### MCP server URL

https://splunk-mcp.stg.splunkcloud.com:8089/services/mcp

### Where does the issue occur?

When pressing the Connect button

### Transport used

streamable-http

### Client registration type

Static

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

Not applicable (my server does not use SSE)

### When did you last reproduce this?

15 July 2026 5:50 PM IST

### Browser and OS

Chrome 149 on MacOS 26.5

### Describe the issue

Claude.ai custom connector fails to connect to the Splunk MCP Server using static client credentials for OAuth connection.
We are able to connect to the Splunk MCP Server using the same OAuth server using other MCP clients such as Claude Code CLI and Cursor.

I can confirm that the Splunk MCP Server is OAuth spec compliant

1. Unauthenticated request to the MCP endpoint returns 401 Unauthorized error along with WWW-Authenticate header.
```
curl -i -s -X POST 'https://splunk-mcp.stg.splunkcloud.com:8089/services/mcp' \
-H 'Content-Type: application/json' \
-d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' | grep -iE 'HTTP/|www-authenticate'
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer resource_metadata="https://splunk-mcp.stg.splunkcloud.com:8089/services/.well-known/oauth-protected-resource"
```

2. PRM endpoint contains resource and authorization_servers fields.
```
curl -sk https://splunk-mcp.stg.splunkcloud.com:8089/services/.well-known/oauth-protected-resource | jq
{
"resource": "https://splunk-mcp.stg.splunkcloud.com:8089/services/mcp",
"authorization_servers": [
"https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155"
]
}
```

3. Authorization server details are present at the following endpoint
```
curl -sk https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/.well-known/openid-configuration | jq
{
"authorization_endpoint": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/oauth2/authorize",
"device_authorization_endpoint": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/oauth2/device/authorize",
"end_session_endpoint": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/oauth2/logout",
"grant_types_supported": [
"authorization_code",
"urn:ietf:params:oauth:grant-type:device_code",
"refresh_token"
],
"id_token_signing_alg_values_supported": [
"RS256"
],
"issuer": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155",
"jwks_uri": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/.well-known/jwks.json",
"registration_endpoint": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/oauth2/clients",
"response_types_supported": [
"code"
],
"scopes_supported": [
"openid",
"email",
"groups",
"offline_access",
"fedtype"
],
"subject_types_supported": [
"public"
],
"token_endpoint": "https://splunk-mcp.api.staging.scs.splunk.com/splunk-mcp/sis/v1/rg/rg2cfcb08b070f21ef69fdc992654e2155/oauth2/token",
"token_endpoint_auth_methods_supported": [
"private_key_jwt",
"client_secret_basic",
"client_secret_post"
]
}
```

### Issue details

Connection to the Splunk MCP Server using OAuth static credentials is failing with following error during connection phase.
```
Couldn't reach the MCP server. You can check the server URL and verify the server is running. If this persists, share this reference with support: "ofid_2ad90411b999a302"
```

Tracking the requests made during the connection phase, we can see that the connectivity is failing with **start_error**.
```
https://claude.ai/new?&server=f0f4e49a-cb3d-41fc-beff-038e9a4c7e48&step=start_error&flow_id=ofid_2ad90411b999a302
```

Flow IDs
- ofid_b05f38ab6830182a
- ofid_df387f26fb40153f
- ofid_7dedf22d214088f6

### Expected behavior

Claude.ai custom connector should be able to connect to the Splunk MCP Server using OAuth static client credentials.