[Auth/Connect]: Claude custom connector can't connect to Salesforce Sandbox.

### MCP client

Claude (Web)

### If "Other", which client?

_No response_

### Client version

_No response_

### Authentication flow attempted

Authorization Code (browser)

### OAuth scopes granted

api sfap_api refresh_token einstein_gpt_api

### Org Type

Sandbox

### Network environment

- [ ] Corporate proxy or egress filtering may apply
- [ ] VPN in use
- [ ] mTLS or custom CA certs involved
- [ ] None of the above

### Endpoint configuration

```text
https://api.salesforce.com/platform/mcp/v1-beta.2/sandbox/platform/sobject-all
https://api.salesforce.com/platform/mcp/v1-beta.2/sobjects
```

### Token acquisition steps

```shell

```

### Error messages

```shell
https://claude.ai/settings/connectors?&server=9de4c397-c7ef-4cbd-8dfa-481c22d703bf&step=end_error&oauth_error=unsupported_grant_type&oauth_error_description=grant%20type%20not%20supported
```

### I have already checked the following common causes

- [x] Incorrect audience / resource / scope parameters
- [x] Using a Connected App instead of External Client App
- [x] Token issued for wrong environment (e.g., login vs test domain)
- [x] Proxy stripping auth headers
- [x] mTLS handshake failures (cert chain/hostnames)
- [x] Mis-typed server URL or path

### What I’ve tried

After initially getting this working in a developer environment, I then tried to get the Claude AI working with a Salesforce sandbox environment. I was able to get it working with a local client but using the remote salesforce mcp server connection didn't work. Tried changing scopes, changing networks, used different users, using different solutions proposed by Claude but nothing on the server side has worked.
Tried different endpoints but it made no difference.
https://api.salesforce.com/platform/mcp/v1-beta.2/sandbox/platform/sobject-all
https://api.salesforce.com/platform/mcp/v1-beta.2/sobjects

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