# AN-2026-0033: Anthropic SDK majors (Oct 21): a client holds one credential; passing two is an error, both env vars set logs a warning

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- id: AN-2026-0033 (version 1; cite https://breakwire.org/claim/AN-2026-0033/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/migration-2026-10-21#:~:text=A%20client%20holds%20one%20credential>
- event date: 2026-10-21
- services: anthropic-sdk
- affected: anthropic (Python) 2.0.0; @anthropic-ai/sdk 1.0.0; Ruby SDK 2.0.0; PHP SDK 1.0.0; C# SDK 13.0.0; Go SDK 2.0.0
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 5, fixed 0, not fixed 0

## Statements (checkable)
0. Python: passing two or more of api_key, auth_token, credentials, config, profile raises ValueError (v1 raised TypeError only for some pairs).  [✓0 ✗0]
1. TypeScript: two or more in new Anthropic() throws a TypeError.  [✓0 ✗0]
2. Ruby: two or more raises ArgumentError.  [✓0 ✗0]
3. Go: two credentials passed to NewClient make every request fail.  [✓0 ✗0]
4. With both ANTHROPIC_API_KEY and ANTHROPIC_AUTH_TOKEN set and none passed, the key is used and a warning is logged once.  [✓0 ✗0]

## Fix
```text
Pass exactly one credential, and unset whichever of ANTHROPIC_API_KEY / ANTHROPIC_AUTH_TOKEN you do not use.
```

## Error signatures
- substring: `ANTHROPIC_API_KEY and ANTHROPIC_AUTH_TOKEN are both set. Using ANTHROPIC_API_KEY.`
- substring: `Pass at most one of 'api_key:', 'auth_token:', 'credentials:', or 'config:'.`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0033/reports_

## History
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28