# AN-2026-0844: anthropic Python SDK 2.5.0 (Nov 4, 2026): OverloadedError and ServiceUnavailableError now subclass InternalServerError; 'except InternalServerError' placed first now also catches 529/503

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- id: AN-2026-0844 (version 1; cite https://breakwire.org/claim/AN-2026-0844/v/1)
- kind: breaking · status: active · review: editor
- source: <https://github.com/anthropics/anthropic-sdk-python/releases/tag/v2.5.0>
- more sources: <https://github.com/anthropics/anthropic-sdk-python/blob/v2.5.0/src/anthropic/_exceptions.py>
- event date: 2026-11-04
- services: anthropic-sdk, anthropic-api
- affected: pypi:anthropic >=2.5.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The anthropic-sdk-python v2.5.0 release notes (published 2026-11-04) list under Bug Fixes: 'client: make OverloadedError and ServiceUnavailableError subclasses of InternalServerError'. The same release adds error classes for HTTP statuses and API error codes, and error_code, details and api_message on APIStatusError.  [✓0 ✗0]
1. In anthropic 2.4.0 src/anthropic/_exceptions.py, OverloadedError and ServiceUnavailableError subclass APIStatusError directly; in 2.5.0 both subclass InternalServerError (checked 2026-11-04 by installing both wheels).  [✓0 ✗0]
2. Consequence: Python matches except clauses in order, so code with 'except anthropic.InternalServerError' before 'except anthropic.OverloadedError' / 'ServiceUnavailableError' now routes overloaded (529) and unavailable (503) errors to the InternalServerError branch, and isinstance(e, InternalServerError) is now True for them.  [✓0 ✗0]

## Fix
```text
Put except anthropic.OverloadedError and anthropic.ServiceUnavailableError before except anthropic.InternalServerError (or branch on e.status_code / the new e.error_code), and re-check retry/backoff logic that treated InternalServerError as non-retryable.
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0844/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28