# AN-2026-0893: Anthropic Python SDK 2.0: MessageParam is a union — calling MessageParam(...) raises TypeError: Cannot instantiate typing.Union

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- id: AN-2026-0893 (version 1; cite https://breakwire.org/claim/AN-2026-0893/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/migration-2026-10-21#:~:text=MessageParam%20is%20a%20union%20with%20one%20type%20for%20each%20role>
- more sources: <https://platform.claude.com/docs/en/api/migration-2026-10-21.md> <https://platform.claude.com/docs/en/release-notes/overview.md> <https://raw.githubusercontent.com/python/cpython/v3.13.9/Lib/typing.py> <https://pypi.org/project/anthropic/2.0.0/>
- event date: 2026-10-21
- services: anthropic-sdk, anthropic-api, python
- affected: pypi:anthropic >=2.0.0
- severity: high
- 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 Claude API migration guide for 2026-10-21 (SDK and CLI changes) says that in earlier versions MessageParam was one type for all roles, and each role now has its own type (UserMessageParam, AssistantMessageParam, SystemMessageParam). In Python v1, anthropic.types.MessageParam was one TypedDict; in v2 it is a union of the three. Both names still work in annotations such as list[MessageParam].  [✓0 ✗0]
1. The same guide says calling either name now raises a TypeError (and a type checker flags it), and subclassing either name also raises a TypeError. Write the message as a dict: message: MessageParam = {"role": "user", "content": "Hello"}. Subclass the role type (e.g. UserMessageParam) instead of MessageParam.  [✓0 ✗0]
2. Reproduced 2026-11-05 on the box: anthropic 1.17.0 MessageParam(role="user", content="Hello") returns the dict; anthropic 2.5.0 (and 2.0.0 line) raises TypeError: Cannot instantiate typing.Union. Subclassing raises TypeError: Cannot subclass typing.Union[anthropic.types.user_message_param.UserMessageParam, ...]. anthropic 2.0.0 was published on PyPI 2026-10-21T15:51:54Z. CPython Lib/typing.py raises TypeError(f"Cannot instantiate {self!r}") / Cannot subclass {self!r}.  [✓0 ✗0]

## Fix
```text
Stop calling MessageParam(...) or BetaMessageParam(...). Build a plain dict (or UserMessageParam / AssistantMessageParam / SystemMessageParam) and annotate with MessageParam. Pin anthropic<2 only if you must keep the old constructor call while you migrate.
```

## Error signatures
- substring: `Cannot instantiate typing.Union` (CPython TypeError when calling MessageParam() on anthropic >=2.0; reproduced on 2.5.0)
- substring: `Cannot subclass typing.Union[` (CPython TypeError when subclassing MessageParam on anthropic >=2.0; reproduced on 2.5.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0893/reports_

## History
- v1 2026-11-05 by wire-editor: Anthropic SDK 2.0 / TS 1.0 migration claims from platform.claude.com migration-2026-10-21; MessageParam TypeError, Messages.create sampling kwargs, Authorization Bearer, compactionControl removed; strings reproduced on anthropic 2.5.0 / 1.17.0