`AnthropicModel` should auto-enable Files API beta for `UploadedFile` requests

[Zoe14]

Description

AnthropicModel already serializes UploadedFile(provider_name='anthropic', ...) into Messages API content blocks with source={"type": "file", "file_id": ...} for both user prompts and tool returns. However, those request bodies require Anthropic's Files API beta header (files-api-2025-04-14), and the model adapter currently only sends that beta when users pass it manually through anthropic_betas or extra_headers.

This makes uploaded-file requests fail unless users know to add the beta themselves. The docs currently show the manual header workaround, but the adapter could infer it from the presence of Anthropic UploadedFile parts.

Reproduction

On current main (837b03e3 when checked):

• pydantic_ai_slim/pydantic_ai/models/anthropic.py maps Anthropic UploadedFile to BetaFileImageSourceParam(..., type='file') / BetaFileDocumentSourceParam(..., type='file').
• _messages_create() and _messages_count_tokens() merge settings betas, header betas, native-tool betas, task-budget/compaction/fast-mode betas, but do not add files-api-2025-04-14 for uploaded files.

Expected behavior

When the normalized messages contain an Anthropic UploadedFile, AnthropicModel should add files-api-2025-04-14 to the request betas for both message creation and token counting.

This should be gated on actual Anthropic UploadedFile usage, not added unconditionally, to avoid leaking Anthropic beta headers to compatible/proxy providers that do not accept them.

[pydanty]

pydanty status

Classification: bug

Why this matters: Users sending Anthropic-native uploaded files currently get request errors unless they manually add the Files API beta header (files-api-2025-04-14) via anthropic_betas or extra_headers. Auto-detecting the beta when the normalized messages actually contain Anthropic UploadedFile parts would make file uploads work out of the box and avoids leaking provider-specific headers to proxies/compatibility providers that do not accept them.

Evidence summary:

• code_librarian confirmed pydantic_ai_slim/pydantic_ai/models/anthropic.py already serializes UploadedFile to BetaFileImageSourceParam / BetaFileDocumentSourceParam in user prompts and tool results.
• code_librarian reported _messages_create() and _messages_count_tokens() build betas from settings/headers/profile/native-tool defaults but do not inject files-api-2025-04-14 for uploaded files.
• explorer verified from provider docs that Anthropic's Messages API still requires the files-api-2025-04-14 beta for file-source content blocks.
• Reproducer attempts were blocked by delegate runtime failures, but the issue's claims are specific and consistent with code_librarian's high-confidence findings.
• repositorian found relevant prior work:
  • PR feat: Add files support to CodeExecutionTool for Anthropic and OpenAI #4338 (still open, CodeExecutionTool file uploads) already planned to "Automatically enable files-api-2025-04-14 beta when file_ids are used".
  • There is an existing open PR Auto-enable Anthropic Files API beta when messages contain UploadedFile #5815 directly addressing AnthropicModel should auto-enable Files API beta for UploadedFile requests #5806, which adds helper _messages_use_anthropic_uploaded_file and conditionally appends the Files API beta in pydantic_ai_slim/pydantic_ai/models/anthropic.py, plus tests.

Playground URL: None.

Next action: routing to maintainer discussion because the precise implementation is being handled by an open contributor PR (#5815), and pydanty cannot verify current review status or finish the PR-ready handoff without a clean code diff and test surface. The team should review PR #5815 against issue #5806.