fix(anthropic): use native structured output for Sonnet 5

[le-shi]

TLDR

Problem this solves:

• Sonnet 5 structured responses can leak tool envelopes

How it solves it:

• Route Sonnet 5 through Anthropic native output_format

Relevant issues

Fixes #35677

Linear ticket

Pre-Submission checklist

• I have added meaningful tests
• My PR passes all CI/CD checks (e.g., lint, format, unit tests)
• My PR's scope is as isolated as possible; it only solves 1 specific problem
• I have received a Greptile Confidence Score of at least 4/5 before requesting a maintainer review (Greptile reviews automatically once the PR is opened; only comment @greptileai to re-request a review after pushing changes)

Delays in PR merge?

If you're seeing a delay in your PR being merged, ping the LiteLLM Team on Slack (#pr-review)

Screenshots / Proof of Fix

Not available in this environment because Anthropic credentials are not configured

Type

Bug Fix

Changes

Adds supports_native_structured_output to the direct Anthropic claude-sonnet-5 entry in both model maps and adds a regression test covering Responses structured output routing

Final Attestation

• The tests check the right things, including the edge cases, and regressions in the respective real-world customer use-cases are not possible after this PR

[greptile-apps]

Greptile Summary

Adds native structured-output capability metadata for the direct Anthropic claude-sonnet-5 model so JSON-schema requests use Anthropic's output_format instead of the tool workaround.

• Keeps the primary and bundled backup model maps synchronized.
• Adds a focused regression test asserting native output-format routing without generated tools or tool choice.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The synchronized capability metadata is consumed by the existing Anthropic structured-output mapping, and the focused regression test verifies that the exact Sonnet 5 model selects native output formatting rather than the tool workaround.

Important Files Changed

Filename	Overview
model_prices_and_context_window.json	Marks the direct Anthropic Sonnet 5 entry as supporting native structured output.
litellm/model_prices_and_context_window_backup.json	Mirrors the Sonnet 5 capability flag in the bundled backup model map.
tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py	Adds a regression test confirming Sonnet 5 maps JSON-schema response formats to native output format without tool fallback.

_{Reviews (1): Last reviewed commit: "fix(anthropic): use native structured ou..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[codspeed]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing le-shi:fix-anthropic-sonnet-5-structured-output (26f1a3b) with litellm_internal_staging (b66d4e6)}