[Anish Mehta (anishmehta24)]

Motivation & Context

AnthropicClient._prepare_response_format set additionalProperties = False directly on the schema dict it was given. That dict belongs to the caller, so after one request their response_format was changed: a schema that allowed additional properties now forbade them for every later request that reused it, including with other providers.

Description & Review Guide

• What are the major changes? Build a copy with {**schema, "additionalProperties": False} instead of assigning into the caller's dict. The pydantic-model branch already works on a freshly generated schema, so it's unchanged.
• What is the impact of these changes? Same request payload as before; the caller's response_format is no longer modified.
• What do you want reviewers to focus on? Nothing beyond that one line.

Related Issue

No existing issue.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible

Test: test_prepare_response_format_does_not_modify_caller_schema in tests/test_anthropic_client.py; it fails on main. The anthropic tests pass and the repo's pre-commit checks are clean.

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